Engine runs normal until taken off choke then runs very fast

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Rock Crusher

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I just picked up my first river runner. It is a 1995 G3 with a 1996 Yamaha 25HP outboard. First let me say that this boat came with a tank of gas that was about full so I have no idea how old it is. My engine idles great at half choke but the second I take the choke off it starts running at an extremily high speed. It revs up so high that my wife took off running because she thought it was going to blow up. In reality it was probably only revving to maybe 3500 RPM or so but it scared her none the less and it surprised me too. I let the engine run for about 10 minutes on muffs in my back yard before I ever took the engine off of choke. So please help me understand why my engine revs up so high when I take it off of half choke???
 
2 cylinder 2 stroke twin carb 25 I assume. If that's the case...more than likely someone let the fuel get old in the carbs at one point and the pilot jets became restricted. Then they tried to use it and realized that it won't idle....and promptly turned the idle up so it would idle. Not uncommon.

Look for air leaks at the joint between the intake & carb. also if it's oil injected, look at the oil lines & make sure they're attatched to their nipples on the intake manifold/reed block. Could be other air leaks too but those are the most common.

If those are good...pull the carbs, clean 'em, reinstall. I'm somewhat familiar with that motor and I can walk you through synchronizing them and checking the timing once the carbs are clean.

If it's a single carb C25...I am not that familiar with them other than they run rough, they're loud, harder to work on than the twin carb 25's.
 
I would first look at the throttle plate to see if it's stuck open. It sounds like with the choke closed, the choke is acting like the throttle plate.
The problem could be as simple as a bad throttle spring not allowing the throttle plate to close.
 
turbotodd said:
2 cylinder 2 stroke twin carb 25 I assume. If that's the case...more than likely someone let the fuel get old in the carbs at one point and the pilot jets became restricted. Then they tried to use it and realized that it won't idle....and promptly turned the idle up so it would idle. Not uncommon.

Look for air leaks at the joint between the intake & carb. also if it's oil injected, look at the oil lines & make sure they're attatched to their nipples on the intake manifold/reed block. Could be other air leaks too but those are the most common.

If those are good...pull the carbs, clean 'em, reinstall. I'm somewhat familiar with that motor and I can walk you through synchronizing them and checking the timing once the carbs are clean.

If it's a single carb C25...I am not that familiar with them other than they run rough, they're loud, harder to work on than the twin carb 25's.
+1 if idle not turned up/throttle stuck open, then vacuum leak somewhere.
 
Thanks for the replies everyone. I took the front cover to the carb off and seen one plate that is open (I assume this is the choke?), behind it was another plate that I could move with a lever and it opened and closed just fine. The spring seemed o be good and shu the plate on its own.

I will check the vaume lines on it tomorrow after work. Maybe I will go ahead and order a rebuild kit for the carb. I thought about running a can or two of seafoam through it and seeing if anything improved.

I've never rebuilt a carb before but it appears to be simple enough. Any advice on the best place to purchase a rebuild kit from?

My engine is the C25MSHT 1995 single carb model.
 
I was afraid it'd be a C25. I hated those things!! Still do. It's a 25 HP and luckily the C25's had the exact same powerhead as the C30 did (498cc, IIRC)...so I assume it'd be possible to swap the carb and maybe the exhaust tuner, and you'd have a 30. I have never attempted it. Most people that had problems with a C25 chunked them and either got a 25 triple or a twin carb 25. I did have the misfortune of having to clean a carb on a C25 about 2 months ago. Got it running in the tank and forgot how badly they idle compared to a triple 25 or a twin carb 25. I kept fooling with it to get it to idle....then opened up the Marine spec book (the Yamaha book) and read the specs: Idle speed 1000-1100 RPM. Timing 0° BTDC. Set it at that and sent it home. Couldn't get it to idle much lower anyway, not without it dieing.

Gettign the carb off isn't impossible but it's not nearly as easy as a twin carb 25.

The kits are everywhere. Yamaha has an excellent one.
 

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