what would limit my top speed, truck wont sustain 64mph?

[324matt]

I finally got my truck all put back together, and, besides installing 2 new fuel tanks and sending units and the fuel gauge not functioning properly, I cannot seem to get the truck up to speed. It runs good through 1st 2nd and 3rd gears. It starts right up and idles great. it pulls good for a stock 350 with 118k, right up until I get going about 62mph. Then it sputters. Then at 63 or 64mph it starts dying out like it is starving for fuel. If I let off the gas pedal it comes back alive and will start to slow down. I waited until I coasted down to 50mph and then floored the pedal and it raced right back up to 63 or 64 and started to die then loose speed and come back on. gain speed, die, come back on. Can anyone give me clue on what to adjust? or check? is there a rev limiter or a governor? its a 85 k20 with qjet 350 and th400. 4.10 gears. all stock. stock tires.

Im stumped.

[greg64]

Matt, how about the fuel filter? It or the tank sock could be plugged up bad enough to limit flow.

[324matt]

Yes. The fuel filter... im pulling it now. And the sending units and tanks are brand new so there should be absolutely no dirt in those tanks
Thanks man

[greg64]

Sure sounds like fuel, doesn't it. I guess the fuel pump could be weak from age and not keeping up with demand also.

[donut]

first thing I always check, gas cap/s. Loosen them, if it runs better then it's a vent issue.
Is it rpm or speed based? Can you hold it in 2nd gear and replicate the problem?

I only ask because my timing chain is stretched and that throws timing all wonky. 118k, wouldn't surprise me.
Have you pumped fuel from your fuel line to a can to see how much and if it's pumping well?

[324matt]

not yet. ill try that.

[86c20]

sounds just like what my c20 did with a bad pump. ran fine started ok pulled hard till about 50 then would mess up. lasted that way over a month then i could only do 20.

[franken]

Not enough air (stuck choke?), fuel (pump or filter), or timing?
Probably a better explanation of parts on the vehicle and a troubleshooting plan would help.

[324matt]

Well. I'm going to get a fuel pump and a fuel filter tomorrow. And im seriously considering taking all the emissions stuff off and leaving it off. It has to be removed to get to the fuel pump anyways.

[franken]

Randomly replacing parts is rarely a good way to solve a problem.

[Oldriginal86]

I'm with franken, get a fuel pressure/vacuum combo gauge and check the fuel pump first. Emission equipment doesn't have to come off to replace the pump. In addition to other things mentioned before, check the catalytic converter for plugging if equipped.

[Gregski]

'85 is the sweet spot for the god awful semi electronic carburetors what do you mean when you say "I finally got my truck all put back together" engine? transmission?

let us know how the new fuel pump performs, after that I would check timing (since it's easy and it doesn't cost you anything) perhaps inspect the cap and rotor, then if that doesn't fix it time to rebuild the carb

[324matt]

Getting my truck back together meant i pulled the bed. Sand blasted the rear frame. New springs shocks and tanks sending units. Re wired the tail lights. I didnt mess with anything under the hood.

[carboncrew]

Quickly check the fuel line right before the fuel pump. It does a pretty sharp bend into the fuel pump. They seem to get old and pinch/collapse - like a garden house.

I know this sounds simple but I've seen it on my own pickup and since then have suggested it to several others and they found the same problem.

[Dead Parrot]

While this cries "fuel flow problem", don't overlook the ignition side. I managed to screw up a new set of plugs during a carburetor swap on a Camaro. Went from a too rich setting to a too lean setting and managed to produce the "High Speed Glazing" often shown toward the bottom of the spark plug conditions charts. It wouldn't go faster than 65 without missing and cutting out. More throttle made it worse. Replaced the plugs, problem went away.

[MikeB]

Originally Posted by donut

Is it rpm or speed based? Can you hold it in 2nd gear and replicate the problem?

That would be the first thing I'd try. But it does sound like it's running out of gas at some RPM or load.

[turbosix]

I'd also check the timing. I've seen retarded timing limit rpms.

[324matt]

I changed the fuel filter in the carb. That was the culprit. Seems to be fine now with improved acceleration and better engine function. The old filter had a orange rust paste inside it. Probably the original filter.

[donut]

Cool, glad you found it and you posted back.

[324matt]

My neighbor is a chevelle aficionado so i had him help me out