Low idle in hot weather

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Righty Tighty

Righty Tighty

Blame it on the dog
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Hey friends, this is probably a very basic question, but here goes. '68 Barracuda with a slant, 1 barrel carb, 3 speed manual (A903).

The engine starts with 2 pumps of gas after sitting for weeks, and runs great. I've noticed that in hotter weather (100F +), the engine idles quite low. I don't have a tach on the car and haven't confirmed with a timing light, but it sounds like around 500 RPM or lower. It's so low that it sounds like it wants to die, but stays idling. Doesn't happen in cooler weather, only hot temps.

It still runs fine, but I'm wondering what causes this. I'm still learning the nuances of tuning carburetors, and this feels to me like that's what this is. I don't think it's fuel boiling, because it happens on a fresh start when the engine is cold.

Is this a problem that could be solved with some adjustments?
 
I'd start simple. Heat can cause hoses and gaskets to expand, potentially leading to small vacuum leaks that affect idle stability. You can check that With the engine running, spray carb cleaner around the vacuum hoses, intake base gasket and carburetor base. Listen for RPM changes when doing this.
 
You may be in the early stages of vapor lock and or problem might be air density.hot air is less dense and the motor may need a little more air through the idle circuit. try isolating fuel line and pump from heat . you can slice open a piece 3/8 fuel line and cover hard line.
 
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Some carbs had a compensator. Basically a little strip of bimetal/ thermostat with a rubber similar to a needle/ seat, that when hot actually opened a bit and allowed an air leak, to lean down the mixture. I don't keep up on carbs at all anymore, and don't remember which do or don't.

Just making sure the idle is a bit on the high side and VERY correctly adjusted helps a lot

Info:

Hot Idle Compensator

www.carburetor-parts.com www.carburetor-parts.com

allcarbs.com

Hot Air Idle Compensator Rochester Holley - Allstate Carburetor

New Hot Air Idle Compensator is Rochester brand but can be used in many applications. This part fits on Rochester Quadrajets, Rochester Mono Jets, Holley
allcarbs.com allcarbs.com

LOLOL I just learned something: Look at this off a Ford:

Compensator Valve - Hot Idle - Used ~ 1967 - 1968 Mercury Cougar / 1964 - 1968 Ford Mustang

Compensator Valve - Hot Idle - Used ~ 1967 - 1968 Mercury Cougar / 1964 - 1968 Ford Mustang
secure.cougarpartscatalog.com secure.cougarpartscatalog.com
 
67Dart273 said:
Some carbs had a compensator. Basically a little strip of bimetal/ thermostat with a rubber similar to a needle/ seat, that when hot actually opened a bit and allowed an air leak, to lean down the mixture. I don't keep up on carbs at all anymore, and don't remember which do or don't.

Just making sure the idle is a bit on the high side and VERY correctly adjusted helps a lot

Info:

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LOLOL I just learned something: Look at this off a Ford:
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Click to expand...
I like that. I was thinking maybe his choke butterfly could be partially closed when normal temps and then opening completely when it's real hot out making his linkage drop off high idle lobe. I know. It's a long shot.
 
Next time it gets close to empty, fill it with non ethanol fuel and see if that helps.
 
67Dart273 said:
Some carbs had a compensator. Basically a little strip of bimetal/ thermostat with a rubber similar to a needle/ seat, that when hot actually opened a bit and allowed an air leak, to lean down the mixture. I don't keep up on carbs at all anymore, and don't remember which do or don't.

Just making sure the idle is a bit on the high side and VERY correctly adjusted helps a lot

Info:

Hot Idle Compensator

www.carburetor-parts.com www.carburetor-parts.com

allcarbs.com

Hot Air Idle Compensator Rochester Holley - Allstate Carburetor

New Hot Air Idle Compensator is Rochester brand but can be used in many applications. This part fits on Rochester Quadrajets, Rochester Mono Jets, Holley
allcarbs.com allcarbs.com

LOLOL I just learned something: Look at this off a Ford:

Compensator Valve - Hot Idle - Used ~ 1967 - 1968 Mercury Cougar / 1964 - 1968 Ford Mustang

Compensator Valve - Hot Idle - Used ~ 1967 - 1968 Mercury Cougar / 1964 - 1968 Ford Mustang
secure.cougarpartscatalog.com secure.cougarpartscatalog.com
Click to expand...
I did not know that
 
Thanks, everyone. I looked, and didn't immediately see a hot idle compensator. I actually don't really know if it would be readily visible in the first place?

I gave the carb a once over (should've done this before posting) and it was actually loosely mounted to the intake. Snugged it up, started the car and it idled beautifully without even a pump of the gas. However, the car was in the garage, so I pulled it into the sun to warm it up a bit. We'll see how it acts when she's nice and hot.

I like the ethanol-free suggestion, the closest station is about 45 minutes from me.
 
Slant-6-centred carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted here for free download.

Tune-up parts and technique suggestions in this post.

As soon as you can, get the three books listed in this thread.
 
The low idle speed in hot weather is caused by the mixture becoming richer. The air has less weight as it heats up, which does two things: it makes the mixture richer, now no longer the optimal ratio of air + fuel. And less oxygen by weight reduces the hp, so rpm drops.

Try adjusting the mixture screw a tad leaner.
 
RustyRatRod said:
Next time it gets close to empty, fill it with non ethanol fuel and see if that helps.
Click to expand...
I'm jealous of you guys that have that option. I have to drive 40ish miles each way into the next state to have that option
 
volaredon said:
I'm jealous of you guys that have that option. I have to drive 40ish miles each way into the next state to have that option
Click to expand...
I admit, we've got it better than lots of guys and gals across the country and it shouldn't be like that. Gray is a town of only about 2500 or so on paper. I'd "guess" about at least twice that many work there during the day. There are about six in Gray and all but ONE carry non ethanol gas.......and it MIGHT, I've just not looked at every single pump.
 
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Fuel was my 1st guess only because I had such a problem with it in my 67 2 brl V8. So after my cure was to swap intake, carb, fuel line routing and filter location to 73 up, problem solved. Sometime later several local stations stated offering ethanol free. Not only did I not need to change the 67 but haven't had the first problem with primer bulbs or anything I own that wears a carburater.
failed to quote 40 mile trip comment. Was gonna ask, GOT CRUISE CONTROL? LOL
 
67Dart273 said:
Some carbs had a compensator. Basically a little strip of bimetal/ thermostat with a rubber similar to a needle/ seat, that when hot actually opened a bit and allowed an air leak, to lean down the mixture. I don't keep up on carbs at all anymore, and don't remember which do or don't.
Click to expand...

Hot-idle compensators were on '70-'71 Carter BBS and Holley 1920 carbs, in response to higher underhood temperatures brought on by emission control strategies such as retarded base timing and higher-temp thermostats.

They worked, but added cost, so they were deleted. GM (Rochester Carburetor) offered an aftermarket add-on item called the "CarbAIRator"; see one here (whole instruction sheet is presented in the photos).

OP's car is not exhibiting the described symptoms because it lacks a hot idle compensator.
 
I appreciate all the input, gents. Naturally, I got really busy right after posting, so I haven't had time to tinker.

I hope to get out there today or tomorrow and try out your suggestions. I just have to remember to pull the car into the sun early enough to get it hot (while the engine is cold), so I can repeat the conditions in which I'm experiencing the issue.
 
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