Inconsistant RPM's

[Harold Joyce]

I have a 1990 Yamaha 225 with approx. 800 hours. Due to the "ethanol" discussions and poor idling, I elected to rebuild the carbs. Surprisingly the carbs were clean. However, replaced the original parts with the new parts that was supplied with the rebuild kit. The engine starts well and idles without problems. Once underway and up on plane, and after approx. 30 seconds, the engine rpms vary up and down by a rate of 100-200 rpms. I suspect a fuel supply problem.

Any suggestions?

[boats.net]

Check for air leaks in fuel hose connections, fuel filters. Check for pinched fuel lines or restrictions in fuel system.

[Operaman]

I had a similar issue. New fuel lines, fuel pumps, Yamaha water seperator and it is barely there anymore.

I too am gonna check my lines/fittings.

I also removed the quick connect at the engine.

Now, a few weeks ago, I noticed much more drastic drops, and even lost my top end, which was new. Then it would come back. I had abbout 2 seasons ago, a thermostat cover fail, allowing water to run down each spark plug. Bought 2, only changed 1. Why? Wish I knew. Well, while winterizing, I saw corrosion on the other bank of plugs. I took off the thermostat cover and an ear fell off. It was leaking, just not as bad as the first time.
Can not wait for spring to try it out!

[rkmartin]

Yes. It sounds like a vacum leak to me.