engine not getting over 3500rpm

[yamaha701986]

my yamaha 70 1986 runs fine up to 3500RPM. After that it wants to cut out. It still keeps on running, but I pull back on the throttle to make it run smooth. All I can get out of it is 3500RPM. I cleaned the fuel filter, installed a new primer bulb on fuel line, check the fire on all cylinders and drained the gas out of the two carbs. (I could not get to the bottom carb.) It ran fine last summer, but I put a new impeller in last week because the horn went off, the first time I put the boat in this summer. After installing a new primer bulb the bulb still is not very hard. Any help is greatly appreciated. thanks

 

[timster]

Re: engine not getting over 3500rpm

I would check your fuel lines under the cowling and also your fuel filter. You might be getting air in your fuel lines.

 

[99yam40]

Re: engine not getting over 3500rpm

If you overheated the motor I would check compression and spark just to make sure that is OK.

You need to check the fuel pump for hole in diaphragm.

Pull carbs, clean, and adjust properly

 

[rodbolt]

Re: engine not getting over 3500rpm

what 99yamaha says and in that order.

 

[yamaha701986]

Re: engine not getting over 3500rpm

Thanks for replies. Today I ran the boat and got 3800 rpm's. Then I pulled th plugs and compression was 100 lbs. all 3 cylinders. I am getting spark on all 3 cylinders. I ordered a diaphram and will replace diaphram
Do you think this is connected to the engine overheating and the horn going off when the boat was first put in the lake this yesr? Is there any temperature sensor that could be defective?

 

[mopar56]

Re: engine not getting over 3500rpm

I wouldnt waste my time with a diaphram, if it needs a pump replace it, the cost is not that much different, remove the pump with line still attached, and squeeze the primer bulb if fuel comes out the hole for the pulse replace the pump, if not, dont, then take the boat on a sea trial with two people, one drives up to problem rpm, the other is at the motor with the plastic carb cover off squirting fuel from an oil can directly into each carb one at a time if the motor responds and picks up rpm remove the carbs,(all of them) and overhaul them, make sure to remove the jets completely and use good engine tuner for cleaner,NOT GAS!, very common problem after engine has been sitting for long periods of time, also probabley no overheat damage if compression is even although 100lbs is a litlle weak, Ed

 

[yamaha701986]

Re: engine not getting over 3500rpm

Edd, thanks. Good idea. I will try it Saturday when I return to the lake. I am not sure I understand exactly what the pulse is. Could you please explain?
Thanks,
David

 

[mopar56]

Re: engine not getting over 3500rpm

Sorry to take so long to get back, unfourtuneatly work gets in the way of fun!, the pulse is how the pump works, it is bolted to one of the cyls, as the piston travels back and forth a "pulse" is created behind the pump causing the diaphram to move back and forth or "pulse" causing it to suck or draw fuel, like wise if you pull the spark plugs and the plug in the cyl that has the pump beside it is wetter than the rest it is also a good indication the pump is bad, hope that helps, Ed

 

[yamaha701986]

Re: engine not getting over 3500rpm

Ed, thanks for the explanation. I am going to the lake this Saturday and will put the new diaphragm on and if that does not work I am going to squirt gas down the carbs while my wife drives the boat. I appreciate your help and will let you know what happens.

 

[yamaha701986]

Re: engine not getting over 3500rpm

Thanks to alll of you who have responded to my question. Problem with the boat is still not fixed but better. Today I went to the lake and tried out somethings. I had ordered a diaphragm, but when I got there to get it --was the wrong one. I squirted gas in the number one and two cylinders and no change in rpm. After squirtting gas in the number three cylinder rpm picked up. Went back to dock and pulled all 3 carbs and removed the jets, cleaned them. Put carbs back on and boat is now running smooth at 4500. Shot gas in cylinder 3 again and rpm's picked up to 5300. My wife has agreed that she will drive the boat and I will stand back there and squirt gas while it runs. Seriously I think I am going to return the diaphragm that was the wrong one and instead order a new fuel pump. What do you think about that? Do you have any further advice?