Rough idle and takeoff 250 inline

[68gmcdude]

Originally Posted by '68OrangeSunshine

68gmcdude -- What's your Odometer reading?

Just over 55k original miles.

[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?

['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.

[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.

[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.

[68gmcdude]

Engine is idling worse and is getting worse under load. I advanced running to 8⁰ from 6⁰ and then increased to 10⁰. No improvement. I took the belt off of the smog pump so it wouldn't run and nothing improved. The carb has a nice stream of gas shooting into the carb when I pump the accelerator. I'm thinking the old truck didn't like the long 400 mile trip on the highway. I had never driven it that far in the highway before.
I'm going to do a compression check next. Any other suggestions? I'm thinking it's just tied and needs rebuilt at this point. Only 56k miles on it.

[Steeveedee]

C'mon people, there has to be someone local to him who can sort this out!

['68OrangeSunshine]

68gmcdude -- They sell little brass plugs that thread into your head to seal off any leakage from an inactive A.I.R. system. Also the steel tubing, pump, and bracket are dead weight now. It never did much to improve air quality. It just had the _appearance of doing something positive_ about air pollution. They were political, not engineering, ''improvements.''
I appreciate your desire to keep the truck original.
At only 56,000 your rings should still be tight. L6s are bombproof.
I ran my second 292 L6 to 250,000 -- by then I was blowing big blue smoke rings of Valvoline 50 weight.
Do the compression check, and get back to us.
Good luck.

[68gmcdude]

Originally Posted by '68OrangeSunshine

68gmcdude -- They sell little brass plugs that thread into your head to seal off any leakage from an inactive A.I.R. system. Also the steel tubimg, the pump, and bracket are dead weight now. It never did much to improve air quality. It just had the _appearance of doing something positive_ about air pollution. They were political, not engineering, ''improvements.''
I appreciate your desire to keep the truck original.
At only 56,000 your rings should still be tight. L6s are bombproof.
I ran my second 292 L6 to 250,000 -- by then I was blowing big blue smoke rings of Valvoline 50 weight.
Do the compression check, and get back to us.
Good luck.

Where are the brass plugs available that seal off the holes in the head?

[Steeveedee]

Air pollution arguments notwithstanding, we'll need to see a picture of your engine to determine the type of plug you need after the remove the AIR injection manifold. The threads are straight pipe with a shape of double flare 5/16" b-nut like a brake line fitting. I'll go on record right here with stating that the AIR system on any vehicle produced with it did not cause rough idle or take off. I'd like to hear credible stories of people who had this problem. Millions of vehicles were produced across ALL vehicle manufacturer's products where no complaints were lodged. I worked on those vehicles back in the day as a certified mechanic, and this was not an established issue, ever.

[geezer#99]

Redo the lifter adjustment.
Only do 1/4 turn from zero lash.

['68OrangeSunshine]

Originally Posted by 68gmcdude

Where are the brass plugs available that seal off the holes in the head?

I haven't needed them in decades, but I'll research it. On one head, I got a brass set from the local Hot Rod shop. Another head, I found set screws the right diameter and pitch.
I'll look around. Roughly about 3/8'' IRC.