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02-13-2017, 12:14 PM,
#1
Rear Cylinder Misfire / Dead
Hello everyone. I'm new to posting here but have been reading for a couple months as I've tried to diagnose a problem with my 2002 VStar 650.

I purchased this bike a few months back with a reported problem of a gummed up carb. Since the price was low it seemed like a good deal. After cleaning the carbs a couple times it still had the same problem, the rear cylinder fires intermittently. That's when I started hitting forums and watching youtube videos.

The bike, as mentioned above, is a 2002 VStar 650. It has around 30,000 miles on it. The exhaust pipes have been replaced with Vance and Hines pipes. It still has the AIS system on it. The pilot and main jets appear to be the original sizes. A compression test on the rear cylinder indicates that it's within spec. The valve clearances are all within spec. I've changed the plugs. Switched out the coils, even though they were within spec. The pickup checks out fine. I can't find anything wrong with it, and with the plug out of the cylinder and a timing light attached it seems to fire just fine. The problems start when I put the plug into the cylinder. At first it seems to run OK and the timing light flashes in a regular pattern, then over maybe 30 seconds it starts to flash less and less with an irregular pattern. Eventually, within a few minutes, it pretty much stops. When the plug comes out it's wet and blackish.

Is it possible that I have too rich a fuel mixture going in (BTW - The air fuel mixture screws haven't been tampered with), which is causing the plug to short out and not burn gas, or is it another problem causing it not to fire, which then results in a wet plug?

I've read a lot of posts with similar sounding issues but none ever seem to reach a conclusion where someone indicates that they finally fixed it.

It's driving me crazy so any suggestions would be greatly appreciated.

Thanks
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02-22-2017, 11:08 AM,
#2
RE: Rear Cylinder Misfire / Dead
(02-13-2017, 12:14 PM)womble Wrote: Hello everyone. I'm new to posting here but have been reading for a couple months as I've tried to diagnose a problem with my 2002 VStar 650.

I purchased this bike a few months back with a reported problem of a gummed up carb. Since the price was low it seemed like a good deal. After cleaning the carbs a couple times it still had the same problem, the rear cylinder fires intermittently. That's when I started hitting forums and watching youtube videos.

The bike, as mentioned above, is a 2002 VStar 650. It has around 30,000 miles on it. The exhaust pipes have been replaced with Vance and Hines pipes. It still has the AIS system on it. The pilot and main jets appear to be the original sizes. A compression test on the rear cylinder indicates that it's within spec. The valve clearances are all within spec. I've changed the plugs. Switched out the coils, even though they were within spec. The pickup checks out fine. I can't find anything wrong with it, and with the plug out of the cylinder and a timing light attached it seems to fire just fine. The problems start when I put the plug into the cylinder. At first it seems to run OK and the timing light flashes in a regular pattern, then over maybe 30 seconds it starts to flash less and less with an irregular pattern. Eventually, within a few minutes, it pretty much stops. When the plug comes out it's wet and blackish.

Is it possible that I have too rich a fuel mixture going in (BTW - The air fuel mixture screws haven't been tampered with), which is causing the plug to short out and not burn gas, or is it another problem causing it not to fire, which then results in a wet plug?

I've read a lot of posts with similar sounding issues but none ever seem to reach a conclusion where someone indicates that they finally fixed it.

It's driving me crazy so any suggestions would be greatly appreciated.

Thanks
Since no one has responded to my post, and there seems to be a lot of similar posts out there, I'm wondering if this is a known issue with these bikes, and is something that I'm ultimately not going to be able to fix. Not to be beaten by this I've gone out and procured another working VStar of similar vintage. My plan over the next few months, time permitting, is to systematically switch components between the two bikes (while reporting those findings here) until I either get the first one working or kill the second one. Let's hope it doesn't result in the latter. Fingers crossed.
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02-22-2017, 11:22 AM,
#3
RE: Rear Cylinder Misfire / Dead
well reading thru the symptons.you are either loosing spark but you switched the coils.. then i'll assume the problem is in the idle circuit.i'm wondering if the pms screw is set right.i had a messed up screw that would not turn in all the way so when i backed it out 3 turns it was really 4.
2007 1100 custom,cobra longs and pods,doubled clutch spring.
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02-22-2017, 03:06 PM,
#4
RE: Rear Cylinder Misfire / Dead
(02-22-2017, 11:22 AM)nick59 Wrote: well reading thru the symptons.you are either loosing spark but you switched the coils.. then i'll assume the problem is in the idle circuit.i'm wondering if the pms screw is set right.i had a messed up screw that would not turn in all the way so when i backed it out 3 turns it was really 4.

