25 hp 1989 yamaha runs fine in neutral/idling, but can't get high revs in gear

[miffyjack]

I've got a 1989 25hp Yamaha autolube. It fires first or second pull and I can get it to full revs in neutral. When I put it in gear it'll idle fine, but struggles at speed. It'll run for a while, but not at full revs. It sounds a bit like it's running on one cylinder.

I thought it was some kind of fuel issue. I've changed the spark plugs. Cleaned out the carb (it seemed clean). Cleared out the bowl filter which was clear anyway. I've tried pumping the fuel in while it's running, but the ball stays hard and it doesn't make any difference.

Could it have anything to do with the oil? Should I try running it on mixed fuel?

Thanks in advance for the help.

 

[99yam40]

Re: 25 hp 1989 yamaha runs fine in neutral/idling, but can't get high revs in gear

Compression, spark, and timing along with fuel pressure/vacuum are all easy to test
If fuel pump has a hole it will over rich a cylinder and not fire properly

But sounds like you are running lean on fuel at higher RPM

 

[Toddboat]

Re: 25 hp 1989 yamaha runs fine in neutral/idling, but can't get high revs in gear

I'm no mechanic, but it seems that if it runs well in neutral but struggles in gear, it may be the gears that are putting unnecessary strain on the motor. Have you drained the lower unit oil to see how things look down there? It could be very gunked up in there, or if signs of water exist in the oil, there could be rusted gears and such. I would rule out the gearbox and driveshaft by making sure they are well lubed first, and make sure the prop spins very easily by hand. If the lower unit doesn't seem to be the problem, perhaps try cleaning the carbs more thoroughly. You said they seemed clean, indicating that you may not have taken their cleaning too seriously. My carbs looked pristine but my motor ran rough. Once I gave them a thorough cleaning, it made all the difference in the world. I used nearly a fill can of carb cleaner on just 3 carbs to give you some idea of how well I cleaned every hole.

 

[miffyjack]

Re: 25 hp 1989 yamaha runs fine in neutral/idling, but can't get high revs in gear

Thanks for the tips folks.

I changed the gearbox oil last week, but it was fine. The prop spins easily as well.

"Compression, spark, and timing along with fuel pressure/vacuum are all easy to test
If fuel pump has a hole it will over rich a cylinder and not fire properly"

The spark is fine. I'm assuming fuel pressure is OK as pumping it with the bulb pump makes no difference and it stays hard. I'm going to change the fuel line to be sure (I have a spare one anyway). I'm afraid I don't know how to check the compression, timing or fuel vacuum. I searched the forums without success. Can anyone point me in the right direction or tell me how to do these things? Apologies if this is a really stupid question.

Thanks again in advance!

One symptom update is that it seems to run fine flat out in gear for a couple of minutes and then struggles. That makes me think it's a fuel issue, or possibly dirt in the carb. Do you guys agree?

 

[99yam40]

Re: 25 hp 1989 yamaha runs fine in neutral/idling, but can't get high revs in gear

lots of good info here
Engine Frequently Asked Questions (FAQ)