Port Engine goes to idle

[lovestocruise]

Ok, this is driving me nuts. I have a 97 330 Sundancer with twin I/O's powered by Merc Cruiser 7.4 Big blocks with 4 barrel carbs.
The port enging was rebuilt two years ago and only has around 20 hours.
Here is the problem. I will be cruising at 3100 rpm's and all of a sudden the port engine goes from 3100 to 1000 rpm's for a split second and then comes back up to 3100.
Also if at the dock in neutral it will go fro 1000 rpm to around 500 and then back up all for a split second.
Once it actually staled at idle.
I was told it could be the low oil sensor or water in the tank or a bad battery etc,
any ideas on where to start?

 

[Chris Dauth]

Sounds like a sick tacho, had the same problem. Disconnect the grey wire from the negative side of the coil and see how it goes.

 

[lovestocruise]

The tach is just registering what is going wrong . The engine actually goes out. Almost like it is going to stall. So I am pretty sure it is the engine and not the Tach.

 

[fc3]

Could be an ignition problem of intermittent wiring, bad coil, bad cap and rotor, bad plug wires, or the ignition system could be damp. Or it could be the carb with a sticky float valve, bad float adjustment, dirty passages, etc. Or you could have a dirty or plugged fuel filter. Or on a hot day you could be having a vapor lock issue. So many things to choose from! So little information!

 

[tdappleman]

How easy is it to replicate? You mention the oil pressure - if the sensor thinks it loses pressure it will cut power to the fuel pump until it sees pressure again - I think the minimum is about 8psi. You can clean the wire connections in case it is an intermittent connection - maybe loose and losing connection when you hit a wave, etc. You can always swap sensors and see if it changes sides.

 

[Vince_nj1]

My my old 260, there was an ignition cut-off switch driven by the transmission cables that cut power to the engine for a moment when you switched from forward to reverse. I assume you have a similar set-up. Any chance that the ignition cut-off switch is out of alignment and kicking in when your driving?

 

[lovestocruise]

All Good information, but I should say the I just had the engines serviced the week before. all new plugs wires filters etc. also did compression check and that was all good.
It will actually do it sitting at the dock. The engines will be on and then you can actually see and hear the port engine powering down then power back up in a second.
When you cruising it feels like a brake was put on when it powers down then like you hit the gas when it comes back up. the only guage that moves is the tach. Oil presure volts temp all stay the same.

 

[boatn]

Can you duplicate the problem or does it just happen?

 

[PlayDate]

Since the tune up was recently done....I'm suspecting a loose primary ignition wire. I would have the guy who tuned it up reset all the connectors he disturbed. Specifically, he should be looking for a broken wire or loose connection. A simple coil output test will check the condition of the coil. Does your mechanic have an engine analyser which will check the secondary (high voltage) ignition components?

It could be fuel pressure or a stuck float but it seems to be coming back up to speed too quickly for a pressure problem.

-John

 

[Chris Dauth]

If you get a short in the tacho, it will pull the ignition system down and then no spark. Like I said, I had this problem and all the experts had me pulling everything off and checking. Another old guy told me to disconnect the grey wire from the coil and it never faulted. Eventually I replaced the tacho. Been OK ever since.

 

[lovestocruise]

ok, Chris and pladate I am going to try both suggestions. I think one of them could be the issue. Will let you know what happens.
Thanks for the interest and ideas.

 

[lovestocruise]

Just reviving this post to let those that gave all that great advise know that I used the boat this weekend and guess what! No problem at all. the engine ride smooth and no fluctualtion at all. Must have been something but it cleared itself up.
thanks to all for the response.:smt021