1968 GMC 250 Inline 6 4.1L Chugging

[JAV6]

I went through a similar problem with my 292, replaced many parts just as you during the process of elimination. All of the items helped, HEI, fuel pump, filter, carb rebuild, etc. but problem would return after awhile. Turned out the fuel tank had rust buildup that would come loose, plug a carb jet or eventually plug the fuel filter. Pulled the carb apart several times to just find more rust getting past filter. Cleaned it all out and ran great! Just my 2 cents.

Originally Posted by 68gmcdude

UPDATE..

I have done the following:

Replaced the fuel pump
Replaced the sock filter on the fuel sending unit
Replaced the fuel filter in the carb
Blew out fuel line from sending unit to fuel pump
Blew out fuel line from fuel pump to carb
Replaced rubber fuel line from sending unit steel line to fuel pump
Replaced vacuum line from distributor to carb
Replaced vacuum line from smog pump to carb
Confirmed vacuum advance canister is good
Confirmed vacuum advance arm is working
Checked vacuum at carb
Confirmed manual choke works properly
Confirmed points/dwell set properly
Confirmed timing
Confirmed RPMs
New exhaust (it was needed)

Going to have the carb checked again (was rebuilt last year). I don't know where to go from here...

[68gmcdude]

RICHARDJ

One question or yours that I forgot to answer. The Intake/Exhaust gaskets were replaced last year. Still look good. No black soot around this area.

[68gmcdude]

RICHARDJ

One question of yours I forgot to answer. The Intake/Exhaust gaskets were replaced last year. They still look good and there is no black soot around that area. Thanks.

[68gmcdude]

UPDATE:

Carburetor came back from the shop and everything checks out ok.

I pulled the distributor and put new points and condensor in even though points looked good and condensor was a year old.

New issue. I marked the distributor and motor so I could just drop it back in. I also took a picture of which way the rotor was pointing. When I try to reinstall, the distributor will only seat with the rotor pointing another direction. Why? Should it not go back in the way it was? The truck was not moved and or cranked while distributor was out.

First pic is before I pulled it. Second pic is how it now seats... uuugh!

Help!

[RichardJ]

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Confirmed vacuum advance arm is working -- vac adv should be connected to Ported Vacuum port with no advance @ idle with 0 in.hg.
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Checked vacuum at carb -- Vacuum = ? in.hg. @ rpm, checked WHERE ? Port on manifold or does you carb have manifold vacuum port?
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Confirmed points/dwell set properly -- With vacuum hose plugged, does dwell stay @31 when engine is revved, With engine @idle, use hand vacuum pump to operate vacuum advance through its full range. Watch timing on balancer with timing light. Again check that dwell stays @ the 31
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Confirmed RPMs -- What is the Idle rpm with choke off and engine warm
New exhaust -- Did you check the Intake/Exhaust gasket against the head? Look for black soot on the block below the manifolds.

NEW Exhaust? Did you get the LMC exhaust?

[68gmcdude]

Originally Posted by RichardJ

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Confirmed vacuum advance arm is working -- vac adv should be connected to Ported Vacuum port with no advance @ idle with 0 in.hg.
*
Checked vacuum at carb -- Vacuum = ? in.hg. @ rpm, checked WHERE ? Port on manifold or does you carb have manifold vacuum port?
*
Confirmed points/dwell set properly -- With vacuum hose plugged, does dwell stay @31 when engine is revved, With engine @idle, use hand vacuum pump to operate vacuum advance through its full range. Watch timing on balancer with timing light. Again check that dwell stays @ the 31
*
Confirmed RPMs -- What is the Idle rpm with choke off and engine warm
New exhaust -- Did you check the Intake/Exhaust gasket against the head? Look for black soot on the block below the manifolds.

NEW Exhaust? Did you get the LMC exhaust?

I'm a do it yourself mechanic so please forgive me if my answers don't make sense or I'm not understanding something..

Vacuum advance is connected to port on carb in the picture I have attached. When disconnecting and checking with vacuum guage and engine at idle the guage read 0. When revving the engine the guage would read up to 20. I did not have a meter hooked up for RPMs when revving the engine.
Idle for this motor with choke off and warmed up is 700rpm. I believe normally it would be 400rpm but according to the shop manual it should be 700rpm because mine has the A.I.R. smog pump.
I have not checked your third suggestion. Will do that.
I ended up getting the LMC exhaust. Ended up working out good.
Thanks for everyone's help... Hope to get this figured out soon.

[tim_mc]

Sounds similar to my issue last year: Truck had an occasional backfire or load up when driving. It progressively got worse to where it would start OK, but would die when put into gear. It would idle and rev OK, but ran badly under a load, or when stopping at a stoplight. I had to put it in neutral a few times to keep it from dying at intersections. Later it would lean out and backfire after it warmed up and eventually would not restart when warm.
Fix: New coil and module. GM HEI was only a few years old, but as we know that doesn't matter. Upgraded to DUI coil and module and have been running great since.

[72SB]

If the 6 cyl is like the V8, you use a long screwdriver after pulling the dizzy back out and turn the oil pump slightly. Looks like you want to turn it slightly to the right.

Sometimes when you pull the dizzy it turns the pump some so putting it back in same orientation the slot end of dizzy won't engage the pump unless it goes where the pump now is.

[Steeveedee]

Looks like you are one tooth off. Do you have before and after pictures of the crank pulley and timing marks?