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?