The Stovebolt Site Forums The Shop Area Engine Shop Serious "pause" at about 3000rpm

I have a '68 Chevy with a 350. During my last post here, y'all gave me some good thoughts on my intermittent over heating and collapsing upper hose following the replacement of the water pump. I replaced the thermostat and radiator cap, and problem is now solved. I think the collapsed hose was a result of a vacuum forming (the radiator cap did not have a vacuum relief) after the thermostat finally opened and the coolant dropped in temp.

Anyway, my new problem is a serious loss of power at about 3000 rmp, usually around 65 mph, but sometimes more or less. I am concerned that by running it very hot a few times I might have done some damage?

The power loss is like running out of gas - a complete cough for several seconds until the rpm lowers and it kicks back in. If I just push it, it pulses, but gets worse.

Thoughts?

MAER,

What kind of ignition do you have on your truck, and what kind of carburetor?

Stuart

sounds like fuel delivery or ignition issues.

Define "running hot"

Scott

Start by changing the fuel filter, particularly if it's one of those small ones at the carburetor inlet fitting. They're notorious for getting clogged up just enough to restrict the fuel flow at or near full throttle.
Jerry

Answers to follow up questions:

The truck has HEI ignition with a 650 Edelbrock carb.

By running hot I mean before I replaced the t-stat there were several occasions when the radiator was cool but the needle was solidly up against the H side of the gauge before the t-stat opened and it went down. The first time it happened I was about 50 miles from home, so I ran it hot.

hot is still an arbitrary value in this case. H on the gauge corresponds to ? If you blew the cap (assuming a 7psi cap) you were at 235F with a good mixture of coolant, if you didnt blow the cap, you were under 230, which is fine, shouldnt have done much damage.

Check the HEI module along with the cap/rotor and plugs. Then move to the carb.

I changed the fuel filter, but it still continued to kind of cut out at about 65 mph. This morning I put in some '93 octane gas to see if it would run better, and it got much worse.

MAER,

I looked at an exploded view of your carburetor but it isn't clear to me if there is any internal filter beneath the carburetor inlet. I think this is the one Jerry (HRL) mentioned changing.

If you are dead certain that the octane of the fuel made a difference, it would tend to make me suspicious of your HEI unit, or at least some facet of the ignition system.

Have you tried running the truck up a steep hill and lugging the engine a bit. I don't know what kind of gearing you have, but can you get the rig above this problem, or does it get worse and continue to buck and cut out?

It isn't clear yet (to me anyway) if the problem is the ignition or a starved carburetor.

Stuart

The filter I changed is in the line just before the carb.

I am down to about 1/4 tank of the '93 octane, and will refill with '87 to confirm the fuel difference, but I am pretty sure it made a difference, unless there is some other variable I overlooked.

When I try to push it past the cough, it gets worse. If I do push it like that, it gets so bad I need to take it down to an idle to bring it back. Otherwise it gets better as soon as I back off.

Remember that the truck had overheated badly a few times last month - I have been worried that it might be partially blown head gasket? The oil and coolant look fine.

If you didn,t have a major lose of coolant I don,t think the o-heat caused any damage. If the head gasket was bad or a cracked head yiu would get white smoke out the tail pipes.
Check underneath the truck. Sometimes guys put a inline filter under neath.(I do). Check the flex line. You could have a bad fuel pump. Some models have a bag filter on the pickup tube inside the gas tank. It sounds like a fuel problem to me, but could possibly be the ECM in the dist.

MAER,

The little filter that is mentioned isn't visible from the outside. It is inside the carburetor housing, right in back of the fuel inlet nipple...this is provided your carburetor has one.

It sure does sound like fuel. A overtemp on the engine should have done nothing to affect the fuel system. On the other hand, a electronic component could be affected by this heat deal.

I had a pickup with a 350 in it that was severely overheated by the previous owner. It had a funny miss as idle and it was traced to some valve springs that had lost their temper due to the meltdown.

Although this sounds like the classic fuel starvation, I'm going with the dark horse, the HEI module that took a hit from the heat. Dig around and prove me wrong!

Stuart

Problem solved! (or at least symptoms gone).

I found another filter in the line under the truck. When I replaced it, the fuel from the old one had a ton of rust and stuff that dumped out.

At the same time, I looked at the HEI, and found one of the terminals and its ignition wire fouled black. So I cleaned that off.

After those two changes, it runs smooth to high speed. Not sure what solved the problem, or if I got to the bottom of the root cause, but I am happy anyway. Good to have all my horses back.

Thank you all for the help!