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 Posted: 11-04-2016 04:00 pm
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Bfitz241
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Posted this in my thread, but thought this was a more appropriate location

Took a look at my carburetors yesterday. Dellorto 45s.
I'll list what I found for those interested:
Cold start jet 95 [that seems excessive to me]
Main air corrector 230
Main emulsion tube 7772.7
Main jet 158

Idle jet holder/air corrector 7850.9
Idle Jet 58

Auxiliary venturi 8011.1
Chokes 36mm

Admittedly I'm not a Dellorto guru, but based on the book and the presumed level of engine modification [not much] this setup seems to be all over the place. To me it looks like he was trying for high rpm hi flow high power but at the same time trying to keep the low speed lean for economy.

I'll say this, it sooted up a new set of plugs in about 20 mins idle time and that's with an MSD 6 AL.

Comments appreciated

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 Posted: 11-04-2016 08:56 pm
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NigelK
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Hi Brian

I was trying to post a scan from the Lotus workshop manual, which shows DHLA45 jetting for the various "specs" of Lotus engines from 907 through to 912HC. But I can't because Photobucket isn't working at the moment. In the meantime I'll annotate your original post with the jetting from Lotus 907 engine spec 1, which is more or less contemporaneous with the last of the J-H 907 engines but with a higher CR (9.5:1 vs. 8.0:1), increased cam duration and lift (D cams vs. C cams) and increased port size in the cylinder head (25.5mm vs. 24mm)

Starter jet 70
Main air corrector jet 110
Main emulsion tube 7772-5
Main jet 142
Idle jet holder 7850-1
Idle Jet 55L
Auxiliary venturi (figure not provided)
Chokes 38mm

Hope this helps.

Best wishes,
Nigel

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 Posted: 11-05-2016 12:25 am
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Bfitz241
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If I may, I would like to ask for clarification on one point. As I understand the text of Des Hammil's book, when an emulsion tube is referred to as "richer", it will have either fewer, or smaller, emulsion holes than a tube which is labelled "leaner". Am I correct in that assumption?

Nigel, thanks for your help once again.

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 Posted: 11-05-2016 06:34 am
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NigelK
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Ah, Brian, now there you've got me. I can pass on carburettor info from workshop manuals, but to actually explain it is beyond me. I'm just a geologist... :-)

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 Posted: 11-05-2016 09:30 pm
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subwoofer
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Bfitz241 wrote:
If I may, I would like to ask for clarification on one point. As I understand the text of Des Hammil's book, when an emulsion tube is referred to as "richer", it will have either fewer, or smaller, emulsion holes than a tube which is labelled "leaner". Am I correct in that assumption?


I have Des Hammill's book, and IMNSHO it is a piece of c%&#, its only justification is that it gives a good stripdown and rebuild procedure. Since I am curious by nature, I have tried to piece together some kind of mental image of how the emulsion tube works, but that book was not any help whatsoever. John Connolly at aircooled.net has written a few bits and bobs about the subject spread around the net, the best one was at the Innovate forums - AFAICT those are gone.

So here we go:

The emulsion tube controls the fueling curve from vacuum begins to build in the main venturies and until the vacuum (somewhat proportional to airflow) is high enough to completely drain the emulsion tube well. At that point fueling is controlled by the main jets and air correctors only.

Three main factors control the shape of the curve while the emulsion tube is active;

1. Fuel level in the float chamber, controlled by the float and needle valve. I believe a higher fuel level will richen the mixture across the board, but I am not positive. A change might add or it might multiply fuel flow, I don't know.

2. Placement, size and angle of the holes in the emulsion tube. If my thinking isn't completely off, larger hole area means less fuel and more air, i.e. leaner mixture. Holes high up probably affect the whole curve, low down it will affect the curve only when the well is almost dry (at close to max airflow).

3. Diameter of the emulsion tube. This controls the amount of fuel in the well, but how exactly this translates in tuning is still beyond me.

Most likely, all of these three parameters have interactions with the main and air corrector jets, and possibly even with the idle circuit.

