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Discussion Starter · #1 · (Edited)
Its been 10 years, boy has it been along time with my girl.

Just lookin' to see if you guys can think of anything I might of missed on this. Down to the last few things I can narrow it down too.

So I have a 1998 V-star Classic 650 with a KJS carb conversion. I have been tryin' to narrow down an issue with the rear cylinder running more rich than the front on idle.

I have checked the valve clearances 5 times. over the past year.
I have checked and re-seated the intake manifold 3 or 4 times to 100% verify for no leaks. It definitely has no leaks.
I increased the jetting on the idle to see if it made a change, it just made the system more rich overall, but no noticeable indications that is was fuel/air related from the carb.
AIS has been plugged using a AIS Plug kit So no leaks there.
Compression comes back as balanced and within range. (lower end of the spec but still within)
Runs fine on the road. However, once stopped, it will coughs into the intake or pops from the rear cylinder exhaust.
Front cylinder can carry the bike by itself with rear coil unplugged. Stalls and back fires from the rear cylinder if I pull the front coil plug.
Checked about a dozen times for a rear exhaust leak. Found a few sometimes, but doesn't seem to fix the issue.

Only thing left I can think of is to rebuild the cams/valves to 100% make sure it isn't them....

So before I take that step, Anything you guys can think of I may have missed?

I have been working on this minor issue for over a few years. It's quite an enjoyable puzzle. lol
 

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Nightfly
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If it's coughing, popping and backfiring you're engine is probably running lean, not rich. Could be ignition as well. If you haven't cured the problem in a few years you have more issues than a rough running engine. Or maybe you just enjoy playing doctor.
 

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Yeah, sometimes playing Doctor is fun. But I think I would have got tired of it by now and a real hard look at the coils and plug wires.
 

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Yeah, sometimes playing Doctor is fun. But I think I would have got tired of it by now and had a real hard look at the coils and plug wires.
 

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Discussion Starter · #5 ·
Well she is hiding in the garage for most of the winter here. I'll pull the coils and test them both with an ohm meter later to see what they come back with. And Ketchboy, didn't think about the wires, i'll look into replacing them to see if their is any change.

Thanks guys.
 

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Discussion Starter · #6 ·
If it's coughing, popping and backfiring you're engine is probably running lean, not rich. Could be ignition as well. If you haven't cured the problem in a few years you have more issues than a rough running engine. Or maybe you just enjoy playing doctor.
Last year I bought a few jets larger for the main and pilot jet to see if it would make a difference in lean/rich quality. The Main is running beautiful now (Over 80kph), and the pilot I upped by two sizes, where as the front cylinder was running lean it is now running fine, but now the rear is running rich. When starting from cold, the rear kicks the bike up right away, rev's high, then dies. With a few shots of starter fluid and a few more cycles the bike will start running... This may help you..

Before pilot jet increase Rear cylinder was fine, Front was lean (Rear spark plug would get dirty from a ride, Front would have fuel on it, or be clean.)
After Pilot jet increase Front cylinder is fine, Rear is now rich (Rear spark plug is black and wet, Front is a nice dark brown.)

With a laser temperature tool...

Temp out side = 0c
Once bike starts to kick, temp on rear exhaust port is 20c ~ 5c Front is still 0 ~ 5

Once bike starts running Rear exhaust port is 70c ~ 5c Front is now 30c ~ 5c

(Putting hand behind exhaust tips to feel the exhaust pressure and to get an idea of how the cylinders are firing...)
Bike is starting to warm up, rear exhaust is popping out of rear exhaust tips every 20 or so cycles or coughing into intake (I can feel the pressure drop or blow out the exhaust)
Front cylinder is normal now from being carried by the rear running now, pressure feels even out of its tip.
Temps are now Rear 100c, front 70

Bike is now at operating temputure after running for about 5 -7 minutes. Rear is at 100c ~ 10c Front is at 100c ~ 10c

Front is still coughing/popping every now and then.

Mikuni HS40 Flat side Carb
 

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The Mikuni HS40 Flat side Carb is way too large for that tiny, underpowered bike:surprise:

If anything, the carb should have the leaner set of jets installed and the slide circlip raised to lower the needle to make it leaner.:smile:

I assume you have just one Mikuni mounted and if so, there should be no disparity in fueling between the front and rear cylinder. Your problem is maybe a vacuum leak or it's electrical:sad:

I would call the Mikuni distributors help desk and explain your problem and ask for help.:smile_big:

Go back to stock and enjoy the thing:grin:

Sam:nerd:
 

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Ghost in the machine
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Sounds to me like it could be a failing coil pick-up. This is a well known issue for the V-Star 650. Last time I changed one out, I accidentally reversed the 2 wires on the new one and it caused a situation that sounds like the issue you are having.
 

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Swamp Rat Rider
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Sounds to me like it could be a failing coil pick-up. This is a well known issue for the V-Star 650. Last time I changed one out, I accidentally reversed the 2 wires on the new one and it caused a situation that sounds like the issue you are having.
Would definitely be checking the Ignition part of it myself as well ..
 
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