1956 Carburator idle drops at stops

[CtCarGuy]

I had my carb rebuilt last winter. It is the stock 1956 TeaPot. I am running off an electric pump at 3.5 psi. When cold, car ideas great and RPM consistent. After driving for awhile, when I stop for a light, the idle is fine and then will suddenly start to decrease and run rough. Going into neutral and giving it gas, the engine response smoothly. Most of the time when the idle drops, when I accelerate from a stop, it does so smoothly with no hesitation. Sometime when I stop, it will loose 300 RPM, run rough and stall. I checked vacuum, adjusted the idle mix using a tachometer. Someone though it might be my MSD coil but from my experience, they quit and then work again when cool. Ideas?

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[74 Harley]

I think I would check the float level first, doesn't sound like a coil to me.
Does your car have power brakes? It could also be a brake booster going bad.

 

[CtCarGuy]

Yes it has power brakes with a rebuilt booster a few years ago. I will disconnect it to see if it makes any difference. Good suggestion. I will also have the float level checked.

 

[4VE]

I had my carb rebuilt last winter. It is the stock 1956 TeaPot. I am running off an electric pump at 3.5 psi. When cold, car ideas great and RPM consistent. After driving for awhile, when I stop for a light, the idle is fine and then will suddenly start to decrease and run rough. Going into neutral and giving it gas, the engine response smoothly. Most of the time when the idle drops, when I accelerate from a stop, it does so smoothly with no hesitation. Sometime when I stop, it will loose 300 RPM, run rough and stall. I checked vacuum, adjusted the idle mix using a tachometer. Someone though it might be my MSD coil but from my experience, they quit and then work again when cool. Ideas?

Perhaps it is an electrical issue. I had a crack in my distributor cap that only messed up when hot. One trick is to take the car into a totally dark garage or during a very dark night, open the hood to look for stray leaking electricity. If you see that, replace all ignition leads and certainly the dist. cap.

 

[Ward 57]

Does it feel or smell like it's running rich when this happens? It's either fuel or ignition. I can't think of anything ignition related with such incident related running problems.
A loose choke flapper getting sucked closed would cause those issues.

 

[Shawnski]

I had my carb rebuilt last winter. It is the stock 1956 TeaPot. I am running off an electric pump at 3.5 psi. When cold, car ideas great and RPM consistent. After driving for awhile, when I stop for a light, the idle is fine and then will suddenly start to decrease and run rough. Going into neutral and giving it gas, the engine response smoothly. Most of the time when the idle drops, when I accelerate from a stop, it does so smoothly with no hesitation. Sometime when I stop, it will loose 300 RPM, run rough and stall. I checked vacuum, adjusted the idle mix using a tachometer. Someone though it might be my MSD coil but from my experience, they quit and then work again when cool. Ideas?

Kind of sounds like your dash pot isn’t adjusted correctly or it has failed. If your sitting in the driveway, cannot will it replicate?
You can have the car idle in the driveway and watch to see you f the dash pot device fails by allowing the Throttle to drop and cause the rough idle or stall.

 

[Ward 57]

Kind of sounds like your dash pot isn’t adjusted correctly or it has failed. If your sitting in the driveway, cannot will it replicate?
You can have the car idle in the driveway and watch to see you f the dash pot device fails by allowing the Throttle to drop and cause the rough idle or stall.

I thought of that also. With that and the idle set before the dashpot fully retracted so the final idle setting was too low.

 

[CtCarGuy]

Kind of sounds like your dash pot isn’t adjusted correctly or it has failed. If your sitting in the driveway, cannot will it replicate?
You can have the car idle in the driveway and watch to see you f the dash pot device fails by allowing the Throttle to drop and cause the rough idle or stall.

Does not seem to happen in the driveway under no load. I am going to check the float level. When I put the carb on, the electric pump was not regulated and pushed 12 psi. Shut it off quick but I wonder if the float level changed due to the pressure. Pressure is now 3.5.

 

[Frankie the Fink]

Sounds like fuel starvation to me.....I'm more familiar with Chevy vintage engines that like fuel pressure in the 4.5 to 5.5 range......

 

[Ward 57]

The dashpot is designed to slow the return to idle speed. You get fuel starvation when coming off speed to Idle too quickly.
When in the driveway run it up to about 2000rpm and quickly release. That may then exhibit your symptoms. A faulty or mal adjusted dashpot is most likely your issue.

 

[emansur]

I have this issue where the car just loses 300-500rpm from the idle speed at random times and sometimes runs better than perfect. I had the carb rebuilt twice now and am not sure how to fix it anymore, therefore I was wondering if I should go with Retrofitting 4150 or 4160 or just EFI all the way. Do you guys have any suggestions?

Thanks!

