Let me run this by Y'all

[Rob Z]

I've been having an issue on my 69 w/302 4v C4. I drive it everyday about 70 miles a day on the highway about 70 - 75mph. I find that if I run at 80 or higher it will eventually begin to run rough, backfires out the exhaust and will hardly idel. Well this happen this last Fri, and as I pulled into the driveway it was running like :rant. I opened the hood and pulled the air cleaner. The carb was gurgelling gas down the throat. Not constant but about a teaspoon burp every few seconds. I have a glass fuel filter and noticed bubbles comming up the line just as the carb would burp fuel.

So here is my thought. I can find nothing out of place and everything I've checked looks good. The fuel pump is of an unknown age (mechanical). My thought is a 70 the pump works good. When the RPMs go up the pump begins to overheat. This is where I believe the bubbles were comming from. If the ppump is having a problem at the higher temp, overheats it is vaporizing the fuel. This causes the bubbles and higher fuel line pressure. With the higher pressure it is over powering the float causing the fuel burping into the carb. When driving with this happening it would be running rich which causes the exhaust backfireing.

Does this sound plausable to Y'all? :shrug

 

[Mach1Rider]

First place I would look is ALL of the rubber hoses and connections.
The bubbles are air and it has to be coming from somewhere, IE; dried rubber lines allowing air to be sucked in.
The high rpm runs add to air being drawn in at a faster rate.
The bubbles are looking to get out, so each time the float lowers for more fuel to be delivered, the air purges and causes the fuel to overflow into the carb/intake.

Heat will cause the fuel to vapor and starve the carb, then stalling is your problem.

 

[Rob Z]

Well it looks as if the mechanical fuel pump was the problem. When I removed it there was fuel on both sides of the diaphram. I could see no visable holes but it was cracking. Once removed I manually operated the pump and could feel and hear a metalic grinding. Not sure where it was occuring, but it was enough that at speed it could have caused some heat.

Anyway, I replaced the pump over Thanksgiving and have driven it a little hard for a couple of weeks with no issues.

One problem down

 

[6t6red]

Make sure you check your oil for any smell of gas and change it if you have gas in it.

 

[Rob Z]

Paul,
Thanks, I did change the oil. I did not check to confirm if it had gas in it or not. But, I did not notice it being thiner then expected.