Poor top end. 1986 70 hp Yamaha

cmtstudios

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Feb 25, 2002
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I bought my boat last year and on the first outing, i found that as soon as I reached top speed, the engine acted starved for fuel. This winter I replaced the fuel tank, fuel line and fuel pump. I took the boat out last weekend and the **** thing is still doing it. It cuts out and bogs down. Even if I let off on the throttle, it won't smooth out unless I take her to neutral and start over. While at top speed when the motor is cutting out, if I squeeze on the fuel line primer bulb, she jumps right back up to running great for a few seconds. <br /> Also, the overheat alarm sounds every time I run it at idle. If I increase the idle RPM, within 20 seconds or so the alarm goes off and I can put it in gear and go. As long as the alarm is going off, I can put it in gear but the engine won't perform, it just cuts out. Once the alarm goes off, it runs fine. Except for the top end problem. Is this normal? <br /> :confused:
 

JB

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Mar 25, 2001
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45,907
Re: Poor top end. 1986 70 hp Yamaha

Howdy, CMT.<br /><br />You may have more than one problem.<br /><br />You have symptoms of a weak fuel pump or restricted fuel supply. Check the venting of your tank and all lines to the pump. If you don't find anything, overhaul the fuel pump.<br /><br />You also have symptoms of a failing water pump. Replace the impeller.<br /><br />Let us know how you do. :)
 

cmtstudios

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Feb 25, 2002
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Re: Poor top end. 1986 70 hp Yamaha

Thanks JB.<br />I have a brand new fuel pump, tank and line.<br /> :confused:
 

Kuni

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Apr 7, 2002
Messages
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Re: Poor top end. 1986 70 hp Yamaha

ok the cutting out when the alarm is on is the protection circuit in the engine trying to stop you cooking your engine.<br />it will restrict your throttle and make your motor skip like a spastic frog if you open the throttle to far.<br />1st i would suggest you deal with you overheat problem ,sounds like your impellor is shot,try that it may help your engine stay cool at high rpm too!!!!.
 

cmtstudios

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Re: Poor top end. 1986 70 hp Yamaha

Thanks Kuni!<br />I'll replace the impeller. But, what do you think is my top end problem. I don't get the alarm at top end, only at idle. It is simply starved for fuel at top end. Like I said before, when I pump the bulb, it picks right back up. I don't know what else to check! Fuel pump is new, fuel line is new and fuel tank is new! The pump works off of vacuum. What internal problem could cause lack of vacuum at top end?<br />Thanks.<br /> :confused:
 

LOLHJ

Seaman Apprentice
Joined
Jan 19, 2002
Messages
46
Re: Poor top end. 1986 70 hp Yamaha

You need to make sure all fuel fittings are tight, make sure they are not leaking air.<br /><br />A friends 80hp Merc did this and it was the fittings leaking air. Pumping the bulb would help the problem, there was no issue at idle.<br /><br />Replace the impeller and then check the fuel fittings, make sure you are using good grade fittings.<br />-John
 
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