Engine stalls when throttle returns to idle

[Wisakedjack]

I am having the following issue with my engine now. It starts fine and idles ok between 1000 and 1100 rpm. This was the case all last season and at some point I'd like to figure out how to make it idle closer to 800 rpm, but this is not a current issue. When I open throttle the engine accelerates fine. However when I bring throttle back to idle my engine stalls. Some times if I move throttle slowly the rpm will drop to 800 there is some hesitation after which idle rpm returns to 1000+. I don't know if this is the aftermath of water in the engine issue that I had earlier in the spring:

http://www.moyermarineforum.com/forums/showthread.php?t=11059

Today I tried to figure out whats going on, so here is what I tried and some observations.

1. I have good fuel pressure after pump (there is a gauge there).
2. I drained some fuel from the carb and it looks clean.
3. I cleaned up the carburetor and replaced gasket between two halves.

At this point I run engine again and still had the same issue. I also noticed that only large adjustments to the idle screw had any noticeable effects.

4. Checked and cleaned spark plugs. Turned out the gap was about 0.02, which I have adjusted to 0.04. This improved things a bit, but did not eliminate issue completely.

I am hoping someone could help me identify the issue and suggest what I could try next. Any help would be greatly appreciated.

[ndutton]

3. I cleaned up the carburetor and replaced gasket between two halves.

Did you run a small wire through the tiny ports in the carburetor throat to roto-rooter them out?

[Wisakedjack]

Yes I did.

[ndutton]

PCV system?

[Wisakedjack]

I did put PCV from Indigo in place 2 seasons ago and it seemed to work well. I did not think about it...

[ndutton]

As a test try plugging the vacuum hose.

[Dave Neptune]

Did you check to see that the C-advance was functioning nicely?

When replacing the bowl gasket did you check between the halves of the carb for warpage? This warpage can cause idle problems if the emulsion well in the center is not sealed!

Are you sure it isn't the throttle linkage holding the carb off of the idle stop? Worth a check.

Dave Neptune

[Wisakedjack]

Dave what is C advance? Are you talking about timing? If thats the case then no I did not change anything with ignition. I went to the boat today and tried the test that Neil has suggested. I had a helper holding a thumb to close hose that we have disconnected from the PCV valve. Still had the same issue. PCV valve rattles, so I don't think its stuck. As far as the carburetor is concerned I was looking for a vacuum leak, which is why I changed the gasket between halves. Old one did look somewhat warped. Dave, what did you mean when you said that:

"the throttle linkage holding the carb off of the idle stop"

How do I check for that? I looked at MM manual and only suggestions I am seeing are basically to clean up carburetor. Given my water intrusion earlier dirt in the carburetor is possible, but I've cleaned it after twice (one full and one partial). I wonder if anything else could be at play here. I really liked the possibility that it was caused by PCV, but I think my test ruled it out, right? That would have been a really easy fix

[indigo]

Vacuum Leak

Now that you have ruled out the PCV valve itself, try carefully squirting a little starting fluid around the two gasketed joints on the PCV spacer plate with the engine idling. If the engine speed changes when squirting, then there is a vacuum leak in one of the joints. You might also squirt a little around the throttle shaft where it enter the carb body to check the shaft seal.

Tom

[Wisakedjack]

I don't know whether this may be at play here... I've noticed that MM carburetor rebuild kit contains what look like rubber gaskets. I don't recall seeing any in my carburetor when I cleaned it. But I had this engine for almost 5 years now and it worked well before. I am just curious now whether gaskets are needed? Are there ones that go on the throttle lever shaft by any chance?

[ndutton]

I went to the boat today and tried the test that Neil has suggested. I had a helper holding a thumb to close hose that we have disconnected from the PCV valve. Still had the same issue.

Good test, eliminates one possibility. Just to confirm, the hose you plugged was the one that connects to the spacer between the carburetor and manifold, correct?

If you bought the Zenith factory carb kit (a recent addition to the catalog by the way), the small black rubber donuts are throttle and choke shaft seals.

[Wisakedjack]

Good test, eliminates one possibility. Just to confirm, the hose you plugged was the one that connects to the spacer between the carburetor and manifold, correct?

Yes, thats the one we plugged - one that connects to the aluminum spacer.

If you bought the Zenith factory carb kit (a recent addition to the catalog by the way), the small black rubber donuts are throttle and choke shaft seals.

Good to know. I've not bought the carb kit recently, but I looked at the picture and wondered what those rubber gaskets are for since I've not encountered any in my own carburetor. I don't know if its time to rebuild yet. I cleaned my carburetor every fall after the haul out and it does look pretty clean. I will do the tests that Tom has suggested to see whether there is a vacuum leak at the flange or the shaft.

[Wisakedjack]

I should have mentioned one more detail. When cleaning my carburetor I was not able to remove an idle jet. The slot shows some minor damage. The MM manual said its better to leave it in place, which I've been doing for years now. I blasted it with carb cleaner and compressed air. I guess I will only need to worry about its removal if I have to rebuild using MM kit. But I would be curious to know how to do it without damaging the carburetor itself.

[capnward]

no idle? check idle jet.

You're right, you should have mentioned that. I once had a problem that the engine wouldn't idle. I couldn't see that the idle jet was partially obstructed until I removed it, held it up to the light, and looked through it. It has been recommended that you file a screwdriver to just the right size for the idle jet, so you won't damage it. It should come out, but if you can't do that, run a thin wire from a wire brush around in it, while it's in the carb. Don Moyer showed that wire trick in his superb video, after removing the jet from the carb. The junk in it may not dislodge with carb cleaner or air. That tiny orifice is so important to keep clean. If there is junk in your fuel, the idle jet will be the most likely place in the carb to catch it. If the opening of it is really mangled, I guess that may affect it as well. Once the jet was clear, the idle came back. Now I take it out every year and check it for crud. That to me is the most important part of carburetor cleaning.
Now I know that there are supposed to be rubber seals around the throttle and choke shafts. I haven't noticed them before. I wonder if I have them...

[Wisakedjack]

Here is a quick update on my issue. I sprayed some starter fluid around the carburetor (throttle shaft, between two halves and around manifold) and did not get any increase in RPM. I had new gaskets put earlier, so for me only the shaft was a suspect. I brought carburetor home and will clean it again with special attention paid to the idle jet (thanks capnward). Will let you guys know what happens when I fire the engine again. In the meantime any other suggestions on what I could try or test would be greatly appreciated.