High RPM Backfire??

[RTCarroll]

Well I just founf my self a nice new-to me 86 with a toyed around with mill (edelbrock head to toe including a preformer carb(1406) and other misc surface mods)

At idle she's fine... then hit the throttle and bout 2,500 rpm its nothin but dog ass with a hint of backfire.... through the carb. Checked timing and adjusted/ cleaned the carb. all seems right with the world. I could really use some suggestions... time is plenty and money is in short supply so i really dont wana bring her in, i'd rather fix her right myself, but i'll be honest she's startin to get to me.

ANY SUGGESTIONS OR ADVICE IS VERY APPRECIATED!

 

[SoCal'75]

Hook up a vacuum guage and watch the needle. It actually sounds like a weak or broken valve spring. Look in your repair manual for the diagnosis on the vac guage. Good luck.
-Lance

 

[Chevy305]

sounds almost like its not getting enough air, like the coke is partially closed.

 

[RTCarroll]

I was going to be looking at the valve train next... not too much experience there. As for the choke... it has an electric choke on the carb that i think has sh8t the bed.... but i've played with it when the engine is warm with the choke off aswell... still just as bad.

How should i go about looking at the valve springs to know if one's shot?

Thanks for the responses so far... I used to live on this forum years ago with my 2 K5's... Always the best GM truck forum EVER!

 

[throttleissues]

Strange as it sounds, I had a Chevy SB do the same thing to me. Turned out one of the pegs fell apart in the dist. cap. In other words, the plug wire still hooked to the to the top, but the bottom half fell off. Misfired and slight backfire only with load and usually over 2000 rpm. Good luck whatever it is.

 

[Oilbrnr]

I'm wondering if one of the lifters is adjusted just a *bit* too tight.

 

[454k30]

tell us a little more about the engine, carb, etc... I had a similar problem on mine and it ended up being that the step up springs in the carb were to weak.

 

[RTCarroll]

Well originally it looks like the truck came with a 305, not too sure if its a 305 or 350 in her now, been too busy to really check and i normally am working on her in the middle of the night. She looks mostly stock minus the exterior. Accel HEI distributor cap and coil, MSD plug wires, connected to ACCEL shorty header spark plugs. The base of the distributor looks stock though. edelbrock valve covers (not sure bout underneath'em looks stock to me), performer intake and performer carb (1406 which is the 600 cfm fuel effecient model) other than that she a stocker it looks like. the carb has an electric choke that i dont trust soo much but it seems to open a bit after a few. I've run the engine with both the choke engaged and disengaged and seems to be f'd up ops normal either way. I'm in the coast guard in maine and we just got in from a short patrol but it's been too wet and cold to work on anything outside this week so i really havent touched the truck at all this week. Hoping for snow soon.. I'd rather be damp and cold than drenched and cold. Anything else? Believe me I'm all ears at this point.

 

[Chevy305]

if its a 305, I've heard that sometimes they have the tendancy for the cam lobes to wipe out.

 

[onetonbb74]

id check the timing with a gun, my old 350 did this, i checked the timing and noticed that it wasnt returning to the same spot consistantly, found out it was a bad timing chain.