Unusual symptoms: Aisan power valve??? (1 Viewer)

[rchalmers3]

I'm working on an '83 2F fitted with a non-US Aisan carburetor. It has a very strange issue I'd like to discuss.

On start up and during warm up it runs fine. After it's warmed up, and if I rev the engine hard a few times it begins to issue black smoke and the idle quality deteriorates. The engine revs well on the extra fuel, but makes way too much smoke to be acceptable. If I shut off the engine, wait a minute and then restart the engine, it runs clean and smooth again until the engine is revved.

I am able to adjust the idle mixture and idle speed with excellent results during the "normal" mode of operation. I can run the idle mixture screw all the way in without results during the black smoking. The fuel level is in the center of the viewing window. There are no droplets coming out of the mains at idle. This engine just received a new head gasket, valve job and tune-up. Vacuum readings at idle are excellent until the overly rich mixture presents itself.

I believe the power valve plunger in the air horn is a possible culprit. I imagine that during a low vacuum signal, the plunger is sticking in the down position, thus forcing the power valve constantly activated. I have suctioned out the fuel bowl, installed a new power valve with jet, and spray cleaned out the plunger, spring, plunger bore and connecting circuits. These changes made no difference.

I'm seeking advise! Has anyone ran into a issue like this before? What did you find?

Thanks,

Rick

 

[FJ40Jim]

The power valve circuit is irrelevant at idle. The PV could be removed entirely with no effect on idle quality.

This symptom is indicative of a secondary butterfly that is not quite fully closing after being knocked open by WOT operation. Secondary slow circuit is much richer than primary slow circuit to compensate for the lack of secondary accelerator pump. Unstick & lube the secondary throttle shaft and problem will go away.