1985 yamaha 70 hp lost top end

[CHank]

two weeks ago my buddy's boat was going wideopen across the lake then suddenly slowed and died<br />fired right back up but will not tack above 1200rpm he has had the boat 2 years.we replaced the plugs,fuel filter,with no success,the motor will tack up all the way as long as it is not under load,checked the timing advance and it turns up all the way when the throtle is opened up<br />can not find anything that has slipped...motor seems to run fine at idle up to 1200rpm.any ideas for a backyard mechanic or do we need to take it to a pro?

 

[fish4970]

Re: 1985 yamaha 70 hp lost top end

did the motor overheat<br />do you have a good water stream from the tatle-tale<br />i would do a compression test

 

[jim dozier]

Re: 1985 yamaha 70 hp lost top end

Sounds very much like you dropped a cylinder. You need to pull plug wires 1 at a time (best to ground the loose wire) and find out which cylinder isn't working. Then you can check the quality of spark with an inexpensive spark tester (available at most auto parts stores) and narrow it down to ignition or fuel issues. Could be bum ignition component (as simple as a bad spark plug) or a clogged carb main jet requiring a carb rebuild. Now would be a good time to buy a manual for that motor (click on icon at bottom of this page).<br /><br />They will rev fine in neutral on only 1 cylinder so that doesn't really tell you anything.

 

[CHank]

Re: 1985 yamaha 70 hp lost top end

motor did not overheat good compression on all three,we had just stoped and filled the tank up with gas,could something get by the fuel filter that would have clogged the main jet in a carb?<br />this all happened in a 10 second span of time.<br />one minute running fine wideopen then died now 1200 rpm could a sensor have gone out that limits the revs? thanks

 

[mrn714]

Re: 1985 yamaha 70 hp lost top end

Check the prop hub.

 

[jim dozier]

Re: 1985 yamaha 70 hp lost top end

I still think you need to check to see if you have dropped a cylinder. You need to pull plug wires 1 at a time (ground the loose wire) and find out which (if any) cylinder isn't working. <br /><br />Then you can check the quality of spark with an inexpensive spark tester (available at most auto parts stores) and narrow it down to ignition or fuel issues. Could be bum ignition component or a clogged carb main jet requiring a carb rebuild. If you can find the bad cylinder and it has spark, squirt some premix in the carb throat and see if it picks up when under load. Then you'll know its a clogged carb.<br /><br />If its an ignition component or a clogged jet its entirely possible that it would be OK one minute and not the next.