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Old 04-25-2018, 11:17 AM   #1
CP241
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Default 91 G9 sputters and dies - stumped!

Hello all, hopefully someone can help me narrow down or has experienced this before because i'm probably missing something stupid that is right in front of my face... I do have a lot of small engine experience.

Anyway... I'm a little stumped on this thing... sorry in advance for the long post, wanted to put as much info up front as i can.

I picked up a Yamaha G9 basket case for cheap. I've been wanting a gas cart for a while, just don't want to pay a fortune for one. Compression is perfect at ~170psi, no noises, smoke, or anything bad. Did not run, no spark. Wiring looked like the guy was hungry for spaghetti and happy to use a pair of cutters. basically completely trashed... brown wire to the regulator/rectifier was melted and wrapped around the airbox somehow... Step 1 was to strip the wiring harness and repair it. no problem there.

Anyway, no spark. Aside from the wiring mess, coil was obviously fried, ignitor was full of rust and corrosion and a brand new spark plug (wrong one for the cart). Bought a new regulator/rectifier, ignitor, and ignition coil. Lesson learned, don't buy the cheap ebay coil lol. after chasing my own tail with a *very* weak spark, picked up another coil at the local cart shop. Plugged it in, and immediately had a nice fat blue spark. Good to go, right? hooked everything back up, fired right up and ran great. I only ran it for 20-30 seconds but it ran. No idea how long the oil had been in there but it was black and nasty so didn't want to push my luck.

next day, change the oil, air up the tires and take it for a ride. This is where i'm stuck... it runs GREAT for the first 2-3 minutes. Then it becomes angry. It starts sputtering, backfiring through the carb, and loses any and all power and dies. If i take my foot off the gas for 10 seconds, hit it again it instantly fires up, will run great for about 10-20 feet, sputter, pop, and die again. IT will continue to do that until it sits for an hour or so. Then, lather, rinse, and repeat the cycle.

I've adjusted the valves (.004 intake and exh). It had a "mokini" carb on it (brand new) but luckily he left the original Mikuni carb in the front pocket. changed the fuel lines and filter, pulled the carb off and cleaned it out. a little trash in the bowl but nothing major. Put it back on, same thing. So I cleaned the Mikuni carb really well and put it on. No change at all.

When it dies, i've maintained pedal pressure, and confirmed spark is still big, fat, and blue so i'd rule out ignitor and coil (typical to fail when hot). No voltage drop at the coil or ignitor. Pickup coil reads about 560 ohms. Also, I've ruled out compression, still 170psi when it dies (checked compression right after it died). Pulling on the choke only kills it faster. I've also tried a shot of starting fluid when it fails and it doesn't even pretend to want to start so it's getting plenty of gas.

I can only come to the conclusion that it's running pig rich. Haven't been able to run it long enough for it to foul the plug or see anything there, but i've pretty much covered all the other bases. I've read about the airbox being picky and etc. but i don't think that's the issue because it runs great the first couple minutes, only does it after it warms up to operating temp. IT will also do it even if you're holding the pedal WOT. I can put it in neutral (shifter straight up) and manually hold the throttle wide open and doesn't make a difference there either. Also, removing the airbox completely (running an open carb) also does not make a difference, still same issue.

I'm open to any ideas you guru's may have
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Old 04-25-2018, 01:03 PM   #2
Spicyunicorn
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Default Re: 91 G9 sputters and dies - stumped!

When it acts up pull the plug right away and see if it's wet when you has the carb off did you check float level? Blow air through the inlet and raise the float air flow should stop when float is level how's the pulse line from motor to fuel pump make sure it is clear they can build up junk in them over time also make sure there's no droops in that line... Sounds like you've covered a lot of the basics someone else will chime in with another idea
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Old 04-25-2018, 01:58 PM   #3
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Default Re: 91 G9 sputters and dies - stumped!

Quote:
Originally Posted by Spicyunicorn View Post
When it acts up pull the plug right away and see if it's wet when you has the carb off did you check float level? Blow air through the inlet and raise the float air flow should stop when float is level how's the pulse line from motor to fuel pump make sure it is clear they can build up junk in them over time also make sure there's no droops in that line... Sounds like you've covered a lot of the basics someone else will chime in with another idea
I didn't check to see if it was level, i blew air through it and it does shut off the flow when i raise the float with my finger.

The spark plug wasn't wet but had a bit of gas odor to it. I didn't pull it immediately as it was acting up, but within a couple minutes. It wasn't soaked by any means like it would be if there was no spark.

I haven't checked the pulse line going to the fuel pump, but the new fuel filter is full of gas, and the carb bowl is always full when i take the carb off. I did spin the motor over with the fuel line disconnected from the carb into a catch can and I did confirm it is pumping fuel. I also replaced the fuel line with a new one, and the breather line going from the valve cover to the airbox. both were chewed on (but not through) by furry little friends. No idea how long it was sitting, but when i hit it with a pressure washer the seat turned white, i thought it was gray :)

I could see the needle valve sticking or possibly the float being out of adjustment causing issues at idle or low speeds, but like i said it gets angry even running it wide open. I wouldn't expect something like that to have an effect running wide open like that.