Yep, I've switched in two different coils and neither seem to make any difference. Both have been checked for resistance, and work fine if connected to the front cylinder.

Definitely planning to switch carbs on these bikes, so if its a fuel problem I can then dig into the carb once more. It would be so nice if it were something (relatively) simple like that.

Thanks for your feedback!

Once I get organized I'll post some pictures.
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02-22-2017, 04:07 PM,
#5
RE: Rear Cylinder Misfire / Dead
https://www.google.com/url?sa=t&rct=j&q=...DeNwalvPxQ this is a problem i've seen many times on forums, everyone says pickup coil. maybe there is a weak wire that breaks down when heated up.hope the link works.
2007 1100 custom,cobra longs and pods,doubled clutch spring.
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02-25-2017, 08:55 AM,
#6
RE: Rear Cylinder Misfire / Dead
Are the carbs in sync?
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03-23-2017, 05:04 PM,
#7
RE: Rear Cylinder Misfire / Dead
(02-25-2017, 08:55 AM)RuggedNorth Wrote: Are the carbs in sync?

Honestly, not sure. But shouldn't the spark plug keep firing in a regular pattern whether or not the carbs are synced?
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03-23-2017, 05:37 PM, (This post was last modified: 03-23-2017, 05:41 PM by womble.)
#8
RE: Rear Cylinder Misfire / Dead
So it's been a while since I posted the initial problem on this forum. Since then we've completed a 3000 mile tour of the south east (which is what I purchased the first bike for). Obviously we didn't take the bike with issues, but I'm happy to report that the second VStar ran without any problems. Now that that's over I can get back to trying to figure out why the first one doesn't run properly. My plan, as mentioned in a previous post, is to switch components between the two bikes to try and isolate the problem. I'll start with major components and depending upon the outcome (and cost to replace) keep going until I figure this out.

The bike on paper is the non running 2002 VStar. The other is a running 2005.


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03-24-2017, 04:20 AM,
#9
RE: Rear Cylinder Misfire / Dead
(03-23-2017, 05:04 PM)womble Wrote:
(02-25-2017, 08:55 AM)RuggedNorth Wrote: Are the carbs in sync?

Honestly, not sure. But shouldn't the spark plug keep firing in a regular pattern whether or not the carbs are synced?

Also, did you clean float valves in carbs (with cotton swab + Farecla F3 + drill you get the valve cylinder clean and polished)? I wonder if too much fuel because of a flood or unsynced carbs could cause carbon deposits/wet spark plug that then cause a closed circuit = no spark. Carb sync should be checked/adjusted once or twice a year anyway so why not rule out a sync issue.
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03-24-2017, 09:34 AM,
#10
RE: Rear Cylinder Misfire / Dead
(03-24-2017, 04:20 AM)RuggedNorth Wrote:
(03-23-2017, 05:04 PM)womble Wrote:
(02-25-2017, 08:55 AM)RuggedNorth Wrote: Are the carbs in sync?

Honestly, not sure. But shouldn't the spark plug keep firing in a regular pattern whether or not the carbs are synced?

Also, did you clean float valves in carbs (with cotton swab + Farecla F3 + drill you get the valve cylinder clean and polished)? I wonder if too much fuel because of a flood or unsynced carbs could cause carbon deposits/wet spark plug that then cause a closed circuit = no spark. Carb sync should be checked/adjusted once or twice a year anyway so why not rule out a sync issue.

I did clean the carbs several times (and they needed it) but I'm very interested in your idea about unsynced carbs causing wet spark plug. It's a chicken and egg thing. I wasn't sure if they were getting wet because something else was causing the spark to stop and the fuel not get burnt, or if too much fuel was causing them to get wet and then not fire. Hopefully the entire carb assembly from the 2005 bike is compatible with that of the 2002 bike. I'll try to switch those around this weekend to see what happens. Thanks for your input.
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