Carb idle circuit issues

[sublimemike]

Been a while since I posted. Thought I would throw this out for carb gurus. Carb is Holley 4296AAS an L88 carb on a big block. Lumpy roller cam. Been happening a while and Ive been trying to cure. Carb runs fine on any short trips but is unhappy when it reaches heat saturation ..ie after 20 miles plus and properly warmed up. Below 1000 revs she just drops completely out and dies - when I come up to lights, roundabout etc . I set the idle running revs about 800 rpm and this is all fine. I've tried richening the idle circuit up with the screws either side thinking it's maybe a bit lean . Started 1and half turns out and tried a 1/4 turn at a time , up to 2 and half turns and not making a difference. Car restarts but need to keep above 1000 revs. Thought about the secondary throttle stop screw? Just want to get some ideas before I maybe have to pull the carb off as this adjustement is done from underneath. I don't have a vac guage or AFM but can get access If I need be. Thanks all.

 

[Oneball]

Have you tried screwing the idles in?

How have you set it to idle at 800 if it won’t run below 1000rpm?

 

[Roscobbc]

Is that carb you're using a vac sec or DP?
Does it have 4 corner idle screws?
Does it have replaceable idle bleed jets?
Old school method if running a rumpity cam with an earlier minimally adjustable Holley was to drill a few 1/8" holes in the primary throttle plate to improve idle air flow through the carb Venturi and avoid opening the throttle plate too far in an attempt to try and achieve a faster and more stable idle. This would prevent the throttle plate uncovering the idle to main jet transition 'slot' right at the base of the choke.

 

[sublimemike]

Odd ball When I first investigated the idles were : LHS completely screwed in , RHS out half a turn. A drag racer had rebuilt the carb but when he set it up he was in a hurry and don't reckon he did a thorough job. When it cold and not heat saturated it runs fine , ticks over 800 etc. Problem is when its hot. I think the issue is on the initial idle circuit. I could wind the idle screw up but it would rev its nuts off cold and would diesel on shutting down.

 

[Oneball]

Running ok at cold but not warm could be it’s too rich.

If you can screw the idles in all the way and it still runs then your problem is as Ross has described, it’s not running on the idle circuit as the throttles are open too much.

Could also be an ignition timing issue. Big cams need a lot of advance at idle.

 

[sublimemike]

Ross, I'm not expert on carbs but have had in pieces and done needle valve O rings as bowls were overflowing etc , also had the metering assembly off etc. Google Holley 4296 for full spec . No only 2 idle screws. I've heard about that trick about drilling the primaries, however the guy who had it new in the states didn't have an issue and im running the same gas as him LL101 avgas so don't see why I should need to resort to drilling hence me thinking about secondary throttle stop screw to increase air and make the idle screw more responsive?

 

[sublimemike]

One ball.. good point about the timing, I did back it off a bit in an attempt to run it on 99 with octane booster a while ago ...it never liked it so hence went onto Avgas. Could try a bit more advance. As you say, It's not running on the idle circuit.

 

[Roscobbc]

What Oneballs says is correct - example - my 489 runs 22 degrees advance at idle. I should have looked at spec's of your carb before replying previously - your carb's a vacuum secondary, fairly typical Holley fitted to many late 60's muscle cars - it will be a 780 cfm unit as fitted to a hi-po 396 or L72 427 (the L88 was 850 cfm) - TBH its a fairly basic carb and won't have any of the 'toys' that later and aftermarket carbs often feature.
TBH your issues re. running problems once the engine is hot seem like they could be more of a percolation issue. This typically is caused by either heat transference through the inlet manifold and up through the carb, effectively boiling off the fuel in the float bowls. A clue can be obtained when you switch the engine off.......do you see fuel dripping/leaking into the carb chokes. Perhaps your fuel line is too close to the exhaust manifold or headers. Does it have a fuel return line to the tank?
Is your inlet manifold stock? - do you have a heat insulator gasket between the carb and manifold? do you know what spec' your cam is and how the ignition timing is set-up (bearing in mind Oneball's comment) - an engine will run hot if ignition is too retarded. One final check - is the primary power valve OK? - an engine backfire can rupture its diaphragm and create running issues. I would be inclined to do as you have thought - tweak open the secondaries a little to increase low rpm airflow......you might struggle to do this however as the secondary stop screw may be 'staked' over........preventing easy changes.

