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Old 08-26-2008, 11:31 PM
VSB VSB is offline
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Misfire & Backfire

Hi guys,

I'm new here and new to vulcans. I just got a '93 VN750. I really enjoy the bike and riding, but it has two small problems. The bike doesn't want to idle until the engine is fully up to operating temperature and it is hesitates like it has a misfire problem. Today one cylinder totally cut out so the bike was just running/driving on the other cylinder. After the bike got up to operating temperature both cylinders were working/firing. The bike was sitting out in the rain for only 5 to 10 minutes the night before.

It seems now and then the battery is having a hard time cranking the engine starter. The battery is just three days old and was fully charged before installation.

And besides this "dead cylinder" that came back to life; the engine always feels like it has no spark on one cylinder or spark coil. This is causing the engine to backfire and sputter some.

Ideas of what it can be?

Some things of what i think it may be:
bad spark plug wire/s,
fouled spark plug/s,
airbox is not sealed airtight,
battery cables corroded or worn out,
bad or corroded electrical ground,
short circuit in the wire harness somewhere,
bad ignition coil,
vacuum leak near carbs,
stator not working correctly.
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Old 08-27-2008, 12:44 PM
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weh44att weh44att is offline
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Is the battery a lead acid or MF battery? Mf batteries give you more cranking power on cold starts. How good is the battery charging, check voltage. Try cleaning up all your electrical connections. How do the spark plugs look when you pull them. Everything you mentioned could be the cause of the cylinder cutting out.
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Old 10-09-2008, 01:10 PM
unclephil unclephil is offline
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I've got something similar on my "new" '92, the front cylinder cuts in/out.
Rode in to work today on one cyl, hope it starts OK tonight like that...

The plugs were clean last time I checked, but I can smell raw gas at stoplights and the backfiring is definitely in the right exhaust only. So that narrows it down to (most likely) a front spark issue.

If you can narrow down the possibilities to front/back, fuel/nofuel, you'll probably git 'er fixed faster.
Good luck.
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Old 10-09-2008, 01:29 PM
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Once again, try Seafoam. It's available at many auto part stores and many Wal-marts are now carrying it.

Could also be the carbs need some attention (cleaned/check for bad hoses)
Neither of ya mention how many miles are on your bikes or what kind of general condition they're in.
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Old 10-09-2008, 02:18 PM
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2x on the seafoam, that may be the place to start. some have had battery issues that made the bike run bad. maby you can check the battery with a multimeter or you can take the bike to autozone where they will check it for free. good luck
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Old 10-09-2008, 04:17 PM
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As with the rest of the gang, make sure it is a Maintenance Free battery (it doesn't have anywhere to add water). All the parts stores (including walmart) show that a wet cell battery will fit our bikes, but in reality, they may only work for a couple of weeks at the most. You'll need a Maintenance Free (MF), also called an AGM battery.

Assuming that it's not the battery, it could very well be your charging system.

As to the misfiring, you'll probably need to clean the carbs. I'd try doing a search in the box above for gumout, and review the procedure to "backwash" the crud out. Here's the link: http://vn750.com/forum/showthread.php?t=8345
If that doesn't work, you may need to pull the carbs. Personally, I'd recommend letting the shop do it, unless you are REAL HANDY with mechanical stuff.

BUT, before trying the gumout and carb cleaning, try the SeaFoam as per HyperBuzzin. There's nothing like it, so don't use just any gas treatment. Add about an ounce per gallon. You can get it at most auto parts stores, it looks like this:


Run through a tank with it, and see if it helps.
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Old 10-09-2008, 05:11 PM
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Usually when back fire is mention along with rain or washing the bike. In many cases it is water in the fuel. drain both carbs and add some kind of water remover in the tank and try it again.
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Old 10-09-2008, 07:50 PM
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If the bikes sat for a long time before you acquired them, you could have a lot of crud in the carb bowls. Seafoam treatments have worked for most but if that doesn't work, pulling and cleaning the carbs is in order. It doesn't take much to clog one of the very small jets in the carb. One quick fix which has worked for me it to take a 4mm allen wrench and drain the carb bowls. You'd be surprised how much gunk comes out with the gas. Once the bowls are drained, close the drain plugs and restart the bike. It may help get rid of what ever crud is blocking the jets. This trick has worked for me.

If the bike is misfiring and you can smell raw fuel, it's a carb problem. If the cylinder is cutting out in a certain speed range, it could be an ignition problem but it could also be a carb problem with a clogged or missing jet. I had the high speed jet come completely out of its mount and drop to the bottom of the carb bowl which made the rear cylinder stop running at anything over 3k RPM.
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Old 10-09-2008, 09:35 PM
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I agree with the Seafoam suggestion. I had similar issues with another bike that was just a crudded carb. Also- chase the ground connections on your battery to the frame and then the frame to the engine. Remove the screws and clean the terminals where they contact. A dirty ground will prevent charging and make the bike slow to turn over. Sandpaper is good for the cleaning.

Good luck!
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