Low idle in hot weather

[Righty Tighty]

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?

 

[OttawaCharger]

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.

 

[Kendog 170]

Possible choke adjustment? IDK

 

[Dart Frog]

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.

 

[67Dart273]

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

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

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

 

[Kendog 170]

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:

http://[URL]https://www.carburetor-...DR7ioTvEgM8rI2QowRQoj4Sjjgvza-vUT79u1xRdZSjme

[/URL]

http://[URL]https://allcarbs.com/product/rochester-hot-air-idle-compensator-rochester-holley/

[/URL]

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

http://[URL]https://secure.cougarpa...-mercury-cougar-/-1964-1968-ford-mustang.html

[/URL]

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.

 

[RustyRatRod]

Next time it gets close to empty, fill it with non ethanol fuel and see if that helps.

 

[Icetech]

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

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

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

I did not know that

 

[Righty Tighty]

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.

 

[slantsixdan]

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.

 

[Bewy]

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.

 

[volaredon]

Next time it gets close to empty, fill it with non ethanol fuel and see if that helps.

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

 

[RustyRatRod]

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

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.

 

[RedFish]

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

 

[slantsixdan]

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.

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.

 

[Righty Tighty]

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.

 

[Righty Tighty]

I forgot to update. I've tuned the carb to the best of my ability and using @slantsixdan 's links, with no real noticeable change. I got to thinking -- I've been having a low charge situation as well on this car, ever since I bought it.

I know, I know, just get out there and fix it! But, my free time has been slim to none the last year or so (too long of a story to explain), so I haven't had the time for it.

Yesterday, I moved the care back into the garage after I finished a project, and the engine turned over slowly, and I could tell the battery's getting low again. It did start, but then idled quite low. It made me realize that I may be experiencing low idle from the low voltage, and that I didn't connect the two earlier.

I tried solving the low voltage problem in the past, but per Righty Tighty protocol, got busy again and left the mystery unsolved. I've got 6 hours slated at the shop today, but when I get home, I'm forcing myself to do some troubleshooting.

 

[MadScientistMat]

Alternatively, the low voltage may come from low idle speed. If you give it a little throttle to bring the speed up, does the voltage come up too?

 

[Righty Tighty]

It doesn't. In fact, let's say I take it to the grocery store 3 times in a week. By the end of the week, the battery will be just about dead. Every trip drains the battery little by little. Does seem to be getting a charge.

 

[MadScientistMat]

Can you get a volt meter on there and see how the voltage moves with RPM?

What alternator and voltage regulator do you have?

 

[Righty Tighty]

For sure, I'll do that in a few hours once I'm home. Am I looking at alternator output or battery voltage? I've tried searching here for threads on similar issues, but haven't quite been able to figure it out.

The alternator is a round back from Rock Auto, and the VR is a newish mechanical unit. I can get the part numbers when I'm home.

 

[MadScientistMat]

Better to check at the battery. There's typically some voltage drop between the alternator and battery, though, so checking the voltage at the alternator output terminal will give more to go on. Check both idling and at ~1200-1500 RPM.

 

[MopaR&D]

Might just be hot enough air that the density goes down to the point where it needs more volume (idle speed adjusted up). I've found this to be the case on my Duster, when the weather gets hot the idle AFR from my wideband O2 doesn't change much but it idles noticeably lower and I have to give the idle speed screw a little tweak to get it back up.

I've also found recently that fuel quality makes a noticable difference in AFR consistency and resistance to boiling in the bowls. For that car I now get gas from Exxon, Shell or BP stations instead of whatever cheapest place is close by.

 

[Righty Tighty]

Might just be hot enough air that the density goes down to the point where it needs more volume (idle speed adjusted up). I've found this to be the case on my Duster, when the weather gets hot the idle AFR from my wideband O2 doesn't change much but it idles noticeably lower and I have to give the idle speed screw a little tweak to get it back up.

I've also found recently that fuel quality makes a noticable difference in AFR consistency and resistance to boiling in the bowls. For that car I now get gas from Exxon, Shell or BP stations instead of whatever cheapest place is close by.

Thanks. I've decided to address the charging issue first, and then get to the idle.

When I got home yesterday, battery voltage was 11.74V before starting. After starting, it fell to 11.10 and climbed to just 11.14V revving to around 2500 RPM. Checked alternator output stud and the other multimeter lead to chassis ground, got 0.0V. One lead to the output stud, and the other to the alternator field terminal, got 17V.

Shut the engine off and looked over the wiring again, noticed the terminal at the ignition side of the VR was melted.

I also noticed a wire at the bulkhead that was pretty ugly as well. I believe it's an ignition wire, because it's got a fusible link, but it's reversed positions if I'm reading the factory diagram correctly.

Anyhow, I've known the car has needed attention for a while, so I'm just going to get to it.

 

[OttawaCharger]

look for something called "Bulkhead Disconnect" on your wiring diagram. The letters will match up with what your highlighting here but they should map out correctly. At least they do for my Dart