250 straight six starving?

[Steve Van Gent]

Trying to get my sons 71 stepside with 250 straight six running. Here is thread to see truck - http://67-72chevytrucks.com/vboard/s...d.php?t=427824

Last owner had 6 in poor condition, bad fuel pump, mating gasket on intake/exhaust burnt out, bad alternator, greasy mess. We started working on this weekend, got running ok, found coolant leaks every where. Bad one, rear freeze plug. So we pulled motor, trying to replace plug-also degreasing motor and compartment.

Here is question- While motor was running, motor idles great, timing is correct, plenty of fuel, new inline fuel filters for catching rust, etc. Motor hesitates and backfires when I try to accelerate. Once I get motor past 2000 RPMS, runs fine. BUt the initial acceleration always results in backfire, trying to stall, hesitation. I pulled carb, it looks fairly recent manufactured carb, and is labeled as a Napa reman carb. Looked clean inside. Now I am stumped, not sure why it is stumbling....any ideas? Seems to have good spark, lots of fuel, no vacuuum leaks, new gaskets on all manifolds.

????

[LandonL]

I know you said the timing was fine, but I had a very similar issue with my 250 six. It would run barely and stumble pop and backfire, found out that the distributor hold down clamp bolt had loosened up and the dist was moving however it pleased. To me it just sounds like a timing issue.. always could be wrong though lol. Usually am

[orangek5]

Originally Posted by LandonL

I know you said the timing was fine, but I had a very similar issue with my 250 six. It would run barely and stumble pop and backfire, found out that the distributor hold down clamp bolt had loosened up and the dist was moving however it pleased. To me it just sounds like a timing issue.. always could be wrong though lol. Usually am

I agree. sounds like a worn timing chain.

[Shyguy]

I don't think a L6 has timing chain, pretty sure it is gear drive.

[mr48chev]

A 250 has timing gears and no chain. Have you checked the accelerator pump to make sure it is working right?

One of the thinks that throws guys is the new, just replaced, rebuilt thing.
just because it had been rebuilt at one time doesn't mean it's working right.

Along with that check for kinked, flattened or twisted fuel lines and if any brass fittings
appear to have been change check them to make sure that they are the correct size.

Back in the early 70's I bought a 57 Panel with a built 327 with a dual 4 early Corvette intake on it. The thing would run like crazy for 50 ft and fall on it's face and the go again after I let off the gas. Turns out that the guy who put it together used a natural gas fitting with a 1/8 inch orifice in the line.

My guess is still that your accelerator pump isn't working right.

[oldgold70c10]

x2 for the accelerator pump.

[Dark Knight]

I also blame the accelerator pump. Depending on when it was re-built, the ethanol in today's gas might have eaten up the rubber flap on the plunger.

[mrsmothers]

3x the accelerator pump I had an f150 that did the same thing.

[Steve Van Gent]

Thanks guys. Will find out soon, I have installed new accel pump while I had apart. I doubt timing, I have it slightly loose, turning it while it running...makes no difference. It runs fine at low speed, runs fine at high speed, just has problem getting from low to high....

[Steve Van Gent]

I should note, must wait til next weekend, motor is out right now, I am replacing rear freeze plug-what a pain...!

[cdowns]

i'd replace all freeze plugs if i had the engine out//preventive maintenance

[Wrenchbender Ret]

Did you check the float level while the carb was down. I agree because it was a reman doesn't mean it is right.

[70STOVEBOLT]

I had an issue similar to this recently. I have a manual choke and it would run fine with the choke pulled out about 3/4, but with the choke pushed in it would hesitate and backfire through the carb. I could not find a vacuum leak anywhere on the intake or carb,
so I pulled the pcv valve hose off and plugged it and it ran better. So I replaced the pcv valve and now it runs fine. You might check your pcv valve also, could be a vacuum leak there.

[PanelDeland]

Might also check the vacuum advance.Mine did the same thing but quit when I replaced the vacuum actuator in the dist.

[stsalvage]

Originally Posted by Steve Van Gent

I should note, must wait til next weekend, motor is out right now, I am replacing rear freeze plug-what a pain...!

I hear that from a lot of guys trying to put freeze plugs in while the engine is in the truck Trust me the inline 6 is so easy to take out and put in spend the weekend and do it right and you even get to clean and paint it while its out and check to see if you need to change the pan gasket the inline 6 engines i hated for the longest time until i found a 1971 with a 250 with a 700R4 with the 4 bb intake with a edalbrock on it and man it runs super

[Roadmarks]

What Panel Deland said! What I would do , however is to hit a local salvage yard and pick up a nice HEI for cheap and toss the point set-up all together. That one change alone will make more difference than you will believe. HEI is THE way to go. 1976 up Chevy car or truck L6 is where you get the distributor. New plug wires and plugs for HEI set to .045. Then look over the gaskets on the intake /exhaust and you'll be good to go!

Later,

Les

[70STOVEBOLT]

What was the outcome on this?

[Steve Van Gent]

Well, the outcome was that I never reinstalled motor after rebuilding carb and replacing freeze plug. Instead I have been spending time building up a Firebird Gen3 TPI V8 and 700R4 to replace old power plant....My bad....

[70STOVEBOLT]

Oh, cool, good luck with the repower.