Help with Idle problem

[mm58]

Hello, Hoping someone can shed some light...

Engine was just put back in truck after a top-end overhaul/cleanup.
After tweaking carbs and timing, truck ran pretty normal for the first week or so.
Everything seemed great. Life was good.

Now I have a problem that is absolutely baffling me.

Engine does NOT want to idle at a stable rpm.

Example: After letting off of the gas pedal and putting gearbox in neutral (say turning off the road and into a parking lot)
engine may rev up to over 2000 rpm. Stabbing the gas pedal might return to 7-800 rpm, but then all by itself, the rpm
will slowly increase back up to over 2000. All without any input from me.

The other thing with this, is that the engine will surge or lurch when driving slow (again, parking lot speed in 2nd or 3rd
gear). Pull into parking space and let off pedal, gear in neutral, and rpm again will climb by itself to over 2000.

All carb linkages have tight springs, so throttle plates are definitely snapping back after letting off pedal.

Accelerator pumps have new fresh seals.

At higher road speeds in 4th /5th gear, it seems to run pretty normal.

All cylinders are firing, and a check of the plugs showed cyl's 1-2-4 dry with slight black soot. #3 was slightly wet.

I have Pertronix ignition, with a new civi dist cap, new civi plug wires, and the stock mil-coil

Compression is high, and leak-down test was outstanding.

Mixture screws are backed out 1-1/2 turns out from full in

I have checked carbs repeatedly for any obstructions to any jets or orifices. Timing is set about 1/2 to 1 inch advance.
But the timing is difficult to set because I can't get it to idle steadily around 8-900 rpm

The tiniest little adjustment to the idle stop screws can make a big change to rpm. (Don't know if that's normal for these engines)


Carb sync tool goes full scale even when I can manage to get it to idle 8-900 rpm for a few seconds

I have checked float levels and both carbs show between the bottom of the plate and the bottom of the clip.

To me, this seems like a carb issue, but I have no idea what else to adjust or look at

Any help appreciated!

Mike

[undysworld]

Maybe an air leak in the intake runners or the hose between them?

[mm58]

Manifolds are tight with new gaskets, and balance hose is solidly attached and intact.

[rmel]

Has the earmarks of a vacuum leak, but you just indicated hoses are tight
Check the hose and fittings on the Break booster too.

[mm58]

Ok, I will check other vacuum lines tomorrow. Thanks

[Andre]

Spray carb cleaner at throttle shafts while running and listen for change in rpm.

[mm58]

Spray carb cleaner at throttle shafts while running and listen for change in rpm.

Will check that too... thanks!

1976 Pinzgauer 710K

[Jimm391730]

All carb linkages have tight springs, so throttle plates are definitely snapping back after letting off pedal.

If the throttle linkage to one carb is too tight, it might close completely after stabbing the pedal but the engine vibrations may slowly open the throttle of one carb enough to bring up the idle. You can check this with carb balancer, or disconnect the throttle linkages and see if it still increases in rpm. The throttle itself may be sticking and when finally starts to move, opens the throttle enough to create the surge or lurch.

The tiniest little adjustment to the idle stop screws can make a big change to rpm.

IIRC a quarter turn can change the rpm by a couple hundred. Does this match your experience?

[mm58]

The return springs on the carbs are the oem springs. I shouldn't have used the word "tight", to infer that the springs were overly strong. My bad.
I've been making 1/8 to 1/4 turns on the idle stops to make my adjustments.

1976 Pinzgauer 710K

[mm58]

Another question while we're at it... when turning the mixture screws out (counter-clockwise), am I making it more rich or lean?

1976 Pinzgauer 710K

[Jimm391730]

When I said "tight", I meant that while the throttle return springs close the throttle and rotates the shaft, the pushrod linkages and ball joints may be a bit too long on one carb so that rod is trying to push the throttle open, while the rod to the other carb is trying to pull that throttle closed. Usually you can leave one rod in place, then test fit the second rod up to the ball joint and adjust the linkage length by screwing the ball end on or off the rod until it is the exact correct length to fit on the ball without moving the throttle shaft even a tiny bit.

If one rod is even slightly trying to push the throttle open, a quick snap of the throttle may allow enough momentum of the throttle shafts to close properly even with a slight bit of force against the rod. But engine vibrations may allow the rod to work that throttle open over time (a few seconds).

CCW mixture screws should make the idle richer.

[mm58]

When I said "tight", I meant that while the throttle return springs close the throttle and rotates the shaft, the pushrod linkages and ball joints may be a bit too long on one carb so that rod is trying to push the throttle open, while the rod to the other carb is trying to pull that throttle closed. Usually you can leave one rod in place, then test fit the second rod up to the ball joint and adjust the linkage length by screwing the ball end on or off the rod until it is the exact correct length to fit on the ball without moving the throttle shaft even a tiny bit.

If one rod is even slightly trying to push the throttle open, a quick snap of the throttle may allow enough momentum of the throttle shafts to close properly even with a slight bit of force against the rod. But engine vibrations may allow the rod to work that throttle open over time (a few seconds).

CCW mixture screws should make the idle richer.

Ok, I see what you mean now. It's highly possible that is the source of the problem. I replaced a couple of the rod ends as part of my overhaul and not sure if I got the lengths exactly right. Will check and report back...

1976 Pinzgauer 710K

[mm58]

Eureka! Jim, your suggestion about the throttle linkage rods was spot on! That's exactly what It turned out to be.
Once again, source of problem as simple as ever, elusive to find, yet hiding in plain sight.
Engine is idling quite fine now, but I still will have to go back in and fine tune timing and carbs...

Thanks to Jim and everyone else who chimed in!

[Jimm391730]

I got lucky. You mentioned EVERYTHING ELSE that you did or checked except changing of the linkage rod ends. But I find it interesting that as detailed as we can all be in describing all the things that were done, the one thing that isn't mentioned is (almost always) the culprit! Must be part of Murphy's Law.

Keep up the good work. Now if I can only figure out why my brakes are worse after I bled them than before... yes, I got air in the lines, but I just can't figure out why... I'll probably have to change the throttle linkage rod ends to get the air out!