Help Motor doesnt run right. - The 1947 - Present Chevrolet & GMC Truck Message Board Network

ryhan87 · 09-15-2009, 11:49 PM

I just bought a 72 Gmc 1/2 ton, but it wont run right. It has the 250ci I6. With a single barrel factory carb on it. It idles pretty good, a little rough but not bad. But when you give it throttle it coughs and backfires and hesitates. But if you give it full throttle it will rev right up without any problems. On the highway i can get it into fourth gear but its like it has no power. I can get about 105km/h out of it but i have to hold it right to the floor and it burns gas like crazy. Im not an expert on carbs but i think thats the problem, does that sound right. Also it has a new cap, rotor, wires and plugs however im not sure if the points are set up right. I have tried pulling plug wires off one at a time and it seems to be firing on all cylinders and i cant smell unburnt fuel so i think thats fine.

texanidiot25 · 09-16-2009, 12:32 AM

What's the timing at? Sounds like an ignition problem to me. Back firing out the carb or exhaust pipes?

Wrenchbender Ret · 09-16-2009, 04:15 AM

I'm with Tex. Your timing may be slow. Set the points to .016 with the rubbing block on the high edge of the dist cam. Set the timing to 6-8 degrees BTDC with the vacuum advance hose off. Make sure the vacuum advance is working.

1970 Bowtie · 09-16-2009, 05:35 AM

I agree with tex and wrench sounds like ignition timing.I had a similar problem with my small block and the old points dist. it was so worn out it was impossible to keep it timed right and it used gas like crazy.I upgraded to a new hei and it runs great and much better mpg.

cdowns · 09-16-2009, 05:54 AM

changing air and fuel filters,cleaning changing gapping spark plugs, cleaning changing setting gap/dwell of points, checking all vaccume lines.

that's all routine maintence for old vehicles to run properly

PanelDeland · 09-16-2009, 07:31 AM

I think cdowns has pointed you right but I would add checking the dist vac advance.I would also suggest you look to upgrade to a HEI,it made a world oof difference in my 250.

geezer#99 · 09-16-2009, 09:16 AM

Definetly switch to an HEI. My 250 was cold blooded just like yours till I switched. Like difference between night and day.

mclairmo · 09-16-2009, 02:08 PM

As the old adage goes, most carb problems are ignition related. Check ignition first. I had a bad condenser once and acted like yours.

unclewatts · 09-17-2009, 04:29 AM

Definitely sounds like timing to me. Get an HEI Dizzy. They're really not that expensive if you shop around.

69GMCLonghorn · 09-17-2009, 10:23 AM

I think it's a defective piston return spring on cylinder #3, very common on the straight 6's...

airdale94 · 09-18-2009, 01:06 AM

-defective piston return spring.

Ignition doesn't get better when you rev it up it gets worse. I'd say it's fuel/carb related. Might just be bad gas if you just bought it and it's been sitting.

texanidiot25 · 09-18-2009, 03:08 AM

Quote:

Originally Posted by airdale94

Ignition doesn't get better when you rev it up it gets worse. I'd say it's fuel/carb related. Might just be bad gas if you just bought it and it's been sitting.

Unless there is a problem with the vacuum advance.

ryhan87 · 09-23-2009, 09:26 PM

Ok so I looked over a bunch of stuff to try and get it running right. Heres what I did. Set points, and checked timing. Timing was set at 4deg. Compression test. 160-140 on four holes, 2 holes where as low as 115. Changed fuel line, vac lines. Pulled valve cover and set all the valves. Checked crankcase vent. I assume you would be able to here blowby when you remove vent and listen so think rings are good. Ran carb cleaner through. Also checked over intake manifold for any vac leaks. Its running better than it was but still not great. Still backfires a bit till its warmed right up. Also doesnt seem to have much power in lower rpm range. Any other suggestions? Also what should this motor rev too? It seems to start almost backfiring or something at around 4000 rpm? Im thinking valves and seats now?

ryhan87 · 09-23-2009, 09:51 PM

Also it is backfiring through the carb

09-15-2009, 11:49 PM	#1
ryhan87 Registered User Join Date: Sep 2009 Location: Lloydminster Alberta Posts: 32	Help Motor doesnt run right. I just bought a 72 Gmc 1/2 ton, but it wont run right. It has the 250ci I6. With a single barrel factory carb on it. It idles pretty good, a little rough but not bad. But when you give it throttle it coughs and backfires and hesitates. But if you give it full throttle it will rev right up without any problems. On the highway i can get it into fourth gear but its like it has no power. I can get about 105km/h out of it but i have to hold it right to the floor and it burns gas like crazy. Im not an expert on carbs but i think thats the problem, does that sound right. Also it has a new cap, rotor, wires and plugs however im not sure if the points are set up right. I have tried pulling plug wires off one at a time and it seems to be firing on all cylinders and i cant smell unburnt fuel so i think thats fine.

