Yamaha g9 predator vtwin conversion

[sho305]

Originally Posted by BLACK DEATH

So with a stock 13hp or 22hp clone at governed should be what rpm?
With gov removed what they usually rev to, safely?
By using the speed calculator the speeds are off it seems but it's just a rough estimate tool.

I think 28 mph is pretty quick or good enough for me with 12-1 stock gears and 22/23" tall tires

Most all lawn engines are governed to 3600 max. Usually you can tweak the governor to get up to 4K rpm, this is useful because many GC clutches shift at 3600-3800 and if your governor is starting to cut throttle you lose power, you don't want the governor to start limiting until after 3600 or where your clutch shifts. Many GC do not have a governor on the engine because the clutch keeps rpm safe and the trans speed governor never lets it get to full shift and max rpm that would be maybe 20mph a GC should never properly see. A lawn engine set to run 3600 max may start cutting throttle at 3300 or something, 3600 is where it is cut back enough to keep the rpm steady.

The stock 420 clones I put in would hit about 4300 rpm all stock with no governor, floating the valves. You don't want to do this much its not good for them. You can put HD springs in and it will rev a little more and not float them, they are cheap to buy, and you have no worries but still would never recommend screaming it for extended periods of time. If you need that speed get HS gears or build the engine up...its just a lawn engine and the rod does not oil well at high rpm. I have run my stock rod over 5K rpm many times but not daily. The problem is after about 4500 the stock cam is all done anyway and does not make much power, while it will rev up there it is slow to go faster. With HD springs, 4 degree timing key, 1.3 ratio rockers, larger exhaust and carb, ported and .050 cut head, thin head gasket, my 420 will only rev to 5200-5400 best I can tell on a flat road because after 5200 the tach would get funny but it seemed to go a little faster after that (38mph gps the time I had a gps with me). I never pegged it going down a hill for more rpm I was not wanting to blow it up on purpose.

The clone twin is a different story there are none or limited parts for it, it really needs a rev kit. If you look on youtube there is a video of a guy doing a dyno on one with a gokart and he revs it past 4K on the dyno with no problems, so I'm guessing it would run to 4500 here and there with no issue but it should have a rev kit (springs or shims at least). Unless someone has a kit for it now I don't know. Its supposedly a clone of the old gx honda twin. Last I knew some of the mud motor guys were working on getting more rpm out of them and I look for someone soon to have parts for them the price is too good for it not to happen.

The single clones gx390/420/440/460 have tons of parts cheap for them, the gx390 was raced for many years so the parts for clones were already there. So they are much easier to build up, the only more popular engine is likely the gx200 and briggs 5hp variants due to gokart racing. The vanny twin is used in racing too but cheap they are not and parts are more too.

[Hevyford2]

Hey I'm doing the same build and would love some info. Right now I'm not sure on what clutch to get. And do i need to turn down the crank to make the clutch fit. Also can you send some pics of the mod for the engine brackets.My email is hevyford2@yahoo.com thsnks.

[sho305]

Most people either get an adapter to run the stock cart clutch (30mm taper fit) on a 1" (most common on lawn engines) shaft. You can get some other taper clutches, and only taper clutches have the gen pulley on them. But you can get a pulley to put behind clutches, Vegas has one. Otherwise people get a 1" clutch to put on their new 1" shaft engine. There are a few 1" clutches out there some are sold by the cart places. Lot of people here like the 780 comet. I have a 94c comet, though its actually a taper fit ezgo replacement clutch.