Oscillating idle when hot

[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?

[SBSerpent]

Originally Posted by Unique427

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

I looked at some reviews on this product and there were two instances where the poly core spacer had started to deteriorate after a short period of use. In both instances, the owners removed the carburetor and saw that the material inside of the spacer was coming apart and shedding particles into the intake manifold.

I'm going with my original plan of 1/2" phenolic spacer (with sealing gaskets on both sides of the spacer). The reduction of conductive heat transfer from an aluminum intake manifold into an aluminum bodied carburetor should be quite substantial given that right now, all I have that separates the two is a paper thin gasket.

I gotta start somewhere and being an aerospace thermal engineer by occupation, I've gotta believe that cutting down the conductive heat transfer from manifold to carb by using a low conductivity phenolic will provide some gains. Stay tuned.