Since I have been spending a lot more time tweaking EFI than trying to figure out carbs, I'm not able to give a better explanation and caveat emptor; this is my perception of it from trying to wrap my head around the concept - I may be mistaken.

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Joachim

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 Posted: 11-06-2016 02:34 pm
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subwoofer
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David Vizard explains how to tweak emulsion tubes: https://www.youtube.com/watch?v=1pkFSA_rRFI

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Joachim

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 Posted: 11-10-2016 01:50 pm
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Bfitz241
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Joachim,
Thanks for the replies. I'm actually a big David Vizard fan and have read several of his books.
I also have an Innovate LM 2 and have done my share of tuning on Holleys and Demons. It's a shame Innovate shut down the forum.

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 Posted: 11-12-2016 03:16 am
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Esprit2
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NigelK wrote:
In the meantime I'll annotate your original post with the jetting from Lotus 907 engine spec 1, which is more or less contemporaneous with the last of the J-H 907 enginesI don't recommend using either Spec 1 or Spec 3 as a template for jetting Dellorto DHLA 45s on a near-stock 2.0 litre 907.

To begin with, there's a philosophical difference between the J-H and Lotus versions of the 907. They targeted different markets. The J-H is aimed more at the gentleman's GT market. That doesn't mean that's where 'you' are, but that was the target. A civilized crowd more interested in a nice drive in the country than balls out, berzerker tearing up some roads. Driveability is generally more of a concern on J-H forum discussions than is peak horsepower and lowered lap times. Not to offend anyone, and I'm sure there are some berzerker J-H owners, but there are such differences present in the engines. And installing 107 cams as an 'upgrade' just leans further in that low-end driveability, civilized direction.

Those factory Jensen-Healeys equipped with Dellortos used 40mm throats with 35mm chokes. Lotus, on the other hand, went straight to 45mm throats and 38m chokes. Smaller throats & chokes make for an engine with a stronger, smoother low end torque curve that is easier to drive in traffic... all at the expense of top end power. Larger throats & chokes help the engine scream right past redline and make lots of top end power, but at the expense of low end tractability. The two goals are in opposition.

Lotus' 1974 "Spec 1" 907 and the Elite were developed by the same blokes who spent years bringing us little over-achiever berzerkers like the Seven, Elite, Elan & Europa. It never occurred to them that the clientelle for their new 'gentleman's express' wouldn't like a car that was so nutzo. The press heavily criticized the Elite for it's lack of driveability, and service centers were inundated by requests to make the car "run better". It was 'great' (!) at full throttle, but it's 'gentlemen' owners never went there.

For 1975, Lotus knee-jerk over-reacted, and issued "Spec 3" with small 35mm chokes (Spec 2 was for Zenith-Strombergs). The car was much easier to drive in traffic, but it's mild manners came at great cost to top end performance. Customers now complained about that not being "Lotus-Like". You just can't please everybody.

Finally, for 1976, Lotus issued "Spec 5" with 36mm chokes, and that was about right for the Lotus market. And that's where I think you should start.

Maybe you're building a track day car, and you want the big chokes offered by Spec 1. But if not, and if you'll be street driving it most of the time, then Spec 5 is the best Lotus Dellorto set-up for a basically stock 2.0 litre 907 (ignoring the fact that Lotus had higher compression, bigger ports, and the other detail differences). Spec 5 is more spirited than the stock J-H set-up and Spec 3, but not as berzerker as Spec 1.

So what about Spec 3, in the middle with the small 35mm chokes. They will give you a very docile engine that is easy (easier) to drive in traffic, but with a very restricted top end... if that's what you want. However, 35mm chokes are really getting to the minimum of works well in 45mm throat Dellorto DHLAs. If you want 35mm chokes, then DHLA 40 carbs would be the better place to start. Which gets you back closer to where J-H started out in the first place.

The following chart shows the jetting for Spec 5 (2.0 907), Spec 9 (2.2 912LC), and Bfitz241's mystery carb... which doesn't look all that far off from Spec 5. The idle jet is large, which might account for the sooty plugs. The main jets are small, which won't help top end.