09-16-2009, 12:32 AM	#2
texanidiot25 Eat My Rust Join Date: Jan 2007 Location: Cypress, Texas Posts: 3,362	Re: Help Motor doesnt run right. What's the timing at? Sounds like an ignition problem to me. Back firing out the carb or exhaust pipes? Last edited by texanidiot25; 09-16-2009 at 12:33 AM.

09-16-2009, 05:35 AM	#4
1970 Bowtie It's all in the timing Join Date: May 2009 Location: Lake Mary,Florida Posts: 600	Re: Help Motor doesnt run right. I agree with tex and wrench sounds like ignition timing.I had a similar problem with my small block and the old points dist. it was so worn out it was impossible to keep it timed right and it used gas like crazy.I upgraded to a new hei and it runs great and much better mpg.

09-16-2009, 05:54 AM	#5
cdowns Senior Member Join Date: Jan 2003 Location: daytonabeach Posts: 22,956	Re: Help Motor doesnt run right. changing air and fuel filters,cleaning changing gapping spark plugs, cleaning changing setting gap/dwell of points, checking all vaccume lines. that's all routine maintence for old vehicles to run properly __________________ 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

09-16-2009, 07:31 AM	#6
PanelDeland I am a Referee of life. Join Date: Jun 2000 Location: Greensboro N.C. Posts: 13,992	Re: Help Motor doesnt run right. I think cdowns has pointed you right but I would add checking the dist vac advance.I would also suggest you look to upgrade to a HEI,it made a world oof difference in my 250. __________________ 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" Last edited by PanelDeland; 09-16-2009 at 07:31 AM.

09-16-2009, 04:15 AM	#3
Wrenchbender Ret Registered User Join Date: Oct 2005 Location: Overland Park, Ks. Posts: 5,229	Re: Help Motor doesnt run right. I'm with Tex. Your timing may be slow. Set the points to .016 with the rubbing block on the high edge of the dist cam. Set the timing to 6-8 degrees BTDC with the vacuum advance hose off. Make sure the vacuum advance is working.

09-16-2009, 09:16 AM	#7
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,751	Re: Help Motor doesnt run right. Definetly switch to an HEI. My 250 was cold blooded just like yours till I switched. Like difference between night and day.

09-16-2009, 02:08 PM	#8
mclairmo Registered User Join Date: Jan 2006 Location: Take Off Posts: 1,908	Re: Help Motor doesnt run right. As the old adage goes, most carb problems are ignition related. Check ignition first. I had a bad condenser once and acted like yours.

09-17-2009, 04:29 AM	#9
unclewatts 70 Chevrolet=Obsession Join Date: Jun 2009 Location: nice ca Posts: 1,067	Re: Help Motor doesnt run right. Definitely sounds like timing to me. Get an HEI Dizzy. They're really not that expensive if you shop around. __________________ I'm Just like my truck. Old, Ugly and Grouchy. Except my truck starts easily in the morning and doesn't smoke! IT'S NOT EASY BEING GREEN! -Kermit the frog 1970 Chevrolet C-10 Longbed Stepside : 350, Muncie M20, 3.08 GM corp. positraction Forest green exterior/light green interior(Where all my money goes) 1996 Toyota Corolla (Parts runner)

09-17-2009, 10:23 AM	#10
69GMCLonghorn Registered User Join Date: Dec 2006 Location: Porter Ranch, CA Posts: 978	Re: Help Motor doesnt run right. I think it's a defective piston return spring on cylinder #3, very common on the straight 6's...

09-18-2009, 01:06 AM	#11
airdale94 Registered User Join Date: Jan 2007 Location: Topeka,KS Posts: 778	Re: Help Motor doesnt run right. -defective piston return spring. Ignition doesn't get better when you rev it up it gets worse. I'd say it's fuel/carb related. Might just be bad gas if you just bought it and it's been sitting. Last edited by airdale94; 09-19-2009 at 02:40 AM. Reason: add

09-23-2009, 09:26 PM	#13
ryhan87 Registered User Join Date: Sep 2009 Location: Lloydminster Alberta Posts: 32	Re: Help Motor doesnt run right. Ok so I looked over a bunch of stuff to try and get it running right. Heres what I did. Set points, and checked timing. Timing was set at 4deg. Compression test. 160-140 on four holes, 2 holes where as low as 115. Changed fuel line, vac lines. Pulled valve cover and set all the valves. Checked crankcase vent. I assume you would be able to here blowby when you remove vent and listen so think rings are good. Ran carb cleaner through. Also checked over intake manifold for any vac leaks. Its running better than it was but still not great. Still backfires a bit till its warmed right up. Also doesnt seem to have much power in lower rpm range. Any other suggestions? Also what should this motor rev too? It seems to start almost backfiring or something at around 4000 rpm? Im thinking valves and seats now?

09-23-2009, 09:51 PM	#14
ryhan87 Registered User Join Date: Sep 2009 Location: Lloydminster Alberta Posts: 32	Re: Help Motor doesnt run right. Also it is backfiring through the carb



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