Old 10-24-2020, 01:51 PM
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Default 90HP Yamaha 2 Stroke Won't Rev Past 4K Under Load

First time posting after lurking for a number of years. This is a great board and I have learned a lot and been able to keep my boats runing well due to you guys so thanks. I can't find what I am looking for so I'll ask for some assistance please.

I have a 2009 CS 178 DLV with a same year 90 HP Yamaha 2 stroke (TLR) on it. Took it out 3 weeks ago and it ran great for 40 min with full range of throttle up to near 6000 RPMs. On the way back, I dropped speed in a no wake zone and upon coming off of the other side I throttled up again and the engine wouldn’t rev up past 4200 RPM and the boat wouldn't get on a plane. Once hitting 4200, further throttle produces no additional results.

This is higher than the 2000 RPM limp mode and there were no warning lights or buzzers so I don’t believe it is an issue with the oil level sensor (plenty of oil) or temp sensor (water is flowing full stream through the engine) – both of which would send it into limp. The boat would rev higher in neutral, but not under load. (I know that these are 2 different operating conditions, but just making the observation) Seemed like a fuel issue. Since the boat is newer to me and I had no baseline of repair history to work from, I did the following:
  • Replaced the fuel filter
  • Replaced fuel water separator (no water in the old one when checked),
  • Replaced spark plugs
  • Cleaned and rebuilt all 3 carbs with new kits.
I could not get the float needle on the bottom carb to seat properly, and after taking it apart, cleaning it and reassembling it 3 times, I got fed up and took it to the mechanic to fix that issue and look into the original problem. I was also told the fuel enrichment lever was broken so it was replaced. Picked the boat up on Tuesday and took the boat out Wednesday night and it ran great for 3.5 hours at various speeds from idle to full throttle with no issues. Took the boat out today (same body of water) and it ran great for an hour and then same thing - can't get over 4200 RPM to get on a plane.

Am I dropping a cylinder? Maybe not getting spark somewhere? I would think that I wouldn't be able to rev it up in neutral either if that was the case. What should I be looking at next?

Last edited by vbskiff; 10-24-2020 at 04:23 PM.