1968 GMC 250 Inline 6 4.1L Chugging
 

I've reached the end of my patience... ��. My 250 inline 6 starts and idles fine. It will run down the road like a champ for a mile or two then start chugging and backfiring real bad. When I let off the gas it idles fine then it starts chugging and backfiring again a little ways down the road. Here is everything that has been done...

New gas tank and sending unit 3 years ago
Carburetor professionally rebuilt fall of 2020
Distributor professionally rebuilt fall of 2020
New cap, rotor and points
New AC Delco plugs and wires last summer
New coil this spring
New fuel pump (last summer and again yesterday)
New fuel lines
New carburetor fuel filter (last summer and yesterday). Old ones looked good.
New vacuum advance line for distributor to carb

I'm going to pull the sending unit out of the gas tank. It's been suggested that maybe the sock filter has collapsed (?).

Any other suggestions or possible issues are greatly appreciated! Thank you!

Re: 1968 GMC 250 Inline 6 4.1L Chugging
 

How often is it run? The 1 bbl on my original 70 would gum up when it set a lot. I would pull off the top, run some carb cleaner thru it and it would run fine until it set again. You may have a sticky power valve or jets from the gas. Only other thing I can think to check is the actual vacuum advance unit to see if it is sticking or not moving.

Re: 1968 GMC 250 Inline 6 4.1L Chugging
 

I run the truck 1-2 times a week to local car shows. About 10-15 total miles weekly.

Re: 1968 GMC 250 Inline 6 4.1L Chugging
 

We used to shoot a little low pressure compressed air into the line not a lot just a quick shot to blow out crap in the sock . Leave the cap off while doing it .

My 68 327 did this when going up hills and at near WOT . Turned out the rubber line going from the pump to the frame hard line was collapsing

Re: 1968 GMC 250 Inline 6 4.1L Chugging
 

OP

You sure your choke is fully opening once engine warms up??

Re: 1968 GMC 250 Inline 6 4.1L Chugging
 

I will try that. Thanks!

Re: 1968 GMC 250 Inline 6 4.1L Chugging
 

I actually just thought of that right before reading this. Going to try disconnecting the manual choke and wire it open, then run it down the road to see what it does. Thanks!

Re: 1968 GMC 250 Inline 6 4.1L Chugging
 

well manual choke should be open fully when pushed back in....so check choke linkage to confirm it does in fact open it fully. You don't need to drive it to do this

fuel pressure may be getting compromised. Maybe a kink in line somewhere??

an inline FP gauge comes in handy

Re: 1968 GMC 250 Inline 6 4.1L Chugging
 

When a condenser goes bad in the distributor it will act like a fuel problem.

The quality of them is not that great any more.

It's cheap and easy to replace. I'd give it a try.
Good Luck

Re: 1968 GMC 250 Inline 6 4.1L Chugging
 

IMHO, dich points all together and put in a Pertronix kit regarding stock dizzy.

with that said IDK if OP's issue is ignition related

Re: 1968 GMC 250 Inline 6 4.1L Chugging
 

Never a good idea to introduce a new variable when trying to troubleshoot a problem.

A year ago you had questions about doing a tune up. You were advised to find a Dwell Meter rather than rely on setting the gap manually. A year later the points are possibly corroded, pitted and in need of cleaning and readjusted. File with small metal file.

After point adjustment, check and reset timing.

Spray carb cleaner around intake manifold while running and check for vacuum leak.

Re: 1968 GMC 250 Inline 6 4.1L Chugging
 

Choke is working properly

Re: 1968 GMC 250 Inline 6 4.1L Chugging
 

I'll be the first to admit that I am not a certified mechanic, but what does setting the points with a dwell meter vs manually have to do with them becoming corroded and pitted?

Re: 1968 GMC 250 Inline 6 4.1L Chugging
 

I've held off commenting, but my 1968 with 250 has basically the same problem.

I've always thought it was a fuel problem. I have HEI and have rebuilt carb a couple times. Truck also has a new fuel sending unit/pickup and the tank doesn't look like it has trash in it.

