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.

[68gmcdude]

I have changed the points, condenser and spark plugs. Point were set to. 019 gap according to the manual. Timing was off and is good now. Seems to be idling better but have yet to drive it. Hooked up a dwell meter and it is only reading 24. Should be 31-34 according to the manual. I keep reading that the dwell/ points should be set with the motor cranking, not with a feeler gauge. I have never done this. How do you do this by yourself? And how do you keep the motor cranking while adjusting the dwell / points?.... Help? Thanks!

[RichardJ]

Originally Posted by 68gmcdude

I have changed the points, condenser and spark plugs. Point were set to. 019 gap according to the manual. Timing was off and is good now. Seems to be idling better but have yet to drive it. Hooked up a dwell meter and it is only reading 24. Should be 31-34 according to the manual. I keep reading that the dwell/ points should be set with the motor cranking, not with a feeler gauge. I have never done this. How do you do this by yourself? And how do you keep the motor cranking while adjusting the dwell / points?.... Help? Thanks!

Really?

They mean with a dwell meter. You could check dwell cranking, but I always put the cap back on and start the engine to check dwell.

Dwell angle is the period of time the points are CLOSED, point gap is checked when points are OPEN. When dwell reads low, it indicates you have gap set TOO wide.
You can not be ham fisted when setting gap. It takes a delicate touch. The point spring is relatively soft and the feeler can act as a lever, prying the points apart, especially if you are using the entire pack of feeler strips. Separate the pack and use the single feeler strip.

[68gmcdude]

Originally Posted by RichardJ

They mean with a dwell meter. You could check dwell cranking, but I always put the cap back on and start the engine to check dwell.

Dwell angle is the period of time the points are CLOSED, point gap is checked when points are OPEN. When dwell reads low, it indicates you have gap set TOO wide.
You can not be ham fisted when setting gap. It takes a delicate touch. The point spring is relatively soft and the feeler can act as a lever, prying the points apart, especially if you are using the entire pack of feeler strips. Separate the pack and use the single feeler strip.

I will just keep adjusting the gap and checking using your method. I'm sure it will take a few times to get it right. What I'm hearing in your explanation is I need to lessen the gap to increase the dwell reading, correct? Thanks...

[68gmcdude]

Got the dwell adjusted to 33 (recommended range is 31-34). RPMs are where they should be and timing is dead on. It's idling great. Will take for a test drive tomorrow. Fingers crossed...

[68gmcdude]

Took the truck for a ride. It idles fine and runs great when up to speed. It's still a little rich upon takeoff and under load with an occasional backfire. The timing is dead on, dwell is good, rpm's are good. I checked the vacuum lines and they are good. Replaced the points, condenser, spark plugs. What else would cause this?

[Steeveedee]

Bad plug wire or two. You may have one open or just high resistance.

[68gmcdude]

Went to change the fuel filter in the carb, even though I changed it in the spring. Just happened to look at the carb diagram in the manual and noticed there is supposed to be a spring in the filter inlet. Wonder if this is the issue? Could the solid backside of the filter that is supposed to rest against the spring be blocking the flow of the fuel if it's being pushed against the back of the inlet?

[68gmcdude]

I am at a loss. Still running rough at idle and takeoff with an occasional backfire. Here's what I've done...

Replaced condenser
Replaced points and set with dwell meter. Should be 31-34. Set at 33.
New spark plugs
New spark plug wires
Carb disassembled, cleaned and rebuilt with new accelerator pump, springs, gaskets, etc.
New fuel filter in carb
New vacuum lines
Set timing
Set RPMs

One note... I had a "clacking" sound so I adjusted the valves while the motor was running. I backed off the nuts until each valve started clacking, then slowly tightened until the clacking stopped. I then tightened three (3) 1/4 turns, waiting 15 seconds between each 1/4 turn to allow for adjustment. All seemed ok.
Two days later, I drove the truck to a show, about 350 miles round trip. Truck ran great on the way down. First started to stumble some on the way home but was fine up to speed on the freeway. Got progressively worse after that.
Could I have adjusted the valves too tight? Could this be causing my issue?

[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.

[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.

[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.

['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 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?

[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.

[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?

[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)