Oscillating idle when hot

[SBSerpent]

On my Ford Racing Motor 392 CID that came with my BDR, I have this issue with an oscillating idle that occurs after the car sits for a short period of time after driving it for a lengthy period of time. I take the car for a long run, reaching operating temperature. Runs fine and idles smoothly at 8K - 9K RPM. Turn the car off and let it sit for a short period of time (10 minutes or more). Fire it back up and the idle speed is oscillating from about 9K to as low as 3K. If I don't feather the gas peddle, it will eventually stall. If I feather the gas peddle for a period of time while idling (say about 5 minutes), then the idle becomes more consistent and steady. Motor is the stock Ford Racing Motor circa 2010 that came with my BDR - with about 7K miles on it. Could this be something related to the PCV valve?

[Gaz64]

Carb percolating.

What carb do you have?

[Tommy]

I agree with Gary. Particularly in regions where the gasoline formula is poor, heat from the resting engine can cause gasoline in the float bowls of a Holley carb to boil. It can take a while for cooler fuel from the back of the car to reach the carb and bring the temp back down after the engine is restarted. . . . As an aside, a fellow on another forum with a different sports car installed a computer cooling fan under his hood to blow on the carb after he shuts down. It solved his problem but seems an unsightly solution.

[Tom Wells]

SB,

I agree with the above posts about the substance we laughingly call gasoline. The 10% alcohol seems to have a boiling point of about 110 degrees F or so...

One thing that would be fairly easy to try - maybe, depending on your location - would be no alcohol gasoline. It's available in lots of places and costs very little more.

Other than that, I faced the same problems for over a year with "gasoline" boiling in the float bowls and causing problems. I tried lots of things: return lines, insulators, heat shields etc etc. Finally it looked like my options came down to aiming an air conditioner vent at the float bowls, or switching to fuel injection.

I selected the latter.

Good luck with finding a satisfactory solution,

Tom

[SBSerpent]

Originally Posted by Gaz64

Carb percolating.

What carb do you have?

Holley Street Avenger 770CFM - aluminum body. Came stock with the motor.

As far as the gas composition, I'll have to look that up. I live in Southern California. Highest available octane is 91, which is what I use.

Did a search for alcohol free gasoline and nothing shows up near me. Closest is about 150 miles away.

[NROTOXIN]

Do you have electric radiator fans that you can manually turn on for around 5 to 10 minutes to cool your whole engine compartment down? How about a phenolic spacer under the carburetor? There are 1/4" spacers that would arguably work to isolate the carb from from the manifold. The two mentioned together might mitigate your issue.

[SBSerpent]

Originally Posted by NROTOXIN

Do you have electric radiator fans that you can manually turn on for around 5 to 10 minutes to cool your whole engine compartment down? How about a phenolic spacer under the carburetor? There are 1/4" spacers that would arguably work to isolate the carb from from the manifold. The two mentioned together might mitigate your issue.

I do have a toggle switch to turn on the electric puller fan after turning off the car. Opening the hood also probably helps with venting the hot air from underneath the hood.

The phenolic spacer is also a good idea for reducing the conductive heat transfer from the intake manifold to the carb. I will need to see how thick of a spacer I can use being that the air cleaner cover sits very close to the cowl - maybe 1/4" gap there.

[MKS427]

I'd start with the phenolic spacer previously mentioned. If clearance is an issue, solid engine mounts or a drop down air cleaner base may be the solution.

[Tom Wells]

SB,

Been down the "try to make the carb stop percolating" rabbit hole.

Insulators, heat shields, return lines for continuous fuel recirculation, drop base filters (made it worse by shrouding the float bowls), you name it.

Nothing worked.

Depending on the chemical makeup, gasoline can boil at as low as 100 degrees F. The petroleum companies seem to like to hide their facts, so we may never know exactly. At elevated altitudes the problem is aggravated.

What I'm certain of is that it boils at a relatively low temperature; way lower than the leaded gasoline that was standard when these carbureted cars were originally produced. Even then they had some problems...

I've driven one of my carbureted cars with the hood and air filter removed. After it warmed up, the geysers coming out of the float bowl vents were an impressive sight. Right before they made tight u-turns and got sucked into the venturis - then everything shut off until the gas could evaporate. All's well now with EFI.

Just saying...

Tom

[NROTOXIN]

Tom,
No "root cause analysis" has really been done here, only internet troubleshooting. What has worked for some does not always work for others. At the same time, these are low cost things to try. Maybe it's not boiling at all and it is something else.......nothing here so far has this been identified 100% fuel boil, only symptoms. Let's treat the symptoms with a couple of low cost parts and then take it from there. YMMV

[patrickt]

Originally Posted by NROTOXIN

TLet's treat the symptoms with a couple of low cost parts and then take it from there. YMMV

I agree with that. You could do a pretty quick test with just aluminum foil and ice packs around the carburetor. If doing that will temporarily make the problem vanish then you're on the right track.

[Tom Wells]

NRO and Patrick,

You guys are probably right

My experience is only with my two cars.

