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Unread 03-20-2020, 02:16 PM   #1
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Default Yamaha G9 18hp - top speed only 28mph. Help

I have a Yamaha G9 eith 18hp vanguard. Have 22 tires and high rev kit. I read everyone saying they are getting mid 30s on top speed. Im only able to pull 28mph with just one person onboard. Any help with this? Do I need to adjust the clutches or springs? I have the Governor removed from the rear transaxle my belt is 3 years old. I only drive about 30 hours a year at the river.

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Unread 03-20-2020, 04:07 PM   #2
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Default Re: Yamaha G9 18hp - top speed only 28mph. Help

Probably in the clutches. Make sure they're completely shifting out. Also proper belt size is essential when doing these big block conversions. A belt that is too short would prevent a full shift. Do you have a tach to confirm you can turn 5500 rpms?
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Unread 03-20-2020, 05:19 PM   #3
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Default Re: Yamaha G9 18hp - top speed only 28mph. Help

I have a G14 with a 18hp vanguard stock tires and just installed a new 780 comet and new stock driven clutch with a Gmax big block belt and with governor removed didnt want to push it too hard as I have stock valve springs so at 4100 rpm it runs 21mph per gps
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Unread 03-21-2020, 09:26 AM   #4
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Default Re: Yamaha G9 18hp - top speed only 28mph. Help

Yeah, I'd be curious to know what the rpm is at 28 mph. Mine at 4000 rpm is around 25 mph, same setup as yours. With that transaxle and those tires at 6000 you should be at roughly 40 mph. I figured my final drive ratio is 10:1 with my setup. Use your actual numbers and plug them into this calculator.

I don't know how accurate this calculator is because with my VC460 engine and the 16hp Vanguard, with 22" tires at 6000 rpm this cart does 40 mph tested by gps on two different devices. This puts the final drive ratio at 10:1. I always thought the stock ratio was 13:1.

Edit: After some reading, the final drive ratio is calculated using the diameter of both clutches at full shift/high gear. Most will just use a 0.8 overdrive calculation. But if your primary clutch is not stock, it could be a different diameter.

So in my case, if I use 13:1 stock gearing x 0.8 overdrive ratio in high gear, that comes out to 10.4:1 final drive ratio. Much closer to being correct.

Edit: I just found a site that says stock G2/G9 gear ratio is 12:1. So new final drive ratio is 9.6:1. That puts the speed right on base on my GPS.

So the trick in the calculator, you need to find out your "actual" gear ratio, then use 0.8 overdrive, and this will be close to "final drive ratio".

Edit: Also for tire size, use rear tires, and use actual diameter, not the size on the label. My tire says 22", but it measures at 21.75". Must use "actual" tire height/diameter.
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