 

[sublimemike]

Thanks for the expansive reply Ross. the stamping on the carb is 3955205. I get what you say about the perculation and the heat transfer and I have looked at many pics of L88 motors. I'm confident the pipe from the fuel filter is routed sufficiently away from heat that I don't have the perc issue. There is a return line. It's a Winters inlet manifold. I had the OK Kustom headers ceramic coated to keep engine temps down. There is a gasket but's it's not a 1" spacer like I have on the cuda. It's got the usual L88 set up, Harrison rad, viscous fan with no shroud that does diddly squat. Usefully the second owner tigged a powerful elecy fan onto the fan surround that seems to keep it cool but I'd never drive it in the city I am inclined to go back and look at what's been changed rather than try and reinvent the wheel. It's 12.5 compression, in reality a bit nutty for the road. Sleathy reckoned if I knocked back the timing and used torco additive it would have road manners. We did that but I think I need to retrace how the timing was originally and put it back and have it running on AVgas. From these posts I've a plan to advance the timing first then check the secondary stop screw. While the the carb is off I'll change the accelerator pump gasket (again).
22 degrees is a lot of advance initial , what's your final or have you locked it out on the MSD?

 

[Oneball]

I’ve got 25 degrees initial advance and 35 degrees full advance on mine but it is a race engine. Runs ok on Esso super unleaded

 

[sublimemike]

What compression ratio?

 

[Roscobbc]

Thanks for the expansive reply Ross. the stamping on the carb is 3955205. I get what you say about the perculation and the heat transfer and I have looked at many pics of L88 motors. I'm confident the pipe from the fuel filter is routed sufficiently away from heat that I don't have the perc issue. There is a return line. It's a Winters inlet manifold. I had the OK Kustom headers ceramic coated to keep engine temps down. There is a gasket but's it's not a 1" spacer like I have on the cuda. It's got the usual L88 set up, Harrison rad, viscous fan with no shroud that does diddly squat. Usefully the second owner tigged a powerful elecy fan onto the fan surround that seems to keep it cool but I'd never drive it in the city I am inclined to go back and look at what's been changed rather than try and reinvent the wheel. It's 12.5 compression, in reality a bit nutty for the road. Sleathy reckoned if I knocked back the timing and used torco additive it would have road manners. We did that but I think I need to retrace how the timing was originally and put it back and have it running on AVgas. From these posts I've a plan to advance the timing first then check the secondary stop screw. While the the carb is off I'll change the accelerator pump gasket (again).
22 degrees is a lot of advance initial , what's your final or have you locked it out on the MSD?

 

[sublimemike]

Sorry, can't see reply- only what I wrote

 

[Roscobbc]

38 degrees total. 10.5 cr. Have always used Shell VPower (as suggested by John when he build my engine) - tend to use Esso non ethanol 99 now.
TBH Mine gets a bit tetchy once temp goes over 190 degrees and although it never overheats as such it does run close to 200 on hot days and gets lumpy.

 

[sublimemike]

38... quite a bit , JS would know best. Yes I have moved to Esso non ethanol as well. Will have a play tomorrow. A friend has a timing gun that reads revs at the same time. I'm guessing my initial has been put back to 15 , will push it up to 17/18 and see how she feels. too much and I'll get a kick back.

 

[Roscobbc]

38... quite a bit , JS would know best. Yes I have moved to Esso non ethanol as well. Will have a play tomorrow. A friend has a timing gun that reads revs at the same time. I'm guessing my initial has been put back to 15 , will push it up to 17/18 and see how she feels. too much and I'll get a kick back.

