250 straight six starving? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Steve Van Gent · 03-13-2011, 08:40 PM

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 · 03-13-2011, 09:02 PM

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 · 03-13-2011, 09:22 PM

Quote:

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 · 03-13-2011, 10:34 PM

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

**mr48chev** · 03-13-2011, 10:47 PM

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** · 03-13-2011, 10:54 PM

x2 for the accelerator pump.

Dark Knight · 03-13-2011, 11:24 PM

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 · 03-13-2011, 11:43 PM

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

Steve Van Gent · 03-14-2011, 01:11 AM

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 · 03-14-2011, 01:12 AM

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

cdowns · 03-14-2011, 08:00 AM

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

Wrenchbender Ret · 03-14-2011, 09:37 AM

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** · 03-14-2011, 01:01 PM

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 · 03-14-2011, 02:43 PM

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

stsalvage · 03-15-2011, 03:31 PM

Quote:

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 · 03-15-2011, 08:59 PM

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** · 06-14-2011, 09:51 AM

What was the outcome on this?

Steve Van Gent · 06-14-2011, 11:06 AM

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** · 06-14-2011, 11:32 AM

Oh, cool, good luck with the repower.

03-13-2011, 08:40 PM	#1
Steve Van Gent Registered User Join Date: Feb 2009 Location: Redding, Ca Posts: 333	250 straight six starving? 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. ????

03-13-2011, 09:02 PM	#2
LandonL Registered User Join Date: Nov 2009 Location: Georgia Posts: 440	Re: 250 straight six starving? 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 __________________ ---\| 1967 GMC Stepside \|-- 250 L6 .030 over ---\| 1985 Porsche 944/1 \|--- Build Thread: '67 GMC http://67-72chevytrucks.com/vboard/s...d.php?t=378640

03-13-2011, 10:34 PM	#4
Shyguy Junior Member Join Date: Dec 2006 Location: Southeast Missouri Posts: 2,436	Re: 250 straight six starving? I don't think a L6 has timing chain, pretty sure it is gear drive. __________________ '67 Chevy C-20 short stepper - build complete, 454/SM-465. '75 C-30 Single Cab DRW-350 small block/NP-435. '77 GMC-6500 Dump Truck, 427 Tall Deck. '92 GMC K-3500 Duallie, 454/4L80E.

03-13-2011, 10:47 PM	#5
mr48chev Registered User Join Date: Jul 2007 Location: Toppenish, WA Posts: 15,708	Re: 250 straight six starving? 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.

03-13-2011, 10:54 PM	#6
oldgold70c10 Who's got steelies? Join Date: Feb 2006 Location: Morrison, Oklahoma Posts: 1,025	Re: 250 straight six starving? x2 for the accelerator pump. __________________ 1969 C10 LWB W/ 250 I6 & 3spd 1970 C10 with '67 small window cab, 68 front end, blue; 305/TH350, 3.08 limited slip rear end, manual brakes, power steering; & 4-wheel drums-My daily driver. 1975 Chevelle Malibu Classic Coupe 350/th350 mint green 1970 C20 Suburban 350/700R4 4.10 gearing green & white 1978 Big 10 Silverado 350/th350, working ac, 2 tone blue-My summer daily driver 1983 GMC 1500 Sierra 305/SM465 2.73 rear end How to add a trip odometer to your 67-72 stock speedometer How to rebuild your Q-Jet My Truck Page My Youtube Channel

03-13-2011, 11:24 PM	#7
Dark Knight Registered User Join Date: Dec 2008 Location: Pleak, Texas Posts: 420	Re: 250 straight six starving? 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. __________________ -1967 Chevrolet Fleetside LWB- 350 V8 w/thumpr cam, NV3500, 3.08 gears, A/C, PS -1983 Chevy Silverado C1500- 383 stroker, TH400 -1989 Chevy Silverado K1500 SWB- 5.3L swap, 700R4, 3.42 gears -And a '66 Ford Mustang- 200 I6, C4 trans, rust bucket "The only thing we have to fear is fear itself." -FDR (and maybe rust too) You've gotta be able to stop and steer before you can go fast!

03-13-2011, 11:43 PM	#8
mrsmothers Registered User Join Date: Oct 2010 Location: ruston la Posts: 169	Re: 250 straight six starving? 3x the accelerator pump I had an f150 that did the same thing.

03-14-2011, 01:11 AM	#9
Steve Van Gent Registered User Join Date: Feb 2009 Location: Redding, Ca Posts: 333	Re: 250 straight six starving? 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....

