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post #1 of (permalink) Old 01-30-2004, 10:30 AM
SHINTON
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HELP 350 wont run under load!

Ok, my GMC Jimmy either HATES me or LOVES me too much, I have just about had it sold twice, and this time a guy drives almost 7 hours one way and she won't run right.

Here is a short version, 2 months ago, crank crank crank, not even trying to start (no spark) we replace distrib cap and rotor and all works till this last weekend.

Sunday, snowing like mad in NC, I go outside to crank it up, the guy is on the way to buy and crank crank crank, not even trying. I get some advice and decide to replace the ignition coil on top of my distrib cap. The Jimmy DOES crank after a while (wet electric issues from snow blowing into engine bay while replacing)

It finally starts, and I let it run/idle for 20+ minutes hoping all will dry out. I try to take it for a test drive and as soon as gas is given under load it stumbles and wants to die, let off and ok (ie idle no load)

I can race the engine in neutral/park, and is fine. (If I goose the gas real hard the carb/engine is backfiring some, carb makes a pistol shot sound and spits up a bit of gas vapor)

Based on some advice, I take it a mechanic who know more than me, but we are still having issues.

He has checked the timing and it is fine, he replaced the module (under the rotor) and checked the pickup coil which he said is ok.

You still cannot put a load on it, or it dies, I havent asked him about the backfire.

Some MISC info, the stupid exhaust manifold on drivers side IS cracked and is part of the backfire issue I am sure, you can hear the leak.

His next suggestion is he is going to swap the carb off his Blazer onto mine to see if it is the carb causing the issue.

If not, boy I am really scratching my head on this one, what do I look for? This has to be either electric or fuel right? Since I "had" an electric issue that I fixed and this problem started right after, doesnt that mean I still have an electric issue?

HELP!!!

Sam Hinton
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post #2 of (permalink) Old 01-30-2004, 12:06 PM
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Re: HELP 350 wont run under load!

A whole bunch of stuff can cause this. Why didn't you replace the plug wires?

An EGR valve can cause this.

Low voltage to the distributor can cause this.

A massive vacuum leak can cause this.

A stuck choke can cause this.

A smoked ICM can cause this because you didn't put heat sink compound under it.

A bad ignition coil can cause this.


You didn't even bother to tell us what year vehicle this is to be able to tell if it has an APT carburetor, an EGR valve, horrible vacuum lines, or anything.
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post #3 of (permalink) Old 01-30-2004, 12:24 PM
SHINTON
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Re: HELP 350 wont run under load!

Sorry, this is a 1972 GMC Jimmy with an upgraded ignition to HEI. (Done before I bought it in 94) I dont think it has most of that stuff, but I could be wrong?

I am collecting things to look for, vacuum seems to be the most popular!

Sam
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post #4 of (permalink) Old 01-30-2004, 06:12 PM
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Re: HELP 350 wont run under load!

change your fuel filter
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post #5 of (permalink) Old 01-31-2004, 03:52 AM
 
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Re: HELP 350 wont run under load!

This may seem kind of simple but check to make sure you have the plug wires on in the correct order 1-8-4-3-6-5-7-2. I had some thing similar when I accidently reversed the #7 and #2 wires.
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post #6 of (permalink) Old 01-31-2004, 08:20 AM
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Re: HELP 350 wont run under load!

Upgraded?

The biggest thing overlooked when upgrading to HEI is they forget to eliminate the ignition resistor. The resistor is in the wire, not a separate discreate resistor.

Check it: Key ON - Engine Off - make sure you have a full 12 volts at the + Batt terminal on the distributor. If not, drag a new 10 gauge wire from the ignition switch to that terminal.
Without it, the secondary is weak, possibly causing your wet trouble.

But - back to the question. The plug wires - are they still running down along under the head in the mud and grease? Grease and dirt is conductive, especially when you mix a little water in. It takes much more than 20 minutes to dry them out.
One at a time, wipe the wires down with paint thinner - wipe, not soak. Then re-route them where it's clean. Or if it's time, put on a new set.

The backfiring is an indication of either cross induction or crossfiring (spark jumping from one to the other) or a lean condition.

Whenever it backfires out the top it burns the fuel in the intake, starving the others - making them lean too, then they backfire, burning more in the intake. So crossfiring on just 2 cylinders often makes it very hard to start or run.

To check for lean, simply hold your hand, or a CLEAN rag over the carb to choke it off. Restricting some air makes it richer. If it runs better covered, it was lean. A trick useful at any speed.

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