If that timing gun has a dial-back facility you can 'log' the timing degrees at specific revs and better understand the timing events from idle through to max advance.
I have been tempted to buy a adjustable MSD timing retarder to plug-in to my current MSD stuff. Ideal being when starting it reduces initial timing by 10 or 15 degrees if I recall - once engine passes 1000 rpm it reverts back to normal setting. I did loose a starter tooth once on a kick back. Even with a relatively new starter mine does not always like starting when hot.
look at this - MSD 8984 MSD Starter Saver with Signal Stabilizer
or this - MSD 8987 Start and Step Timing Control
perhaps this - MSD 8680 Adjustable Timing Control

 

[Roscobbc]

38... quite a bit , JS would know best. Yes I have moved to Esso non ethanol as well. Will have a play tomorrow. A friend has a timing gun that reads revs at the same time. I'm guessing my initial has been put back to 15 , will push it up to 17/18 and see how she feels. too much and I'll get a kick back.

I ran my stock L36 427 with 15 degrees initial and all-in at 2500 rpm.........it ran fantastic (but there again it was a 'cooking' engine)

 

[teamzr1]

Been a while since I posted. Thought I would throw this out for carb gurus. Carb is Holley 4296AAS an L88 carb on a big block. Lumpy roller cam. Been happening a while and Ive been trying to cure. Carb runs fine on any short trips but is unhappy when it reaches heat saturation ..ie after 20 miles plus and properly warmed up. Below 1000 revs she just drops completely out and dies - when I come up to lights, roundabout etc . I set the idle running revs about 800 rpm and this is all fine. I've tried richening the idle circuit up with the screws either side thinking it's maybe a bit lean . Started 1and half turns out and tried a 1/4 turn at a time , up to 2 and half turns and not making a difference. Car restarts but need to keep above 1000 revs. Thought about the secondary throttle stop screw? Just want to get some ideas before I maybe have to pull the carb off as this adjustement is done from underneath. I don't have a vac guage or AFM but can get access If I need be. Thanks all.

You did not mention what year this is ?

What is the color and gap of the sparkplugs and are they correct ones for the makeup of the engine ?
The hotter engine, air and fuel gets the leaner it would be
Checked out the distributor, weights, springs, vac hose ?
Check for vacuum leaks, use a smoker if need be

Find someone or a shop that has an exhaust analyzer, like used for smog testing and see how the AFR changes
from open to closed loop

 

[Forrest Gump]

I’ve got 25 degrees initial advance and 35 degrees full advance on mine but it is a race engine. Runs ok on Esso super unleaded

Thats exactly what timing I’m running on my 383 with a performance but not silly cam. The stock distributor was giving 22 degrees of mech advance. I modified it to give just 10 degs mech advance to see if it made any difference using much higher initial. Result - yes it has improved the idle, it’s actually got more power right off idle revs and idles nicely. It starts fine, no kick back tendencies. I‘ve noticed too that from a cold start (and I don’t have a choke) that the car will be drivable virtually straight away without having to wait for it to warm up for a few minutes.
Mike, I thought the higher the octane fuel, the more advance you can run.
I used to run 38 but have just backed it off a bit to allow for using lower octane fuel if need be - you never really know what is coming out of the pumps.

 

[sublimemike]

Well got on it late today , carb off and got on the little screw underneath to tweak the secondaries. They were very slightly open , gave the allen screw one turn, about the thickness of a piece of card. all back together, not surprisingly idle went up to 1500 ish to mess by idle stop solenoid set up. Seemed to get some response from the idle screws but not sure where to plug the vacuum gauge in. Car wasn't really warmed up properly anyway and started to rain. Need to go for a proper run. Got let down with the timing gun but I'll save that for If it' still the same then I'll get on it for more advance, watch my fingers with that viscous fan and no shroud. Ross I have one of those timing controls on the mopar with the MSD6AL , struggled to get on with it - probably me. Some one mentioned you can lock out with MSD your final advance? What's that all about? JR yes I thought originally a possible vacuum leak, checked and all good. I think it's a case of going through things that have been changed / messed with one at time until I fix it. Away this week but hopefully back on it next weekend if it stays dry. Thanks all for helpful advice