72 f100 390 running on 4 cyliders

[MtStealth]

To begin with, the truck in question is a hammered 72 f100, originally a 360 4x4 manual, currently 390 4x4 manual with edelbrock 1405 and edelbrock manifold. Until 4 months ago it had a 750 cfm edelbrock on it that was dumping too much fuel but i used it for dump runs and didn't care. Finally got around to putting a 600cfm on it and chose edelbrock as it was readily available and the truck was already equipped, nothing against holley Id rather have one but this was $280 new vs 400, and as i bought the truck for 400 I wasn't wanting to dump too much into it at a time (i have put roughly 1200 into it including tires in 8yrs).

Now onto the issue at hand, I was driving down the highway the other day (about 65mph) truck doesn't like going much faster than that, when i got off the high way at idle it is running very rough. limped her home and started trying to figure out the issue. Started with the firing order and timing to make sure it was still set right seems fine, about 10 degrees advanced which it has been for a long time with no issues. check to make sure Im receiving fuel, it has an electric pump as one of the bolt holes stripped out for the manual pump and there are enough threads to hold a block off plate but not trusting it to hold the pump (already had one work itself out). Have plenty of fuel and the regulator is set to 5psi before the carburetor.

Trying to check which cylinders affect rpm, reveals that cylinders 2,3,5 and 8 have no affect on the rpm at idle. The truck seems to run at 1500 rpm plus but at idle seems i have nothing. pulling the plugs after idling they are all clean and dry. Looking at the carb without filter and when i activate the throttle can see fuel enter the chamber. i believe the intake is a performer 2105. all 4 of these cylinders are fed by the same side of the carb and are connected by the same runner or so it appears.

Any thoughts on what could be making these 4 cylinders not run would be appreciated, currently the best i can come up with is either the venturi that feeds this side is plugged and fuel is only getting through when the accelerator pump is activated, or the intake runner has burned through and unwanted exhaust is blowing out the spark. the latter seems unlikely as then i should have wet plugs. I have drained the oil to see if i am getting fuel in the oil and it doesn't smell strongly of fuel.

[Ranchero50]

It's an Edelbrock, I have no idea beyond pop the top and give it a thorough cleaning.

[colnago]

Do you still have the old carb? You could swap it back on and run the same checks, just to make sure it's the carb and not some blockage with the intake. I also have a 1405, and I've never had any problems. Maybe the jet on that side is bent, or the spring is broken, so that side never opens. That shouldn't affect idle, though. I'll have to think about it this afternoon. So far, the only realistic thing that comes to mind is a good cleaning, like Ranchero50 suggested.

Joseph

[Ranchero50]

I'm pretty sure the idle circuit is similar, either adjusting the fuel or air signal to the fuel at idle. Google should bring up plenty of info on tuning and rebuilding Edel's.

[MtStealth]

Yeah I am about to throw the 750 back on and see if it runs "better" but i dont want to leave that on there. I will pull the top off tonight and see if it has debris in the jets, i changed them when i put it in, ~4000ft elevation. it has 3 filters between tank and carb so it shouldnt have debris but who knows. the pump is 2 months old and the carb was new when installed late january. As to adjusting the idle circuit, no effect

[BobbyFord]

Pull the top off the carb and clean the primary venturi booster tubes. I'll bet one side is clogged.
Scroll down to page 21 ---> http://www.edelbrock.com/automotive/mis ... manual.pdf
#32 is the boosters. The debris will be in the small tube that points downward.

[MtStealth]

Problem resolved, I didnt want to think the issue was in the carb as it was so new but turned out the fuel line to the carb was deteriorating. found tiny chunks of rubber in the fuel bowl and that is what was plugging the venturi. all clean now, time to fix exhaust leak.

[Manny]

, time to fix exhaust leak.

Welcome to fordification and good luck on the exhaust leak.

[colnago]

Glad you're back on the road! Always feels good to find/fix a problem with your 50-year-old ride!

Oh, and to the board.

Joseph