MR-1 loses RPM when warm

[AuburnVFR]

I have a 2008 FXHO with the MR-1 HO motor that we don’t use that much since getting the boat. This spring I changed the oil and spark plugs and noticed it loses rpm and speed after riding for 5-10 minutes. When cold it will turn 10000+ rpm and burn 12gph at wide open throttle. Once warm, it drops down to 8500-9000 rpm at WOT burning 10-11gph. Sometimes it surges back and forth between full and reduce rpm, but mainly stays in the reduced range. I thought it was old gas and burned through that tank a few weeks ago. I put fresh gas in today but still got the same results.

Any ideas why it would do this only after warm up? I figured if it was a spark plug or a coil issue, it would do it all the time. Could oil level cause this issue? It looked right during the cold check on the trailer (at the L mark).

Thanks

 

[zipper]

Check the spark plugs gaps. You may have one out of speck after it gets hot. Make sure the plug wires are seated. Where did you source the plugs. There have been counterfeit ngk's being sold on the internet.

Counterfeit Ngk Spark Plugs | Consumer Alert

How to identify fake Ngk Spark Plugs

thecounterfeitreport.com

 

[Babin Farms]

Double check and make sure all of the spark plugs are tight in the head, not overly tight but just tight. Sounds like one may have worked loose.

 

[Fishwater2002]

I am working through a similar issue & believe that I have it pin pointed to the TPS switch. Looking at it with the YDS it is out of spec & showing the throttle opening at 4.2 degrees with the engine cold & not running. My boat would run at max rpm until it got hot then the RPM’s would drop 2k RPM’s fluctuating back & forth until it either stalled or I shut it off. Then once it cooled down no issues until it warmed up again.

 

[AuburnVFR]

Thanks for the replies.

I purchased the current NGK CR9EB plugs through Amazon. The previous set was the iridium version and were in there for a few years (not a lot of hours though). I think I have an extra set lying around and will swap them out if there is no obvious issue with the fitment, gaps, or sloppy workmanship (too much anti-seize).

Hopefully its not the TPS since I've not had a stalling issue.

 

[AuburnVFR]

Update…

Spark plugs changed, plug gap was checked and was correct. The removed plugs looked normal (pictured below). Still got 10k initially, then once warmed up it would not go over 9k.

The oil level was way too high for some reason. I swear I only filled it to the low mark on the dipstick when it was cold on the trailer (at my house, away from the lake). I checked it on the water after running and took about a quart and a half out to get it to just under full (hot). It would still not run over 9k when hot.

The aftermarket reusable metal mesh air filter (R&D) showed no signs of motor oil contamination. I ran it without the air filter anyway. It initially went over 10k for a few minutes, then back to 9k.

It idles and runs smoothly, it’s just not making full rpm once it’s warmed up. Any other known residual effects from too much oil? Any other ideas or something to trouble shoot?

Thanks

 

[zipper]

As far as the oil level on the mr-1 dry sump motors...when cold, fill to the bottom of the dip stick, just touching. Filling to the low mark is too much. I always checked mine while the engine was running at idle or within seconds of shutting down. It will not blow out. The oil will leak down from the dry sump over time and show an incorrect reading. Check again and add if needed right after shutdown if you are not comfortable checking while running.

 

[buckbuck]

Is it possible you have an exhaust leak and are starving the engine for oxygen?

 

[Murf'n'surf]

Do the wave runners have air filters? If so, try running without it.

The older boats with the MR-1's were a mixed bag of results on top rpm level. Some people were able to peg the motors at 10K, some only got 10K on one motor. Elevation, humidity and air temp are probably the biggest contributor to the RPM offsets.

 

[AuburnVFR]

Is it possible you have an exhaust leak and are starving the engine for oxygen?

Interesting idea. Next time I'll run without the rear portion of the seat in place.

 

[AuburnVFR]

Do the wave runners have air filters? If so, try running without it.

The older boats with the MR-1's were a mixed bag of results on top rpm level. Some people were able to peg the motors at 10K, some only got 10K on one motor. Elevation, humidity and air temp are probably the biggest contributor to the RPM offsets.

Already tried it w/o the air filter.

It does run just over 10k from a cold start, only loses rpm after running a few minutes once the engine is warm.

 

[Murf'n'surf]

Already tried it w/o the air filter.

It does run just over 10k from a cold start, only loses rpm after running a few minutes once the engine is warm.

That is consistent with my prior observations of temperature.

 

[Murf'n'surf]

Have you smelled your oil? You said it was overfull, just making sure you don't have injectors stuck open and dumping fuel into the oil.