Rough idle and takeoff 250 inline

[68gmcdude]

Good evening....
My '68 GMC has an inline 250. It's been running beautifully all summer, including this past Saturday when I took it on a 400 mile, all highway trip to a show. Yesterday I went to go for a ride and now it's idling rough and is rough on takeoff with an occasional backfire. Once up to speed it still runs fine. I changed the points tonight since they looked pretty bad (after only a year) and it didn't help. Tomorrow I'm changing the plugs. What else could it possibly be? Last year I changed everything... Plugs, wires, cap, rotor, points, condenser, coil, carb rebuilt, timed, etc... Thanks!

[Palf70Step]

Check/CHange/claen your fuel filter. You may mav gotten a piece of crude in it that's partially clogging things.

[68gmcdude]

Originally Posted by Palf70Step

Check/CHange/claen your fuel filter. You may mav gotten a piece of crude in it that's partially clogging things.

I'll try that. Just changed a couple months ago but it's possible. Thanks!

[Stanco]

Check for a vacuum leak then check the timing.

[68gmcdude]

Originally Posted by Stanco

Check for a vacuum leak then check the timing.

Thank you... I will do that.

[HO455]

I would break out my vacuum guage and see what it told me. If your not familiar with using one here is a basic primer. The vacuum guage should point you in the right direction in your troubleshooting.

There are numerous other articles as well as videos to help on the interweb.

https://www.onallcylinders.com/2015/...gine-problems/

Good luck. I hope it turns out to be an easy fix.

[geezer#99]

What did you set the timing at?

[68gmcdude]

Originally Posted by geezer#99

What did you set the timing at?

0 degrees TDC... According to the manual, that is the setting for the 250 with the A.I.R. system (smog pump). RPMs @ 700.

[67 twins]

Originally Posted by 68gmcdude

0 degrees TDC... According to the manual, that is the setting for the 250 with the A.I.R. system (smog pump). RPMs @ 700.

You know I'm willing to bet your favorite beverage that if you set that to a reasonable number like 10-14 all your problems will go away.

[68gmcdude]

Originally Posted by 67 twins

You know I'm willing to bet your favorite beverage that if you set that to a reasonable number like 10-14 all your problems will go away.

Set the timing to 10-14 before TDC?

['68OrangeSunshine]

Originally Posted by 68gmcdude

0 degrees TDC... According to the manual, that is the setting for the 250 with the A.I.R. system (smog pump). RPMs @ 700.

Do you still have the A.I.R. pump working on the engine?
I bought my '68 C/10 Stepside in 1973. I discarded the A.I.R. gadget in '74. I was no longer in California. It hit the bottom of a GI can with a satisfying CLUNK.
I have a 292 L6, bored .030 over, I set my timing at 12*.

[68gmcdude]

Originally Posted by '68OrangeSunshine

Do you still have the A.I.R. pump working on the engine?
I bought my '68 C/10 Stepside in 1973. I discarded the A.I.R. gadget in '74. I was no longer in California. It hit the bottom of a GI can with a satisfying CLUNK.
I have a 292 L6, bored .030 over, I set my timing at 12*.

Yes. I still have the A I.R. smog system on it. I'm trying to keep this truck all original. I'll try adjusting the timing to 10-14 as suggested by you and others.

[geezer#99]

Really!
I’ve run my last two at 14 initial.

If you bump it up the idle will increase and you’ll be able to reduce the idle back to 600 rpm. Another plus to closing the throttle plate more is you’ll likely be idling on the idle circuit and not the power circuit.

[RichardJ]

>>I'm trying to keep this truck all original.<<

YA. Ya. Ya. As if anybody gives a damn. ggg

Seriously, many people who want to keep the original appearance, gut the pump and plug all the passages so that it looks original, but doesn't function.

Those AIR systems caused idle and backfire problems from the get-go and that's one reason why so many were removed. If you look at a '66 system, the first year, and compare it to the versions in the years after that, you will find dozens of changes GM made to try to get them to work better.