You didn't mention the float weight or height. Fuel level is key/ foundational to jetting, and for the moment it's an unknown. But presuming it is as noted in Spec 5, or even Spec 9, below...

In my experience, the 7850-7 Idle Jet Holder (aka, Idle Air Corrector) is several steps richer than the 7850.9 in your carb, and in my experience, both the 2.0 and 2.2 liter engines like the -7 way better... ie, Spec 5.

Spec 9 came later, and Euro emissions were more strict by then. So the leaner 7850-9 pumps in more air for an overall leaner condition in the idle circuit (which feeds the engine up to ~3200 rpm), then keeps it all going with a larger Idle Jet. The better solution is to use the richer 7850-7 idle jet holder (less air), with a smaller Idle Jet necessary to balance the mixture. A 50 Idle Jet is still 'emissions-lean', but something in the 52-54 range should work... not 58 (sooty plugs country).

Personally, I'd recommend Spec 5 with a little richer Idle Jet. Start there and see what it does for you.

Engine Model: ........ 2.0 907 ....... 2.2 912LC ..... Bfitz241's
Dellorto Specs: ...... Spec-5, ....... Spec-9 ......... Unknown
Carb Type .............. DHLA 45E .... DHLA 45E ..... DHLA 45 (45, or 45E ?)
Choke ................... 36 mm ........ 37 mm ......... 36 mm
Main Jet ................ 160 ............. 160 .............. 158
Main Air Corrector .. 230 ............. 230 .............. 230
Main Emulsion Tube, 7772-8 ........ 7772-8 ......... 7772.7
Idle Jet .................. 50 .............. 55 Esprit/ ...... 58
................................................. 58 4-seaters
Idle Jet Holder ....... 7850-7 ........ 7850-9 .......... 7850.9
Float Weight ........... 10 gr .......... 8.5 gr ............ ______
Float Height ........... 16.5-17mm.. 14.5-15mm ..... ______
Float Needle Valve .. 170 ............ 170 ................ ______
Pump Jet ............... 38V 42H ...... 50H ............... ______
Starter Jet ............. 70 .............. 95 .................. 95
Starter Emul Tube .. 7482-1 ........ 7482-3 ........... ______

Why the difference in 912 Idle Jets between the Esprit and the 4-seaters? Weight. Or the potential for the 4-seaters to be loaded down with more beef and luggage, making them heavier off the line (ie, more likely to bog down). The Jensen-Healey is a light car and shouldn't need the richer idle mixture.

Dellorto floats are available in several different weights, and the float height is closely linked to the float weight. So while the Spec-9 floats (8.5g) are set higher than the Spec-5 floats (10g), it's not correct to assume they're set richer. "For a given fluid level", a lighter float will naturally ride higher. In marine terms, more "freeboard".

I have 8.5g floats in my carbs. I've played around with the height setting and settled on 14.5mm as working best.

Before you set your floats, be sure you know the float weight (it should be engraved on top of the float). There were 7g, 8.5g and 10.0g floats, but they're not all available any more. If you must change float weight due to availability, then also change to the float height to whatever is correct for that weight. The fuel level in the float bowl is the real goal, and any float weight can get you there if adjusted to the appropriate height. Said another way, each of the three float weights, when set to their individual float heights, produced the same fuel level. The difference wasn't a richer or leaner condition; but rather a matter of responsiveness and stability due to float weight/ mass.

I hope you find something useful in all that bandwidth. Good luck sorting out the carbs.

Regards,
Tim Engel

Last edited on 01-08-2017 11:09 pm by Esprit2

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 Posted: 11-12-2016 02:08 pm
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Bfitz241
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Tim,
In depth and informative.
I especially appreciate the info on the idle air correctors.
I have no interest in beating the snot out this thing...I've got another car for that, tractable, easy to live with are the order of the day.
I haven't weighed the floats...however I do have a scale that can do that, nor have I checked float level...sounds easy enough to do.
I have no idea what was done to the engine outside of it has LOTUS pulleys on the cams and auxiliary drive. My understanding is that is not how Jensens came.
It also has a cast iron manifold. I don't know how well they flow but if you can't get the air out you can't get it in.
I will say this about the engine, it starts easily, idles well and will idle down to 850 rpm. Maybe that can help your analysis.

