115 missing at high RPM

[redog]

I have a 2000 115 Johnson with a jet pump. Recently it started missing at higher rpm's. The first time I noticed it, it was just a slight burble. It seemed to be worse the last time I took it out. What are some things that I should check for before taking it to a boat mechanic? I do have a service manual but am not a boat mechanic. Thanks in advance.

 

[Texasmark]

Re: 115 missing at high RPM

Spark plugs are known to cause missing at WOT. Fuel problems would be more like complete cutting out or rpm's dropping off rather than a cut out of a cylinder here and there.

Mark

 

[funpilot]

Re: 115 missing at high RPM

I don't know if this is the case with Johnson Jet Pumps but it is on Mercurys. Look in the engine wiring diagram for a rev limiter. Jet pumps can allow an engine to over rev before they load up with water and offer enough resistance. The rev limiter cuts the ignition at higher revs to prevent this. They can fail intermittantly and cut the ignition at lower revs than intended. Typically, you can disconnect them and run without to trouble shoot. Just accelerate smoothly and avoid jumping wakes with it disconnected.

fp

 

[ezeke]

Re: 115 missing at high RPM

Make sure that all of your fuel lines are tight. Look for any place that air could get into the system. Check the pump pulse line carefully as well as the membrane.

A common occurrence is that fuel tanks with small amounts of fuel may allow air to enter on tight, fast turns. This results in erratic missing somewhat after the turn so that it is not considered to be related.

Air entering the fuel system results in a potentially very damaging lean condition.

 

[redog]

Re: 115 missing at high RPM

Bought some new spark plugs today and installed them. Will let you know if that was the problem. The boat dealer said it is a common problem. He said a plug might be fouled with oil.

 

[Stumedic]

Re: 115 missing at high RPM

I'm with Mark. Every high speed problem I've had was plug related. Stu

 

[Captain Jeff]

Re: 115 missing at high RPM

In addition to what everyone else has said, get a timing light and clip it on to one spark plug wire at a time. Have someone drive the boat and watch the light when the problem is occuring. It sounds like you may have an electrical problem.

If it flickers erratically on all 4 wires then you have a faulty optical sensor or power pack. If it only flickers on one or two then you may have a bad coil pack. It should normally flicker rapidly but smoothly and almost be constant on at high rpm. Good luck.

 

[redog]

Re: 115 missing at high RPM

Took the boat out last evening with the new plugs installed. Problem solved. Thanks guys for the help. Had me worried.