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?

First off, revving it in neutral is no test, forget doing that, check spark on all cylinders when it's playing up, all cylinders, not just one the 3 cylinder Yamaha has a unique ignition system! if you have dirty fuel and you clean the carbs, and run the boat, the carbs can get "dirty" very quick, clean them properly again and run off a remote tank complete with fresh fuel and hose.

I should also add that the fuel tank was drained and refilled between incidents with stabilizer added.

That's nice, stabiliser will not "fix" your issue.

I have same engine. I would also check spark for all 3 cylinders. Seems like an intermittent issue and those can be hard. Good luck. Tighten all your connections and ground.

Didn't think that stabilizer would "fix" anything here, but I thought anything done would be worth mentioning.

Sometimes stabilizers and additives with scrub a dirty fuel tank clean. I have a 3 cylinder Yamaha that sat unused before I bought it and I had to clean the carbs twice when whatever crud in the tank dislodged and made it’s way to my carbs.

Clean coils ground connections.

Yeah you can do that....but will it help if spark is not the issue? Trouble shoot, fix the issue, don't root around with "stuff"

Agreed. I haven't touched anything since it started acting up again today. Going to take it out again tomorrow with someone else behind the wheel so I can pull the plug boots while under load.

Did you buy this boat recently?

Stator. Mine did the same thing. Bad rectifier killed the high speed windings.

Were the symptoms intermittent for you?

yes and only after heating up at wide open. I just realized this is a Yamaha though. It did it for my Mercury 2 stroke. Does it surge at 4k or just go to 4 smoothly and stay there?

DING, DING, DING! We have a winner!

The other thing to recommend is to make sure your battery uses clamps or nuts and not wingnuts. Clean all the connections from battery to engine as well after you diagnose the regulator and stator. Those high resistance connections or dropped connections at the battery can kill your electronics.

That was one of the first things that I did - check and clean all connections. I came to a standstill with my own diagnostic capabilities so I took it to my mechanic. His assessment was that the stator, coils, and regulator all tested okay (within spec) and he is leaning towards the CDI unit.

CDI Unit
 

The answer is: CDI unit was bad. stator, coils, rectifier all tested okay. I replaced the CDI and have full RPM range now.