One thing I would like clarified, you refer to redline...I take that to mean 7,000, am I correct? To be more clear I think you are referring to the same level of rpm for both the Lotus and JH engines
Now, admittedly I haven't driven this thing yet...mostly because it was not capable of being driven. But its mechanical issues are being corrected so that will happen.
I hadn't planned on revving it higher than 6,000.
A related question, what rpm was the horsepower rated?
Thanks for your help
BF

Last edited on 11-12-2016 04:28 pm by Bfitz241

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 Posted: 11-12-2016 03:14 pm
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DonBurns
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All-
This info is very helpful, and I am glad there is an active sting on this topic since figuring out Dellortos is my main mission in life at the moment. I rebuilt my first engine myself in the late 70's (14313) but on my current JH (15800) turned it over to a performance dyno shop for rebuild and setup. I'm not entirely happy with the set up part and have decided to learn about tuning Dellortos on my own. I rebuilt Strombergs a few times, but these are my first Dellortos

My engine is 2.2, 104 / 107 combination, 9.6 compression, Big Valves (West Coast Cylinder Head). The 45 Dellotos are as supplied by the club store. Perhaps Greg could publish all of the carb specs for 2.0 and 2.2 jetting on these? Maybe spec 5 and spec 9 respectively?

I have so far not opened the carbs. As delivered the timing was off spec (way advanced) so I corrected that. I have a Carbtune Pro, and ColorTune, and have been playing with those and have the engine running pretty well, but doesn't feel perfect. Also running hot in spite of new 3-row, largest available oil cooler which could suggest running lean? It seems like air flow, flame color and RPM are not reacting as much to mixture screw adjustments as the Des Hammill book suggests they should. So I am looking at getting an Innovative LM-2 or LC-2 (not sure where I would mount the gauge).

Anyway, I am determined to get this right, and have a question. When the dyno tuner delivered the car, he made a comment that he had to "open up" jets to get it running right. That would be OK I suppose if it made it run right, but the engine as delivered ran like cr%p. So my question is does someone have jet specs for what hole sizes each jet should have? I would like examine the jets and check with pin gauges to see what these yahoos did to me, and correct if necessary.

BTW- I have read through the Hammill book a few times and have learned a lot, but bottom line seems to be that it would be great to have a collection of all of the jets, chokes, venturis, etc to trial and error the fine tune. Who has that? Does anybody know a carb tuning shop on the west coast that can do this?

Thanks-
DAB

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 Posted: 11-12-2016 04:27 pm
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Bfitz241
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I like the LM 2 and Logworks software is a very in depth program to analyze your tuning efforts.
Weld a bung in the pipe and see what it's really doing.

It sounds to me like your dyno operator just set the thing up for max horsepower on the dyno.  That's the easy way.  Part throttle and idle are the hardest to tune.

Did he give you the dyno printout?  If it has the air fuel ratio column you can get an idea what the engine was doing.

Last edited on 11-12-2016 04:32 pm by Bfitz241

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 Posted: 11-12-2016 04:59 pm
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DonBurns
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BF-

Here is dyno. This is out of car so maybe things got messed up during installation. They ended up firing the kid who did the engine installation and final set up. There were many things wrong when they sent me on my way, including forgetting to put oil in my newly rebuilt 4-speed.

Where did you locate the bung? Looks like there is a path just back of the headers.

DB

Attachment: 2016 02 Dyno Test-1.jpg (Downloaded 134 times)

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 Posted: 11-12-2016 05:26 pm
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Bfitz241
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As I was told the O2 sensor needs to be within 12" of the collector. I put mine about 8" back.

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 Posted: 11-12-2016 08:45 pm
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Esprit2
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Bfitz241 wrote:
I haven't weighed the floats...however I do have a scale that can do that, nor have I checked float level...sounds easy enough to do.

