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post #1 of 17 (permalink) Old 05-13-2017, 04:56 PM Thread Starter
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Having trouble tuning dual Weber setup

Completing a restoration of an 81 Spider and now trying to get the car running well. Car has dual 40DCOEs. I can get the car to idle very smoothly but in order to do so the idle mixture screws have to come out at least 2 to 3 turns which is too much according to my setup documents. According to the tuneup instructions, this indicates the car has a lean idle circuit, and needs different jets. I'm not a carb expert so I'm not sure if that is the proper diagnosis. More info: Carbs are properly in sync according to Uni-syn... Any application of the gas causes the car to stumble as if it is not getting enough fuel. No matter how many little tweaks I do to the idle mixture screws, I cannot get the car to rev without stumbling severely.

FWIW, we just completed a restoration of a 79 with the same dual 40DCOE configuration and I had no trouble getting that car running. This 81 is much trickier for some reason.

Any tips or advice is greatly appreciated, thanks!

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post #2 of 17 (permalink) Old 05-13-2017, 05:45 PM
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What is the full model number of the DCOE's? Most of the information that you are likely to find only refers to models up to 40DCOE32. There are significant differences in later models, particularly in the idle/progression circuits.

Ed Prytherch
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post #3 of 17 (permalink) Old 05-13-2017, 06:40 PM
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In anticipation of what Alfaparticle will advise...

Nothing I read suggested the idle mixture screws on my DCOE 72/73s on my 1975 Alfetta GT 1.8 should be screwed as far out as they were (5 turns, plus or minus), but when I wound them in to what the tuning books said, the car wasn't happy.

On Alfaparticle's advice, I relaxed and left them 'out there', with minor tweaks only after, and left the jets alone.

Also, I assume your accelerator pump jets are supplying a good squirt?
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post #4 of 17 (permalink) Old 05-13-2017, 09:58 PM
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Here's my 'non-expert by a long shot' experience... my 75 GT was a total pita to get into any kind of running order, and after taking care of the electric (tiny fracture in distributor cap drove me mad till I found the pin loose on its base) and the valve clearances (adventures in shim sourcing and gap size guesstimating), came the DCOE 40's :
1. CLEANING OFF GRIME. Old fuel had settled into the tank and the carbs leaving a thick film of calcified grime... generous overnight soaking in carb cleaners is a must for car that has been sitting for long.
2. 6 REVOLUTIONS FROM SHUT. After setting jets and all the internals according to the reco's on this forum and elsewhere, car was still hesitating as you describe, was lucky to have an experienced friend visiting, he basically closed the idle screws completely shut then turned them open individually till he reached what he felt was optimum setting. We counted exactly '6' complete revolutions. Downside is that it kept dying at sub-900 rpm idle so took that upto 1400 rpm.
I have since exchanged the 40's for 45's (the thinking was to better feed high compression pistons and high lift cams and ported intakes, etc...). The engine does breath a little better but the difference is really very marginal compared to well cleaned and tuned 40's... wasted money really.

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post #5 of 17 (permalink) Old 05-14-2017, 05:37 AM
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Quote:
Originally Posted by Ranz View Post
In anticipation of what Alfaparticle will advise...

Nothing I read suggested the idle mixture screws on my DCOE 72/73s on my 1975 Alfetta GT 1.8 should be screwed as far out as they were (5 turns, plus or minus), but when I wound them in to what the tuning books said, the car wasn't happy.

On Alfaparticle's advice, I relaxed and left them 'out there', with minor tweaks only after, and left the jets alone.

Also, I assume your accelerator pump jets are supplying a good squirt?
Yep, 5 - 5.5 turns sounds about right. Usually have to wind them in a turn to get the car through an MOT but the idle goes lumpy so I just wind them out again afterwards....
If it stumbles as soon as you hit the gas it's most likely to be the accelerator pump jets (either blocked or not getting fuel in the first place)

Ian

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post #6 of 17 (permalink) Old 05-14-2017, 05:52 AM
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Quote:
Originally Posted by alfaparticle View Post
What is the full model number of the DCOE's?
The answer to this question can mean all the difference.
Up to 40 DCOE 32, the mixture screws are turned out about 3/4 turn from closed. Carbs 44/45 and up the screws are turned out multiple full turns. One reason for this difference is the thread pitch and taper of the needles.

Jim

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post #7 of 17 (permalink) Old 05-14-2017, 07:05 AM Thread Starter
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Thanks for the responses so far! I was definitely approaching 5 turns or so when I was starting to get results.

