missing at high rpms?

[tecton]

truck seems to want to miss/sputter/backfire around 4-5000rpm

 

[micah]

Could be fuel delivery problem. The stock manual fuel pump has a hard time keeping up at those rpm's. Or it could be time to rebuild the distributor.

 

[1-ton]

I would vote for ignition since it is happening at higher RPM's, but I don't count fuel system out either.

 

[gm guy]

Check your timing at idle and overal @ 3500 rpm. Maybe the vacume advance on your dist is not funtioning. /forums/images/graemlins/k5.gif

 

[Can Can]

I'm willing to bet you have one or two dying wires. They always show up under load at higher RPMs...

 

[pauly383]

Are you running stock HEI with stock replacement parts? Stock HEI isn't too good at high RPM's. To get the best RPM range out of HEI you must upgrade the coil. /forums/images/graemlins/k5.gif

 

[gm guy]

Good tip. I'll get a better coil too!

 

[diesel4me]

I hate to be a pessimist,but if its a chevy and it breaks up at high rpm and its not the plugs,wires,cap,rotor,or a fuel problem as already stated--its time to check for a round lobe on your camshaft--nearly every small block I've owned had(or still has! /forums/images/graemlins/whistling.gif)a bad lobe.Usually its an exhaust lobe that wears.This causes a backup of exhaust gas in that cylinder,and when the intake valve opens,the leftover exhaust tries to escape into the intake and out the carb,or throttle body--and it screws up the other cylinders by interfering with the flow of fuel/air mixture to the other cylinders.If you take off the air cleaner and rev it up high in neutral,you'll hear a gargling sound,and perhaps backfiring through the carb--that means its time for a new camshaft and lifters.Hope I'm not right in your case--Good Luck! /forums/images/graemlins/crazy.gif

 

[tecton]

the engine just turned 20,000miles since it was built two days ago

cam would wear out that fast? /forums/images/graemlins/thinking.gif

distributor was replaced some time ago....mabye end of last year

 

[gm4x]

[ QUOTE ]
the engine just turned 20,000miles since it was built two days ago

cam would wear out that fast? /forums/images/graemlins/thinking.gif

distributor was replaced some time ago....mabye end of last year

[/ QUOTE ]

What about valve job at time of rebuild? Valve springs (rate) checked (or springs replaced?) Possible valve "float" at higher RPM?

20,000 miles on motor after a 2 day ago rebuild? /forums/images/graemlins/yikes.gif

 

[GMCLegacy]

he is saying: two days ago it turned over 20,000 miles since the rebuild....i think