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11-14-2017, 03:36 PM | #1 |
Not Yet Wild
Join Date: Dec 2016
Posts: 28
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Yamaha G2 fresh frebuild no speed
So I was recently able to buy a Yamaha g2 in decent shape for 300$. It was burning oil and low compression , yet still ran decently. Had the head bored out, replaced and installed appropriately oversized piston and rings. Now the compression tested consistently at 175 psi. The problem is that it only starts if you baby the throttle very softly at first then slowly accelerate and even then it only follows to a point. I also get a weird carb backfire if I try to accelerate to fast, as the motor dies out. I’ve been able to get max 2500-2700 rpm out of the motor in neutral. How high do these 4 strokes rev? I know it has a limiter. I’m trying to figure out if it is a clutching problem or motor. It barely does 8 mph maybe 10. It was faster than that before I rebuilt the motor which is why I don’t think it’s a clutching issues. Sorta stumped. Let’s know what you guys think, I’m sure there’s lots of folks on here with more experience on there 4 strokers than myself. Thanks
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11-14-2017, 05:21 PM | #2 |
Gone Wild
Join Date: Jun 2015
Location: NW Iowa
Posts: 1,669
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Re: Yamaha G2 fresh frebuild no speed
Was the cam timed properly is valve lash set to .004" ?
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11-14-2017, 06:53 PM | #3 |
VC 460 CLONE GONE WILD
Join Date: Oct 2014
Location: Ray BY The Bay NJ
Posts: 10,633
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Re: Yamaha G2 fresh frebuild no speed
You did zip tie the governor on the transaxle ? My G14 used to wend its way up to around 4500 rpms.
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11-14-2017, 09:35 PM | #4 |
Vegas modded 420
Join Date: Jun 2009
Location: West MI
Posts: 15,443
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Re: Yamaha G2 fresh frebuild no speed
I'd guess you have an intake leak, carb problem, maybe timing issue but I would look at mixture first. If it is then it should run better if you can hold the choke on part way if you find the right spot it will go better. Yamaha are very picky about any change in airflow so any leak in the airbox, changes to it, leak in the intake spacer, will make them run poorly and foul the plug also. Could also be related to rebuild like timing off, leaking heat gasket or something strange like that. Though you say its got good compression it should not be a leak then.
Carb backfire could be mixture or mechanical, usually cam timing or valvetrain problem if mechanical. With 22s G9 would run 30mph on good day must have been around 4200-4400 range with no governor, but G2 has a rev limit iirc it is about 4K rpm not sure. Should be around 20mph or close with stock tires. They have good power, I could spin the tire on pavement with stock tires. If stock clutch with the big gold cover look for cracks in the cover, if a weight breaks they hit and crack it on side. You can take the cover off and inspect them they usually break at the pins on the wide part of the weight. Make sure it slides in/out easily etc. Normally this will limit speed because it will not shift much, but if your motor has problems take care of that first. It will not shift right with no power either, it has to reach a certain rpm to shift. |
11-15-2017, 01:47 AM | #5 |
Not Yet Wild
Join Date: Dec 2016
Posts: 28
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Re: Yamaha G2 fresh frebuild no speed
Okay, I’m gonna try to tear into it again today then. Seems like timing is coming up more than once, so I’ll check it again after I set the clearance for the valves and fiddle with the carb. I must have done that at least 10 times already. Almost doesn’t matter how the carb is set, it either doesn’t run, or it runs poorly. I’ll check back in later with results. Thanks for the suggestions so far.
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11-15-2017, 01:53 AM | #6 |
Not Yet Wild
Join Date: Dec 2016
Posts: 28
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Re: Yamaha G2 fresh frebuild no speed
And as for the governor, I’ll check as well. I could just remove the drive belt and try reving it that way, see how many rpms I get. I’m telling ya, it moved faster before though, at least 15, so I doubt it. But good suggestion, it was only in neutral. Maybe I moved the governer durning the reinstallation
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11-16-2017, 03:42 PM | #7 |
Not Yet Wild
Join Date: Dec 2016
Posts: 28
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Re: Yamaha G2 fresh frebuild no speed
So, this is the result so far. I was bearing some of my weight on the air box and noticed the rpms jumped way up. Rpms initially spike to 3800 then settle at 3400, and won’t go higher. Tightened down the air box, and presto I’m going 14-15 mph. Still not as fast as I’d like, but a lot better. So I decided to adjust the valve clearance to .0004. Took it out for a test drive around the block. 10-12 mph uphill 14-15 mph on a flat 16-18 downhill. Upon returning I tested the compression. Now I’m getting 160 psi. A couple drops of oil in the cylinder, and still 160 psi. This makes me think that my valve adjustment hurt my compression. Any tips on how to do it close to perfect? If the psi still doesn’t go back, what could be the cause? I drove less than 2 miles in all. Could it be that the timing is off and I somehow accidentally had the valves set perfectly to compensate for the timing being off? Is that a possible scenario? Thanks for all your help guys. I’m charging the battery right now just to eliminate as many variables as possible
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11-16-2017, 03:50 PM | #8 |
Not Yet Wild
Join Date: Dec 2016
Posts: 28
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Re: Yamaha G2 fresh frebuild no speed
Just tested again with battery fully charged. I was reading the tester wrong
It was reading 165psi before charging and now it’s doing 170ish psi again. So I think I’m good. Any ideas for more speed? Is 3400 rpm good? And what can account for 3800 rpm jump initially. Can I bypass the governer and directly connect carb to pedal? Thanks for any suggestions |
11-17-2017, 01:04 PM | #9 |
Gone Wild
Join Date: Jun 2015
Location: NW Iowa
Posts: 1,669
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Re: Yamaha G2 fresh frebuild no speed
G2 has a rev limiter if you bypass governor it will likely bounce of the revlimiter and its very annoying
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11-20-2017, 03:59 PM | #10 |
Over This Interview Is...
Join Date: Jun 2012
Location: AZ
Posts: 17,449
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Re: Yamaha G2 fresh frebuild no speed
Governor nut fully adjusted in?
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