The Stovebolt Site Forums The Shop Area The HiPo Shop sbc runs bad after 5 minutes

Swapped out my 283 for a stock 350 crate engine. Had to pull it out to balance at 1800 miles. Nothing else changed or modified. Starts, runs fine, gets up to temp(180 deg.)while driving, after about 5 minutes it starts to bog, and then starts to backfire when ever I try to get into it some, and keeps getting worse. Will cruise, but nothing more. It was a year before it got back into the vehicle and restarted. I thought maybe it was running out of gas and causing a lean situation. Checked fuel flow from tank, new fuel filter, new pump. Carb looks clean inside. Up untill it starts to act up, it will do what ever you want, so I kind of ruled out mechanical issues. Other than the balance issue, it run fine before, and nothing has changed. I am running an Edelbrock Performaer carb, and Petronix in the distributor. If it was the carb, why start then. I knew I was going to tear it down, so I run a tank of Marvel Mystery oil thru it , and then topped it off and added some more. Can't imagine that would affect anything.Any suggestions are greatly appreciated. Thank you for your help.

Take the gas cap off for a few minutes and see if that makes a difference. Could be stopped up enough now that it is letting the fuel pump create a vacuum on the gas tank. If so then with the cap off it should continue to run.

Another thing may be the coil getting too hot. How long does it have to set before it will restart and run?

When it acts like this check the spark to the plugs to see if it's bright blue. If not then something is failing in the ignition system.

I have a seperate hose that vents the gas tank, but I will check to see if it is open. The motor does not stop running, it will run alright in "cruise mode", but when ever I try to get agressive on the throttle, is when it is not happy. Before it starts acting up, I can floor it with no problem. I am driving it currently with the hood off, so I can't imagine it is a vapor lock type of thing. I thought about the coil, but have not checked to see if it's getting hot. Thanks for your reply.

Checked the coil?

Loosen up the exhaust pipe ahead of the muffler. The symptoms you're describing are classic for an exhaust system restriction such as a clogged muffler or catalytic converter if you have one. Another diagnostic procedure involves connecting a vacuum gauge to a source of straight manifold vacuum such as the brake booster line or the transmission modulator and hold the gauge down with a windshield wiper so you can watch it as you drive. When the bogging-down symptoms begin, the vacuum will drop to nearly zero and stay there anytime you're trying to accelerate. Loosen an exhaust pipe flange somewhere ahead of the muffler and see if the performance improves.
Jerry

FWIW.. I had a '70 Nova back in the mid 70's that exhibited similar symptoms to what you are describing.. it was a cracked distributor cap.

I vote coil also, I would think plugged exhaust would show up right away, not after five minutes.

Cracked Dizzy cap... I forgot about that one (and it was only 5 years ago) but yeah that would do it too.
It seems electrical to me whatever it is.

A new Distributor Cap was the first thing that I tried, no help.

All the things mentioned above. One step at a time. Do something, like the dist cap, then test. Make a list and go through it. Add items as needed. Add this one just for kicks, Vacuum leak. So far:
1, Vent at tank.
2, Coil.
3, Cap, done.
4. Exhaust restriction.
5,Fuel pump.
You get the idea. One step at a time. Let us know what you find.

If nothing else shows up, try pulling the pertronix and put in points and condenser.

By the way, is the backfire through the exhaust or through the carburetor?

Jon.

It mainly backfires thru the carb, but if I get on it hard, it does both.

tried dist cap, nothing
fuel filter, nothing
fuel flow from tank, I can fill a 2 pound coffee can in about 20 sec.
new fuel pump, nothing
took out jets, metering rods etc, clean
exhaust flowed fine when I took motor out, unless theres a mouse in there, but I wouldn't think that would change with time or temp.
I think I have another coil, I can stick it in and try it.
My fuel line is all hard plumbed, except from frame to pump, which is only a year old, so I don't see any kind of a fuel line collapse issue.
Thank you for your input. I'll keep you posted.

Venting the exhaust only involves loosening two or three bolts at a manifold flange- - - -might be worth a try before you reject the idea, as I've encountered the problem numerous times over the past 40-something years.
Jerry

Through the carb is usually a lean condition. I've seen a few problems with the eddy carbs. Check for leaks around the intake and carb.

I always thought a back fire was usually electrical and not fuel but
what do I know.....I had a mallory on my 235 and the truck would act crazy at times.. it would bog down, then run fine for awhile, then at hiway speed it would drop down to almost nothing and start up again, it would sometime back fire, etc, but ended up being in the electronic module....

1st choice would be the already mentioned clogged exhaust. I've seen it on a couple occasions. IN both case the engine would start and idle fine but cough,backfire and miss badly when accelerating.
Both were 80's model SBC's....

2nd swag would be one or more valves are too tight and once the engine warms up they aren't closing fully.

Good luck
Dave

Well, it turned out to be the coil. At least I didn't have to start tearing into things inside.
Thanks everybody for your help and input.

What coil were you using? I've melted a brand new msd blaster3 on initial start up. Sometimes ya get faulty parts. Glad you found the problem. I now run a blaster2. No issues in 14 years.

Msd 6al - 7000rpm pill
Blaster2 coil
Msd boost timing master
Msd billet distributor
10mm Taylor wires

It was a Petronix coil, worked fine for 6 years, and someting happened while it sat for a year waiting to be put back in. New one is the same thing, works fine.

Thanks for the reply. Happy motoring