03-14-2011, 01:12 AM	#10
Steve Van Gent Registered User Join Date: Feb 2009 Location: Redding, Ca Posts: 333	Re: 250 straight six starving? I should note, must wait til next weekend, motor is out right now, I am replacing rear freeze plug-what a pain...!

03-14-2011, 08:00 AM	#11
cdowns Senior Member Join Date: Jan 2003 Location: daytonabeach Posts: 22,956	Re: 250 straight six starving? i'd replace all freeze plugs if i had the engine out//preventive maintenance __________________ 71c-10 350/2004r/4:11 lowered3/4 longbed/dead by hurricane MEANING OF DEATH::::: SOMEBODY ELSE GETS YOUR STUFF DONT BELIEVE EVERYTHING YOU THINK TAKE MY ADVISE;I DON'T USE IT ANYWAY

03-14-2011, 09:37 AM	#12
Wrenchbender Ret Registered User Join Date: Oct 2005 Location: Overland Park, Ks. Posts: 5,229	Re: 250 straight six starving? Did you check the float level while the carb was down. I agree because it was a reman doesn't mean it is right.

03-14-2011, 01:01 PM	#13
70STOVEBOLT Senior Member Join Date: Jun 2005 Location: Winona Lake, IN Posts: 6,414	Re: 250 straight six starving? 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. __________________ 70 C/10 SWB 402/TH400/3.73 "The Needy Beast" 200,000 Mile Club Disc Brake Club Owner installed options: Front Sway Bar Power Steering Power Brakes Cigar Lighter Courtesy Lights Deluxe Side Markers Wiper Delay Sliding Rear Window Power Windows Power Locks Sniper EFI 2015 Silverado 1500 LS 4.3/6L80/3.23 lowered 2" front & rear

03-14-2011, 02:43 PM	#14
PanelDeland I am a Referee of life. Join Date: Jun 2000 Location: Greensboro N.C. Posts: 13,992	Re: 250 straight six starving? Might also check the vacuum advance.Mine did the same thing but quit when I replaced the vacuum actuator in the dist. __________________ The 47-present Chevrolet and GMC Truck Message Board Network,it's owners,moderators,members,and associates of any type should not be held responsible for my opinion. You can't fix stupid,not even with duct tape. "My appearance is due to the fact that "GOD" does punish you for having too much fun!" Barrett-Jackson has perfected alchemy,they make rust into gold! "You can lead a horse to water but you can't saddle a duck" "Cleverly disguised as a 'Responsible Adult' "Sometimes your Knight in shining armor is just a retard in tinfoil"

03-15-2011, 08:59 PM	#16
Roadmarks Registered User Join Date: Apr 2010 Location: Raleigh NC Posts: 157	Re: 250 straight six starving? 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

06-14-2011, 09:51 AM	#17
70STOVEBOLT Senior Member Join Date: Jun 2005 Location: Winona Lake, IN Posts: 6,414	Re: 250 straight six starving? What was the outcome on this? __________________ 70 C/10 SWB 402/TH400/3.73 "The Needy Beast" 200,000 Mile Club Disc Brake Club Owner installed options: Front Sway Bar Power Steering Power Brakes Cigar Lighter Courtesy Lights Deluxe Side Markers Wiper Delay Sliding Rear Window Power Windows Power Locks Sniper EFI 2015 Silverado 1500 LS 4.3/6L80/3.23 lowered 2" front & rear



Register or Log In To remove these advertisements.

06-14-2011, 11:06 AM	#18
Steve Van Gent Registered User Join Date: Feb 2009 Location: Redding, Ca Posts: 333	Re: 250 straight six starving? 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....

06-14-2011, 11:32 AM	#19
70STOVEBOLT Senior Member Join Date: Jun 2005 Location: Winona Lake, IN Posts: 6,414	Re: 250 straight six starving? Oh, cool, good luck with the repower. __________________ 70 C/10 SWB 402/TH400/3.73 "The Needy Beast" 200,000 Mile Club Disc Brake Club Owner installed options: Front Sway Bar Power Steering Power Brakes Cigar Lighter Courtesy Lights Deluxe Side Markers Wiper Delay Sliding Rear Window Power Windows Power Locks Sniper EFI 2015 Silverado 1500 LS 4.3/6L80/3.23 lowered 2" front & rear

Register or Log In To remove these advertisements.