Backfires and 29 MPG.....

[jaclaz]

Hi Jac here, new to the forum, new to the Vulcan 500. Just bought a 2001 Ltd. It backfires. It's blued the left pipe. It gets only 29 mpg. It has under 14K miles. The previous owner of 1.5 years says he just re-built the carbs and had them professionally adjusted. Well I went back to that pro and he adjusted them again saying "they were too lean.. That the set screw was self winding since the spring that keeps it SET was 14 years old and wearing out requiring new stronger springs." YET after his adjust it still backfires. And now the owner won't respond to my questions about the re-build.... like did you re-jett the carbs? From past experience I know that causes backfires when the air fuel mixture is out of wack, larger jetts require larger exhaust flow both in and out.... By the way there is a K&N air filter.
Soooo, any ideas?

Thanks for your help....

[charnotx]

I think you are on the right path. A few questions:
1) What spark plugs are you running? D8EA vs D9EA?
- depending on your location and climate, and fuel type, one will likely be better than the other
2) What octane fuel do you use? Does it have ethanol? If so, how much?
- this can also affect the heat levels of your engine. The higher ethanol %, the hotter then engine will run. It will also junk up your carbs, which will likely need regular fuel treatment like Seafoam. This can be combatted with what spark plugs you use (or at least, this is have been the best method I have found).
3) What are you idling at?
- if you have it idling too rich, then it will cause issues as well

Outside of those quick factors, the most likely cause based on personal experience, would be that the previous owner re-jetted the vulcan to a higher stage without making the necessary changes with the rest of the system.

I'm sure a couple other people will hop on and make some suggestions, but this is my 2-cents

P.S. This post over on vulcanforums.com might help with ideas: (http://www.vulcanforums.com/forums/50-v ... ost1666682)

[jaclaz]

Thanks Mr. "C"...
Thanks for replying so quickly... I finally got a responce from the previous owner that these are the stock jets... And I have no idea what plugs are in it since it's only days old in my ownership. I use premium pump fuel, 76 mostly... 92 octane with 10% ethanol here in Oregon. By the way my elevation here is 2,000' above sea level. I rode it up in the mountains to 5,000' ...no problems, other than backfiring. And there's no Tach so I have no idea at what rpm I'm idling at. Sorry I'm not more helpful...
Has anyone else had backfiring issues, but even more to the point is the major cut in MPG to only 29.....

[Triangles]

With 29mpg something is seriously borked up. There are so many things that could be wrong I'll try to not write a book.

The first thing that jumps out at me with one pipe being blued is that the inner exhaust pipe has failed and the hot exhaust gasses have overheated the external pipe. Hard to tell without a pic. Also there's usually a rattle associated with this. This usually doesn't happen until somewhere between 20 and 25K miles. I suppose it's possible an unscrupulous PO replaced your speedo with one that has fewer miles. The pipes being double walled usually don't blue on this bike but I suppose it's possible if the carbs are screwed up for many many miles.

Backfiring rolling off the throttle is very normal. The OEM mufflers do a good job of quieting this. If your bike has aftermarket mufflers the backfiring may be annoying and loud. Backfiring does not necessarily indicate a problem. Mine backfires thru the carb sometimes until the engine warms up. This is cause by the factory tune being a little lean and can be eliminated by using the choke.

By K&N filter do you mean you have pod filters or is there a K&N filter in the OEM intake box? If you mean the latter, then don't worry, the K&N filter isn't much different from the OEM filter which is essentially an oil soaked sponge. In other words the K&N filter won't affect the carb tuning. Now if you've got K&N pod filters in place of the OEM intake box then stock jets in the carb will not work.

Since you didn't mention it I assume you have unmodified stock exhaust/mufflers?

Ok now that I've got that out... Here's a few first steps I would take to point me where I need to look to fix things.
The first thing I would do is verify both spark plugs are firing. If you start the cold bike and one side warms up noticeable faster than the other or one side doesn't get warm, that's a good indication that you may be running on one cylinder and a spark plug may not be firing. You can verify that a spark plug is firing by pulling it and holding the plug with the metal threads in contact with the motor. Pull the other plug too and/or simply unhook the wire from the other plug (you don't want that other cylinder to fire). Also be careful to keep the plug grounded while cranking the engine. I'm always afraid I'm gonna zap my self when I do this. I haven't zapped myself in decades since I was dumb enough to pull the coil wire from the distributor on an old car. I"m still not sure what possessed me to do that but it was not pleasant to say the least.
Next and/or at the same time I'd check to see if the carbs are remotely synced. I'd do this by removing the carbs enough to be able to look down them to verify both butterfly valves close simultaneously. Alternately and possibly a better idea would be to build a cheap manometer to synchronize the carbs. Someday I plan to build something like this except for 2 bottles since we only have 2 cylinders:


If all that checks out ok, I'd next make sure the float valves are sealing. A quick and dirty way to check this is to park the bike such that the front wheel is higher than the rear. Making sure the engine is cool and there are no potential sources of ignition in the vicinity, turn the fuel valve to "PRI" and let it sit for a while. "PRI" is prime. it lets fuel flow even when the engine is off. In the reserve or on position, engine vacuum must be present to allow fuel to flow. If I observe fuel leaking out the air box then the float valves are bad. The reason I have the front wheel elevated is that if you don't it's possible for the fuel to run into your engine, wash down the cylinder walls and dilute the oil.

I would also check/adjust your valves. Who knows if that's ever been done. Also change the oil if you haven't already.

Give us more information on the stock/modified nature of the intake/exhaust. Maybe even some picts. Also, you're wasting money using anything higher than 87 octane fuel. Unless you're adding a turbo or jacking up the compression there's no need for higher octane.

[jaclaz]

OK I took it to "my" mechanic and he found the carb re-build was botched.... jets weren't seated well or tightened and carb wasn't especially clean as in float valves.... He ordered another jet to replace one and we're waiting for it's arrival... He says the backfire is fixed and the hesitation he felt (not me) should be fixed.. The bike is stock except for the K&N Air filter. I also want to adjust my rear brake pedal and my clutch engages right near the end of the lever travel....
And you're right regular fuel is adequate. I've run both no difference.
Any way thanks for all the feedback!
Cheers, Jac

[Triangles]

FYI it is normal for the bike to have a flat spot about mid throttle, ie you will start to accelerate and it will stumble and then take off like a banshee. I can only guess this is gas residue in some of the tiny passages in the carb. It usually only appears when the bike has set or more than a month and depending on how bad it is, it will usually go away after a few tanks of gas. If this is only a minor annoyance and not causing driveablity issues, I wouldn't worry about it until you've gotten a few tanks thru the bike and it hasn't improved or gone away.

I hope he's putting in new float valves in for you. They are cheap, should only be about $15-$20. The sealing tip is rubber and for the age of the bike they should probably be replaced if they haven't been already. If he's already got the carbs apart it's practically no extra work to replace these.

If your mechanic isn't already doing this. I'd ask him to check the level of gas in the float bowls when he gets it put back together. There usually isn't any (or much) adjustment needed but it's a good quick way to see if the float valve is sealing when closed.