severe backfiring - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 27 (permalink) Old 04-27-2007, 01:28 PM Thread Starter
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severe backfiring

Hello everyone,
i'm new to this, so i hope i can explain to you what my problem is. i have a 92 vulcan 750. the bike runs great under full throttle, but stumbles and misses under light acceleration in the 3500 to 4000 rpm range, and when i "level off speed around 60 mph and mostly in 5th gear ( it will occaisionaly do it in 4th gear when bike is hot ). it only does this on the left side exhaust ( rear head ) and backfires alot on decelleration, again only on the left side exhaust ( rear head ) bike did not do this prior to carb rebuild, i have taken bike back to shop 7 or 8 times to get it fixed ( they rebuilt carbs ) they have re-jetted several times, but will not go away. the only mods to this bike other than rejetting are v&h cruizers and the removal of the Ca. emmsions. bike was doing this prior to pipes and emossions. also, i keep reading about "marbleing the coasters" to prevent backfires. can someone explain this to me? did i mention that i am only getting about 30 mpg? is this a clue?

thanks in advance for all of your help

Jerry
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post #2 of 27 (permalink) Old 04-27-2007, 01:45 PM
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Sounds like it is rich or sticking float.
Have you pulled a plug to see what they look like?
MArbleing or coastering is plugging the air inlet which is part of the emisions.
The reed valves are on the side of the motor, Left side on the rear, righ tin the front behind the Ear.
By plugging them and adjusting the air mix screws on the carb helps with that.
Have you looked for a Vacum leak?
Is the vent hose still attached to the right Ear?

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post #3 of 27 (permalink) Old 04-27-2007, 05:31 PM
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Considering the low gas mileage and the popping, sounds like a rich condition where unburned fuel is igniting when it get's into the exhaust system when it hits some air. First off, I think there is something causing too much fuel to get into the chamber. Possibly a float stuck or float needle crudded up. This would allow too much fuel to build up in the bowls and run into the cyl. A vacuum leak would generally cause a lean condition so I'm ruling that out for now.

Why did they rejet the carbs? Aftermarket pipes don't usually call for rejetting, unless you change the air intake system, i.e. ear shave. I'm concerned that they changed jets to larger ones but your air intake hasn't changed which causes a rich running condition in all rpms except for idle.

We'll need more info to be able to help you, as in other mods (ear shave) etc. Also, exhaust leaks can cause backfiring too as fresh air helps unburnt fuel ignite. I wouldn't do marbling yet. We need to figure out why you are running rich before bandaiding that by marbling. Reguardless, I'd dose the tank with seafoam and let it run so it gets into the carbs and shut it off to let it soak overnight. That might help dissolve any crap that might be keeping the float valve open. Let us know what else has been done. We'll try to get your baby happy again!

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post #4 of 27 (permalink) Old 04-28-2007, 10:19 AM Thread Starter
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Hi all,

I haven't done any mods to bike, except remove the ca. emissions ( I changed fuel tanks from a 92 ca model to a 95 standard model. ) Not sure why the shop re-jetted the bike. i went by yesterday to ask what size jets they installed, but they didn't know. My home recently burnt down, so i lost all of my receipts showing what they had done to it. I have treated with seafoam, a full bottle to 1/2 tank of prem. gas. still, no help. can you tell me on the air idle screws which way is lean, in or out? and is there an easy way to pull the carbs to inspect the floats and jets? i have also swapped the coils to see if 1 was weak, but no change there either.

thanks
Jerry
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post #5 of 27 (permalink) Old 04-28-2007, 12:39 PM
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I've not had the carbs off this bike, but there are others here that can help you with that. I recall someone saying you can change the jets without removing the carbs though, so I hope someone speaks up to help you there too. I know the air mix screws are idle mix and won't have much if any effect on the engine running beyond idle. I know I'm not much help with your questions, but I'm hoping someone will help you soon!

Fergy
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post #6 of 27 (permalink) Old 04-28-2007, 02:09 PM
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For the air adjust screws, in is lean, out is rich - the factory setting is 1 5/8s, but Fergy's right, that that shouldn't be the source of your backfiring while running. If it's just the left side, check for any loose bolts on the left side exhaust - Chad, I think, noted that an exhaust leak will cause backfiring, and that's the easiest place to start. I'm trying to think, though, if you pulled the emissions stuff, the hoses must've been pulled off of the reed valve covers (the reed valves introduce fresh air into the exhaust to burn off any unburnt fuel), and the rear vacuum port. And there's a hose coming out of the airbox that hooks into the emissions set-up,too. Jon Landau, are you out there??? Did you coaster or marble the reed valves after you shaved? And how did you handle the rear port?

Also, while it's theoretically possible to check your jets with the carbs on the bike, I don't think it's practical. There's just no space in there to work, really.

C
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post #7 of 27 (permalink) Old 04-28-2007, 02:22 PM
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There could also be vacuum caps over the reed valves and vacuum port on one of the carbs (right side). That's how I capped mine when I removed the rotten emission hoses.

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post #8 of 27 (permalink) Old 04-30-2007, 08:51 AM Thread Starter
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Wow, Thanks for all the imput. I checked the bolts on l/s exhaust, ok there, in regards to the the hoses coming off the reed valves, I left the big 4 way vavle on the bike ( the one right under the rear edge of the tank ) and left all hoses connected to it. However, i cannot find any hoses running to either of the ears. could it run somewhere else? Please don't forget, not only does it backfire, but it also runs bad at 3500-4000 at hwy speed, misses on left side.

thanks again, you guys are great !!

Jerry
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post #9 of 27 (permalink) Old 04-30-2007, 03:45 PM
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Quote:
Originally Posted by jprather View Post
Wow, Thanks for all the imput. I checked the bolts on l/s exhaust, ok there, in regards to the the hoses coming off the reed valves, I left the big 4 way vavle on the bike ( the one right under the rear edge of the tank ) and left all hoses connected to it. However, i cannot find any hoses running to either of the ears. could it run somewhere else? Please don't forget, not only does it backfire, but it also runs bad at 3500-4000 at hwy speed, misses on left side.

thanks again, you guys are great !!

Jerry
Did you block off the vacuum port on the rear carb? One of the emissions lines runs to that brass nipple.

C
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post #10 of 27 (permalink) Old 05-01-2007, 09:43 AM Thread Starter
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I don't think i blocked off any vacum lines. the only one i could find went to the 4 way valve. i took off the 4 way valve and the bike ran terrible, so i put that piece back on. i have also looked for the line that goes to the left ear, but cannot find one, nor a place it would hook into.... anybody wat to buy a bike ?
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