sputtering and backfiring - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
Carbs and Fuel System
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post #1 of 8 (permalink) Old 05-14-2007, 08:20 AM Thread Starter
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sputtering and backfiring

Have a 1995 vn750 with 12,000 plus miles for two years now. Only mods are v&h exhaust when I got the bike. Been reading all your forums before signig up with nothing working so far, so I thought I put my info In maybe I missed something. Took bike out a week ago for ride and was sputtering and backfiring from idle up to 5000 rpm could not get going would die. Only doing from left exhaust which is the rear cylinder head. Frist thing I put seafoam in started up sat over night no luck. So far replaced rear spark plugs with new, old ones were black and wet. Also checked for good spark on all 4 cylinders. By installing a good spark plug in each cylinder spark plug wire and grounding to head and cranking over bike. R&R carbs (7 times now) disassemble, cleaned blew out any hole I could find (4 times now), inspected for torn or cracked diaphrams, checked and reset fuel levels (4 times now) tried different float levels, made things worse at times.It's at original setting again. Vacuum checked all hoses for leaks. Only found vacuum hose going from carb to air reed valve, replaced same. Tried new battery from my other bike no luck. Swapped front and rear coils no change, still are swapped. Pulled all electrical plugs, found corrosion on regultor/rectifier below battery with corrosoion, cleaned same. Checked rubber boots from carbs to cylinders can't see anything wrong. Only thing I've seen in this forum that's odd on my bike. Is that the vent hose from the carbs to the right ear (air filter). If it is in to far it can get blocked and stall the engine. On my if you put your finger over the end of the hose, the engine will speed up and hold at a higher rpm. Can't figure how it is ding that being a vent hose. So far the only thing I have gained from this which is a feat in itself. I can remove and install the carbs in 20 minutes ansd start the bike.
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post #2 of 8 (permalink) Old 05-14-2007, 08:42 AM
 
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Wow! Sounds like you've been through a run of the standard for issues. Maybe I should run down to VA beach and have you show me the tricks to getting the carbs on and off quick.

Have you checked your compression for the rear cylinder? Check the exhaust system on that side for leaks, Check timing for the rear cylinder. You didn't mention mix screw adjustments, have you checked them? Sounds as if you've more or less rebuilt the carbs. Have you synced them? Just a few thoughts. I am by no means very experienced with the mechanics of my VN750 yet. These are a few ideas I found in my troubleshooting section of the Clymers manual.
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post #3 of 8 (permalink) Old 05-14-2007, 08:51 AM
 
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Sounds like one of your vacuum hoses is going to the right ear instead of the vent line. The vacuum lines should go to the fuel valve and the smog valve (they come from the lower outside of the carbs). The vent lines that t and go to the right ear come from the top between the carbs you can't see them without pulling the carbs. You shouldn't have any noticable change in RPMs by blocking the vent line for a short period, blocking it for any length of time will cause the engine to stall.
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post #4 of 8 (permalink) Old 05-14-2007, 09:19 AM
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Quote:
Originally Posted by wwmkwood View Post
Sounds like one of your vacuum hoses is going to the right ear instead of the vent line. The vacuum lines should go to the fuel valve and the smog valve (they come from the lower outside of the carbs). The vent lines that t and go to the right ear come from the top between the carbs you can't see them without pulling the carbs. You shouldn't have any noticable change in RPMs by blocking the vent line for a short period, blocking it for any length of time will cause the engine to stall.
You probably have your carb parts memorized by now, but here's link to a pic I did during one of my many carb pulls that shows what wmkwood is talking about [ https://www.vn750.com/photopost/showp...o=680&cat=548]. The little brass nipple (in this case for the front cyl) is your vacuum line - you have a similar one on the rear cyl, just above the fuel/air adjust screw. The vacuum hose to that rear cyl nipple should come out of the emissions canister that's located behind the gas tank. Also, you said you have a vacuum hose going from your carb to your reed valve - that reed valve hose shouldn't come straight from your carb, but also off of the canister. Failing all that, trying pulling the cover off your reed valves- on my project bike, PO had installed one of 'em backwards (not sure what damage that may have caused, 'cept it was black as tar inside the cover).

C

Last edited by Crobins365; 05-14-2007 at 09:21 AM. Reason: Couldn't pick up link properly
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post #5 of 8 (permalink) Old 05-14-2007, 11:01 AM
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The way you describe the "vent" hose - it's the vacuum hose as others have stated. In a stock setup - that hose should go back to a canister under the seat. You should be able to see the nipple the hose attatches to on the right side carb.

The hose that you want going to the ear is one you can't see where it connects. If it's going back to the canister and the vacuum hose is going to the ear, it's a wonder your bike is running at all.

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post #6 of 8 (permalink) Old 05-14-2007, 08:06 PM
 
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I have this exact same problem, and have been trying to track it down. The engine sputters at low rpm, and backfires pretty bad (especially when cold). Sometimes it sounds like its missing, too. I haven't done much with it yet, but am looking for ideas. I'm planning to clean the carbs and put in new plugs, since the bike has been sitting around for a while (I just bought it used).

It runs great at higher rpm (maybe 3K or more) but just putting around town its really sputtering bad. What are a couple of quick fixes to try? Plugs, clean the carb, then what?
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post #7 of 8 (permalink) Old 05-15-2007, 12:10 AM
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I had similar sputtering problems and inconsistent idle. The first time that happend on my bike it was dirty carbs, so I thought that's what it would be this time. In the process of draining the carbs, and filling them with cleaner, I noticed that the right side vacuum hose was cracked.

I replaced it with a new hose and my poor idle and back firing disappeared.

So I'd say, check the vacuum hoses (right side to the emissions device under the seat, left side up to the petcock). 6 feet of silicon vacuum hose cost me $10, so cheap enough to replace.
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post #8 of 8 (permalink) Old 05-15-2007, 05:58 AM
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Quote:
Originally Posted by The_Spaceboy View Post
I have this exact same problem, and have been trying to track it down. The engine sputters at low rpm, and backfires pretty bad (especially when cold). Sometimes it sounds like its missing, too. I haven't done much with it yet, but am looking for ideas. I'm planning to clean the carbs and put in new plugs, since the bike has been sitting around for a while (I just bought it used).

It runs great at higher rpm (maybe 3K or more) but just putting around town its really sputtering bad. What are a couple of quick fixes to try? Plugs, clean the carb, then what?
A carb sync probably woulnd't hurt, either - the out o' sync seems more noticeable at lower rpms, although the benefits of the fix run you throughout the range.

C
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