'72 GMC I6 to V8, motor won't stay running, wiring issue?

[grancuda]

I just swapped out my 250 I6 to have a 350 V8 w/HEI. I got if fired up but it only stays running for a couple of seconds and I am curious if some of the factory wiring needs to be connected to something. Originally there were 2 wires running to a solenoid on the carb, 1 wire running to the idle solenoid on the carb, 1 wire running to a temp sensor on the head and 1 wire running to a temp sensor on the lower part of the thermostat housing. Do any of these need to be connected to something for the motor to run? I am running an aftermarket gauge for the temp so I really don't need the temp one but didn't know if the temp sensor under the thermostat needed to be grounded out possibly. pretty much just cut the wires off and taped them up. For the HEI wiring I removed the yellow from the starter soleinoid, cut the pink wire where it comes out of the bulk head about 1" from the bulk head and soldered a new heavy gauge wire to it and ran it to the batt term. The truck will start but dies imediately. I am thinking it may be one of the engine wires but don't know for sure.

[Tx Firefighter]

It really sounds like your new ignition wire isn't staying "hot" when you release the key. Use a test light and verify that it is hot when the key is in both "start" and "run" positions. I bet you find your problem there.

[grancuda]

Is there suppose to be a ground wire running to the HEI?

[glock35ipsc]

Originally Posted by grancuda

Is there suppose to be a ground wire running to the HEI?

No. You can test the HEI by temporarily running a wire straight from the battery to the plug on the cap.

Otherwise, check the usual suspects; are you getting fuel, or enough fuel? Timing? Idle set too low? Vacuum leak?

[sandking]

What are you using for fuel delivery?

[grancuda]

A Holley Blue Pump. I have been running it for about 3 months.

[69halfton]

agreed with tx firefighter.

I had the ignition modual go in my HEI Dist. the truck would fire and run as long as you kept the key in the start position. as soon as you let go, it was litterally like shutting it down.

A whole new MSD dizzy later and she was running again

[JP2120]

There is a purple wire coming out of the bulkhead on the firewall. This wire goes to the s terminal on the starter. The yellow wire comes off the starter at the r terminal and goes to the hei on the bat terminal

Some of the wires to the points type ignitions were a resistor type wire so make sure you have good battery voltage at the yellow wire coming off the starter

[LockDoc]

Originally Posted by JP2120

There is a purple wire coming out of the bulkhead on the firewall. This wire goes to the s terminal on the starter. The yellow wire comes off the starter at the r terminal and goes to the hei on the bat terminal

Some of the wires to the points type ignitions were a resistor type wire so make sure you have good battery voltage at the yellow wire coming off the starter

With an HEI you do not use the yellow wire at all. It is a full voltage wire that is hot only when the key is in the start position and is only used for a points distributor....

You need a 12 gauge wire from the "Unfused Ignition" terminal on the fuse block to the positive terminal on the HEI distributor.

LockDoc

[hilandr452]

Originally Posted by LockDoc

With an HEI you do not use the yellow wire at all. It is a full voltage wire that is hot only when the key is in the start position and is only used for a points distributor....

You need a 12 gauge wire from the "Unfused Ignition" terminal on the fuse block to the positive terminal on the HEI distributor.

LockDoc

He's right,

I did the points/HEI swap, just hook up that 12 gauge wire to switched source and you're probably good to go.



Dan

[JP2120]

Originally Posted by hilandr452

He's right,

I did the points/HEI swap, just hook up that 12 gauge wire to switched source and you're probably good to go.



Dan

No argument there. My yellow wire from the starter was already 12 gauge from the starter with great voltage so it was most likely changed out when I got the truck. My yellow wire is also copper strand and I have noticed that the resistor wire was a silver stand wire. I like to save my unfused ign terminals on the fuse block for accessories and have even added a couple of protected female piggybacks to this terminal since my vintage air system, stereo system have used them up. You cant go overboard on the piggybacks tho due to circuit integrety. I do use inline fuses on everything from the unfused source.
JP

[grancuda]

I think I have an intake that isn't sealing up. Now that theexhaust is on it sounds like it is turning about 2500-2800 rpm and if it goes below that it won't recover by gassing it, it just dies. It runs great and pulls good once it is running.

[JP2120]

Try holding something over the top of the carb to restrict the airflow like a large shop towel. If it starts to idle better then you have a lean condition which can be caused by intake leak, brake power booster leak or even idle circuit in carb.

Also pull a couple of plugs and see what color they are. Really black is an over rich condition.
Try these things and get back to me.

What kind of carb are you running ?
JP

[grancuda]

I pulled the plugs when I got home from the muffler shop and they are tan. Now after starting it alot over the past 2 days the plugs are black.

The truck won't idle at all so I can't do the rag over the carb trick, it just starts, runs for like 2 seconds then sputters and like it is still getting some fire and gas or if I hold the throttle open about 1/4 way on start up it fires right up to about 2500 and I can let off to about 1800-2000 but if I let off too much it will die and it doesn't matter if I pump it, it won't recover from dropping too low in the rpm's. I put a vacuum gauge on the port on the front pass side on the carb and it shows 20lbs until it dies, when it starts to die it just drops to nothing. I have a 600 cfm Edelbrock on it and I borrowed a buddies new carb he bought for his project and it does the same thing so I have ruled out the carb. Next I am having the HEI module tested at lunch to see if maybe something is wrong with it. The truck runs great from about 1800-2000 all the way up and makes good power and pulls nice. It is just the low end that doesn't work. I am thinking of pulling the intake which is a Edelbrock Performer RPM and seeing if maybe something is wrong there and maybe a vacuum leak but I don't see how it would be. I am really at a loss for what is going on here.
A week ago the motor was in a different truck, running great and now all I have done is pulled the intake, cleaned up the motor and put it in mine and put the Performer RPM on and I put it back together and now it is doing this. Anyone have any ideas?

[glock35ipsc]

Originally Posted by grancuda

A week ago the motor was in a different truck, running great and now all I have done is pulled the intake, cleaned up the motor and put it in mine and put the Performer RPM on and I put it back together and now it is doing this. Anyone have any ideas?

You just found the things you need to rule out then. If it ran good in the other truck, and now won't run in your, look at what changed. Are you using the distributor that was on the motor, or a different one? I don't recall if you said you ran a new wire to the HEI, one that is a solid 12v from a switched source?

If you can get it to run at a constant RPM, like adjusting the idle screw to keep it there, go around the intake gasket with a can of ether or carb cleaner or something to see if the idle changes. That would help to pinpoint a vacuum leak from the intake. Is the intake new or used? What gaskets did you use?

Double check the distributor also to be sure it's not a tooth out.

If all you changed was the intake, carb, and distributor.... you have ruled out the carb it sounds like. Rule out the distributor (either a mechanical/electrical problem, or one tooth off), then check the manifold.

[JP2120]

Call Me 512-268-5750 ask for Duke

[grancuda]

Fixed it, it ended up being the module in the HEI. It was the low rpm side of it.
Thanks for everyones help, the truck runs awesome and is way more fun than the 250.