Rough Idle and Stalling

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grwindmill97

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Ryan
Truck Year
1984
Truck Model
K10
Engine Size
350
Apologies in advance, this may be rather long.

I have a 1984 K10 that has a rough idle and stalls. I’m not an expert at this by any means, but I bought the truck 3 years ago and have done all maintenance and repairs, except for rebuilding the transmission, which I felt was well beyond my capabilities. I’ve been working on this problem for at least a month and I’m ready to throw in the flag, so this is my last resort before I take it to a real mechanic. Which I hate to do, because I feel like I’m just one step away from fixing it, I just can’t quite get there.

- 350 motor: supposedly rebuilt about 5 years ago by the previous owner.

- Rochester Quadrajet: rebuilt by me using a kit from Cliff about 100 miles ago. It’s a M4ME, with the double pumper disconnected by the PO.

- 700r4: rebuilt about 3,000 miles ago, installed by me with a new torque converter.

-Also, the PO deleted anything relating to emissions, aside from the PCV which I have replaced.

To explain everything I have to start in November of last year. I took it on a trip of 600 miles all together. Prior to the trip I replaced the distributor, but I can’t remember the symptoms that prompted me to do so, but as I recall, it fixed the issue. On the way to my destination, the truck ran well, but I noticed it did run slightly hot at highway speeds. On my return trip, I had trouble maintaining speeds above 70 mph. The problem with the acceleration started getting worse, it felt exactly like the carb was struggling for fuel during hard acceleration and under a load, ie. up a hill or highway speeds. Eventually it got to the point where I had to drive it home going 5 mph up a hill near where I live.

I replaced the fuel pump and rebuilt the carb, which returned the acceleration to what it was during my trip home, but it didn’t fix the problem. The previous owner had removed the passenger side tank, so I bought a new one and installed it. I also installed a new selector valve and switch. During this process I found an obstruction in the return line, which I cleared.

I noticed when using the new tank, that the acceleration problem did not exist. So, I started using that tank as the primary until I could troubleshoot the drivers side. Shortly after this, I filled up the new tank all the way for the first time and inadvertently overfilled it, because the pump didn’t shut off. Gas started leaking from somewhere under the truck, I was not able to determine the exact location, but it has not occurred since. While driving it home, the truck started running rough and stalling. When I got home I verified that no gas had gone into the charcoal canister, blew the line back for good measure and siphoned some fuel from the tank. Since then the only times I have had the truck out have been on test drives after adjusting/replacing something. One particular time I stalled so many times and flooded the engine so many times that the battery almost died on the side of the road from running the starter.

Now that I’ve covered all that stuff you probably didn’t need to know, here’s what I’ve done since.

Replaced spark plugs, wires, ignition coil, ICM, fuel pump (again), all soft fuel lines, PCV, brake booster grommet, blocked off EGR, verified float height, verified needle seats properly.

I’ve set the timing, adjusted the carburetor, swapped the carb with a brand new Edelbrock 1406 I had on the shelf and the problem persists. The idle is rough running off of the bowl on both carbs, so unless it is a fuel quality issue, I can’t imagine it’s fuel related. The tank has been drained, refilled and treated with seafoam.

I have a cylinder shorting tachometer and there doesn’t seem to be an issue with one cylinder in particular. Compression was tested last year and each cylinder was around 140 with little variation.

Any advice is greatly appreciated, I apologize if I overlooked a similar issue in another thread. If you have any questions, I’m more than happy to answer.
 

Ricko1966

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Drive it without the gas cap see if your problem goes away. You know you cannot compress a liquid right? It doesn't give like air, I wonder if your tanks not venting and with the tank full there's no air to provide a little give. Also check vacuum, if you Rev it and hold it revved what does your gauge do? At idle what does your gauge do?
 

grwindmill97

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Ryan
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1984
Truck Model
K10
Engine Size
350
Drive it without the gas cap see if your problem goes away. You know you cannot compress a liquid right? It doesn't give like air, I wonder if your tanks not venting and with the tank full there's no air to provide a little give. Also check vacuum, if you Rev it and hold it revved what does your gauge do? At idle what does your gauge do?
I took the cap off and brought it up to operating temperature, still has a rough idle. The tank is about 3/4 full now.

At idle it’s pulling 18in, needle bounces a little. Depending on how hard I rev, it’s holding between 19-20in. When I let off of the throttle, the vacuum drops to about 16, then picks back up.
 

Ricko1966

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I took the cap off and brought it up to operating temperature, still has a rough idle. The tank is about 3/4 full now.

At idle it’s pulling 18in, needle bounces a little. Depending on how hard I rev, it’s holding between 19-20in. When I let off of the throttle, the vacuum drops to about 16, then picks back up.
Did it quit stalling out?
 

fast 99

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When throttle is closed vacuum should increase not decrease. Know you preformed dynamic compression but how accurate was it? Have you checked for an intake gasket leaking underneath? Try dribbling fuel in each primary side of the carb when running any change?
 

75gmck25

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K25 Camper Special TH350 NP203
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5.7
16” is pretty low vacuum for an engine with a stock cam. I would have Firestone check for vacuum leaks around the entire intake, using a leak detector (propane, etc.).

Also, how did you manage to switch from a Quadrajet (spreadbore) to an Edelbrock (squarebore) without changing the intake? Are you using a dual pattern intake, and do you have the right gaskets to make sure the carburetor baseplate is sealing properly to the manifold?
 

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