Tom

[SBSerpent]

I will try the phenolic spacer route. Right now, it appears that I just have a 1/16" (?) paper (?) gasket at the carburetor base. I will get at least a 1/4" phenolic spacer to start and see if that helps the situation. I will have to swap out the oval "Cobra" air cleaner because once I raise the carburetor, I will have interference issues between the air cleaner and the cowl. A conventional round air cleaner will solve the clearance issues.

On another note, how does the increased thickness of spacers impact performance? Is thicker better, not just from an insulation standpoint, but from a performance standpoint. Also, do I go with a spacer that has a four hole center or one with a large one hole center?

[Unique427]

Originally Posted by SBSerpent

I will try the phenolic spacer route. Right now, it appears that I just have a 1/16" (?) paper (?) gasket at the carburetor base. I will get at least a 1/4" phenolic spacer to start and see if that helps the situation. I will have to swap out the oval "Cobra" air cleaner because once I raise the carburetor, I will have interference issues between the air cleaner and the cowl. A conventional round air cleaner will solve the clearance issues.

On another note, how does the increased thickness of spacers impact performance? Is thicker better, not just from an insulation standpoint, but from a performance standpoint. Also, do I go with a spacer that has a four hole center or one with a large one hole center?

To combat vapor lock issues there's two ways to fight it.
Cooling the fuel AND raising the chosen fuel's boiling point.

A low cost experiment would be to eliminate all ethanol blends and try
burning only unleaded non-oxygenated fuel.

To find unleaded non-oxy fuel near you consult: www.pure-gas.org.

Also, unleaded non-oxy race gas (Sunoco 260GTX) has a much lower
Reid Vapor Pressure rating than pump gas and will be more resistant to boiling.
Unleaded non-oxy race gas mixes with unleaded non-oxy pump gas if you
wish to blend to a specific octane rating as well.

Might be worth a try before breaking out the wrenches and checkbook.

[SBSerpent]

Originally Posted by Unique427

To combat vapor lock issues there's two ways to fight it.
Cooling the fuel AND raising the chosen fuel's boiling point.

A low cost experiment would be to eliminate all ethanol blends and try
burning only unleaded non-oxygenated fuel.

To find unleaded non-oxy fuel near you consult: www.pure-gas.org.

Also, unleaded non-oxy race gas (Sunoco 260GTX) has a much lower
Reid Vapor Pressure rating than pump gas and will be more resistant to boiling.
Unleaded non-oxy race gas mixes with unleaded non-oxy pump gas if you
wish to blend to a specific octane rating as well.

Might be worth a try before breaking out the wrenches and checkbook.

Thanks for the tip, but being that the closest station is over 100 miles from me, this option is pretty much a non-starter for me.

Just trying to figure out whether a 1/4 phenolic spacer is good enough or should I go with 1/2"?

[Unique427]

Originally Posted by SBSerpent

Thanks for the tip, but being that the closest station is over 100 miles from me, this option is pretty much a non-starter for me.

Just trying to figure out whether a 1/4 phenolic spacer is good enough or should I go with 1/2"?

A few guys here have used Quick Fuel's 300-4013QFT
"Quick Cool Carburetor Insulator Kit" with reported success.
In addition to creating a manifold/carb thermal barrier, it
provides a heat shield under the fuel bowls too.
Installation height is 0.50".

https://www.summitracing.com/parts/q...SABEgKlQPD_BwE

[Gaz64]

I knew I had seen that plate before somewhere:

Photos -

So now Quick Fuel sells it with their sticker on it?

[fastd]

SBSerpent,
Did you search for Sunoco race fuels gas stations specifically? Also, you may be able to find some race shop locally that sells higher octane fuel.

I had the same issue: phenolic spacer helped a bit; when I added fuel injection it fixed it; I’m sure the return line helped. You could add a return line and spacer, find some place to add 5 gallons (or more) of race fuel to a tank.

[Tommy]

I'm a DIYer much of the time and I couldn't justify the cost of the Quick Fuel insulation kit just to find out if it works. So I bought a sheet of aluminum and made my own radiant heat shield. It's not as pretty as theirs, but I was able to test the concept without spending too much. That and an Ebay 1/4" phenolic spacer seemed to help a bit.

[Unique427]

Originally Posted by fastd

SBSerpent,
Did you search for Sunoco race fuels gas stations specifically? Also, you may be able to find some race shop locally that sells higher octane fuel.

I had the same issue: phenolic spacer helped a bit; when I added fuel injection it fixed it; I’m sure the return line helped. You could add a return line and spacer, find some place to add 5 gallons (or more) of race fuel to a tank.

Can also buy online and have it shipped to your house in 5 gallon steel pails.

Sunoco 260GTX unleaded non-oxy has a Reid Vapor Pressure rating
of 4.7 and only a 10% evaporation rate at 182F.

As others have stated, a fuel return line can help in some cases. Where
it's less helpful is when cars have a low hanging metal fuel tank with no
non-metal barrier between it and asphalt. The radiant heat coming
off the road on a hot day can heat fuel in the tank to +100F with enough
road time.


Sunoco's write up on Reid Vapor Pressure if interested:
https://www.sunocoracefuels.com/tech...pressure-vapor

Sunoco 260GTX
https://www.sunocoracefuels.com/fuel/260-gtx