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.

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?

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.

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.

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

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.

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.

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.

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.

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...:lol::flag::flagw::haha:

Re: Help Motor doesnt run right.
 

:haha: :agree: -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.

Re: Help Motor doesnt run right.
 

Unless there is a problem with the vacuum advance.

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?

Re: Help Motor doesnt run right.
 

Also it is backfiring through the carb