Carb keeps boiling over!!!

[mike105105]

Hey guys, I need some help. I thought I fixed it with a rebuild in the begining of the winter, but apparently it was just the colder temps. When my truck gets hot the engine starts cutting out, and in the worst cases dies completely then vapor locks till it cools down. My temp gauge stays in the middle so I know the rest of the engine isnt overheating. If I pull the air cleaner off when it dies I usually see gas bubbling out of the vent tubes on the carb till it goes dry. I do not see why I would need a carb spacer now since the truck went the past 30 years without one.

thanks,
Mike

 

[Can Can]

Are you running a 305?

 

[rcamacho]

Assuming there's nothing wrong with the engine, throw a heat insulating spacer under the carb. The plastic ones have worked well in my experience.

 

[mike105105]

No 305 here....pure 1977 350....with only ~90k miles.

I was thinking of adding a spacer, but like I said, I dont know why it has gone 30 years without one and now it does....me thinks there is a problem somewhere....hope not though lol


thanks,
Mike

 

[Can Can]

When's the last time you swapped in a new fuel pump?

 

[mike105105]

The previous owner replaced it last year I think...looks very new.


thanks,
Mike

 

[Can Can]

Looking new and running new are two different things, brother. And aftermarket mechanical fuel pumps are notorious for breaking down way too soon.

Vapor lock can be caused by a mechanical fuel pump that's on the way out.

 

[dyeager535]

'77, doesn't that have a fuel return?

GM DID have a problem with this on the cars eventually (especially the chev V8 powered monte carlo's) but never heard of anything particular to the trucks, and as you mention, 30 years and no problems, why now?

I'd have to agree with the fuel pump, especially if you've got a return line. Fuel should be constantly circulating with a return line, which is one way GM addressed "vapor lock", which is actually fuel boiling in the line BEFORE the fuel pump. (pump doesn't work with bubbles)

You don't have a fuel pressure regulator on there do you? I'd also verify your engine is as cool as you think it is. Temp gauge or sender might be inaccurate.

 

[JEBSR]

You should also pull the return line and the fuel cap and insure you can blow through the line. If it is clogged, that could be your problem as well.

 

[Metrodps]

I had same problem on a 1974 350 Chevy. We put a thick carb gasket on it and it solved the problem. Besure the gas cap vent is working also.

 

[mike105105]

Thanks for the replies. No return line as far as I have noticed. Line goes into the pump, up to the carb and thats it. Would a dying pump really make this happen? I know the carb is getting a good amount of fuel because I see it all boil out when it dies. I am fairly certain the temp gauge is accurate, it comes up smoothly and hovers around 1/2, i have seen it go higher at times so I know it doesnt get stuck at 1/2. I guess I will have to try a spacer.


thanks guys,
mike

 

[dyeager535]

Is half even the right position? On mine that's indicative of 210*, I believe the sweep on all the temp gauges is the same. Mine hovers right around 11 o'clock on the gauge face at 195*. While not hot, if you are getting 210* or higher, you may actually have a problem. A radiator that isn't working to the best of it's ability would allow the temp to get higher than normal in the engine and maintain that temp.

If it's a cooling system issue, it would explain why a 30 year old system would start having problems, and it would likely be a gradual onset.

Of course, if the cooling system is brand new, then forget all of that.

 

[rcamacho]

Assuming there's nothing wrong with the engine, throw a heat insulating spacer under the carb. The plastic ones have worked well in my experience.

Another thought occured to me - heat riser valve. If you are still running the stock riser valve in the exhaust it could be stuck closed, thus impacting performance and heating the wee out of your intake and carb well past what's required for cold startup performance. I had a similar problem on an old carbureted 402 BB many years ago. A new riser valve solved the problem.