1990 7.4L w/B2 idle problems

[trimjb]

My 1990 7.4L is having some idle issues. This has a Rochester Carb.motor.
I started experiencing idle problems last year. Can fire up but anything below 1000 -1500 rpm stalls out.
Had carb rebuilt by a carb expert that does nothing but carbs. Changed out the electronic sensor (Thunderbolt), rotor and cap. Drained all fuel from tank (thinking that fuel was old, new fuel filter. . Still same issue.
I started the motor between each step to see if progress had been made. Nada.
When I pull back slowly from about 1500-2000 RPM no load the motor will start to stall. Pump the throttle lever and can sometimes get back to high RPM. Going back to idle the motor will hiss thru the carb like the butter fly is opening up. Possible starving for fuel??
My next step will be spark plugs (have not run a compression) and possible new coil.
I have a thought that maybe the fuel pump is not delivering at lower RPM. There is a hard line between the fuel pump and carb. so not sure how to install a pressure gauge unless I remove the hard line and build a soft line. No fuel in the sight tube from fuel pump to carb. One thing I did notice was when I pull the sight tube off the carb while running I get a vacuum on the fuel pump end. Is this normal?
Any thoughts would be helpful.

[Ruffryder]

fuel delivery problem sounds like,
remove the oil fill plug on your mechanical fuel pump to check.

[Nauti_Mike]

The sight tube is connected to the fuel pump between two diaphragms. There are two to isolate fuel from getting into the crankcase and cause an explosive condition. when either fails oil or gas shows up in the tube as a warning. Under normal conditions the tube is dry. Never thought about it but I guess the push pull between the diaphragms may create a vacuum. How many hours are on the engine? Have you check the ignition timing? I would also check the plugs and compression. Also inspect the exhaust to see if water is leaking into the exhaust stream and getting sucked back into the engine.

[builderdude]

Warm it up, pull the plugs, and compression test the engine. This will confirm or eliminate the possibility of an internal mechanical issue. Even if the fuel pump pressure was suspect the amount of fuel in the float bowl should keep it idling for a bit before the bowl was run down. The carb was rebuilt by a reputable guy? Where the fuel lines replaced? They deteriorate on the inside and will clog up a freshly rebuilt carb. Sounds like the idle circuit isn't functioning properly.

[Nauti_Mike]

I would think a fuel pressure issue would show up at higher rpm and not idle.

[builderdude]

I agree Mike

[builderdude]

The "hiss through the carb"
Just for giggles verify the firing order is correct.

[trimjb]

fuel delivery problem sounds like,
remove the oil fill plug on your mechanical fuel pump to check.

Don't know anything about an "Oil Fill Plug" on my fuel pump

[trimjb]

The "hiss through the carb"
Just for giggles verify the firing order is correct.

Nothing has changed on the firing order. I have the same symptoms as before I started replacing components.

[trimjb]

Warm it up, pull the plugs, and compression test the engine. This will confirm or eliminate the possibility of an internal mechanical issue. Even if the fuel pump pressure was suspect the amount of fuel in the float bowl should keep it idling for a bit before the bowl was run down. The carb was rebuilt by a reputable guy? Where the fuel lines replaced? They deteriorate on the inside and will clog up a freshly rebuilt carb. Sounds like the idle circuit isn't functioning properly.

Yes, carb rebuilt by a very reputable shop. All they do is carbs.
New fuel hose from tank to fuel filter/water separator. I was able to inspect the fuel delivery tube in the fuel tank when I removed all the fuel. Steel tube from fuel pump to carb.
I will be changing plugs this weekend.
Idle circuit suspect. I will be tweeking it this week end to see if adjustment helps.

[JThiessen]

Your changing plug wires too I assume? And how about the coil?

[trimjb]

Thanks for all the helpful hints here. Kind of scratching my head. Process of elimination I guess.
I would like to do a fuel pump pressure test. The steel tube from pump to carb make it more challenging. Has anyone changed this pressure line to a CG certified flex line?

[trimjb]

This is the fuel pump. Pretty crusty, Out of site out of mind.

[Chief_Alen]

Choke opening all the way when warm ?

There fuel in that yellow fuel line from the carb to the pump ?

[Don77]

Ck for a vacuum leak! Ck around carb base and at intake manifold to head for vacuum leak!
If it wont idle it must be a bad leak and that would make it easier to find!
Get it to idle down as low as possible when you ck for leaks!
Good luck!
Don

[Nauti_Mike]

I agree Don a vacuum leak will cause this issue.

[builderdude]

With some fittings you could test fuel pressure at the end of the metal tube where it connects to the carb, problem is because she won't idle you'd have to just crank her over and I'm not sure cranking rpm is sufficient to test fuel pressure. You can disconnect the fuel line at the carb and run it into a clear glass bottle to see how it squirts while cranking.
Only do this if your comfortable doing so, don't want to hear about you on the news

[trimjb]

Choke opening all the way when warm ?

There fuel in that yellow fuel line from the carb to the pump ?

Will check the choke. No fuel in the sight tube.

[trimjb]

Ck for a vacuum leak! Ck around carb base and at intake manifold to head for vacuum leak!
If it wont idle it must be a bad leak and that would make it easier to find!
Get it to idle down as low as possible when you ck for leaks!
Good luck!
Don

New gasket on the carb base, torqued to spec. Will check intake manifold to head.