Engine problems when hot, please help

[1970heavychevy]

Any help greatly appreciated. I have a 70' C10 with a 70' 350 Impala motor. Motor was rebuilt 5 years ago when it belonged to my dad. He drove it 2 years before giving him the same problem. I bought it 2 years ago knowing it had a problem. It fires up no problems and seems to idle fine. When it gets hot, after idling for 10 minutes or a short drive, it loses 85% of its power. This is a run down of what I've done in the past week but no luck.

My dad had the fuel tank sock replaced when he had it.
I replaced all the fuel lines, fuel pump, fuel filter, added a regulator, and cleaned the carb (Edelbrock 1405).
Replaced the distributor cap, button, module, coil, and plug wires.(HEI)

What else could it be? I want to get started on the fun stuff of this build but won't until its running.

[WorkinLonghorn]

I would start by looking at a spark plug or two to get an idea of what is going on,but it really sounds like fuel starvation. Check float level,etc.within the carb.or just try another carb.
I dimmly recall having a car once that had the carb attached directly to the manifold without any heat insuator and the gas would boil in the carb and cause the same prob.

[1970heavychevy]

Would air bubbles, when off, in the fuel filter close to the carb. mean anything? All the plugs look good. Gapped and clean.

[WorkinLonghorn]

No, But I'd check (very carefully!) by removing the fuel inlet from the carb after warm-up and during this 85% drop-off,and measuring the flow and pressure.
You might want to check the cylinder cranking pressure too after warm-up.I suppose that if the valves were badly out of adjustment,you might have that problem.

[1970heavychevy]

I need to check about the heat insulator. I am reading about the gas boiling and it makes a lot of sense. The thing that really confuses me is that my dad said the problem came out of no where. Every thing ran strong for about 2 years after getting the rebuilt motor and then it started.

[jay6891]

Have you tried to run a new wire to your HEI? Would be a quick try, run a new wire from fuse box, 12 volt switched, to dizzy. Might be the wire breaking down when it gets warm/hot and causing a bad/weak connection. And your losing volts to distributor.

[1970heavychevy]

Jay6891, that's a cheap thing to try. I need cheap at this point. As far as hot fuel, does anybody have any comments on fuel coolers?

[jay6891]

Yea I like cheap too. First thought was your coil going out for the same reason. But you said that you had already changed it. Chevy are to good to just throw parts at, sometimes its the simple things that make them a problem. Hope you get it fixed. You said that its an old problem, the new parts didn't fix it. So my thought that whatever it is., it will be minor, 50ct/$1 part.

[jay6891]

Hey another thought, gas cap been replace during that time?, might not be a vented type and pulling a vac on fuel tank and starving engine.

[1970heavychevy]

I planned on sorting the wires tonight any way. So replacing that will fall into line. What gauge should I use? And any thoughts on fuel coolers?

[jay6891]

10/12 gauge would be my choice. Just try loosening the fuel cap and see if that changes anything before you go out and buy a new one.

[1970heavychevy]

Thanks, I will be working on this stuff tonight.

[Luvlegs]

does it have a heat flapper thingee at the exit of one of the exhaust manifolds - (opens when the engine warms up). When I was a kid, I had a 400 that would do it until my dad pointed out there was hardly any exhaust coming out that tail pipe

That was so long ago, I can't even remember if I have all the details correct or not.... I might need a nap....

[1970heavychevy]

It has headers, I'm not sure if they do or not. One thing to mention is my engine compartment seems to get really hot.

[Luvlegs]

fan clutch?

[1970heavychevy]

Fan works, just didn't know if a hot engine compartment would cause my original problem.

[OLDIRON69]

old racer buddy of mine puts heater hose over his fuel lines under the hood. Works for him to keep 'em cool.

[1970heavychevy]

I read something like that but I don't have metal lines and the lines I have don't feel hot. If I had the metal ones I would give a try though. Thanks

[Indyuke]

Does it seem to overheat? Puking fluid, that sort of thing.

Does the loss of power seem smooth? Is it all of a sudden, or does it happen over the course of several minutes? When you drive the truck, does it seem to run smoothly despite the loss of power or does it seem somewhat jerky?

Based on what you've said it sounds like a carb issue. Maybe after the choke fully opens...

You said your dad replaced a bunch of stuff when he had it. When did he notice the problem happening? Do you know if he replaced all the stuff because of the problem, or if maybe the problem happened after he did some of the stuff?

Wow! Just realized I asked lots of q's.

[1970heavychevy]

No puking fluid problem. Loss of power seems smooth, no jerking. It will fire up and take off like it should (get a wheel in second (auto trans). But after a few miles it bogs and loses power. My dad only replaced the tank sock after the problem started, I did the rest this past week. I am leaning towards fuel boiling in the carb. from what I've read but the gas cap not venting is another possibility. Just not sure at this point.

[72BLUZ]

check and make sure the intake has not blown a gasket

[leddzepp]

Well, ignition and carburation have been addressed....what about compression? A loss of power that is steady , and not jerking or sudden sounds like a compression loss. If it idles fine and runs without missing, then you do not have a problem with fuel delivery or ignition. Run a compression check on all 8 cylinders. I would be willing to bet you have a major loss of compression on at least one of them. Sounds like a bad head gasket to me, or maybe a bad piston ring.

[Indyuke]

Clogged exhaust possibly? If the carb, ignition and compression seem fine, then a severely plugged exhaust could cause a loss of power after just a few miles.