I was beginning to think maybe the muffler is restricted, but i can feel plenty of exhaust coming out. And when it is cold, it just runs too good for it to be something like that I would think, it just isn't happy once it gets warmed up to operating temp.

Is there some type of sensor or switch (Temp sensor, oil level, etc) that would make it do something that I am missing? The G2 apparently had an oil level switch but I don't see anything about that on the G9
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Old 04-25-2018, 07:36 PM   #4
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Default Re: 91 G9 sputters and dies - stumped!

CP welcome to the club are you able to see the fuel in the lines? Clear or blue lines? Just wondering if you may be running out of fuel at wot?did you remove or replace tank fuel filter?
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Old 04-25-2018, 10:03 PM   #5
Spicyunicorn
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Default Re: 91 G9 sputters and dies - stumped!

There was an oil dummy light but it's just that a light it wouldn't shut it down "The spark plug wasn't wet but had a bit of gas odor to it" this leads me to think the float is set to low or sticking but I'm not sure why it takes so long to restart check out the float and report back since you have adequate flow with the hose off that rules our the fuel pump
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Old 04-25-2018, 10:26 PM   #6
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Default Re: 91 G9 sputters and dies - stumped!

Ok, it's definitely running super rich. It's fouling the plug pretty fast. I just cleaned the plug with a brass brush so it was silver and shiny. Didn't go anywhere just left it in neutral and ran it for 2 min until it started sputtering and backfiring. Pulled the plug quick, it was wet and black.

At this point if I just let off the gas pedal for 5 seconds and the. Press it again, it fires up and runs good for a very short time then starts sputtering and backfiring. I tried this several times, then just switched the plug for a fresh one and it ran for a minute instead of 10nseconds and then sputtered and died. I think it is fouling out the plug too quickly and it is losing spark that way. If I remove even the fouled plug and hold it next to the exhaust pipe it will spark really strong and arc a good 1/4 inch or more to the pipe not even touching it.

When it runs it doesn't smoke at all so I'm sure no oil is getting into the combustion.

What would cause it to run so rich it turns a clean spark plug black in a matter of minutes?

Edit: sorry for typos. I'm terrible at typing on my phone....
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Old 04-26-2018, 06:33 AM   #7
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Default Re: 91 G9 sputters and dies - stumped!

Do you know what your air screw on the carb is set? I always start with 1&1/2 turns out. Anything should run at that setting and you can fine tune from there. Has the jet been changed? Is airbox complete? What plug are you using? Check the oil for fuel dilution? If needle and seat is worn,grooved on original carb?
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Old 04-26-2018, 06:38 AM   #8
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Default Re: 91 G9 sputters and dies - stumped!

Get a new spark plug NGK-BPR2ES
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Old 04-26-2018, 08:58 AM   #9
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Default Re: 91 G9 sputters and dies - stumped!

Thanks for the responses.

I'm running a BPR2ES NGK plug, it is brand new (albeit fouls very quickly).
Needle and seat are not worn. The crappy chinese "Mokini" did the exact same thing and it is brand new...
air mix screw is 1.5 turns, and i've tried adjusting it in and out and does not seem to have any effect.

The airbox is complete. Last night i took it apart and cleaned it out really well to make sure there wasn't a nest or something in the front baffle part of it. I dont have an air filter yet (it's been ordered) but that would only restrict more air and make it run even richer...

Would the governor cause it to behave like this if it's adjusted wrong? is there a certain way it should be adjusted?
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Old 04-26-2018, 01:23 PM   #10
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Default Re: 91 G9 sputters and dies - stumped!

ok... may be on to something here. I pulled out the gas tank and drained it. I did put fresh gas in it when i started because it was empty (or so i thought) and drained out a bunch of greenish gas. I couldn't hear any sloshing in there when i got it, so there couldn't have been much in there. I don't know if there was some diesel in there, 2 stroke premix or maybe just old gas... or maybe just picking up the sludge from whatever evaporated as it sat. i've seen really old gas turn brown many many times, and even turn into black tar (in the case of a 1987 Honda quad i restored a while back) but never green... Also took out the gas tank's fuel filter and it was pretty deteriorated and falling apart. I took it off and threw it out so now the cap down there is just a nipple, which i figure should be okay for now. I've got a brand new fuel filter in line on the new hose anyway. I rinsed the tank out real well, blew it out best i could with an air tip and now it's sitting out to dry for a couple hours. I'll put it back in this afternoon and fill it (again) with fresh gas.

lift kit is showing up today so i can get some decent tires under it, then i'll take it for a spin and see how she does. If it's just bad gas i'm gonna kick myself, but that would explain why it fouls out the spark plug so quickly.
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