Random sputtering and jerking after driving for awhile

[3wheelingCJ]

After fixing up the cooling system and upgrading the carb to 650cfm, every time I drive for at least an hour the jeep starts randomly sputtering, looses power and jerks. Then it will go away after a few minutes, drive fine for 10-15min and then starts running like again.
I tried to figure out if certain load conditions (up hill, flat, residential ) have greater affect than others, but didn't notice anything. It runs great for the first hour or so. No sputtering or loss of power, pulls good and smooth.
The engine does have low compression, 90-100psi, but wouldn't the engine run bad all the time if that was a true cause?

Any help is appreciated. Really would like to use the jeep as daily driver soon.

Thanks.

 

[Torxhead]

Since this did happen after a carb swap, I think you should check that. Make sure the choke is operating correctly and the fuel pump could be a bit weak also.

 

[3wheelingCJ]

Might be a stupid question, but would a lager carb require a stronger fuel pump? Wonder if I get a loose wire powering/grounding the choke...
could timing have anything to do with this?
just really perplexed by how random the symptoms are.

 

[CJim7]

Probably not timing or it wouldn't just do it intermittently. My guess would be vapor lock. Make sure your fuel line isn't too close to a hot spot.

 

[Hedgehog]

This has GOT to be heat related. Even though it's cool now, it sounds like a minor vapor lock. When you attached your fuel line to the new carb. where did you run it? To close to the hot stuff on your engine isn't good at all.

Edit ...... Jim and I were typing at the same time.

 

[Lumpy Grits]

Check for vacuum leaks at the manifold and carb's base plate.
VERY soapy water works well here---
When was the last time the fuel filter was changed?
Next time the 'sputter' starts. Loosen the gas cap, and see what happens.
LG

 

[Saddle Tramp]

All good possibilities above. A vapor lock if that’s the problem would be eliminated for the most part with the return fuel line assuming that the filter is installed with the outside edge outlet place up. From my experience, I would replace the fuel pump first as it sounds like classic early pump failure symptoms to me.

 

[Fastatv]

A 650 CFM carb is very large for a AMC 304 CI engine but I guess you dialed in the jets and metering rods ( if its an Edelbrock ). Do you have the proper carb base gasket ( fairly thick ) because if you don't, the carb could be absorbing too much heat creating vapor lock or perculation.