Yamaha 1991 70 HP two strock has no power when in gear

[freeforest]

Hi I just got a boat with a Yamaha 1991 70HP two stroke. After I cleaned the carbs it is idling fine but when it is in the water and I put it into gear and accelerate it makes a lot of clunking noise, vibrates and dies. It accelerates fine in neutral or when when the engine is in the air (even in gear) but not when in water and in gear. Any ideas?
Thanks,
Jan

 

[JB]

Re: Yamaha 1991 70 HP two strock has no power when in gear

Howdy, freeforest.

Welcome to iboats.

Probably running on only one or two cylinders. They can do that. . . seem fine with no load and fall flat as soon as you ask them to do some work. You can identify which cylinders are making power by pulling spark plug wires (carefully!).

 

[99yam40]

Re: Yamaha 1991 70 HP two strock has no power when in gear

As always first thing to do is check compression just to make sure you have a motor worth working on and then check spark on all cylinders with a spark tester. If all is good the it is back to the fuel

 

[robert graham]

Re: Yamaha 1991 70 HP two strock has no power when in gear

Pull your spark plugs and check them...they can tell you a lot about the motor...maybe bad fuel pump diaphragm...easy fix. Good Luck and let us know what you find!

 

[freeforest]

Re: Yamaha 1991 70 HP two stroke has no power when in gear

Re: Yamaha 1991 70 HP two stroke has no power when in gear

thanks everybody for your responses. I have changed the plugs and that didn't make a difference, the compression seemed fine too, at least it was similar for all three cylinders around 125 on cold engine. Pulling off the spark plug wires killed the engine so I am assuming that means it is running on all three. I haven't gotten into the fuel pump yet but is doesn't seem like a fuel problem because it dies with a lot of rattling and noise if it was just not getting any fuel wouldn't it just stall out right quietly?

 

[99yam40]

Re: Yamaha 1991 70 HP two stroke has no power when in gear

Re: Yamaha 1991 70 HP two stroke has no power when in gear

Motor should run with one plug disabled, you have a problem.
like I said check with a spark tester to make sure all plugs are getting good strong spark.
Check to make sure timing is correct with inductive timing light, this can help look at spark also if you move to the different plug wires.
If good comp and spark and timing then it is a fuel issue. too little or too much
lean fuel/air ratio will pop a piston so be careful.
Probably need to pull and clean carbs

 

[JB]

Re: Yamaha 1991 70 HP two stroke has no power when in gear

Re: Yamaha 1991 70 HP two stroke has no power when in gear

Pulling off the spark plug wires killed the engine so I am assuming that means it is running on all three.

You need to pull the plugs one at a time, freeforest.

Pull one with the engine idling. Does the rpm drop? If so, that cylinder is making power; if not that cylinder is dead. Put the wire back and try the next one.

What you are trying to do, in the quickest, simplest way, is find out if each cylinder is making power. Leave the compression tests, spark tests and other diagnostics for AFTER you know which one is not making power.

 

[freeforest]

Re: Yamaha 1991 70 HP two strock has no power when in gear

OK I figured it out -with some help - the carbs need to be totally rebuild just cleaning them was not enough they were too worn out and need new jets, needles, etc. She should run after that...

 

[99yam40]

Re: Yamaha 1991 70 HP two strock has no power when in gear

The only time I have found the jets needed replacing was when someone had drilled them out to a larger size.
maybe if they were all corroded up would be another reason.
needles and seats stick or can be damaged if they are mistreated just like the jets.

most of the time the carbs just need to be cleaned real well with carb cleaner and maybe an ultrasonic bath clean. and then adjusted properly like the manual states after reinstalling.
It may take a couple of times to get all the passages clean.