Engine stalls when throttle returns to idle

[thatch]

Wisakedjack, Sorry it took so long for the light to finally come on, but, since we're dealing with a Catalina-30 here, you may have a vacuum leak at the back end of the manifold. Here's how it goes. The hot section of a c-30's exhaust runs laterally from the manifold over to the top of the water-lift/muffler, where it drops down and connects, normally with a short length if rubber hose, to the inlet on the top. Engine torque, over time, tends to either crack the water-lift or to loosen up or actually crack the flange at the back of the manifold. Neil and I replaced a manifold on an A4 a couple of years ago that had completely cracked off. The manifold had rusted through and the crack went undetected until after it was removed. Recently I rebuilt my hot section and replaced my manifold because of a crack in the flange area. One way to check for a vacuum leak at the manifold gasket is to spray some WD-40 between the head and the manifold while the engine is running. If there is a leak, the engine rpms should change and some white smoke should show up in the exhaust. I realize that you had recently removed the head, but didn't notice if you removed the manifold or not.
Tom

[Wisakedjack]

Tom, thanks for the suggestion. I did not remove the manifold. I will look at the manifold flange and do the test that you've described.

[Wisakedjack]

I am very happy to report that my carburetor issues have been resolved. While cleaning my carburetor this week I've discovered the following issues:

1. I think the main issue was that the idle screw was not seated completely when I was doing its adjusting. I got the new longer one from MM earlier this season and the fit was pretty tight, so I assumed it was screwed in all the way, when the conical tip was merely showing in the passage opening. In retrospect I should have realized the the spring should be fully compressed when the idle screw is all in. Having done this properly I did 3/4 turn out since I have PCV valve as an initial setting and it pretty much worked well.

2. I carefully cleaned the idle jet with small wire. I was not able to take it off since the screw driver slot is half gone. So, I had to do it in place. By the feel it was not completely clogged, but may have had some residue there.

3. I discovered that choke plate was put incorrectly. As a result the poppet valve was opening to the outside instead of inside. I think that explains why I always needed to open the choke right after the engine was started.

4. Finally, I replaced the throttle plate. The old one was somewhat bent and did not fully obscure the lower of the two idle holes. Whether that had any significant effect on how my carburetor worked I don't know.

After putting carburetor in the engine and minor adjusting I no longer have any issues with engine shutting off when I throttle down. In addition the engine idles nicely at 800 RPM. I don't think it ever sounded this good.

I would like to thank everyone here for advise and helpful suggestions. It was extremely valuable to learn few other possible causes (like PCV valve failure or cracking exhaust flange) even though they were not in play at this time. Thank you guys!

[ndutton]

Check your spring

There were so many different needles and springs used throughout the 4 decades of the late model carburetors that a problem emerges when someone uses an earlier (smaller diameter) spring with one of the later needles.

The proper dimensions are: ID is just under ¼” and the uncompressed length is just over ½”. Turning the factory needle in against the seat, the spring is not quite fully compressed. There is a small shoulder at the outer end of the needle which the spring slips over.

[JOHN COOKSON]

I didn't reread the thread.
If you don't have a "polishing filter" between the fuel pump and the carburetor I would strongly recommend one.

TRUE GRIT

[Wisakedjack]

I do have a polishing filter between the pump and the carburetor

[Wisakedjack]

I did some sea trials this last weekend. Engine does not shut down anymore when I throttle it down. Max RPM at full throttle is close to 2000 now. Opening throttle more has no effect. At full throttle my boat speed was about 5.5 kts, but I had outgoing tide against me. So, its close to hull speed. Makes me very happy! I don't know how I could've done it all without having all your help and access to this awesome community. Thanks a lot guys!

[ndutton]

Did you ever find out why the idle mixture needle didn't seat properly the first time?

[Wisakedjack]

I did not want to apply too much force and thought it was seated. I should have realized that I was wrong since spring was barely compressed. When I realized my mistake half a turn with screwdriver pushed it in from which point turning it rest of the way was easy.