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hi,
79 L82 manual orginal carb ~35000 miles..
Eventual plan is to get carburetor off to Lars (but i can't stop driving it)..
As far as i know no issues with cold startup until last week.. temps drop below freezing, (garage 40-50f ish..) usually 2 pumps then fast idle (longer now with out heat riser) drop while warming and all is well. Warm and any restart within hours OK..even out in freezing temps..high idle right away.
Now starts after 2 pumps (COLD ambient) ..but not high idle.. stumbling smoking (blue black?) barely catch it pump gas rev it up clears out.. then finally high idle..all is well..
So i have a read a LOT of threads.. and a some of Lars forensic rebuilds..
- tightened visible screws and top bolts..and mount bolts. (Gentle snug) all tightened slightly..
- went to choke pull off.. the vacuum line was very original and loose wet.. and kinked.. it is like an S shape from carb to pull off.
- sucked on the end of a good hose on vacuum pull off and it moves but leaks vacuum.. cannot get it to retract fully (out of breath from it leaking).. distributor vacuum holds as a comparison.
questions please
- is that a special 5/32 vacuum hose like an s shape or just replace with a straight hose?
- should the pull off hold vacuum? if so any thoughts on part # to replace?
again i am speculatin this is the issue.. carb has gas and can see it squirt at cold so bowls are full.. its like it is flooded at very cold start..
From: At my Bar drinking and wrenching in Lafayette Colorado
Info for everyone about the choke pulloffs for the M4M (1975+ integral choke) carbs:
The choke pulloff, in addition to cracking the choke open for proper cold-start, also acts as a "damper" for the secondary airvalve. If the secondary airvalve is allowed to "pop open" too quickly, it will cause a secondary tip-in bog/stumble due to a lean condition on the secondary side. By controlling the airvalve opening rate, there is a secondary fuel shot that is induced, duplicating the function of an accelerator pump. It is important that this occur in order for the carb to produce a smooth secondary transition.
Unfortunately, the current aftermarket replacement choke pulloffs do not have the correct bleed delay restriction in them, and they will allow instant opening of the secondary airvalve, with associated stumble and bog problems. So the new pulloffs have to be reworked to install a correctly calibrated air bleed restriction. If you have replaced your pulloff and now have a secondary tip-in problem, you need to modify your pulloff. I've seen many of the replacement pulloffs for the 1968-1970 carbs with the same problem.
Lars
Nothing special about the vacuum hose, except the length.
It is easier for me to attach linkage and hose to assembly. Then move it into the correct position and install the mounting screws last.
From: At my Bar drinking and wrenching in Lafayette Colorado
Your pulloff is defective. If you're sending me the carb, I replace those with a correctly calibrated unit, so don't bother replacing it before you ship the carb. The defective pulloff will affect choke operation as you describe.
Lars
Your pulloff is defective. If you're sending me the carb, I replace those with a correctly calibrated unit, so don't bother replacing it before you ship the carb. The defective pulloff will affect choke operation as you describe.
Lars
From: At my Bar drinking and wrenching in Lafayette Colorado
Info for everyone about the choke pulloffs for the M4M (1975+ integral choke) carbs:
The choke pulloff, in addition to cracking the choke open for proper cold-start, also acts as a "damper" for the secondary airvalve. If the secondary airvalve is allowed to "pop open" too quickly, it will cause a secondary tip-in bog/stumble due to a lean condition on the secondary side. By controlling the airvalve opening rate, there is a secondary fuel shot that is induced, duplicating the function of an accelerator pump. It is important that this occur in order for the carb to produce a smooth secondary transition.
Unfortunately, the current aftermarket replacement choke pulloffs do not have the correct bleed delay restriction in them, and they will allow instant opening of the secondary airvalve, with associated stumble and bog problems. So the new pulloffs have to be reworked to install a correctly calibrated air bleed restriction. If you have replaced your pulloff and now have a secondary tip-in problem, you need to modify your pulloff. I've seen many of the replacement pulloffs for the 1968-1970 carbs with the same problem.
Lars
Since this thread is about pull-offs,
Question:
The Corvette parts and illustration catalog calls out 17050759 for 76-79 L48 M38.
It also calls out 17053201 for the 77 M40 and 75-79 High Performance.
Physically they look the same, is the difference just the way there are calibrated (Bleed Off Delay Time)?
Thanks Lars!
Info for everyone about the choke pulloffs for the M4M (1975+ integral choke) carbs:
... Unfortunately, the current aftermarket replacement choke pulloffs do not have the correct bleed delay restriction in them, and they will allow instant opening of the secondary airvalve, with associated stumble and bog problems. So the new pulloffs have to be reworked to install a correctly calibrated air bleed restriction. If you have replaced your pulloff and now have a secondary tip-in problem, you need to modify your pulloff. I've seen many of the replacement pulloffs for the 1968-1970 carbs with the same problem.
Lars
Great info Lars. So how would I, (a mere novice!) accomplish this? Is there a specific restriction method you could share, and on/off time that we're looking for?
My car is no where near stock, and still figuring out just what I have, while getting ready to drive and enjoy her (at paint&body now, not driveable yet). And while I can't swear that I DO have any tip in problem, given all the rest of this issues this car has, it won't surprise me if I do!
Thanks,
Paul
Last edited by 73sbvert; Jan 12, 2021 at 02:03 PM.
From: At my Bar drinking and wrenching in Lafayette Colorado
If you don't have access to a machine shop for fabrication of custom bleeds, you can put a dab of 5-minute JB weld into the pulloff vacuum nipple to plug it. Just a dab big enough to create a "plug" about 1/8" deep into the vacuum nipple. Once the JB has fully hardened, drill through it with a .022" diameter drill. It is important that the hole be "clean," so make sure the epoxy is fully cured and really hard - don't drill when it's still slightly soft. This wil get you a nice pulloff rate that will provide very good secondary airvalve control.
If you don't have access to a machine shop for fabrication of custom bleeds, you can put a dab of 5-minute JB weld into the pulloff vacuum nipple to plug it. Just a dab big enough to create a "plug" about 1/8" deep into the vacuum nipple. Once the JB has fully hardened, drill through it with a .022" diameter drill. It is important that the hole be "clean," so make sure the epoxy is fully cured and really hard - don't drill when it's still slightly soft. This wil get you a nice pulloff rate that will provide very good secondary airvalve control.
Lars
Thank you sir! A gentleman and a scholar!! I will bookmark this in case I need to do just that!
One more question if I may: the way I was taught to set the mixture and idle screws was to: in small increments, bring the mixture screws (left and right) out until the max vacuum was achieved, and bring down idle speed incrementally if needed to keep it from going too high, but around 1200-1500 or so was acceptable. Then once the max vacuum is done, bring the idle speed down to the approx 750-ish, and button it up.
Does this sound right?
Paul
Last edited by 73sbvert; Jan 12, 2021 at 02:28 PM.
Reason: Ask another question
!! cant wait..
79 carburetor choke pull off question
