At wits end and need help from some of the engine experts here!

81 4spd with emissions stripped, edelbrock eps manifold, 1406 600cfm edelbrock carb, HEI distributor, Champion Alum Radiator with ford 2 spd fan and custom relay kit, factory exhaust manifolds with air tubes plugged, factory style side exhaust. May have a mild street cam from PO rebuild. Custom air cold air intake through K&N panel filter.

Was running great then one day recently started sputtering lugging and popping on moderate to hard acceleration. runs fine on light throttle, I get a little shuddering in first gear off the line on light throttle that feels like a miss.
This condition only happens under load and after engine is up to full operation temp (195+) in neutral it will rev smooth to redline at any temp. Runs like a scared dog until the temp hits 195 - 200 then it starts!

Here is what I have done so far:
Pulled plugs, perfect gap nice dark tan no issues, no cracked insulators.
Pulled Edelbrock fuel filter and cleaned.
New Cap, Rotor, and HEI Coil.
New silicone HP wires with insulators.
Adjusted idle mixture with vac gage per Edelbrock specs, I do have a slight oscillation on gauge at idle.
Set TDC, pulled Distributor and re-timed. Set at 14deg, advance goes to about 32deg.
Compression on all cylinders is in spec., I did not do a leak down.
Ran fuel cleaner through the tank and sprayed in carb. Carb only has about 3000 mi on it and is unmodified.

After all this it still does the same thing , its a little better but still cant hammer down on the pedal.
I thought about flat cam lobe, sticking valve(s), broken spring but ruled those out because it runs fine light/no load and cold.

Any ideas would help, I'm hoping it is something simple.
Greg

 

have you checked with the engine off are you getting a good shot from the accelerator pump? You timed the engine to 14deg with the vacuum disconnected? I would not be reving the engine to redline in neutral.

 

Also signs of an HEI module going bad. May even run fine at first start up but as soon as distributor gets hot it'll do just like you describe

 

I agree with Dave. Sounds like a distributor problem. Swap out the dist with a known good one will save you time nickel and diming all the bits and pieces inside. It could also bea loose wire going to the dist. As the engine torques up, it moves in the engine bay. A loose wire will act up

 

Could be your fuel is percolating on a winter blend on a moderately warm day. Is there a heat insulating gasket between carb and manifold? Running a return line on the fuel pump as well?

 

Just had the same problem with mine doing the same thing. would run fine but when you gave it some throttle it would pop. Valves needed to be readjusted. Not sure if that is your problem but it worked for me.

 

Yes, good shot of gas from the nozzles.
Yes timing done with advance disconnected and vac line plugged.
Figure of speech, just goosing it not to redline!

 

There is a thin phenolic spacer under the carb. This has never happened before even in 100 deg Houston heat.

 

I'll check out the HEI module and either replace it or see if a buddy has a spare HEI dist. I can borrow to swap. I did replace the cap, rotor and coil. No loose wires, I checked all those and did repair a bad connector on the tach. wire.
Thanks for the tip.

 

Ignition module, replace with an ac delco unit.

 

OK it's not the ignition module. Will pull the valve covers Friday and check the springs/adjust the valves.

 

weak fuel pump?

 

Let me know if that works for you. Hopefully that will get you where you need to be. I just got some oil deflectors so myself and a friend are going to readjust mine this weekend. Good luck

 

Everything looks fine with the valvetrain, So I think I might have a bad lifter. Since this only happens under load and hot any ideas on how to isolate and check them. I will try to run an engine cleaner through the crankcase and change the oil as a first effort.

 

Not sure how to isolate that. You could pour some Marvel Mystery Oil down the carb while running it. Rev it a bit. That might free up a hung valve. This one is giving you a run for the money. Could do a compression check as well. If one of your valves is not seating properly you could find it that way.

 

Did a compression check and all is well. I may have a lifter with crud in it or one that is collapsing under load. Since this only happens hot under full load I can't duplicate it in the garage. I'm going to run some crankcase cleaner through it and change the oil. Hopefully this flushes out the suspect lifter.

 

No need for the deflectors. Doing that with the engine running is a mess. Assuming you have hydraulic lifters, Best way to adjust is with the engine cold.
Remove valve covers and plugs, rotate engine same direction as run with crank bolt until TDC, #1 valves will open then close, timing mark on "0" This is #1 firing position.

#1 to Top Dead Center and adjust:
Exh.- 1 – 3 – 4 -8​
Int.- 1- 2 – 5 - 7

Rotate again​ one full turn to timing mark on "0" again, #6 valves will open and close.
#6 to Top Dead Center and adjust:
Exh.- 2 – 5 – 6 – 7​
Int.- 3 – 4- 6 - 8​
​
Loosen nut so pushrod will spin in your fingers, tighten until pushrod will not spin between fingers and then add 1/2 turn (preload) and tighten lock nut.
Some engine builders recommend 1/4 to 1 full turn of preload, depends on your cam, a little less provides more top end, I have had good luck with 1/2 turn preload.

You will only turn the engine 2 times. TDC #1 and TDC #6

 

keep us posted on how it turns out. Hopefully you will get it going. My friend said if it is a collapsed lifter your rocker will be really lose. Seems like this one has been a long haul hopefully you will get it fixed here directly. Good luck

 

With much trepidation, I ran a can of Engine Flush through it, changed the oil with a quart if Slick 50, no more issues. Ran it hard and hot and runs like a scared dog!
Must have had some crud in a lifter.
Oil looked good, no metal, no grit.
WORD OF WARNING>>>>>>
I knew the engine was relatively clean or I would not have chanced running the flush through it.
If you do not know the history of your engine the flush is not a good idea since it can loosen material in the motor that can dislodge after the oil is changed and cause damage to bearings and components.