How to diagnose engine hesitation, stumble, jerking, bucking

[Jts]

When I got my 83, it was in sad shape, broken radiator, doors and windows that wouldn’t open, absolutely no power, wouldn’t idle, and the engine would cut out at random times. I decided to write a bit about my progress so it could help other people out diagnosing their engine hesitation stumble jerking bucking problem. The problem my car had was a total cutting out when mid throttle. When driving around from ¼ to ¾ throttle, the engine will act like its running out of fuel. Flooring the gas pedal causes the engine to run normally.

First off, check the vacuum advance on the distributor. The canister has 2 ports (3rd port being linked to the 2nd). They should hold vacuum individually. There should be no bleed of vacuum between sides. If there are any leaks in the canister, it’s bad. If it tests ok, check to see if the canister is getting vacuum while running with the timing light test. My canister was bad. I replaced it, with no change. Turns out my choke breaker diaphragm was leaking vacuum. I fixed the leak and gained a lot power from the timing advance, but the cutting out was still there

Next check the egr. Unplugging the hose that goes to the egr canister will tell you if the egr is pumping too much exhaust gas into the intake. When I did this, the cutting out improved, but it was still quite bad. I have left the egr blocked for now

Next I blocked off EBCV that is run by the O2 sensor. By unplugged the 2 hoses from the carb that go to the EBVC, you take the o2 sensor and computer out of the equation. Doing this also improved my cutting out, but did not cure it.

So far I have established that my problem is due to a lean fuel condition. Blocking off the egr and EBCV causes the mixture to richen, but not enough to solve my problem. I then chose to rebuild my carb, thinking that a jet must be clogged. I purchased a rebuild kit for $19 (Discount import parts in Beaverton/Portland if anyone is interested). I tore the carb apart and found it to be in pretty good shape. I discovered that the power valve was bad though. It did not pass the tests inside the FSM. It would not allow air when extended or not. I went to a jy and got another power valve. I put the carb back together and installed it. It definitely improved the cutting out problem. If you have checked the egr and EBCV system, I would check the carb for problems. I now get no cutting out of the engine. There is still a very slight stumble when about 3/4 throttle. I am beginning to think that this is caused by the secondary throttle opening. At this point, I am happy with the performance of the engine. It has 316k on it, and all the parts that i've replaced on it were from the junkyard (egr modulator, egr can, vac advance, o2 sensor).

I hope this helps people track down their stumble. For me, the power valve was the big key to fixing the stumble. However, the egr and EBCV system can cause a lot of problems if they are faulty. Some people may find that blocking off these systems will improve the performance of the car enough to drive with it permanently blocked. I almost settled with it, but decided I wanted it to be as good as possible.

[takza]

A method for diagnosing a carb:

"So I appologize if my diagnostic techniques seem dated..

As I recall from being a Datsun mechanic three careers past with the infamous hitachi carb, all carb idle issues can be diagnosed with these steps:

1. tighten all carb screws, and tighten carb-manifold nuts. datsuns love to loosen these.

2. Start engine, Let the engine warm up until idle gets funky. spray WD40 around carb at 4 angles 90 degrees apart in short 1 second bursts. See if idle speed stablizes. if it does, then there is a choke/idle solenoid leak. or vacume hose break, possibly a warped carb."

[4wdchico]

Could I get another definition of what is being called the "power valve" in Jts's original post. Have looked in the FSM exploded diagram and do not see anything called that. Are we talking about the 1st fuel cut solenoid as referred to on pgs. ec56-57 of the FSM?

Thanks

[takza]

On page 136..."Power Valve with Jet"...also called the "Power Piston"?

[gmeddy]

egr modulator, egr can, vac advance, o2 sensor??
on the 83 model?
I have 83 model, i did'nt think there was any 02 sensors in cars this old?! do i have one?
what is this egr stuff? i think mine is pretty much completely stuffed, if it's the thing bolted to the manifold under the carb, that can be removed when doing the Weber install and put a port there for the PCV hose to run to...