Rebuilt 350 start up & idle fine. Smooth drive for 10 min then sputters.

[MC_K20]

Ok guys, firsf things first, names Matt. Drive a 1986 GMC Sierra K20. This is my first post. Ok here goes. I need help badly. Just put a motor in my 86 k20. Carb 90s 350 engine. Basically rebuilt the whole thing before I put it in. Motor has 80k. She starts up fine, idles beautifully. But heres the problem, itll drive fine and smooth for about 10mins then start to sputter.. doesn't die out though just keeps sputtering and has no pep. Ill shut it off wait 5 mins then start up and same thing happens, its fine then sputters after about 10mins. Wtf is the problem ?! I cant figure it out.

[beamn7]

Welcome to the board Matt. From MA originally myself.
Is the Fuel pump good? Float level in carb good? What type of carb? Computer controlled or no? Seem like a fuel issue or ignition issue? HEI distributor? Possible ignition module in distributor? That was a common description for a failing ign module. They warm up and start to fail...truck may run but sputter bad.

[MC_K20]

Thanks for the reply man. I should've been a little more specific about the details. Fuel pump is id say about 2 months old. Delivers fuel to carb fine. Carb is an edlebrock, not sure of the model. I havent checked the float level. Non hei ignition and no computer. Temp gauge doesnt work atm so im not sure if if starts to sputter once warmed up or not. It literally will start to sputter after about the same distance everytime i drive it.

[Dead Parrot]

Odd question. What is the weather like when it does this? If cool and humid, you might be getting carburetor icing. Mine did something similar when the Thermac vacuum pot developed a leak and quit sucking warm air into the cleaner. Mine would start sputtering and losing power about 3 miles from home on the way to work, but only when the weather was cool and drizzly. Which in Oklahoma, makes it an intermittent problem. Adding more choke would keep it running. When I finally noticed and fixed the busted vacuum pot, the problem disappeared.

[MC_K20]

Honestly, just put the motor in and started driving it about a week ago. Weather has been id say 30-50 degrees each day.

[cadillac_al]

Gas tank vent clogged or kinked? Maybe try cracking the gas cap next time you go for a ride.

[MC_K20]

Ive heard the sock (vent) could get clogged, but last time i checked before old motor was pulled, it wasnt. Ill try the gas cap trick tomorrow.

[geezer#99]

Edelbrock carbs don't like more than 5 psi fuel pressure.
It could be flooding when it runs for awhile.

[MC_K20]

Never did it with the old engine. The old engine ran fine with the carb, ended up having low compression in a cylinder. Which was the whole reason i did the swap. People have told me to re time it, try a different carb, furl tank vent. Literally everything is new, from flywheel to exh and intake manifolds to all new gaskets to all fuel related things (tank, lines, pump) to cap + rotor + coil + wires. And tons more. Yous get the idea.

[Dead Parrot]

30-60 degrees was the magic temp range to cause the icing issue in mine.

Have never had over psi issues with my edelbrock when using stock fuel pumps.

[rich weyand]

I like the fuel cap suggestion. If the vent line is plugged or there is no vent line, with an unvented cap, you will see problems after you drive for a while, which will go away after it sits a bit. Easy to check: wait till it acts up, then open the fuel cap. If you get a hiss of inrush air, that's it. Solution: drill the cap.

I also like the carb icing suggestion. Are you running an open-element air cleaner, instead of the thermac? Were you before? In the same weather?

[D13]

Non HEI? Coil or condesnor. Unless this is a deliberate old-school look, toss that point distributor in the trash and put in an HEI. Classic coil failure is run fine until it gets hot, then either dies or gives weak spark. Cool it off and it works fine until it gets hot again.

Or water in the fuel, setting in bottom of carb, wil idle forevere but when fuel starts flowing it gets into the jets and the motor gets water not gas.

[MC_K20]

Running a thermac. Always have even with old motor summer and winter. And yes, non HEI. I plan on doing the gas tank idea today. Then, im going to re time it. If its still doing it, ill upgrade to a HEI. Appreciate the help guys.