1966 F250 with FE390 and a Holley 2300 2BBL Carb. I have been hearing popping from the exhust when I take my foot off the pedal without putting in the clutch for a couple months. I put a vacuum presure guage on and made sure idle screws were set optimally. I was getting stalling last year so replaced the fuel pump thinking there was a fuel delivery issue. Now I am getting stalls after goosing it to get into on coming traffic and still hearing the popping (backfire) when I take my foot off the pedal after accelerating.

I read some threads on the decel backfire possibly being caused by a manifold leak so I checked the manifold bolts and 2 on the passenger side were a little loose. I am now about to move the vacume hose from the distributor and move to the carb to see if that makes a difference. Can anyone help with next troubleshooting steps? I can't tell if this is fuel delivery or vacuum issue.

Thanks in advance!

 

You have two issues that are not related to each other
The popping through the exhaust 99 times out of 100 is caused by an exhaust leak allowing air to be drawn into the exhaust system allowing unburnt fuel to ignite inside the pipes or muffler
Exhaust leaks between the heads and manifolds on FE’s is a well documented issue

As for the stalling I’m not sure what you mean about moving the vacuum line from the distributor to the carburetor

 

Thank you Desroked 450! so it sounds like the popping is something I either learn to live with or perhaps get a different muffler? I had the engine rebuilt last year, I asked them to put on mufflers that were "respectable but would NOT **** off my neighbors", I think they heard "AND" instead of "NOT", so I am guessing a more muffled muffler might decrease the popping noise?

I figured out my option of moving the vacuum line was not actually an option when I got back to my truck. I did notice though the two screws on the vacuum advance connecting to the distributer were loose. I tightened them up. Now I am thinking I need to check vacuum on the entire system. Looking at the carb without the air filter on, I tightened up any hose clamps for hoses connecting to the carb and to the rest of the engine. Was that a good next step? Can vacuum issues cause random stalling?

 

Muffler is not the issue, most likely a small exhaust leak between the head and exhaust manifold is

What’s your definition of stalling, hesitation when you step on the gas or the truck accelerates for a short distance and then noses over
One can be a vacuum issue while the other is most likely a fuel delivery problem

 

Hmm, how do I find a leak between the head and exhaust manifold?

The stalling is that the engine is losing power. I can pump the gas and the engine will start to come back to life but eventually it stops and I have to sit for a bit pumping the gas and turning the engine over to get it back up and running.

 

Vacuum isn’t going to cause that type of stall, if it did the engine would be running badly all the time
I would look at fuel delivery problems, improper float level, plugged fuel filter, bad fuel pump, collapsed rubber fuel line, blockage in the tank

 

The float level was high. I followed manufacturer instructions to check and lower the float level. That would explain why everyone thought my garage smelled of gas! Would this be a cause of stalling after goosing it to get into traffic?

 

Have you recently replaced your gas cap? If you did you have replaced the old cap with a non-vented cap and that could cause your stalling issue. Years back I replaced the gas cap on my 70 F100 and the next day headed to work it wound up stalling on me a few blocks from where I worked and wouldn't start back up. I was running late and left it on the side of the road until evening and when I got back that evening it started right up and I drove it home. The next morning was a repeat of the day before and it quit on me again just about the same place. My gas gauge read about a quarter of a tank but it made me wonder if the gauge was off. I had a piece of hose in the bed and decided to see if there was gas in the tank and when I opened the cap there was loud whooshing sound. The non-vented cap drew enough of a vacuum on the tank to interrupt fuel flow and stall the truck. Replaced the cap with a vented one and the problem was solved.

Another possibility could be vapor locking where the engine is running just hot enough to vaporize the fuel in line to cause it to stall. Had that happen on my 66 F100 several times, the factory temp gauge ran a tad bit on the high side but didn't show it as overheating. The eventual fix was replacing the radiator and that fixed the vapor lock problem.

Just a couple of things to think about and good luck on solving your problem.

 

Thanks for the ideas @wtx66f100 ! I did replace the gas cap with a locking one and have gone back and forth with that and the original. I have also scoped the tank and looked for blockage but that does not seem to be the issue. I am pretty sure the fuel system is good from the gas cap to the fuel pump after inspection so at this point, after modifying the float level, if I get stalls, it sounds like it might be a vapor lock. So thank you for the pointers.

Back to the exhaust popping. Do I need to take off the manifolds to inspect them? if so should I just order some new gaskets? Is that the known issues or is it the manifolds?

 

A lot of times you can see exhaust leaks...a little black streaking. Permatex 81878 Ultra Copper Max Temp RTV silicone will permanently seal leaks in manifold to head surfaces and pipe connections though it is a little messy and not as easy as gaskets.

 

looking closer at the manifold, I actaully do not see a gasket! So any recommendations? I found some on LMC but would it be better to spend a little more with what is at Napa?

 

I don't think they came with gaskets....at least not from the factory. I'd rather go to Napa so I can see the gasket and have an idea that it will fit. And I'd prefer copper or steel...not the paper blow out type.

 

If it is stalling when you let off the fuel I would look at the idle circuit in the carb. Seems like it is leaning out to me. Could have sucked some trash up in a jet.

 

I’ve never used them myself but I hear good things about Remflex exhaust gaskets
Another possible leak point is where the exhaust pipe bolts to the manifold

Have the manifolds ever been off the engine and if so how long ago, breaking the top bolts off in the heads is a well known issue when doing exhaust work on a FE/FT engine

 

Stalling update: I think tuning the float has helped. I took the same route that stalled out many times last week and only stalled once and was able to get it running a lot faster this time. So I think I will tune the float a bit more. I am guessing too much fuel in the bowl is causing the stall?

Popping update: I put a Fel-Pro gasket from Napa on, no change really. The manifolds were off the engine about a year ago when the motor was rebuilt. I did find a video on youtube that showed someone putting a straight edge on the manifold to show how the old one was causing leaks. Unfortunately I did not see that video before I put the new gasket on, that might have been a smart test... so now my dilemma, do I just get a new doughnut gasket for the exhaust pipe to manifold connection? Or do I just break down and get new manifolds?