Yamaha g9 “choking”

[Jwcc89]

Compression test last night was 150, leak down was <2%.

does the stock air box run a cartridge type filter or a cone?
Going to orieleys today to get parts to do the Chrysler swap, but I’m wondering if the real issue is somehow air flow related after the many posts I’ve read.

I only have issues with it running when throttle is >50%, and at WOT it chugs slow bad it won’t run, gradually letting off the pedal back to 1/2 throttle and then it runs smooth again

[CP241]

Skip oreillys. If the oem ignitor works, it’s better than Chrysler conversion. You don’t have a spark issue if it runs fine under 50% throttle. You’ve got an air/fuel problem

The only fix I’ve been 100% successful on is putting in an oem airbox, oem carb, and filter. You could try screwing around with the jetting to get it to run, but I’ve never gotten it to run worth anything without the complete oem setup. Never tried all that hard either, but have played around a bit just out of curiosity.

I’ve seen *most* of the airbox for sale on eBay. They’re pricey. Same for the oem carb. But that’s really the only thing I can suggest that I know works. Skoal82 had a couple parts carts in the buy/sell section, but not sure if he’s got an airbox or not. Worth a shot. Or maybe reach out to plowmans, he may have something as well. Or maybe a carb that would work with an open filter, I really don’t know but might be worth a call. Only other option I could suggest is eBay and a prayer these carts are old and finding some parts, specifically some of the airbox parts long discontinued, can be quite challenging.


Worst case there’s always the option of a 420 swap. But I prefer the stock motor, just my preference.

[Jwcc89]

Thanks for the info, I’ll keep a look out for stock parts, and give plowmans a call.

I don’t plan on doing the 420 swap, this was a cheap cart, and I’d like to keep it that way, I don’t plan on dumping several thousand into it 😁

[CP241]

FYI with minimal fabrication you can drop a 420 into it for about a grand, with the Vegas carts kit. If you DIY the engine cradle and reuse the stock clutch (which involves cutting and tapping the crankshaft down some so it’s shorter) it’s around $400ish. But that’s a lot of work… I still prefer the oem engines. Quieter and runs great when they run right. Just gonna take some hunting and fixing to make it work.

Since you already have a cheap $20 China carb, might be worth trying to hog out the main jet a bit and see if you can get some more fuel into it and get it to run better. A set of the micro hand drills can work well for that and come in tiny tiny sizes. If there’s no airbox it’s running lean. Add more fuel and with enough playing you might get it to go pretty good.

Worst case you’re out like $17 for another mew China carb.

[Jwcc89]

From what I’ve been reading, and with everything I’ve done, I think that is the route I’m going to go.

It seems I am running lean, due to more airflow rather than a real fuel issue. It seems there are a lot of people here that have had good luck going to a 3” k&n and a jet kit. I’d like to keep the intake I have, seeing as it is stock, just missing the bottom of the box, I don’t see a need to change and get an different filter that is even smaller.

I see that the g2 main jet size is 108, I’m guessing that means .0425”? Is this the same for the g9? Only goos information I could find was a stock g9 people were drilling to .0420” (which would mean main jet on g9 is a different size?)

Information from this forum people seem to be getting good results with 125-130, which in my little mind would say drilling out to .0492-.0512.

I have 3 carbs…the original to me one, which has no markings, so could very well be a China knockoff rather than the stock mikuni, and 2 other Chinese knockoffs I bought because they were cheap.

I found a few hand drill sets I can pick up for about $20 on Amazon, which would allow me to drill 1.1, 1.2, 1.3, or 1.4 mm, this is assuming my above assumptions are correct

[CP241]

I wouldn’t try and gauge based on jet size. I’ve found that different companies size their jets a bit differently than each other. Ans since you’ve got all China carbs, any relativity to sizing is out the window. Go by feel. You’re going to have to tune that one manually. You are absolutely correct it’s lean because of extra air.

I’d get the hand drill set. Find which one fits the jet now and go up one size at a time until you get it right. Once it runs good and stops bogging, you’ll be in range but just barely on the leaner side of things. I’d go one or two sizes larger (depending on the steps and range of the bit set you have) just to richen it up a tad more, but not too much. Just enough to put you in the safer zone.

Then, get it good and warmed up. Install a new plug and run it hard. Then pull the plug and inspect it to see how it’s burning.

[Jwcc89]

call me dumb, but what do you mean about checking the plug? you mean to see if its black and starting to fowl?

[CP241]

Google “plug chop” and you’ll have plenty of info to go on with pictures.

Basically you’re looking for the color and condition of the plug to show how it is burning.

[Jwcc89]

It seems I finally fixed the issue!

I took one of the 3 “mikini” carbs that I have (all which ran the same…crappy) and drilled the main jet out, it was at 0.9mm, drilled out to 1.4mm and the cart seems to be running perfect.

I had talked with a few local “mechanics” and had narrowed my problem down to a lack of fuel, or a weak coil, their theory was that maybe with the amount of fuel being delivered at WOT, the coil and spark was just weak enough not to ignite. My troubleshooting started with coking it while driving, the more throttle and the more choke, the better it ran, which lead me to believe the cart was running lean. Since I knew I had good fuel to the carb, the internals of the carb had to be the problem.

I’m still unsure why this problem suddenly started, but as everyone on the forum has stated, these “mikini” carbs are not reliable like the oem mikuni. Maybe some garbage got in the carb and clogged the main jet initially, my initial troubleshooting I missed this.

Next steps, figure out how to wire up a tach since I have the governor bypassed (I may hook it back up, was only unhooked as a troubleshooting step). I also need to google plug chop and check the spark plug, hoping to get around to messing with it this evening if the rain will ever stop 🤦🏻*♂️.

[papadon]

I recently bought a g9 and being brand new to carts I didn't notice until i got home that it had no air filter or airbox. It runs fine without box at all. couldn't tell a difference on the test drive. fyi I'm looking for airbox to see difference