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Possible Idle hesitation.

TJ1978

TJ1978

I have MANY questions
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Truck idles decent (to me) at 750ish RPM. Drives fine, at least to me. 350 sbc, edlebrock 650 avs.

But as it idles, it’s goes smooth...... then what can only be described by me a hesitation.

So think of it like. Idlllllllllllllle, start, idlllllllllllllle, start, idlllllllllllle, start . It is consistent in this. Like a hiccup, then idle. It’s idles for a few a seconds then the hiccup.

Maybe it’s nothing? The air/fuel mixtures screws are about 2 turns open on each. I have not checked with a vacuum gauge as I really don’t know where to stick it to get manifold vacuum. Can the back where the brake booster goes work, assuming the tube will fit?

Anyway, I’ve shown a few people and they have said it’s nothing “sounds fine to me” but, my inexperience may be making this more than it is. I have older video and didn’t do this.

Only strated if memory serves, when I was trying to get the idle rpm down. Carb is brand new-ish and no hesitation when I stomp on it either. Or could it be from me in installing the carb spacer?
 
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Mine has done that. Up'd the idle a little to 600rpm in drive with the brakes on. I think it's about 800rpm in park.


***EDITED to clarify my point:
I realized my comment wasn't as useful as I intended. When I said mine smoothed out at 600rpm in gear, I should have added that I had bumped up the idle about 100rpm. The point I meant to make was that perhaps a slight increase in your idle RPM would help. I didn't mean to make a big deal about the 600 number; only that you might need to up your idle a little.
 
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Could be an ignition miss,or lots of other things--if you have a timing light,hook it up to each plug wire one at a time and watch the flashes (dont look right at the bulb,shine it on something else ),and see if it "skips" a flash when that cylinder's turn to fire happens...a bad plug wire or cap,coil with a carbon track or crack can cause this..usually the engine will do it worse,misfire under a load if its spark related though..

The vacuum gauge will tell a lot,if it might be a vacuum leak or other issue..you can tap it in to any manifold vacuum source..
Sometimes over advanced timing or pre-ignition makes one cylinder fire sooner than the others...loose timing chain can too..one fouled or defective spark plug..the list is long..


Another possibility is a sticking valve ,a worn valve guide,or one cam lobe is worn enough to not open all the valves an equal amount..but if it pulls well under load and only does this at idle it is probably a lean misfire,or vacuum leak ,not any other serious internal problem..
 
diesel4me said:
Could be an ignition miss,or lots of other things--if you have a timing light,hook it up to each plug wire one at a time and watch the flashes (dont look right at the bulb,shine it on something else ),and see if it "skips" a flash when that cylinder's turn to fire happens...a bad plug wire or cap,coil with a carbon track or crack can cause this..usually the engine will do it worse,misfire under a load if its spark related though..

The vacuum gauge will tell a lot,if it might be a vacuum leak or other issue..you can tap it in to any manifold vacuum source..
Sometimes over advanced timing or pre-ignition makes one cylinder fire sooner than the others...loose timing chain can too..one fouled or defective spark plug..the list is long..


Another possibility is a sticking valve ,a worn valve guide,or one cam lobe is worn enough to not open all the valves an equal amount..but if it pulls well under load and only does this at idle it is probably a lean misfire,or vacuum leak ,not any other serious internal problem..
Click to expand...


Engine has like 300 miles on the rebuild. New wires, plugs etc etc. And it only started after I was messing to get the idle RPM down.. it was idle at 1000-1200 rpm.

But where do I hook the vacuum gauge up to get manifold vacuum? Like, exactly where? A picture would help. I keep reading “any manifold vacuum” that may make sense to experienced people. But I have no idea where that would be... is there a port on the manifold itself and if so where? Further what size connector do I need to “tap” into that to hook the gauge up to?

I’ve found no info/video demonstrating this, other than “get a vacuum manifold port” but nobody shows where or how to do it.
 
Where to hook vacuum gauge? The port between the carb and distributor? What adapter fits there?

2A3A8B32-5F91-423A-B64C-37EE9A2344AB.jpeg
 
Any port on the carb below the throttle plates is manifold vacuum --you'll feel suction there with it idling,or test it with the vacuum gauge.. --there might be a plug you can take out of the intake if there is one--you can tee it into the vacuum modulator vacuum port if you have an automatic,or the power brake vaccum hose..the PCV valve hose is best left alone though..
 
diesel4me said:
Any port on the carb below the throttle plates is manifold vacuum --you'll feel suction there with it idling,or test it with the vacuum gauge.. --there might be a plug you can take out of the intake if there is one--you can tee it into the vacuum modulator vacuum port if you have an automatic,or the power brake vaccum hose..the PCV valve hose is best left alone though..
Click to expand...

I can use the back of the carb, where the brake booster goes?

I don’t know how to use a timing light. I have one, just never used it before.
 
On this front one as well?
Sorry to be annoying here. I’m just trying to figure this out.

21891A35-F2A5-4979-9410-F2FE4EA8E918.jpeg
 
You want the one on the bottom right that says "manifold vacuum"..

NOT the "timed vacuum" port,that is ported vacuum,that only gets vacuum past idle or at a certain RPM,usually only the EGR valve or sometimes the vacuum advance uses that port..
 
@diesel4me
Thanks boss... I thought I had a vacuum gauge. But must have misplaced it in the move or something. So I had to order a new one today... will post back once it arrives.
 
I realized my comment wasn't as useful as I intended. When I said mine smoothed out at 600rpm in gear, I should have added that I had bumped up the idle about 100rpm. The point I meant to make was that perhaps a slight increase in your idle RPM would help. I didn't mean to make a big deal about the 600 number; only that you might need to up your idle a little.
 
@mrk5
A guy in my neighborhood who has a couple old Chevys, I figured he may know something... So, I drove over and asked him about my issue also to see his trucks.
He said fuel could be getting old and to bump up the idle a tad.. Also, since I don’t drive it that much my ears may be looking for stuff that aren’t really an issue or it could something, get a few gallons of fresh gas and vacuum gauge and go from there.

I have video of my truck running(the last time it ran before moving about a month ago) and that hiccup wasn’t there. But it was on last start up when I was messing with idle rpm. I took the video after I installed the carb spacer. The vacuum gauge should be tomorrow. I’ll add some fresh fuel from my 5gallon jug and post back.
 
If it sets a lot I would look for gas that doesn't have alcohol in it. Do an internet search for "ethanol carburetor" and you will have a lot of reading.

I have noticed our 2008 Silverado company truck doesn't have a perfect idle, but I never sweat it. Worry about it more with the C10 even tho in reality I should be more forgiving of the C10.
 
Wes Harden said:
So this started, after the spacer was added?
You may a vacuum leak.
Click to expand...

No... it happened a while ago, then nothing. Put spacer on idle was fine. When I drove it from my old house to new one it was fine. Then it started again as I was moving it from the gravel to the drive way (same day)... hahahahaha.

I’m going to mess with it this week once the vacuum gauge arrives.
 
May have some water in the fuel...take a sample and let it settle,see if its cloudy or a layer forms on the bottom..

I've gotten crap gas right fresh from the pump at the corner gas station before--my lawn tractor would barely run on it...dumped it out and went to another station and then it ran great again,after 5 minutes of pooping and backfiring while the crap gas left in the carb got used up..
 
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