View single post by Esprit2
 Posted: 11-12-2016 04:16 am
PM Quote Reply Full Topic
Esprit2

 

Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 289
Status: 
Offline
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 01-09-2017 12:09 am by Esprit2