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My 1986 Fifth Avenue idles rough and stumbles when trying to accelerate under load.
I did a tune-up: Plugs, wires, rotor, cap, PCV valve, air filter, new vacuum lines.
I found that the vapor canister doesn’t hold a vacuum, so I plugged the line from the carb.
The bowl vent is left open to the airThe acceleration problem still exist and it seems like a timing problem to me. Like the timing is firing before TDC causing the engine to stumble.
Light acceleration doesn’t cause this effect and there is lack of power. It’s tough getting onto the highway.Manifold vacuum seems good, but surges at idle. Haven’t checked it lately, though
Just recently, I decided to mess with the ignition computer cave line.
I found that if I disconnect it and plug the line to the carb, the acceleration problem gets worst.
If I pinch-off the vacuum line, while running, I get better acceleration, with less stumble.
The same result is obtained, if I apply a vacuum to the computer input, using a hand pump.
A quick test drive shows better acceleration and better power, under this condition.Could this mean that the computer is bad?
I think the rough idle is partially related to the computer, but the carb probably could use a rebuild, after 30 years.
Thanks,
Matt
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[quote=”Mgoglia” post=175597]I think the rough idle is partially related to the computer, but the carb probably could use a rebuild, after 30 years.[/quote]
Yup, I’d say so.
Other checks to do are a compression check, a leakdown test, and a check of the timing chain’s condition. You say “manifold vacuum seems good, but surges at idle.” You mean the gauge is not steady at idle? That can indicate several different problems, depending on what the gauge needle is doing.
hooked up a timing light and checked timing with vacuum hose removed from computer and plugged. The timing was moving back and forth kind of erraticly what looked like 10 degrees. I tried tweaking the timing a bit and not much improvement in the stumbling when I hit gas. The best performance I get is when I have 10 lbs of vacuum applied manually to the computer sensor.
with no vacuum applied, I can’t accelerate much at all before I get stumble and pinging.
How much does the O2 sensor affect the timing? I have a new one that I will install next.
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