Quick run down of what is new on the truck. HEI, Offy Dual, hedman header, Summit/FiTech throttle body.

Symptoms: Really rough idle with an intermittent miss on at least one cylinder, but I suspect more than one. Vibration at certain rpms from what seems to be the same miss at idle.

What I have checked: Changed the plugs, checked all of the plug wires, tuned every which way possible but still runs rough. No vacuum leaks, 17 hg at idle. I put a laser temp gauge on the header tubes and one second the temp will read 400 about two inces from the flange, then I go back to it and it reads 130. But it does this on 3 cylinders. Changed those plug wires so I know it isn't a bad wire. Timing is set.

I am at a loss and don't want to start throwing parts at it unless needed. When I hooked power up to the HEI I just found a random ignition source from the old systems wiring and used that. I read that HEI's need at least 12v to run right, should I try hooking the HEI up to the "i" on the starter solenoid? Maybe as the coil is firing it is not getting enough voltage, causing a random miss on certain cylinders? I know these 300s are supposed to run like a sewing maching but I'm worried at times mine is going to break a motor mount.

 

I am not up on the after market EFI but here goes my .02
That 17 hg seams a little low I would think it would be up to 20 hg. I never checked mine (stock) so dont know if it can get 20 hg to start with.

What is the timing set to?
Is the HEI a chepie off Ebay?
Can the EFI control the timing if you had the right dist.?
What plugs, make & model you running?
The 3 pipes showing a low temp what happens if you pull the plug wire when running, 1 at a time, off the plug what happens?
You can do this for all of the cyl to see what happens and can be done at the cap but HEI has a lock ring so plugs maybe easier.
When removed the motor should change in sound and run a little ruffer if the hole is making power.

As for the 12 volt feed the "I" stud should not have a full 12 volts unless you are cranking the motor.
That "I" stud was used on points dist. to send a full 12 volts when starting / cranking to give it a hotter spark.
Once running that "I" stud then had reduced volts that feeds the coil.
If you have anything on that stud now measure the volts both cranking and running then remove the wire and see if that stud has power on it ....I bet not.

As a test run a wire from the battery + side to the power of the HEI and see if that changes anything.
To turn off the motor you will need to remove power on this "test" wire as you are "hot wiring" the dist. with full 12 volts.
If this fixes the issue then I would wire in a relay and use the wire you have going to the HEI as the trigger.

Let us know what you get in testing
Dave ----

 

Are you able to provide a picture or diagram of how to wire this relay? I have watched people do it on youtube but cannot remember where. 4 terminal relay? One wire to battery, one to ground, one to HEI with an inline fuse? The fourth wire is what stumps me.

 

This should help got it off Ebay
https://www.ebay.com/itm/16548158170...SABEgJI5_D_BwE
Dave ----

 

Did you ever figure this out? I have the same setup with the same problem..

 

I am new here but I have been lurking a long time. Anyway I hope I'm posting this correctly but I have done a similar setup as you, I have a holly 3310 carb and a summit brand hei distributor in a 1997 f250 hd I have converted to carburetor. We'll I am having the same missfire/vibration as you described here and I was wondering if you ever figured the problem out? Thank you advance for any help.
p.s. I would send you a PM but i haven't posted enough or been here long enough to PM anyone.

 

GM HEI can be voltage sensitive but usually it shows up as a high rpm misfire under load.

 

Did you ever have the motor running with EFI without this problem?
If it never ran before then you need to start at the beginning on a motor health check.
What is the compression like? Maybe a leak down test would be better?
Has the cam gone flat? If valve dont open like they should can cause run issues.
How did the intake manifold seal to the heads? It may look good up top where you can see but if open in the oil valley it can cause a vacuum leak, maybe only on 1 hole with bad gasket? and cause the problem?
When it is running pull 1 plug wire off at a time and see if that changes anything.
What do the plugs look like and what brand / make are they?
Dave -----

 

used to have similar problem with new champion plugs
installed motorcraft missing gone