Oscillating idle when hot

[Tom Wells]

SB,

Not sure how to delicately phrase this, but return lines for a carb fuel system don't do squat.

The only return line that might help would take the fuel directly from the float bowl - not sure how this could be done but I'm willing to listen.

How do I know? Been there, done that. More than once.

Return lines keep the fuel that's in the gas line circulating so it doesn't get warmer by returning it to the tank, typically from a regulator, not from the float bowl.

It's the overheated gasoline (haha) in the toasty warm float bowl that causes the problems. An insulator/heat deflector may help some, but if the car sits, not running, well, good luck with that. Running the fans may help some, but how long does it take to cool the underhood area after the car is stopped?

Tough problem. No easy, simple solution.

Sorry,

Tom

[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.

[SBSerpent]

Originally Posted by Unique427

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

I did a search and found a Sunoco station close by that sells these different grades of fuel. Do they need to be used by themselves or can you mix with conventional fuels (understanding that you dilute the capabilities of the Sunoco fuel in doing so). Do you risk any damage to rubber seals/gaskets/Orings by using these higher grade fuels?

[fastd]

You should add a heat shield also. I like the 100 octane low lead - it’s expensive but you will enjoy the ride. Afaik it’s ok to mix the different grades. There is also a 114 or so octane that you’re not supposed to pump directly into your car - 5 gallons of that in a 15-20 gallon tank would work. I believe the higher grade fuels are actually better than the everyday gas for engine, gaskets , etc.

[Unique427]

Originally Posted by SBSerpent

I did a search and found a Sunoco station close by that sells these different grades of fuel. Do they need to be used by themselves or can you mix with conventional fuels (understanding that you dilute the capabilities of the Sunoco fuel in doing so). Do you risk any damage to rubber seals/gaskets/Orings by using these higher grade fuels?

There will be no ill effects on seals/gaskets/O-rings, etc using unleaded
non-oxygenated race gas. It is PURE gasoline made without ethanol.

Blending race gas with pump gas is no problem. You will need to do
a little math to get the desired octane rating. Example is a 50/50
blend of 91 octane unleaded non-oxy pump gas with Sunoco 260GTX
98 octane unleaded non-oxy produces a blended octane rating of 94.5.

I run the above mix during hot Summer months or car parades.
BBF, carb, mechanical pump, no return line or phenolic spacer.
Fuel line is 1/2" SS braided Aeroquip with air-gap between block.
+90F car parades with lots of stop-n-go but no vapor lock.
Hot restarts have not been an issue during parades as well.
May not work for everyone but this seems to work on my car.

Really hope this helps you out even a little.

Read Sunoco's info about mixing here:
https://www.sunocoracefuels.com/tech...ulating-octane

Bonus: burning race gas your exhaust will smell a bit sweeter!!

[SBSerpent]

Installed a 1/4" spacer with gaskets top and bottom. Has made a dramatic improvement with hot starts. Ditched the oval Cobra air cleaner for a 14" Sprectre air cleaner with a the blue synthetic media around the perimeter AND on the top cover. Looks really good, not to mention much more air flow now. Matches nicely with the blue plug wires.

Tried to upload a photo but the file won't upload, even though the file size is below the 99kb max limit for jpeg files. Have had this recurring issue now for months. Not sure what is going on but it is very frustrating.