Ugly Tech headscratcher...

[Budbeater]

This is going to take some background so it will end up long. Please bear with me because I need the help.

My truck (1971 Chev, 355ci, Edelbrock 650 Carb, HEI ignition) was idling high so about a month ago I messed with the choke (electric) because it seemed like it wasn't set right to disengage. I cranked the setting down and it seemed fixed until the next morning. The next morning I went out to Locomotive Springs (about 1 1/2 hour drive) I stopped in Snowville to top off on gas. About 3/4 mile outside of town the problem started. The truck would be driving fine and then with no warning would violently shudder as the engine would cut out and restart several times in less that a couple of seconds. Then the engine would shut off completely. Sometimes it would just shut down with no warning. Sometimes it would just buck and jump as the engine would cut out and restart rapidly. The lights and the rest of the electrical system would never even flicker and the truck would start back up easily even while we were still at speed. Getting home on the freeway was horrible. In what seemed to be completely at random the truck would rapidly stumble and shut off like described before.

At first I thought I had over adjusted the choke, so I adjusted it back incrementally several times. This has not helped. In fact yesterday I disconnect the electrical input to the choke and the problem stayed exactly the same. Then I thought that I got bad gas at the station in Snowville, so I added some water remover and I have checked my filter, but nothing seems clogged and I have put in 4-5 tanks of gas since. The stalling seems to be too rapid to be fuel loss.

I have noticed that the problem is somewhat temperature related. It doesn't seem to start until the coolant temp guage goes over 180. It also seems to be worse when the air temp is warmer.

Also the problem seems to be slightly worse under some type of load (going up a hill, starting from a stop, accelerating)

I am wondering if the problem is a short in the HEI ignition. I don't know what else it could be, but I don't want to spend money on a new coil, module etc if that isn't the problem.

I have only had this truck about 18months and I am slowly going through it to upgrage and replace anything worn or broken, so I know that I have inherited someone else's issues.

Any Ideas? Any trouble shooting that I have missed?

BBB

 

[great scott]

Its not going to be easy to catch but you need to find out if you are lossing fuel or spark. You could start by hooking up a psi gauge next to the carb and run it out by the windshield so that you can see it as you drive. Be careful not to pinch the fuel line on the gauge or let the hood fly open.

 

[DaveB]

A plugged fuel filter can act that way but it may be electrical . After checking fuel flow look at the ignition system. You should check to make sure that whoever wired the 12 Volt feed for the HEI didn't use the original wire that went to the coil in the old 71. The original wire is a resistive wire which is meant to limit coil current and is also too small of guage for the HEI. On my old 72 chevy that I built up years ago I used the factory feed wire from an HEI donor truck and hooked it to the same fuse block feed that was for the coil wire. The feed is a large wire and can handle the current. On several of the HEI equiped vehicles I have had the spark plug wires cause a very similar problem. They misfire under a load and you would swear that the carb had a problem but it was actually the wires. I had this happen on three different cars. If changing wires doesn't help I would next change out the module in the HEI, they aren't that expensive and if it doesn't solve the problem you will have a trail spare. Next, the coil.
Dave

 

[Budbeater]

Update: I replaced the coil, cap and rotor. I found a corroded terminal on the cap and a bad plug wire. I pulled the bad wire, but I still have the problem.

The module to coil wires appeared fine. I am going to look over the whole length of the power wire. It should be adequate in size since I have been driving the truck 18months with no problem, but it could have been burned on a header tube.

Bud

 

[gijohn40]

I think that when you adjusted the choke you might have messed up the spring on the choke... and when the temp gets hot or the vacuum from a heavy load kicks in the spring is pulling the butterfly down and cutting off the airflow to your engine....

 

[Budbeater]

I think that when you adjusted the choke you might have messed up the spring on the choke... and when the temp gets hot or the vacuum from a heavy load kicks in the spring is pulling the butterfly down and cutting off the airflow to your engine....

How would I inspect/fix this?


Bud

 

[TheBigSgt]

I had this problem with a 88 dodge once and it was the pickup coil inside the distribitor. What you have sounds electrical to me.

 

[Budbeater]

I replaced the fuel filter today. It was alot more gummed up than I thought. I haven't driven it enough to see if it helped, but it did spaz once coming home after the change.

Bud

 

[Budbeater]

Truck still a spaz. Time to replace more parts.

Bud

 

[BCGPER]

I'm just guessing here, but mid 80's HEI GM mag pick-up's act this way. "If" that's what you have, swap out the pick up and rotor. They're dirt cheap, it's it's probablt due anyway.

Truck still a spaz. Time to replace more parts.

Bud

 

[Budbeater]

Finally fixed the problem.

1. Replaced the cap and rotor and burned/corroded plug wire. Replaced coil. Did not fix problem.

2. Replaced fuel filter. Slight, brief relief but then problem persists.

3. Readjustment of carb. No change in problem, but gas mileage getting better.

4. Seems that electric choke spring is broken. Can't fix choke setting. Solution? Replace carb. Choke costs about $50 and I can get a 750 for
$280.
Problem persists, infact got much worse for about 5.5miles and then the truck died completely.

5. Replace fuel pump. NO MORE PROBLEM. Runs a little rich, but new jet and rod change has solved that issue. Truck is currently running better than ever.

It seems that the problem was twofold. Bad choke (also must have been shorting out because when it was reattached I got my dash lights back, and a dying fuel pump. The total cost of the correct parts would have been about $70. I spent much more than that! And a whole lot less than taking it to a mechanic.

BBB

 

[cannoncrawler]

Besides the savings over the mechanic, its good to have a carnal knowedge of your own stuff....




thanx for the update!!!!!