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2006 Merc 90 hp 2 stroke surging when idling

R

Randy G

New member
Over the weekend I noticed that my Merc 90 hp was first making a strange noise when idling, similar to a loud vibration sound, that would just occur then disappear. I also noticed that at idle speed, in no wake zone, the motor seemed to rev up just slightly then drop right back down. At running speed I did not notice anything of any true significance other than there may have been the slightest of sneezing sound. I'm sorry I cant be more descriptive but from the drivers seat it was hard to make out the sound exactly. It seemed to perform fine. I did replace the prop from a 19 pitch to a 15 pitch and refilled the fuel tank prior to going out. I'd be surprised if the prop replacement had any effect and I thought if the fuel was the problem it would be more noticeable at running speeds as well as idle. Any suggestions? Thank you.
 
Why did you make such a drastic change in prop pitch? Did you replace with a "like kind" prop, other than pitch change? Where did you get your new gas? Did you have this problem before this last outing? How long since your previous outing? Do anything on the trip out of the ordinary? Oil tank have adequate oil?
 
I didn't see any noticable jump in RPM''s on the tach but it seems to bounce around slightly regardless. I only had the throttle slighyly past the "N" because the marina was pretty congested Plso I was idling out pretty slow. Again, Im sorry I cant provide more detail. I wish now I wouldve recordrd the sound. The pitch was because one of the blades appeared to be bent on the prop I normally use and I wanted to have it repaired. I had this prop as a backup since I purchased the boat. Gas was from station that I often go to. Had taken out the previous weekend but my son drove the entire time so I didn't recall hearing any differences in motor or assumed he was doing it if I did. Oil level was 3/4's. Was going to check resistant across coils tonight.
 
How did the prop perform when you went to max throttle and got on plane? RPMs steady or jumping around, loosing speed intermittently with increase in RPMs?
 
I did not run it at max throttle because the 15 pt prop would run the tpms too high but did run at 5000 rpms. Tach was fine and boat seemed to perform well. I can't swear that there wasn't a slight difference in sound of motor but it is hard to detect amongst all the other sounds and wind noises. Thanks, by the way, for your help.
 
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