Not sure if I am looking in the wrong place, but I'm seeing the number 151 as the type. Does that sound right? They are stamped 40DCOE 151, with a 7F stamped below.

We did disassemble, clean thoroughly, and put new gaskets in before putting them back on the car. Don't know if it would matter at all, but the car came from Canada. Was curious if the elevation there would have been significantly different enough to affect jetting in Florida?

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Last edited by RenaissanceMan; 05-14-2017 at 07:08 AM.
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post #8 of 17 (permalink) Old 05-14-2017, 07:29 AM
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151 series carbs

You should list all internal component ID #s. It is likely that your issues come from incorrect application of jetting. To help any engine mods particularly camm are a tuning factor. Are they stock? If so what are the casting #s.

Alfaparticle is the guru on these carbs. As well doo a BB search on 151 carbs, and carb tuning. No point in asking questions that have been addressed many times.

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post #9 of 17 (permalink) Old 05-14-2017, 10:29 AM
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151's have long taper idle screws and 5 turns out is not unusual.

Ed Prytherch
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A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke
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post #10 of 17 (permalink) Old 06-04-2017, 06:29 PM Thread Starter
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Originally Posted by ianellisalfa View Post
Yep, 5 - 5.5 turns sounds about right. Usually have to wind them in a turn to get the car through an MOT but the idle goes lumpy so I just wind them out again afterwards....
If it stumbles as soon as you hit the gas it's most likely to be the accelerator pump jets (either blocked or not getting fuel in the first place)
Hey folks, just resurrecting this thread now that my father is back on the carb issues after finishing up some other restoration work on the car.

Wanted to point out that as of now, car starts and idles perfectly, carbs are in perfect synchronization according to the UNI-SYN. As soon as you hit the gas, the car stumbles. ianellisalfa, you mention it is most likely the accelerator pump jets in this scenario. All of the jets have been thoroughly cleaned and blown out. What else could cause accelerator pump jets not to be getting fuel in the first place? Is it possible the jets are incorrect?

Thanks-

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post #11 of 17 (permalink) Old 06-04-2017, 10:51 PM
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Quote:
Originally Posted by RenaissanceMan View Post
Hey folks, just resurrecting this thread now that my father is back on the carb issues after finishing up some other restoration work on the car.

Wanted to point out that as of now, car starts and idles perfectly, carbs are in perfect synchronization according to the UNI-SYN. As soon as you hit the gas, the car stumbles. ianellisalfa, you mention it is most likely the accelerator pump jets in this scenario. All of the jets have been thoroughly cleaned and blown out. What else could cause accelerator pump jets not to be getting fuel in the first place? Is it possible the jets are incorrect?

Thanks-
Sometimes the pump mechanism itself sticks & won't pump the fuel to the jet - if you remove the brass cap covering each pump jet & operate the throttle you should see fuel rise up around the jet. If it doesn't, the jet isn't getting fuel. Remove the top cover from the carb & you can see the plunger that operates the pubp - you can pump this up & down quite quickly by hand (don't use too much force if it resists!) & that's usually enough to get fuel pumping through to the jet again. You then need to check that the tiny hole (0.35mm?) at the bottom of the jet isn't restricted - once it is full of fuel if you blow down it you should get a tiny jet of fuel emitting.

Ian

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post #12 of 17 (permalink) Old 06-05-2017, 02:24 PM
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It is possible to spend a lot of time chasing the Webers when the problem is a weak spark. The AFR can be all over the place as the progression holes are uncovered and a strong spark is important to prevent misfiring.

Ed Prytherch
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post #13 of 17 (permalink) Old 06-05-2017, 02:48 PM
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'81 came usa with a variable cam. I take it you've checked valve clearances and cam timing, what about distributor timing? have you tried changing this? Weak spark as noted could be culprit, Engine performance is susceptible to exhaust leaks, both at manifold and downstream.

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post #14 of 17 (permalink) Old 06-05-2017, 03:04 PM Thread Starter
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Originally Posted by rogerspeed View Post
'81 came usa with a variable cam. I take it you've checked valve clearances and cam timing, what about distributor timing? have you tried changing this? Weak spark as noted could be culprit, Engine performance is susceptible to exhaust leaks, both at manifold and downstream.
This car was originally from Canada... Would that have any bearing on tests?

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post #15 of 17 (permalink) Old 06-05-2017, 03:11 PM
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`81 Canadian car

Should have Euro carb setup with 10548-01 cams.

Tell us what the cam casting #s are. If the timing marks on the cam caps are 99.5/99.5 widen the LSA/LCs to 102 -104 and it will be much better driver.

Richard Jemison
RJR Racing

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