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Discussion Starter · #1 ·
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Hey everyone
Got a Yamaha Vstar 650 classic as my first bike and it has a problem with the rear cylinder not firing. My Dad and I removed the carb and gave it a thorough cleaning, but that didn’t fix it. We replaced the spark plugs too. It was sitting for a bit so the gas is probably bad so our next plan was to dump the tank and trying new gas. The only thing is that it was doing this before it sat for a year. Also replaced the fuel filter and don’t see any trash in it or the tank. We do have spark so we’re wondering if it’s gas related. Anyone have any ideas? Thank you. Any advice is MUCH APPRECIATED!
 

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You're suppose to introduce yourself first, then we can tell you to try switching things from the working cylinder to the non-working one until you find the part that makes the issue appear on the other cylinder.

& Do you really know how to clean a carburetor, or did you just give it your best shot?
 

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Thanks for trying Trials. (y)
 

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Discussion Starter · #4 ·
Hey everyone
I’ve got a 98 Yamaha Vstar 650 classic that is having trouble firing on both cylinders. The rear one is dead, but has spark. I took the carbs apart and cleaned everything in carb cleaner, but it still is having trouble. It was sitting for a few years before I got it and had the issue then. Dumped tank and added new filter, but no luck. Any ideas? Thank you in advance.
 

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Hey everyone
I’ve got a 98 Yamaha Vstar 650 classic that is having trouble firing on both cylinders. The rear one is dead, but has spark. I took the carbs apart and cleaned everything in carb cleaner, but it still is having trouble. It was sitting for a few years before I got it and had the issue then. Dumped tank and added new filter, but no luck. Any ideas? Thank you in advance.
Does it have compression?
 

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Hey everyone
I’ve got a 98 Yamaha Vstar 650 classic that is having trouble firing on both cylinders. The rear one is dead, but has spark. I took the carbs apart and cleaned everything in carb cleaner, but it still is having trouble. It was sitting for a few years before I got it and had the issue then. Dumped tank and added new filter, but no luck. Any ideas? Thank you in advance.
Just a thought/my experience: awhile ago, '02 Vstar 650 did not seem to be firing on both, & would stall. Both seemed to have some spark (weak?) but problem persisted. At some point, finally saw no spark at rear(?) After some research, replaced pick up coil (philips head screws have serious lock tight, used hand held impact driver to remove) w/oe part, & the stator & rectifier w/Rick's parts because had the cover off;)...problem solved. Good luck!
 

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Ace Tuner
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You've got spark, probably fuel, possibly compression? The rear cylinder was not running (er, working) before the bike sat idle.
First guess:
A bad spark plug cap can allow a spark when testing outside of the cylinder but fail to allow a spark under compression with the spark plug is installed.
An inexpensive aftermarket NGK plug cap can be used temporarily to eliminate your factory plug cap as the problem. Most shops should have the aftermarket plug cap in stock at about 4 or 5 bucks.
Unscrew your factory cap, screw in the NGK cap. Cheap, easy troubleshooting. ;)

S F
 

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I've had the same issue happen on my old 650 classic after a carb cleaning. Damn pain in the ass that thing was. For me the cause was out of sync carbs, it was a pretty easy fix though.
 

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Discussion Starter · #12 ·
*Update
We took off the main tank and replaced it with a smaller temporary tank and the bikes revs, but only when we block off the majority of the air going to the carbs with our hand. When we do that, she revs and sounds great. Both cylinders are firing and the pipes are hot. Any ideas on what could be going on? It seems like it’s starving for gas.

also I saw that some people were converting the dual carbs to a single carb 38mm makuni(probably spelled swing) has anyone had luck with that or is it a bad idea?
 

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*Update
We took off the main tank and replaced it with a smaller temporary tank and the bikes revs, but only when we block off the majority of the air going to the carbs with our hand. When we do that, she revs and sounds great. Both cylinders are firing and the pipes are hot. Any ideas on what could be going on? It seems like it’s starving for gas.

also I saw that some people were converting the dual carbs to a single carb 38mm makuni(probably spelled swing) has anyone had luck with that or is it a bad idea?
Don't know about the single carb thing. With the carbs installed, you may be able to remove float bowls & liberally spray carb cleaner (wear a mask) into any jets visible (idle/slow speed jets are most susceptible to clogging when full float bowls are allowed to sit & fuel evaporates . (I think there is a 650 V-Star Forum also, which may help) (Note; my "pick-up coil" failure (tells cyl/plug when to fire) was intermittent as I recall, good consistent spark was not occurring).
I’m a novice so I might of missed something. Is that the floats that were out of sync for you?
Multi carb engines require that each cyl. is drawing nearly an equal amount of vacuum pressure while running in order to be synchronized & run smoothly. One carb is nonadjustable (the base) & the other(s) are adjusted to a similar amount of vacuum draw.
Hope this helps.
 

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I’m a novice so I might of missed something. Is that the floats that were out of sync for you?
You synchronize multiple carburetors by adjusting their fuel/air ratios.
Floats do not require synchronization with anything other then the fuel level in the float bowl. Think of the float needle and seat as being just like the water level control on a toilet tank. That's all it does, maintains the correct level of fuel for the venturi pipes to draw from.
Think of the main jet as being a fly sprayer

There is the basic principals behind a carburetor. It's a toilet tank connected to a fly sprayer that is power by gravity and venturi effect vacuum pressure.
 

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*Update
We took off the main tank and replaced it with a smaller temporary tank and the bikes revs, but only when we block off the majority of the air going to the carbs with our hand. ...
You blocked off the air intake which made the vacuum pressure draw more fuel out of the carburetor because you stopped it from drawing air, it had to suck from somewhere and that somewhere was the holes leading to the float bowl which operates at atmospheric pressure.
 

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& btw you can not synchronize carburetors on an engine that has different compression from one cylinder to the next, so compression testing is a fairly important step.
Vacuum intake pressure is proportional to the cylinders compression, if you have one cylinder with low compression, that cylinder will not draw as much fuel/air mix through the carburetor attached to it.
 

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...let me know what you think of the video I linked because that was 100% the fix in my case. Plus it's just a really neat n easy way to sync carbs old school, but definitely a handy trick to know.
 
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