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Thread: 720 Help!!

  1. #1
    Fast & Loud meeper's Avatar
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    720 Help!!

    a friend has a 720 GTI, 01 or 02 i think....getting spark on both plugs....runs on the cylinder closest to the front but when you take that plug wire off it stalls....will not run on the back cylinder, getting spark, give it gas when running and wants to stall almost and sorta back fires too.....

    is it not getting fuel on the back cylinder, how do we fix?


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    Blackhardt's Avatar
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    Is the back plug wet when you pull it out of the head? If it's wet with gas you will be able to sniff it and tell. If it's wet with water it will look like sludge on the plug.If it's gas on the plug I would first check spark to that cylinder and if you have that I would check the compression. I have a neighbor that had the same sort of problem on another type ski and come to find out that it was getting water past the head gasket. Since water doesn't compress he also chip the piston so now we have to rebuild the top side of the motor atleast. Karl

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    back plug just looks and smells like oil, the front one smells a little like gas and oil.....spark is getting to both cylinders.....i just want to know how to check for fuel...

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    Sounds like you need to remove the carbs and clean them.

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    Fast & Loud meeper's Avatar
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    how do you do that?

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    96XPSS's Avatar
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    Just happen to have the manual for you...

    http://www.mikuni.com/pdf/sbn_manual.pdf

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    I would check your rotary valve timing. Pull the air box. unbolt the carbs and unbolt the rotary valve cover. turn the motor over by hand and watch the valve to make sure it doesnt skip a beat. 720 are famous for stripping the rotary valve gear.

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    96XPSS's Avatar
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    Quote Originally Posted by JOJO567 View Post
    I would check your rotary valve timing. Pull the air box. unbolt the carbs and unbolt the rotary valve cover. turn the motor over by hand and watch the valve to make sure it doesnt skip a beat. 720 are famous for stripping the rotary valve gear.
    Dude...if his RVT is off both spark plugs are going to be wet. You are leading him down the wrong path. Maybe you can come over and take your time to verify it for him?

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    Quote Originally Posted by meeper View Post
    a friend has a 720 GTI, 01 or 02 i think....getting spark on both plugs....runs on the cylinder closest to the front but when you take that plug wire off it stalls....will not run on the back cylinder, getting spark, give it gas when running and wants to stall almost and sorta back fires too.....

    is it not getting fuel on the back cylinder, how do we fix?
    Those are all the syptoms of a stripped rotary gear. I think you are sending him down the wrong path. Those machine have a single carb so both cylinder wouldent run. Anyways if hes gonna have the carb of then its a real easy check.

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    96XPSS's Avatar
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    Quote Originally Posted by JOJO567 View Post
    Those are all the syptoms of a stripped rotary gear. I think you are sending him down the wrong path. Those machine have a single carb so both cylinder wouldent run. Anyways if hes gonna have the carb of then its a real easy check.
    Since when does an engine run on one cylinder with a stripped gear? It doesn't. 720's have two carbs...

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