1931 running lean when hot

[Dave39MD]

My new to me 31 , 60 series  Buick 66S, runs great when the outside temp is around 60 and I make no stops. If I stop to get gas and start out again it starts to backfire thru the carb. If I pull the choke out about 1/2 of an inch it smooth's out and runs fine. I can slowly push the choke back in without problems. My guess is it is something heat related. I would appreciate any suggestions.

 

Thanks

 

Dave

[Dave39MD]

I should add it normally runs at 160 and when I say hot it is up around 170 to 180. I did do a search and found the heat riser can be a problem but would it run well at times and then not with pin holes in the heat riser tubes?

 

Thanks

 

Dave

[DonMicheletti]

Dave,

If there were pin holes in the heat riser tube it would run poorly all the time. Plus the tube would probably fail entirely in short order.

[Mark Shaw]

IMO, the heat riser tube should always be replaced with aluminized exhaust tubing. 

 

And, the heat system should be blocked or at least turned off to eliminate such issues. 

[Dave39MD]

Thanks for the info.

[Dave39MD]

Well I found a crack free heat riser and had new tubes installed. Put it on and it runs great. One difference I noticed was the gasket from Bob's that goes between the riser and the carb was much thicker than the paper thin gasket on the old one.  The new gasket may help with heat transfer and vapor lock problems? The riser I took off was welded and brazed in many places and it looked like the tubes were beginning to fail and patched with something by one of the po's.

Thanks for the help.

Dave

[Dave39MD]

It's back. When the outside temp is above 80 it starts backing firing through the carb after operating temp approaches 180. One thing I noticed is a smell of gas before it starts and I pulled over and noticed gas coming out of the vent on the float bowl. Seems strange with what I am taking as a lean back fire that is cured with more choke. Could it be fuel is boiling out of the bowl?  Any ideas or suggestions are welcome.

 

Thanks

 

Dave

[Mark Shaw]

It could be vapor lock.  If I remember correctly, the original fuel line is routed under the engine from the fuel pump to avoid any hot zones.  Replace the fuel filter and check the fuel line routing around hot zones in the engine compartment. 

[Bob Engle]

32 Buicks had insulation on the line from fuel pump to carb.  I would think 31's might be the same.  There should also be a heat shield between the air cleaner and exhaust manifold.  Be sure to plug the exhaust diverter so no exhaust goes to the carb.

 

Bob Engle

[Dave39MD]

Mark and Bob, thanks for the replies. The fuel line runs behind the engine to the fuel pump on the drivers side. There is a heat shield between the fuel pump and the engine block. My car is a late 31 and has that  air cleaner, I don't see a provision for a heat shield near it unless it is built in to the late air cleaner.  I will insulate the fuel lines and try to block off the heat riser tube. The old heat riser I removed was blocked off and the problem I am describing was much worse.

 

It was a little cooler today and I ran it pretty hard and no backfiring at all, it was a real pleasant ride.

 

Thanks again for the help

 

Dave

[Mark Shaw]

Running ethanol fuel will also increase the potential for vapor lock.

[Bob Engle]

The heat shield was one of those parts that got trashed as they were a nuisance to work around.  More 32 Buicks are missing the heat shield than have them in place.  The photos are of John Scheib's model 57S, but the heat shields are similar on all series.

 

I carry an infrared thermometer whenever I am driving an old car.  It's easy to take temp readings on radiators, heat risers, brake drums, engine blocks etc.  

 

Bob Engle

[Dave39MD]

It doesn't look like the 31 had the mounts for a heat shield. Maybe Buick was trying to cure something in 32.

[Bob Engle]

You are right.  In that case, I would be sure to block heat riser tubes at the exhaust manifold.  You don't want that exhaust heat any where near your carb or air cleaner.

 

Bob Engle

[Dave39MD]

I found a core plug that fit the inner heat riser tube which is the inlet for hot gas. I also made sure the valve was in the off position.  Temp readings after about a 50 minute drive with a stop were 556 f at the exhaust heat valve, 226 at the midpoint of the horizontal tube going to the heat riser, 135 at the heat riser, and 144 at the carb bowl. The carb air horn was 160 f and the rad 156.

 

Do these sound like normal, non vapor locking temps?

 

It ran great but outside temp was only 70.

 

Dave

[Bob Engle]

The real question will come when it is hot and the condition returns.  It will be important to take the same temp readings and see how they compare to good operating conditions.  

That will give you the info to develope  plan of attack.

 

Bob Engle