I have no idea what was done to the engine outside of it has LOTUS pulleys on the cams and auxiliary drive. My understanding is that is not how Jensens came.
You don't have to weigh the floats, the weight should be engraved on the top side. It's easier to read when the floats are out of the carb.
Bfitz241 wrote:
I will say this about the engine, it starts easily, idles well and will idle down to 850 rpm. Maybe that can help your analysis.Don't set the idle too low, even if the engine runs smoothly at low rpm. A little faster idle makes the engine easier to get off the line, and takes a little pressure off the Idle Air Corrector being dead-nuts perfect. It's nice to have a little lattitude. Lotus specifies 900-1000 rpm, and I prefer 1000-1100 rpm.
Bfitz241 wrote:
One thing I would like clarified, you refer to redline...I take that to mean 7,000, am I correct? To be more clear I think you are referring to the same level of rpm for both the Lotus and JH engines
Now, admittedly I haven't driven this thing yet...mostly because it was not capable of being driven. But its mechanical issues are being corrected so that will happen.
I hadn't planned on revving it higher than 6,000.
A related question, what rpm was the horsepower rated?
The stock redline with the chilled cast iron tappets is 7000 rpm. Install a set of steel tappets before revving higher.

The next limiter is the stock connecting rods, at about 7400-7500 rpm. Clean them up, shot-peen and polish them (and do a really good job of balancing the engine) and they're good for 8000 rpm. The Sunbeam-Lotus rally cars raced the 2.2 liter 911, and made 240 Hp at 9,400 rpm with 'prepared' stock rods. With stock cams, there's little point to exceeding 7000 rpm.

The rpm for peak torque and horsepower varied a bit with compression ratio, carbs and (emissions) tuning. In general terms, peak horsepower is at about 6900 rpm, and peak torque at around 5900 rpm.

The engine likes to rev, and limiting your driving to 6000 rpm will leave a lot of fun un-tapped. It trundles around at normal speeds quite well, but it doesn't really 'perform' unless you're willing to bury your right foot and work the gear lever aggressively.

Regards,
Tim Engel

Last edited on 01-08-2017 11:14 pm by Esprit2

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 Posted: 11-12-2016 08:51 pm
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Esprit2
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BTW... The cast iron tappets don't last forever, and I've been seeing a lot of cracked tappets in the last few years. If a tappet fails while the engine is running, worst case can be very much like breaking the timing belt... bent valves.

The next time the cam carriers come off to clearance the valves, consider installing steel tappets. The price of 16 does add up, but there are aftermarket steel tappets that are not only an upgrade, but less expensive than the factory iron tappets. I know JAE and Kemp High Performance Engines (Garry Kemp) stock them, and I presume Dave Bean and SJ Sports Cars do as well. I've not checked any J-H sources, like Delta... call & ask.

Regards,
Tim Engel

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 Posted: 11-12-2016 10:00 pm
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Esprit2
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DonBurns wrote:
My engine is 2.2, 104 / 107 combination, 9.6 compression, Big Valves (West Coast Cylinder Head). The 45 Dellotos are as supplied by the club store. Perhaps Greg could publish all of the carb specs for 2.0 and 2.2 jetting on these? Maybe spec 5 and spec 9 respectively?I don't know how the Dellortos supplied by the JHPS store are set-up. I'm a Lotus guy and not a JHPS member (I crash the gate on the forum), so I can't get into the website far enough to explore those details. But scroll up a few messages. I posted the jetting set-up for Specs 5 (2.0) & 9 (2.2) in message #8.

Both Lotus Spec 5 & 9 set-ups are influenced by emissions standards to increasing degrees (9 more so than 5), and not as rich as the dyno-tuner might like. Especially the Spec 9 Idle Air Correctors. Instead, use the 7850-7 Idle Air Correctors from Spec 5, then tweak the Idle Jets to suit your engine (see below). The rest of Spec 9 should be about right for a street driven 2.2 that's built-up as you describe. For peak track-day performance (like the dyno-tuner was probably trying to achieve) slightly richer jets might be required; but then you'll be trading emissions and fuel economy for a few more horsepower. How bad do you want it for the way you'll be driving the car?

