View single post by Esprit2
 Posted: 01-26-2015 03:48 am
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Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 517
Barthol wrote:To match the single carb barrels i have to open the air correction screw on the stronger barrel 5-6 turns , and this only brings me close , not completely on the same level.I suspect the throttle shafts are twisted. If you have ever put a wrench on the hex on one end, while torquing the hex on the other end (never do that), then the shaft is almost certainly twisted. It's only brass, and about half of it is cut away to create a flat where each butterfly is mounted. It takes very little effort to twist it.

If the throttle shaft is twisted, then one butterfly is open more than the other. In that case, cranking several turns into the air bleed screw is NOT a fix. When all is right, it should only be necessary to barely crack the air bleed screw open on the throat with the stronger vacuum. Half a turn of the screw is a lot.

Remove the progression port covers, and look down through the small, drilled progression holes into the throat. Operate the throttle, and you will be able to see the edge of the butterfly sweep past. Adjust the idle speed screw such that one of the butterlies aligns with the edge of one of the progression holes. Does the other butterfly align the same way with the same progression hole in the other throat? If not, that's a crude, eyeball indication that the shaft is twisted. Fix it, don't try to 'adjust' around it.

The book fix for a twisted throttle shaft is to replace it. However, at this point, you have little to loose by carefully twisting it back into balance. Failing that, replace it.
Barthol wrote:i balanced the carbs by first adjusting the air screws to get highest vacum readings on each single cylinder .

Then I matched the two carbs by measuring the weakest barrel on both front and rear carb, by adjusting the screw on the connection link

At the end I balanced the two barrels on the single carbs by opening the airfield screw on the strongest barrel until it matched the weakest barrel.(Snip)...
Change the order of your steps. Balance each carb individually before attempting to balance one carb to the other. It may be necessary to do an initial coarse adjustment at the coupler screw just to get the engine to run. But when you really start to dial things in accurately, do each carb individually first, then balance the two carbs at the coupler.

1) set the idle speed as slow as possible consistent with a smooth idle.

2) Set the Idle Mixture Screws for the strongest vacuum at slow idle.

3) Balance the two throats of each single carb. Start with all Air Bleed Screws seated closed, then crack open the screw on the throat with the strongest vacuum, weakening it to match the weaker throat. Half a turn is a lot, and a full turn indicates something is not right. Five-six turns...something is really wrong. There could be a significant airleak somewhere (soft mounts?), or the throttle shaft could be twisted.

4) Balance one carb to the other, using the linkage coupler balance screw.

5) Reset the Idle Mixture Screws for strongest vacuum.

6) Reset the Idle rpm for the slowest possible speed consistent with smooth running.

Repeat as required until no further change is noticed due to a subsequent round. When you're satisfied, then reset the idle rpm to normal idle.
Barthol wrote:I idles at 900 rpm but wit a little spitting from one barrel on the rear carb and on one beryl on the front carb.
There will also be an occasional explosion in the exhaust.(Snip)...
While balancing the carbs and setting the Idle Mixture Screws, use the slowest idle speed possible, consistent with smooth running.

For normal idle, the book suggests 900-1000 rpm, but I prefer 1000-1100 rpm.

The spitting and occassional backfire sounds like one barrel per carb is running lean, or is throttled down too low... lower than it's partner. That would indicate both butterflies are not open equally, which indicates the throttle shaft is twisted.
Barthol wrote:If I twist the throttle fast it seems to hesitate.
if I twist the throttle less fast it picks up without problems.

I have not driven the car yet, and I expect the Orings between carbs and manifold to seal ( they are new )

My question is do I have the right jetting , or is it way off?
My mixture screws seems to have most influence from closed to 2 turns out (Snip)...
If you open the throttle quickly and it hesitates, that could indicate the accelerator pump is not adjusted properly. However, I think there's more going on that needs to be sorted out before you address that. Save the accelerator pump until last.

If the engine hesitates just off idle when the throttle is opened slowly, then the idle air corrector jets (ie, idle jet 'holders') are probably too lean. The 7850-1 you're using are midrange, and generic-normal, but the Lotus engines are usually when a little richer. I use the 7850-7.

You can stretch the air corrector's range a ways by going to a richer idle jet than otherwise necessary. But, IMHO, you have an air corrector that's a step or two lean, and an idle jet that is too lean. I'd suspect that the 7950-7 air corrector and a 55-58 idle jet would be better.

After you work out the initial bugs, make a full throttle run in 2nd-3rd gear. If the engine stumbles at around 3200 rpm, then the idle jet is too small. Go richer until the stumble just goes away, but no richer. Probably 55-58.

The soft mount O-rings are a common source of air leaks. Even new O-rings will leak of not installed and adjusted properly. It helps to smear each O-ring with a light film of Hylomar before assembly. The mounting nuts must not be cranked down tight. Instead, use either coil springs (Thackeray washers) or rubber grommets under the nut and washer. Then, there should be about a .030-.040" gap between the carb and the softmount, and between the soft mount and the manifold (ie, a gap on both sides of the soft mount plate).

With the engine running, spray a flammable aerosol (carb cleaner, brake cleaner) all around the O-rings and the intake manifold to head joint. Any increase in idle rpm indicates you've just found an air leak. Fix it before proceeding.

Are the carbs 45Es, or plain 45s? The "E"s (and some other suffix models) are equipped with idle mixture screws that have fine threads and long tapered needles. When the idle jetting is right, and the idle mixture screws are adjusted for strongest vacuum, they will usually end up 3 to 3.75 turns out from seated. The other coarser thread screws will normally end up 2.5 to 3 turns out.

Too few turns out normally indicates the idle jetting is too rich, and the screws are compensating by being turned in too far. But all things considered, I think you have balance and maybe air leak issues that need to be worked out first.

Here's the carb set-ups Lotus used:
Lotus Spec No ........... #5 ......... #9
Engine ...................... 907 ........ 912LC
Compression Ratio ..... 9.5:1 ..... 9.44:1
DHLA Type ................ 45E ........ 45E
Choke ......... (mm) ... 36 .......... 37
Main Jet .................... 160 ........ same
Main Air Corrector ..... 230 ........ same
Main Emulsion Tube ... 7772-8 ... same
Idle Jet ..................... 50 .......... 55
............ Too lean! Try 55-58 ...... 58
Idle Jet Holder .......... 7850-7 .... 7850-9
.............. Emissions lean! Try .... 7850.7
Float Weight .............. 10 gr ...... 8.5 gr
Float Height (mm)... 16.5-17.0 ... 14.5-15.0
Float Needle Valve ..... 170 ........ same
Pump Jet ................... 38V 42H.. 50H
Starter Jet ................. 70 ......... 95
Starter Emulsion Tube, 7482-1.... 7482-3
Idle Speed (rpm) ..... 900-1000... same

Idle Air Corrector Jet Sizes are in random order, not numerical order:
7850.5 .. Leaner, going down the list gets richer.
7850.1 .. Generic Normal ... yours.
7850.3 .. Generic Normal
7850.6 .. .6 & .7 are very similar, almost interchangeable
7850.7 .. "Lotus" Normal
7850.8 .. Richest

Work out any balance and air leak issues before attempting to adjust any jetting.

Good luck,
Tim Engel

Last edited on 01-26-2015 04:38 am by Esprit2