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Left cylinder not firing intermittently

8K views 19 replies 7 participants last post by  mcdowell92 
#1 ·
I was here previously with an intermittent issue with my bike not running properly, and after a lot of work i got tired of messing with it and gave up for a while. I'm now back at it! I have since confirmed it is due to the left cylinder not firing. This has been an intermittent issue for damn near three years, and i have not been able to ride the bike in six months or so. i have new spark plug wires, plugs, and have flipped the coils with no change. Plug is wet after running and left pipe smells like gas. After getting new plug wires (i needed them anyways) and flipping the coils i flipped the plugs as well for giggles with no change. There is definitely spark on the left side (tested at plug cap), and i assume is firing occasionally as left side does get hot. Last time i was able to ride it, it fired right up and ran like a champ for a 30 minute ride or so. Would not run right after starting it the next day. When the issue first bore its head i was riding down the road and the bike just cut to idle. I pulled over and it would bog down when attempting to rev. Initially it would just start working again after 30 seconds or so and would show up every couple of weeks. I have a new wiring harness and fuse box (old one had a hole melted through the box at the plug, everything electric worked except for kickstand switch), new flywheel and stator (i have a 93 and one of the magnets busted) and rebuilt the carbs. Cam position sensor? IC Igniter? Any ideas would be very much appreciated! Thank you!
 
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#4 ·
Sorry to rain on your parade, but the mixture screws control the amount of gas for idle. It has nothing to do with air movement.
Increasing the amount the screws are out deliveries more gas and then less gas when the screws are driven in.

How is the air movement from air box to carbs, with big concern on the sealing of the carb boots.

Maybe wiggle wires while its running to check for the possibility of a bad wire or connection.
 
#5 ·
I have the screws at 3 1/2 turns out and it seems to be running fairly well. I just took it out for a ride with no issues! It is definitely pulling air, i put my hand in front of the intake and it sucked it up with quite a bit of force when i gave it throttle. The boots are sealed all the way around. I wiggled wires with no change. The throttle seems like it may be a bit hesitant to rev, meaning it seems to rev a bit slower than the throttle twist, but it improved after it was warmed up...honestly this has happened before, it runs fine and then starts back up out of nowhere...but it went from definitely running on one cylinder earlier today, and every time i had tried to mess with it for months to how it is now by only opening the mixture more.
 
#7 ·
Bike still running well, took it for a 50 mile ride yesterday and was able to get the mixture screws down to 2 1/4 turns afterwards by setting the correct idle with a digital tach. I believe i had an electrical problem before and fixed it, but was unable to get the bike running after the fix due to the idle being set way too low (noob messing with carbs). This was my first bike and it feels great to have it back on the road. I have been riding a nighthawk 750 for a while and my wife is going to start riding the vulcan as her 650 vstar is not comfortable for her during low speed maneuvers. Thanks to everyone who helped out! Wish me luck!
 
#11 ·
I can basically let it idle and give it throttle every 5 seconds or so and eventually it will either rev normally or deteriorate back to one cylinder not firing at all. I just fired it up again this morning, full choke was fine, half choke was fine, took off choke and idle fine, let me rev a couple of times, then just started refusing to rev and idling a little rough. I am highly confident it is not any debris in the carb. There are new jets in the carb and i had a clear fuel filter on for a while to see if there was any debris coming from the tank, never saw anything in there, but i cleaned the tank anyway. The diaphragm does not have any cracks and is not deformed. I can give it a little gas and the rpm will "build up", but too much gas at once and it stumbles. I have not been able to find any vacuum leaks. I can only assume that the left side is the issue because it is the one that will stop firing completely at times. Just hooked up to a carb sync tool, they show the same vacuum when idling and attempting to rev.
 
#12 ·
Though diagnosis via keyboard is difficult, often misleading...

have you "read" the spark plugs?

Are left and right looking radically different? Is the left side fuel or soot fouling?

I'll submit a likely fuel metering issue...your carbs are not quite as clean or setup as well as you'd like to think.
 
#13 ·
Well, i feel like a royal jackass. I had got it running making the mixture more rich, rode it around a while, then came home and leaned it back out. That's why it wouldn't run the next day. I came home and turned the screws back out and it's running fine again. The bike has a k&n air filter on it so it makes sense it was running lean. At this point, i would just continue running it and check the plugs periodically to get it dialed in, correct?
 
#14 ·
tune each pilot screw for highest idle, best off idle response, left and right, bike warmed...each in its "sweet spot"

choke should yet still be required when cold, but should start instantly at the touch of the button when warmed.
 
#16 ·
sure, repeat, experiment to your satisfaction...the bike will react favorably when you've got it

generally...adjust screw inward, revs will drop...outward till it reaches its highest RPM and no further..

Always wise to monitor your fuel consumption to establish average MPG trends...and to reveal any sudden, radical variation, indicating possible problems.
 
#17 ·
It ended up wanting to be at about 5 turns out. The idle seems smoother now and throttle response is very good. Thank you so much! It had cut out a few times when i took it out to get warmed up before the adjustment, so hopefully that is the last ill see of it. I'm going to try it again tomorrow and update. Again, many thanks!
 
#20 · (Edited)
The screw is underneath the carb and is a flat head screw. The opening is sort of small and the clearance isn't great, so you may have to look around for a screw driver to use. I use the tool that goes to my dremel because it is only about 2 inches long and has a thin flat head on one side.

The only allen screw I can figure you are thinking of would be the drain screw. But the one you want is the bottom bit in this picture, with the arrow reading "idle mixture screw." The bowl doesn't cover it, it is still accessible when everything is assembled and on the bike.

The difference is pretty obvious, there are a lot of areas that feel the same, further out drop off the cylinder running much at all, and there is a pretty distinct sweet spot where the idle goes up due to the ideal ratio of air:fuel
 
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