>>0 degrees TDC... According to the manual, that is the setting for the 250 with the A.I.R. system (smog pump).<<

Does that manual tell you how to test that AIR system to determine it it is even working?

[Rust_never_sleeps]

^^This was my question. Air injection adds another layer of mystery to the stack on top of an unforgiving timing setup.
Not sure how well it handles excessive blowby of an aging engine, or that check valve gets gummed, or exhaust leaks screwing with flow, etc.

Curious that it gets worse over time on a hot engine. What do the plugs look like

['68OrangeSunshine]

68gmcdude -- What's your Odometer reading?

[68gmcdude]

Originally Posted by '68OrangeSunshine

68gmcdude -- What's your Odometer reading?

Just over 55k original miles.

[Rust_never_sleeps]

Insulting question: you're setting the static timing with vacuum advance disconnected and plugged?
Also, vac advance is functional? (moves freely when vacuum is applied to the hose)

[68gmcdude]

Originally Posted by Rust_never_sleeps

Insulting question: you're setting the static timing with vacuum advance disconnected and plugged?
Also, vac advance is functional? (moves freely when vacuum is applied to the hose)

Yes to both questions...

[68gmcdude]

I checked for leaks around the intake manifold by spraying starting fluid then checking the bolts. No leaks found and all bolts were tight.
I advanced the timing to 6⁰ and took for a ride. Idle seems better but still off a little. Takeoffs were good with no backfires. The truck is a 3 speed manual. Seems a little off when I'm cruising at 30mph and I'm in 3rd gear. No problem accelerating from that speed or any other cruising speed though. Runs beautiful on the freeway at 65mph.
So I was always under the impression that advancing the timing was a bad thing. Evidently not as it seems a lot of people do it. Should I advance it even more?

[geezer#99]

Of course!
Jump right in the deep end of the pool.
Go right to 12.

What’s your idle vacuum?

[67 twins]

The only danger in advsncing the timing is going to far & causing detonation. As Geezer#99 said go to 12 & test drive it while listening for detonation on acceleration. If you experience no detonation then your good. Then enjoy the increase in power and trouble free driving.

['68OrangeSunshine]

What Geezer said.

About Advanced Timing. When these trucks were new, American gasoline was the best in the world. The Tetra Ethyl Lead created a cushion for pistons and valves. Zinc in the oil helped the lifters slide on the cam lobes easy. And Timing was good at TDC.
Since we're dealing with a totally different blend of automotive fuels today, with additives and ethanol, we have to compensate by advancing the timing.

[mr48chev]

Ran great until you got home from a 400 mile road trip?

Think back, where did you buy gas on that road trip? I'm a picky bugger and I don't care if I have to pay more but only buy gas at newer high volume stations or at Costco stations and never at old former gas station mini marts or cute/quaint out of the way stations.

My brother in law drove fuel truck for several years and Costco is the absolute pickiest about fuel delivery. They don't let drivers stick the tank because the stick on the truck might have contamination on it. Most high volume truck stop or similar stations move enough gas that they never get water or contaminates.

Before you go nuts run the gas in the tank out and down as low as you feel safe with and go fill up at the local station that you normally fill up and see how it acts.

I wouldn't put it past dirt or crud in the carb but at least think back to where you bought gas on the trip and when it started acting up.

[68gmcdude]

Originally Posted by mr48chev

Ran great until you got home from a 400 mile road trip?

Before you go nuts run the gas in the tank out and down as low as you feel safe with and go fill up at the local station that you normally fill up and see how it acts.

I wouldn't put it past dirt or crud in the carb but at least think back to where you bought gas on the trip and when it started acting up.

I filled up at GetGo before heading home. My buddy drove his truck down with me and filled up at the same place with no issues. Tank was below 1/4 tank when I got home and I filled up where I usually do. Issues didn't improve.
I have also gone through the carb twice and put a new filter in. Carb has a nice steam of fuel when I pump the accelerator.