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Discussion Starter #1
Had my 96 CR stall out twice in traffic today, came home and pulled the plugs and vertical is fouled while horizontal looks great. I had used a colortune previously to dial carbs in, and when I checked today, the mixture screw on the vertical is nearly closed- horizontal was about 2 turns out. Was a little shocked by this.

Bike has FCR39s and otherwise runs well. Compression test not too long ago was right under 130 both cylinders. Where would you start for diagnosing this?
 

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Check the carb sync, it is probably right but you will be wasting your time on any other troubleshooting if its not.
 

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Before assuming bad rings or such, check for good spark , replace the plug, make sure carbs are in adjustment. Then take it for a ride and recheck the plugs. Do you have old coils and wires on it ? New wires / caps made mine run better, and I wasn’t having a problem.
 

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I had the same problem on the vertical cylinder which turned out to be worn valve guides even though there was no blue smoke on start up. Are you sure it is fuel fouling the plug?
 

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if it has the original 60 pilot jets i'd be more curious as to why it's not fouling the horizontal as well. the 60's just don;t work out here. but i did have one bike that was fine with them, then a year or so later it wasn't, and going down to 50 odd fixed it.

how's that for rambling speculation of no worth?

needle and seat? can you check the fuel level in an fcr by putting a clear hose on the drain? don't know that i've ever tried.
 

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Discussion Starter #7
Pulled the plugs if someone could confirm as oil fouled. Left plug is vert cylinder with mix screw on FCRs completely closed. Bike runs well otherwise. If oil assuming valve guides or seals? Compression was fine when checked a few months ago.

988778
988779
988780
 

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Oil fouling usually leaves a much thicker caked on coating. It is hard to know if this is just the beginning of that or it is simply fuel(too rich) . Both plugs look new based on coloring of the hex so how long does it take to get this black? You might want to get a gas tester on it to see what the CO and HC are.
 

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Did you replace the gasket under the needle and seat assembly, or just install the tuning kit?
Did you replace the gasket under the main jet holder ? If either of these gaskets leak the
cylinder will run rich.
 

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Discussion Starter #10
No,have not done anything to the carbs other than install and adjust mixture screws. They were purchased brand new from Power Barn a few months ago. Carbs are FCR39s.

Plugs are brand new, no more than 200 miles on them.
 

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What mixture screws and what settings?
How did you decide on setting?
Have you tried different setting on that carb?

If you soak a plug with the accelerator pumps you can wet foul them and then you need a new plug or wait for the plug to dry. with keihins never let people touch the throttle.
 

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My comment referred to stock Mikunis . As you were.
 

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Discussion Starter #13
What mixture screws and what settings?
How did you decide on setting?
Have you tried different setting on that carb?

If you soak a plug with the accelerator pumps you can wet foul them and then you need a new plug or wait for the plug to dry. with keihins never let people touch the throttle.
The mixture screws on the bottom of the bowls. Used a colortune on to set them, carbs came pre-jetted based on current bike set up (KN, exhaust). Apart from vertical fouling, the bike feels great. No dead spots, stumbles, etc.
 

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Do you know what the settings are at on the fuel screws? Often I find theses close to 1/2 turn out.

"carbs came pre-jetted based on current bike set up (KN, exhaust). "

Yes I know the sales brochures well but as someone with a dyno it is seldom correct. Stock bikes are some times off from one to another so base settings are no more that a starting point, when you start changing exhaust,airbox, etc all bets are off.The Jetting in the carbs are thee same jets the factory put in them in Japan, yes it is ballpark jetting and is a good starting point. But it is the same jetting for any 900ss with an airbox.

All that said the difference is NOT likely due to base jetting as the difference between cylinders is too great. I would try some new plugs and be gareful to run it one throttle position at a time.
Fire bike up and let settle into a idle and then pull plugs, no throttle twisting as you are trying to see where the black is coming from.
If it is fine then next mark the throttle in segments of
1/8th
1/4
1/2
Run the bike going no more than each setting and accelerate very slowly so you do not add excess fuel from the accelerator pump. After each run pull the black plug and check for when it is darkening.
It will be best to have the bike warmed up before you put in your test plugs.
 

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Discussion Starter #15
Thank you, that gives me plenty to keep busy with. If it's not oil, and it's not a carb issue, what else should I be looking at? Bad coil, something else damaged/loose/burnt in ignition circuit? Bike has never smoked either, so hoping it's not oil.
 
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