Nobody can give you a standard, 'canned' jetting combination that will always work. Engines are all unique even if you try to build them the same, and carbs are all unique without the precise repeatability of ECU controlled fuel injection. They're all different... accept that. Start with a carb set-up that's close, like Spec 9, then tune your engine/ carbs/ ignition to be the best it can be as an individual. It's more like a woman than a computer.

DonBurns wrote:
I have so far not opened the carbs. As delivered the timing was off spec (way advanced) so I corrected that. I have a Carbtune Pro, and ColorTune, and have been playing with those and have the engine running pretty well, but doesn't feel perfect. Also running hot in spite of new 3-row, largest available oil cooler which could suggest running lean? It seems like air flow, flame color and RPM are not reacting as much to mixture screw adjustments as the Des Hammill book suggests they should.How much advance are you running. My 2.2 converted 907 with two 104 cams likes 14-16 BTDC for autocross, and 12 BTDC for the street. But then, my centrifugal advance curve is stock 'Federal' Lotus, and pretty tame.

Don't put too much stock in the ColorTune. I have one, tried it, and it's been untouched in the box for years. The instructions are simplisticly short on details, and focus too hard on achieving stoichiometric, blue flame combustion. That's fine for fuel injection controlled by O2 sensors and feedback loops, but most carbs can't support that. With a blue flame, you're toeing the line of burned pistons, and carbs don't have the fine resolution to safely dance that line consistently. Put a little yellow in the flame. Better yet, park the ColorTune, listen to the engine, and tune to where it runs well and makes good power.

DonBurns wrote:
When the dyno tuner delivered the car, he made a comment that he had to "open up" jets to get it running right. That would be OK I suppose if it made it run right, but the engine as delivered ran like cr%p. So my question is does someone have jet specs for what hole sizes each jet should have? I would like examine the jets and check with pin gauges to see what these yahoos did to me, and correct if necessary. I can't help you with dyno-tuners near you. But start with Spec 9 plus 7850-7 Idle Air Correctors. The big valves do require slightly bigger jets, but I don't have specific recommendations for you.

Warm the engine up, and set the idle speed as slow as it will go consistent with smooth running... no stumbling or spitting. Then open the throttle slowly enough that the accelerator pump isn't much of a factor. If it hesitates just off idle, then the Idle Air Correctorr is too lean. If your carbs have the 7850-9 correctors, the engine probably will stumble... switch to 7850.7.

Or, to do it right, if it stumbles, go a step or two richer with the Idle Air Correctors until the stumble just disappears, but no richer. If the engine doesn't stumble, then go a step or two leaner until a hesitation just shows up, then go back a step richer until it just disappears.

Then go for a drive. Idling in the driveway is one thing, but driving in traffic is something else. Just rich enough to kill the hesitation might still be a step or two too lean for the real world.

With the Idle Air Corrector sorted, find a stretch of open road. Accelerate at full throttle in a mid-gear. Not first, since you'll rip through it too quickly. Not fourth/ fifth, since you'll get a ticket. But select a gear, start slow, and accelerate at full throttle to near 4000 rpm. The Idle Circuit hands off to the Main Circuit at 3200 rpm. If the engine stumbles at or a little before 3200 rpm, then the Idle Jet is too lean, and the Idle Circuit is running out of capacity before the mains take over.

Go a step or two richer on the Idle Jet until the stumble just goes away, no more. If the engine doesn't stumble, then go a step of two leaner until a stumble just occurs, then back a step richer until it just disappears again. No richer.

The Idle Jet and Idle Air Corrector affect each other. So once you get one adjusted, you may find the other one is now off. It's an itterative process, and you need to repeat it a few times until the last round results in no further change. Yes, it's putzy work. And yes, a full selection of jets would be very helpful.

Adjusting the main jets and air correctors is best done on a dyno, unless you like getting tickets. But if the engine falls on it's face before reaching redline, the main jets are too small. But Spec 9 is pretty close on the main circuit, so focus your efforts on the idle circuit. Save the mains for the Dyno.