I loved hearing 72c20customcamper say that it was a fuel line collapsing because the symptoms he was describing is just like mine, slight incline, near WOT trying to go 69-70mph on the highway with 4:10 rear gears. It starts bucking and you have to let off the gas and slow down for it to get better. Doesn't always happen, you can run for miles at about 70 then it just chokes.

Interested to hear what you find out. I'm thinking I'll inspect the rubber lines, but I know I've replaced those in the past. Might try something stiffer, like some PTFE lined AN line. I've been neck deep in my other truck and kind of neglecting the other one...

Re: 1968 GMC 250 Inline 6 4.1L Chugging
 

One thing I should mention... The truck came from California and it still has the original Air Injection Reactor (A.I.R.) smog pump. Thought I would mention this in case it could be part of my issue.

I know there will be responses that tell me to remove it but I would like to keep things as close to factory as possible.

Re: 1968 GMC 250 Inline 6 4.1L Chugging
 

The A.I.R. system is not the cause of your problem. Anyone removing the system did it because they either didn't understand the function or didn't like the buzzing the pump made. Or maybe some other reason that I don't know about. But it won't cause your problem.

Re: 1968 GMC 250 Inline 6 4.1L Chugging
 

I wasn't sure that was causing my issues but thank you for confirming this.

Re: 1968 GMC 250 Inline 6 4.1L Chugging
 

My truck was giving the classic vapor lock symptoms . One day it did it and I killed the ignition and pulled over . I had a clear filter on it just to see if there was crude in the system . The filter was dry and then it slowly started to fill . The hose was real mushy from oil soak from the outside . Oil and gas line dont mix not made for it I cringe when I see it used on transmission coolers knowing it will fail. Changed it out and never had the problem again .

Re: 1968 GMC 250 Inline 6 4.1L Chugging
 

To me sounds like something awry in the ignition system. Like a few others have said check your points gap, initial/total timing, confirm your vacuum advance is working properly. Also run your multimeter on the negative side on coil, throttle it up and check if your condenser going bad.

Re: 1968 GMC 250 Inline 6 4.1L Chugging
 

>>but what does setting the points with a dwell meter vs manually have to do with them becoming corroded and pitted?<<

I didn't say it did.

New point contacts are smooth and shinny. After a few miles they take on a course appearance caused by about 300volts arcing across them when the points open. If you don't understand where the 300 volts comes from, you need to read up on the Kettering Induction Ignition System.
In the first image, the enlarged points show a course, gravelly surface. They show a .016" gauge giving a .021" actual gap. Filing with a small metal file will help provide better results and in the end a more reliable dwell. But, they still won't last as long as new before they need attention.

The real problem with using a feeler gauge is that it requires a delicate touch. The point arm spring is relatively soft and the feeler gauge is a metal pry bar. If you don't take the feeler gauge out of the pack, the gauge pack acts like a pry bar handle, giving even worse results.
If you ask 10 guys to set the gap on your dist and check with a dwell meter, you'll probably find 3 guys that set the gap correctly.

The second image shows misaligned contact points that will go out of adjustment very rapidly. Many guys don't bother to check and the Chinese made points are worse than ever.

A couple guys mentioned the condenser. The easiest way to check to see if the condenser should be replaced is to look at the used points. The condenser should be 0.18 - 0.22 micro farads. In the first image you see metal transferred from one contact point to the other. The job of the condenser is to suck up that 300 volts. If the condenser capacity is too large or too small, metal will be transferred from one point contact to the other.

Re: 1968 GMC 250 Inline 6 4.1L Chugging
 

RICHARDJ

Thank you for the information and the pictures. I checked the points tonight. They still look like brand new and the alignment looks good. I hooked up a dwell meter. Should be between 31-34. It registered at 31.

Re: 1968 GMC 250 Inline 6 4.1L Chugging
 

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...

Re: 1968 GMC 250 Inline 6 4.1L Chugging
 

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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?

Re: 1968 GMC 250 Inline 6 4.1L Chugging
 

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.

Re: 1968 GMC 250 Inline 6 4.1L Chugging
 

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.