Merc 40/4 warm cranking problem

[q-t]

I did a search and didn't find anything, but:
We use a couple of 4 stroke Mercury 40's at work. One of them has become hard to crank after it warms up. It will start just fine cold, idles and runs great at speed, and will run all day long as long as you don't shut it down. After it is run a while and warms up it seems to develop vapor lock or something when we shut it down. We can remove the fuel filter on the engine and drain the fuel out, then reinstall and it will usually fire right up and run just fine...until we shut it down again. We have been leaving it running all day long to avoid problems with cranking again. This is a very busy time for us and we can't spare it to go to the shop right now, if possible to avoid it. There is also a water separator filter inline, which is changed regularly.
Any ideas? Thanks

 

[WillyBWright]

Re: Merc 40/4 warm cranking problem

Is this an EFI model or does it have carburetors? First guess with a carbed model would be a leaky primer dribbling gas slowly. It would burn-up in a running motor and dissipate after the motor had been off for a long time. If it's a EFI I'd suspect a sticky injector. Either create a flooded condition which clears over time once the fuel pressure subsides and the residual gas evaporates or leaks out of the cylinder(s).

 

[Silvertip]

Re: Merc 40/4 warm cranking problem

You don't have a cranking issue. You have a warm start issue. Will not crank means the starter does not turn the engine. That clarified, next time it fails to start, immediately pull a couple of the spark plugs and check them for wetness which indicates a flooding condition. You don't mention a model number or whether these are carbed or fuel injected engines. Low compression is one cause of hard starting hot. Do a compression check. Are these high-hour engines which this one may be due for an overhaul.

 

[q-t]

Re: Merc 40/4 warm cranking problem

Thanks for the tips. I'm aware of the difference between cranking and starting; just having a senior moment, I guess. I think the engine is carbed, and probably high hours, too. I just recently started using this equipment, so I'm not sure how many hours are on the engine. I think I heard someone say 900 at the last time it was in the shop, which was probably close to a year back. The engine runs great except being hard to start after it has been running for a while and heated up, then shut down. It will idle smoothly all day long if needed without a problem, accelerates and runs just fine at speed.
I'll post when we determine just what it is.

 

[q-t]

Re: Merc 40/4 warm cranking problem

OK, to bring this back to the top, and hopefully pass on some info for anyone else with a similar problem.
To update: the engine is an EFI unit. After several trips to various repair shops without success, the problem was getting worse. Finally took it to the shop that I deal with for my personal use, and explained to them everything that was going on. The took the extra time to run the boat on a test lake until the problem became apparent (only under WOT, usually), the with diagnosis the problem was identified as a buildup where the water tube from the pump enters the transition plate underneath the engine. A few gaskets, etc and problem solved! This motor has about 1080 hours on it, with quite a bit of time in salt water, which likely contributed to the buildup. As long as the engine was operated at slower speeds, it would run all day long, but at WOT, there wasn't enough water to cool the engine adequately, I guess.