Regards,
Tim Engel

Last edited on 01-08-2017 11:19 pm by Esprit2

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 Posted: 11-14-2016 03:30 am
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Bfitz241
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Don,
2 things I noticed on that dyno sheet...I've read repeatedly these engines are 7000 rpm with max power in the 6500 area.
This guy stopped the pull at 5800, did he say why?
Second is that it says it's pulling 1.3 inhg at 5800...If that is accurate, I'm going to stick my neck out and say your carburetors are too small and are choking it.
As to measuring the jets with pin gauges, I read somewhere that the numbers on the jet represent its size in MM...e.g. a 190 is 1.9mm.

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 Posted: 11-14-2016 06:41 am
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Esprit2
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Bfitz241 wrote:
As to measuring the jets with pin gauges, I read somewhere that the numbers on the jet represent its size in MM...e.g. a 190 is 1.9mm.That's correct.

Regards,
Tim Engel

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 Posted: 11-14-2016 03:35 pm
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DonBurns
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Thanks for all the input. I'm not sure why they only went to 5800. I think they might have been taking it easy on the new engine. This dyno was mainly for set-up - it had just been fired up.

Timing is set to 12 degrees. Saturday I tried something different, just closed all the idle screws, opened to 3.5 turns as suggested in the book, and then opened by quarter turns until it idled smoothly. This turned out to be 4.25 turns. Did not even check for balance. Seemed good at first, but temperature slowly climbed to red, and engine started running worse with some spitting. This is only about 10 miles. Took home and parked for now. I have an LM-2 on order and once I get this set up will try again. Maybe this will lead to enlightenment.

I am assuming or hoping the overheating is tune related, because I don't know what else to do mechanically to improve cooling.

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 Posted: 11-14-2016 07:19 pm
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Esprit2
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Don,

Lean mixture may cause the engine to run a little hot, but it shouldn't cause full blown over-heating. I suspect there's a cooling system issue at work as well. What thermostat is installed?

Balance the carbs first. That's critical, and any tuning on unbalanced carbs is a waste of effort.

Ignoring balance for the sake of conversation...
It sounds like the idle circuit is too lean. When the jetting is correct, tuning the Idle mixture screws for PEAK MANIFOLD VACUUM will result in them being 3.0-3.75 turns out from seated. If they end up out more than that, then the idle circuit is too lean, and you're attempting to (incorrectly) compensate by turning the mixture screws out further. Similarly, if the screws end up further in than that, like 2.0-2.75 turns, then the idle circuit is too rich and you're (incorrectly) compensating by turning the screws in too far.

The Idle Circuit feeds the engine from idle up to 3200 rpm, at which point the Main Circuit takes over. The jets and air correctors contrrol the mixture, and the IDLE Mixture Screws are only for making a small final tweak "AT IDLE". They are not for setting the overall Idle Circuit mixture. Get it right with the jetting first, then see where the IDLE mixture needles end up.

At 4.25 turns out, the tip of the needle is just about fully withdrawn from the orifice. Anything more than 4 turns has gone beyond the effective taper on the needle (ie, you've pulled the needle out of the hole), and will make no additional change. Anything beyond 3 3/4 turns means richer idle jetting is required (Idle Jet and/or Idle Air Corrector). Anything less than 2 - 2 1/2 turns means leaner idle jetting is required.

What Idle Jets and Idle Air Correctors are in ypur carbs now? There's really nothing more to talk about if we don't know where the jetting is now. The jets are all stamped with ID numbers... pull them out and look. A good starting point would be 7850-7 Idle Air Correctors with 52-55 Idle Jets.

But all that is pre-mature. Balance the carbs FIRST, then re-adjust the idle mixture screws for strongest/ peak manifold vacuum. If the mixture screws are still out 4.25 turns, then the idle circuit is too lean, and needs to be re-jetted. I recommend starting with 7850-7 Idle Air Correctors, then seeing where the Idle Jets take you... probably 52-55.

Regards,
Tim Engel

Last edited on 11-14-2016 07:27 pm by Esprit2

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