engine miss at highway speeds

[nvrforgotten52]

hi,
i have a 69 c10 longbed with a 250 ci motor and a th350 tranny, and i have a bad miss at speeds above 55 mph. i can accelerate great up to 55 but when i try to go faster than that the engine feels like it dies but when i let up on the accelerator the engine feels like it turns back on. it has a rebuild carb, brand new head with hardened valve seats, new air cleaner, no vaccum leaks, free flowing exaust. its stumped the crap out of me so far any suggestions?
thanks,
david

[Longhorn Man]

kinda sounds like a fuel restriction, you checked to see if there's a filter? Maybe under the cab?

[dwaite72lnghrn]

I'm not a 6cylinder guy but what is your timing set for and how long since the rebuild. Valves misadjusted can cause the loss of power at higher rpms.

[Karl Hoh]

Is it a miss or a loss of power? Are you still runing points? Are they clean & gapped correctly? Maybe a weak fuel pump? Are the rockers adjusted too tight? All these things can cause problems at higher rpm. You say it dies @55 and if you let off it turns back on. Can you then creep past 55 or not? Will it do it under extended acceleration at this rpm?

[nvrforgotten52]

hey thanks for the replies. i replaced both the fuel filters (under the cab and in the engine compartment). it is kinda wierd because i Can creep past 55 if i really baby the throttle. but anything past 1/8th throttle at 55 mph the engine will miss really badly. it seems like fuel related to me but im not totally sure. maybe the accelerator pump?? i just adjusted the valves and they are fine tunned pretty well... i rebuilt many engines and carbs and have never had this problem. the timing is dialed in too by the way.

[Karl Hoh]

What do you mean by "miss". Does it stumble like bad plugs? Does it "flatten out" like it has no more power? Does it pop it the carb like too lean fuel mixture under load or at high rpm on the driveway like a problem with the high speed fuel circuit or fuel pump? Unless it does it every time you step on the pedal, it's not an accelerator pump problem. What do you mean by "miss"?

[nvrforgotten52]

no it doesnt miss like a retarded ignition when the engine is under load or a lean mixture, its like the engine just cuts out, its really wierd. and it only does it on the freeway i can rev it in the driveway and it wont miss and i can floor it driving around town with out any miss or hezitation at all. its really wierd.

[nvrforgotten52]

im thinkin its gotta be the carb, i have a spare carb that ill try tomarow and see if it helps. i cant think of anything else it could be besides the fuel pump.

[Karl Hoh]

Ok, that is weird. I'm out of ideas at the moment. Does it happen right away when you get up to speed or does it take bit before it starts?

[nvrforgotten52]

no, its just when you get up to 55 and try to go faster without babying the throttle it will just cut out. its pretty wierd, something ive never seen before. but it really seems fuel related to me.

[clif1968]

I had the exact problem on a '74 Chevy with a 350. It ran fine around town and only get 55 on the highway. Above 55 it would fall flat on its face. It ended up being a crimped fuel line between the stock pump and the carb. I guess the restriction would'nt flow enough fuel at constant highway RPM.

[70rs/ss]

I too had this problem in an I6 truck, it was the fuel filter in the carb. Check it, or swap the carb out to see if it "fixes" the problem to isolate where to look. Swap carbs, if not that, then the fuel pump, then lines to see if one is crushed.

[nvrforgotten52]

thanks guys, ill try this stuff out, see if it fixes it. i didnt think of the one filter inside the carb, and i havn't checked for kinked lines so im hopin its one of those!
thanks,
david

[rage'nrat638]

if it's got a hei might be the poll piece?