200-400 RPM fluctuation

[GulfCoast56]

I have a 1999 DX150
Changed plugs, fuel filters
Still have a small change in RPM while running cruise speeds. Idle sounds great, and smooth. Every so often it seems to stop the issure for 5-10 minutes, then resumes.
No misfire is heard, but can hear the change in RPM, and "feel" the boat change speed.
What should I be testing to find the issue
Thanks Mike

[99yam40]

fuel pumps,filters, screens, and injectors.
testing pressures when problem shows up would be best.
along with posting the complete motors model # so everyone know which one you have

[DennisG01]

And...

Check out my thread on what sounds like a pretty darn similar issue. I still haven't solved the problem, but there were a lot of EXCELLENT suggestions and helps in there by guys on this forum. Reading through might (hopefully) help solve your problem. If you do... let me know!!!

http://www.yamahaoutboardparts.com/f...m-th23920.html

[DennisG01]

Thanks, Captain. I agree with every single thing you've said. I've also done all of those things and it's now been about 3 seasons with this issue. I've even ran with a fuel pressure gauge hooked up - although every time I do that, the issue doesn't present itself! Almost like it "knows" it had better behave! Plugs always look good and all look exactly the same. Other than this one little issue, the engine runs absolutely perfect. I don't want to go too far into this in the OP's thread, but I suppose it would all still be helpful info for him? But, again, thank you for taking the time to comment!

[GulfCoast56]

fuel pumps,filters, screens, and injectors.
testing pressures when problem shows up would be best.
along with posting the complete motors model # so everyone know which one you have

Model is DX150TLRP

Thanks for the replys
I am still searching for the answer, but what is the abbreviation VST ? from Cap't Solo reply stand vor?

[DennisG01]

Definitely read through my post - there's a bunch of possibilities presented in there. There's likely no "exact" answer - it's going to take some investigative work.

Vapor Separator Tank. The silver-ish tank thing on the port side of the engine. Contains hi-pressure pump and filter hat.

[99yam40]

Vapor Separating Tank

The P you posted at the end of your model # indicates it is a 1991

DX150TLRX would be 1999

[rodbolt17]

the P would be carbed, the DX indicates OX66.
cannot be both.

[99yam40]

the P is a long way away from the X on a key board, so do not see how he hit the wrong key by mistake, so not sure if he knows what motor he has.

It is a mystery