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Discussion Starter #1
99 Vulcan 1500 classic, carb has been cleaned and rebuilt, K&N filter with cobra speedster longs, with dynojet kit. Bike starts, idles and runs great.....until you let off throttle you get backfiring with flames out the rear. Also, if you just crack it just above idle, you'll get a cough and flame through the carb. Played with the carb a ton and can't get anywhere. Any ideas? I'm leaning on possible timing issue but not sure, if anyone agrees with this can you help me with how to check the timing....I'm learning as I go with this bike. Thanks
 

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Discussion Starter #3
First, I set it at Dynojet kit specs, 138 main with 3turns out on mixture screw, and needle clip on 4th gap from top. At this point, bike ran best at that setting...but still doing all of the aforementioned. I've tried different positions on the needle, turning out the screw more, turning in the screw some...bike just runs worse when I do those things though. The only thing I haven't tried was a different main jet. Dynojet says to use a 138 main, which is what was on it stock. I don't think that would be causing the issue???
 

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1. If you have hi flow air and pipes, your main jet could be too small, however, different jet manufacturers use different numbers, so a Dynojet 138 is the equivalent of a Keihin 150. I recently put a Dynojet 148 in mine since I got a Baron's Big Air kit and got rid of the stock air setup altogether. That's the equivalent of a Keihin 162. Here's a handy chart:

http://img.photobucket.com/albums/v...umbers_Cross_Reference_Sheet-_zpsd2c083ca.jpg

2. If you have to turn your pilot screw out more than 2 to 2-1/2 turns, either your pilot jet is too small or

3. Your carb is not properly seated to the intake. Sucking air behind the carb can cause a fire, which sometimes will come back through the carb and also cause backfiring.

First thing I'd check is whether the carb if properly attached to the intake.
 

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Discussion Starter #6
Well....the bike ran fine when the old man put it up in 2002 LOL then it was completely stock. it sat till this past winter when I began working on getting it un=seized. when he parked it, he was in a bad place in life so it wasn't stored properly at all...anyways, I noticed the exhaust first when I first got it goin, but didn't think much of it cause the baffles were rusted out in the stock exhaust, and I figured it just needed "tweekin" on the carb since I rebuilt it. Carb is good and clean, I know that for a fact, and everything on it is new, and yes...float has been checked. TPS is original though. As far as making sure its seated properly to the intake...pretty sure? the slot is in the groove and I can't push it up any farther. oh....6200 miles.
 

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Discussion Starter #7
forgot to mention, all fuel system is new. new filter, fuel lines, petcock, tank, pump...so on, just in case....
 

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Good. It's always helpful to eliminate the simple things first.
 

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So...ideas for starting points? Try larger main first? Larger pilot? I'm not the most mechanically inclined and this is really starting to get frustrating...especially with all the work and time to take the carb off each time I do anything to it, but, I am starting to get it down to an art! LOL
 

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John at the risk of stepping on others toes, I am going to share with you my own experiences dealing with my '99 classic. When I purchased it several years ago the owner had installed a hyper charger & V&H Pro pipe. The bike backfired, stumbled at low speeds and had a bad vibration at cruising speeds.

Things I learned: timing is not adjustable and will not change unless someone has been
into the engine and did not properly reassemble it.

changing air intake and exhaust requires rejetting

vibrations can be caused by uneven fuel distribution and aftermarket
pipes bolted directly to the frame without an insulator

My recommendations for your problem: Change the primary jet to a #45

Change main jet to a 165

These sizes may be a little large and cause the bike to use more fuel, but that is the least of my worries if the bike runs good.

Another note: if the plugs are original check them
 

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Discussion Starter #12
sfair: plugs are original...but if it will help I will post a pic . Think I know what you're getting at, but since they were original I didn't know if I'd really be able to get a clear picture of the situation. Would it be better to just put new plugs in, run some time on them, then pull them? if so, how long? Bike isn't legal to be on road yet and still needs tires and a clutch lever. Bandit: thanks for the advice, I'll look into it as well. I'm starting to feel like I got in over my head with this and probably did, but its fun, and good to know there's people here willing to help me out!
 

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So...ideas for starting points? Try larger main first? Larger pilot? I'm not the most mechanically inclined and this is really starting to get frustrating...especially with all the work and time to take the carb off each time I do anything to it, but, I am starting to get it down to an art! LOL
Ha ha! I feel your pain, brother. I have it down to about 10-15 minutes to remove the seat, tank, air cleaner and carb. :D

You are at lower elevation, so your bike is going to suck more air than mine does at 2500 feet. The new FI bikes have O2 sensors to make small adjustments for altitude. Carbs are another animal.

I concur with Bandit regarding the jets, however as I mentioned before, particularly with the main, Dynojet, Keihin and Mikuni all use separate numbering systems. Make sure when you buy a main jet, you specify whether it's a Dynojet or equivalent or a Keihin. For instance, a 150 Dynojet will be the same as a 165 Keihin. Because I went from a stock filter to a K&N, but kept the stock airbox, I jumped from a 138 Keihin to a 140 Dynojet, which was 152 Keihin. But then after getting the Baron's Big Air kit, I felt a little lean hesitation at the top end of the throttle. So I recently bumped it to a 148 Dynojet, which is a 162 Keihin, and at my altitude it works great.

 

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Discussion Starter #14
ok, so with the table above the 138 that came with the dynojet is the same as a keihin 150. There were different size mains that came with the dynojet kit, but per instructions I put in the 138. The next step up would be a 140 which is the largest that came in the kit. I can try that...that might help with the backfire on decal, but the carb pop right above idle still has to be something else, correct?
 

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Carb popping on acceleration may be due to lack of fuel from a primary jet that is too small. I encountered that with my bike. Another thing to check is to make sure the accelerator pump is working. With air cleaner removed, bike NOT running, look into the carb and twist the throttle. Each time you should see a stream of fuel pumped into the carb throat. If not you will need to replace the pump diaphragm or clear the ports.
 

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Discussion Starter #16
The primary is a 45 stock. might try goin larger on that and main, but first im gonna go back and start from scratch. Set the carb back as per dynojet specs, brand new plugs, then button it all up, run it for awhile then pull plugs and see. Starting to question if possibly the tps. the carb did set in carb cleaner for a long time over winter and did sometime or another fall in completely, think that could have ruined tps?
 

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Discussion Starter #17
forgot to mention, I'll shoot carb cleaner around the boot while running to check for air leaks, post pics of plugs when done, and make sure exhaust is tight. gonna be a few days though. Not sure if it makes any difference but one thing to remember this is all happening when bike is in neutral not moving, at this point in time bike isn't road ready, the tires are shot and it need a clutch lever
 

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Discussion Starter #19
....so you saying that could me my culprit? anyway to test it? All I know is Clymer manual says not to remove it and have to have some special tool to test it. and a new one has to be put on by dealer cause it has to be set to some spec. is that correct?
 

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Discussion Starter #20
Also, one last note about jets... been lookin around online and having problems finding what I "may need" My main jet is keihin round slotted, all I can find is hex head. Does that make any difference? Also not having much luck on a larger pilot jet. If anyone can steer me to a reputable place would be great, if it can be named on this site. Many thanks
 
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