Motor not restarting after motoring

[4n6fisher]

I have been having occasional issue with my motor not wanting to restart after we cruise around. The issue seems to only crop up when I have been running pretty good (when we motored up to top of Camano) at about 2500-3000 rpm (20-22 mph) then I turn off the motor for 20-30 minutes. When I went to restarts it just chugged a couple times, blew out some white smoke then did nothing. Turns over great and new cap and rotor (so spark is good) but no sign of wanting to start at all. I thought we were going to have to head back on the kicker (would have been a long ride and the pot would have been in water well after 1 PM cutoff). Engine seemed pretty hot still (temp still 120-140) so I left the engine cover open while we started cruising with the kicker. After about 30 minutes the main motor fired back up. It ran good the rest of the day, but we did modify our procedure a little. We started letting the boat idle for about 5 minutes after any significant motoring and that seemed to help but small sample set.
To me it seems like I am either getting some sort of vapor lock or the engine gets flooded when I turn it off right after high rpm running, but not sure how to know or tell which one.
The boat is a 1978 19' Glasply cuddy with a Mercruiser 470.
If anyone has any ideas or has had this issue please let me know, thanks.

[BentRod]

Aren’t Mercruisers fun?!

My first question is in regards to temperature. You said it seemed hot, but temp gauge read normal. How hot are the risers getting? They should roughly be between 100-160 degrees F. You’ll need an IR thermometer to test, but if you can’t keep your hand in them then they’re on the hot side.

[4n6fisher]

Yes just when you think you got it all dialed in something crops up, but it is 41 years old!

Not too hot, just still warm, as it did not cool all the way off during the time we were just hanging out with motor off.

Wondering about something in the carburetor that might be allowing gas to continue to flow after running hard and stopping fast.
Thanks for any tips if there are any more.

[BentRod]

The carb would definitely be my next subject. But, it's difficult to diagnose without actually being there and I'm certainly no expert on the matter, just have some experience with the Mercruisers and engines in general. Although it shouldn't happen, I've seen the automatic choke be a problem if it closes slightly on a warm engine. The solution is to manually open up the choke when starting. Of course that requires removing the top of the carb, which is no fun on the water either. When was the last time the plugs were changed?

[4n6fisher]

New plugs last year, new cap and rotor this year. Good thought on the choke, another friend mentioned that might be an issue. I should probably check the plugs to see what they look like.

[Berlinettaa]

Pull a spark plug after trying to start hot and it dies; if the plug is wet with fuel, the engine is flooding. A few things can cause this like a leaking float seat or overheating the carb bowl. Sometimes a plastic riser/thick gasket is used in between the carb and the engine to discourage heat sink and fuel vaporization. If the engine runs normally all other times I would not suspect the ignition system but it is easy enough to check with a cheap in line spark tester. Another good tool is your nose- does the motors exhaust smell heavily of fuel at idle? This will help it start when cold but can cause hot start problems like you are having. Make sure your idle set screws (the one or two screws with springs on them near the base of the carb that go directly towards the barrel) are set properly. A good starting point is all the way in (until the needle is seated, dont overtighten) then back it out 1 1/2 turns. Adjust with the engine running from there to achieve the highest idle speed, then adjust the idle screw on the main throttle plate if it is too high.

[Tanker5455]

Yeah, I was thinking about the engine being flooded with fuel as well, I had this issue on other engines (mostly motorcycles) and what you described does sounds like it.

[4n6fisher]

No issues this time out with the only change being letting the main motor idle down for 3-5 minutes after running, restarted perfectly every time. Nice to finally have a trip with no mechanical issues.