 

[Ward 57]

I have this issue where the car just loses 300-500rpm from the idle speed at random times and sometimes runs better than perfect. I had the carb rebuilt twice now and am not sure how to fix it anymore, therefore I was wondering if I should go with Retrofitting 4150 or 4160 or just EFI all the way. Do you guys have any suggestions?

Thanks!

What are the other symptoms when the idle drops? How low does it drop from the higher idle? Does it still idle properly when it drops?
Every classic car and especially the carbs have their own personality. If the carb has been rebuilt twice, you can't blame it on the rebuilder or carb. The rebuilders set them to spec.. When reading the shop manual they put in different jets depending on where the end product was going. Different jets when being shipped to high altitudes like Denver or to So. Cal. near sea level.
Each carb has to be fine tuned once installed. Choke settings, High idle when the choke is engaged, warm idle when the choke pulls off, Idle fuel mixture when warmed up. Carbs are not just plug and play. I think your high idle setting is just on the edge of being engaged and not.

 

[jack-in-sac]

I had my carb rebuilt last winter. It is the stock 1956 TeaPot. I am running off an electric pump at 3.5 psi. When cold, car ideas great and RPM consistent. After driving for awhile, when I stop for a light, the idle is fine and then will suddenly start to decrease and run rough. Going into neutral and giving it gas, the engine response smoothly. Most of the time when the idle drops, when I accelerate from a stop, it does so smoothly with no hesitation. Sometime when I stop, it will loose 300 RPM, run rough and stall. I checked vacuum, adjusted the idle mix using a tachometer. Someone though it might be my MSD coil but from my experience, they quit and then work again when cool. Ideas?

That does sound like you are on the low end of fuel pressure. So it doesn't do it in the driveway! Have you visually and mechanically checked that the dashpot is functioning properly? That is what your situation sounds like. Yes it could be ignition problems but not likely. Most of the time when the electronic ignition fails it just stops working. That is not always true. There are times when the failure can be intermittent, but that is uncommon. If you have any suspect wiring connections they can contribute to problems. Did you bypass the ceramic resistor when installing you electronic ignition? Generally electronic ignition does not need or want the ballast resistor. the early 12 volt systems still used what was essentially a 6 volt coil stepped down with the resistor. That continued at least into the 60s. One odd possibility can be the hose that connects from the fuel line on the frame to the mechanical fuel pump can have intermittent pressure failure. This is exacerbated by the current gasoline mixtures that include alcohol and other mixes. This can contribute to fuel delivery problems. By the way how is your choke set? Does it open the butterfly when the engine warms up? If the choke does not move to the running position it can also choke ( no pun intended) the fuel delivery.
In closing this sound more like fuel issue rather than ignition problems. Check all fuel systems hot and cold to see that they are working and adjusted properly for start-up and running. Good luck

 

[emansur]

What are the other symptoms when the idle drops? How low does it drop from the higher idle? Does it still idle properly when it drops?
Every classic car and especially the carbs have their own personality. If the carb has been rebuilt twice, you can't blame it on the rebuilder or carb. The rebuilders set them to spec.. When reading the shop manual they put in different jets depending on where the end product was going. Different jets when being shipped to high altitudes like Denver or to So. Cal. near sea level.
Each carb has to be fine tuned once installed. Choke settings, High idle when the choke is engaged, warm idle when the choke pulls off, Idle fuel mixture when warmed up. Carbs are not just plug and play. I think your high idle setting is just on the edge of being engaged and not.

My carb guy came and fine-tuned it in my location. However, right after each tuning it works perfectly fine. He adjusts the float sometimes, but usually, after a couple of hours, it goes bad again where it cannot idle because it has dropped so low it just dies. I also realized my brake booster might be an issue after reading this post so I am having it looked at soon. I'll see how it goes.

 

[CSPIDY]

Have you checked your fuel for dirt and contamination?
back flush your filter

 

[emansur]

Have you checked your fuel for dirt and contamination?
back flush your filter

Yup took the tank out, cleaned it and added a new gas filter.

 

[Ward 57]

My carb guy came and fine-tuned it in my location. However, right after each tuning it works perfectly fine. He adjusts the float sometimes, but usually, after a couple of hours, it goes bad again where it cannot idle because it has dropped so low it just dies. I also realized my brake booster might be an issue after reading this post so I am having it looked at soon. I'll see how it goes.

Unhook the vacuum hose from the booster and plug it then go for a drive. If it still acts up, that's not it.
I'm thinking you may have a vacuum leak. When fiddling under the dash I broke the age hardened hose to the wiper motor. Ran fine but stumbled and would almost die at Idle and was just retuned. Replaced the hose and now purrs like a kitten. I only discovered it when going through the pre drive check list ( like I'm a pilot ) Testing everything, and the wipers didn't work at all and have been perfect.
Check every vacuum connection and every hose for stiffness or hidden cracks. Sometimes the leaks only happen when warm.