Thread: Stumbling...Missing...

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Howdy all, hope you had a good Thanksgiving weekend.

I'm looking for possible causes for a problem. The truck has a SBC350/TH350 combo. I've been messing with it to get it running right for the past few weeks, and I thought I had it. So I took it to work this morning(about 25 miles) it ran perfect for the first 15-18 miles then stumbling & missing from a standing start and up through the rpm's. After about 4 miles of this it started running good again. It did the same thing on the way home.

The motor: 350 4-bolt, 10:1, cast iron heads(2.02/1.60), rpm air gap intake, Edelbrock 1405 600cfm carb.
Ignition system: MSD pro billet mech adv distributor(set@ 10 idle,35@3200), Mallory HyFire VI-AL, Crane PS92 Ignition Coil.

Any ideas?

I don,t really know what your problem is but am interested in knowing how long you have had the truck in running condition. I have a 65 chevy truck and it did the same thing and like to have drove me nuts. I found out that even thogh i had pulled the gas tank and cleaned it that i still had a problem with the pickup tube. I had to replace the tube to clear the problem. Good luck in finding your problem. Hope someone else can give more help than i can.

Originally Posted by ceh383

Howdy all, hope you had a good Thanksgiving weekend.

I'm looking for possible causes for a problem. The truck has a SBC350/TH350 combo. I've been messing with it to get it running right for the past few weeks, and I thought I had it. So I took it to work this morning(about 25 miles) it ran perfect for the first 15-18 miles then stumbling & missing from a standing start and up through the rpm's. After about 4 miles of this it started running good again. It did the same thing on the way home.

The motor: 350 4-bolt, 10:1, cast iron heads(2.02/1.60), rpm air gap intake, Edelbrock 1405 600cfm carb.
Ignition system: MSD pro billet mech adv distributor(set@ 10 idle,35@3200), Mallory HyFire VI-AL, Crane PS92 Ignition Coil.

Any ideas?

I had the same problem with a shop truck I bought a few years ago.
It turned out someone had mounted another fuel filter in-line and it was plugged a little bit.

Thanks for the input. At this point I don't think it is fuel related. It has a new tank with a 3/8 line out to a LARGE filter then into an electric pump and up to the carb where there is a smaller glass filter that is full, and everything is new. I almost forgot there is a Holley pressure regulator set to 6psi @ idle. I removed the line at the carb hit the switch....pumps like crazy. The thing is it runs great until it warms up fully, then it's hit and miss.....

Originally Posted by ceh383

Thanks for the input. At this point I don't think it is fuel related. It has a new tank with a 3/8 line out to a LARGE filter then into an electric pump and up to the carb where there is a smaller glass filter that is full, and everything is new. I almost forgot there is a Holley pressure regulator set to 6psi @ idle. I removed the line at the carb hit the switch....pumps like crazy. The thing is it runs great until it warms up fully, then it's hit and miss.....

I don't know how warm it is there but do you think you may be getting "vapor lock"?

I don't think that's it either, I have a 3/8 vent in the tank. As for temp.....outside air was about 52 on the way in and 68 on the way home. The temp gauge hit 180 and stayed there. I keep thinking ignition but today I tried initial timing of everything between 5 and 15 degrees. I recurved the distributor for more and less total adv. and in sooner and later......it all makes no difference....

still sounds like a fuel problem to me.... how close to the engine or exhaust is your fuel pump? also i wouldn't want to run a glass filter.... they look nice and make it so you can see the gas.... but they have been known to cause fires, they make clear plastic ones that function the same but dont break so easily

I see your point on the glass filter, I'll probably change it. The pump is at the rear by the tank about 12-14" from the nearest part of the exhaust. The gas line runs in the frame rail away from the exhaust until it gets to the engine compartment. At the closest point it's about 7" from a header I can put a strap on it to move it 2" farther away. I don't think that will help though....

wrap the fuel line close to the exhaust with header wrap.... should solve any heat related problems with the fuel lines if there is any.... good placement on the pump... i've seen a lot of people mount them in the engine bay, which is just too much stress on the pump when it gets hot.....

Thanks, I'll try the header wrap.....Any other ideas?

I think I found the problems....
The first was a bad set of plug wires, brand new MSD super conductor wires arcing to ground. Replaced them and it got a little better, but still not right.
The second issue, you guys kept pointing to fuel, so after going over everything several times and not finding anything I decided to pull the intake. It looks as if I had a leak on the bottom side. I should have it back together tomorrow or the next day.....and we'll see....

i'm glad that your persisted and found the problem..... leaky intakes can sometimes be a hard thing to really find... specially if its leaking on the lower side..... hopefully when you get it back together she'll run like shes supposed to

Well, I got her put back together warmed her up set timing check carb.....every thing's ok. So off for a drive we go.....wow it's running good. I take her home and pop the hood re torque the bolts and I noticed I didn't hook up the alternator(left the wire zip tied to a fender support) so I hook it up and take her for another ride....now it's running like garbage again. Unhook the alt it runs good again.
So now the question is how can the alt make it run so poorly? It's a new 100amp 1 wire deal....

Now that we're back, I thought I'd post what was up....

After reinstalling the intake I gained 1" of vacuum, so I did have a small leak. It wasn't the real issue.
After finding (by accident) it ran fine with the alternator unhooked I replaced it, and guess what.....it still had the same problem.
I came to the conclusion that it had to be the ignition box (a brand new HyFire VI-AL) so I replaced it with an MSD-6al.....It's run flawlessly since.
For what ever reason the ignition box could not handle more than 12 volts once it got warmed up.....go figure.....

Thanks for your time and input......