Misfiring badly, won't run - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 22 (permalink) Old 07-27-2014, 09:48 PM Thread Starter
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Misfiring badly, won't run

I'm already late to leave to see my girlfriend and my bike doesn't want to go anywhere. I got home from over 3000 miles and some bike trouble on the road (died due to not charging well in a long traffic jam), and I started doing a little work on my bike and now she won't run.

Here's what I did:

1. Fixed a short on the rear running light, shoudn't have affected anything
2. Checked condition of spark plugs. They looked pretty good to me (photo below) and I checked torque twice.
3. Changed both pilot jets back to stock, from +1
4. Changed rear main jet to stock, from +1
(not listening to any advice on jets in the future. I don't think I needed to rejet in the first place )

Then it didn't run worth a @*#$. Took the carbs apart, cleaned the float needles. Cleaned everything else in them. Everything looked fine to begin with, but just in case, right? I don't see gas coming through the petcock. No vaccum leaks on the boots or starboard vac port on the carbs.

Still doesn't run worth a #*&$. Cleaned the carbs a second time. Still misfiring badly. It can't be too lean to run just from changing one jet size, especially since choking (enriching on these bikes, of course) doesn't help.

It seems to be misfiring badly and running mainly on one cylinder. If I had to guess, I'd say the front is misfiring, but it's hard to tell. What do I do, pull the carbs a 3rd time just to go back to +1 jet sizes? If that was the problem, it should at least idle great with the choke (enricher) on partially, right?

All of the spark plugs are turning black now. Here's what they looked like before:


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post #2 of 22 (permalink) Old 07-27-2014, 10:17 PM
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Here is what I would do.

First, I would not just pull the carbs unless I was satisfied it was necessary. You can check fuel level on the bike/centerstand, using a short piece of clear hose as per the Kaw manual.

When I work w/motorcycle carburetors, I never clean the needles/seats and reinstall the carbs. I test them. There are a number of ways, I like to hold the carb upside down and draw a vacuum on the fuel inlet using my mityvac. Then I let it set and walk away for a time.

You are right, it should idle.

Those plugs are certainly not too lean.

I will not make a judgement on jets with the info you have furnished here.

HTH.
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post #3 of 22 (permalink) Old 07-27-2014, 10:18 PM Thread Starter
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Drained both bowls. Twice. Both were within 5ml/ 1 tsp of each other, maybe because it's on the kick stand? Not much difference between them anyway so I doubt its the needle valves / flooding.

It ran great before, just a little rich I think due to the jet sizes. It's coastered, ear shaved, open Harley mufflers behind the stock crossover chamber. Nothing else changed on the engine.


Last edited by Hexadecimus; 07-27-2014 at 10:21 PM.
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post #4 of 22 (permalink) Old 07-27-2014, 10:24 PM
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Drained both bowls. Twice. Both were within 5ml/ 1 tsp of each other, maybe because it's on the kick stand? Not much difference between them anyway.
Sounds like Goldilocks. Both are either too high, too low or just right.

I would still check the level. If both are correct, it is not the n/s.
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post #5 of 22 (permalink) Old 07-27-2014, 10:43 PM
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It ran great before, just a little rich I think due to the jet sizes. It's coastered, ear shaved, open Harley mufflers behind the stock crossover chamber. Nothing else changed on the engine.
Ma Kaw does not recommend any oversize for the pilot jets. Only #38 and adjust the pilot air screws. The pilot circuit has a non replaceable #95 air jet.

Ma Kaw does recommend up to a #140 main gas jet w/#100 non replaceable air jet. OEM is #132.

I personally, will buy your preference up to #40/#140.

If it were mine, I would go OEM and start with 2 1/4 turns out on the screws. Then I would decide from there.

HTH. again.

Edit- I never use shims/crutches.

Last edited by wmsonta; 07-27-2014 at 10:46 PM.
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post #6 of 22 (permalink) Old 07-27-2014, 10:44 PM
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Hex, while doubtful that you have the same problem I recently had but I also had a cylinder (rear) that was cutting out and I've also been fiddling with my carbs in an effort to get them just perfect.

At first the miss only occurred for a few hundred yards every 50 miles or so making it impossible to get off the bike and diagnose. As the problem got worse it happened for about 3 miles but after pulling over it suddenly got better. This happened twice and it's hard to diagnose a problem that isn't happening when you get off the bike. A close look at all wiring revealed a loose connector to the rear coil. Have since run it about 130 miles with no problems so hopefully I've found the problem.

Pull your tank, side panels ect and double check all wiring and connections including grounds. Unplug all connections, inspect the terminals and plug back in. Takes an hour to do but in my case it seems to have fixed the problem.

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Last edited by Spencer1; 07-27-2014 at 10:47 PM.
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post #7 of 22 (permalink) Old 07-28-2014, 11:58 AM Thread Starter
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Still missing badly. Changed the fuel in the tank. The plug wires are well connected and sparking. If I disconnect the front cylinder, it runs on the back. But if I disconnect the rear, it runs on the front too without missing much. I checked the sync by eyeball (highly calibrated eyeballs honestly) and it's close enough to run better than this. It was before and I haven't touched it.

No wires off anywhere that I can find. I might swap the jets back just to eliminate that variable. Maybe these are too eroded to atomize well.. But it ran when i bought the bike before I put in the bigger ones.

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post #8 of 22 (permalink) Old 07-28-2014, 12:05 PM
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Maybe these are too eroded to atomize well..
What?
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post #9 of 22 (permalink) Old 07-28-2014, 12:06 PM Thread Starter
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Girl level is correct too
Edit: make that the fuel level. Autocorrect on my phone..

And I doubt the old jets are too eroded after the miles on them, but it's one more variable I could definitely eliminate.

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post #10 of 22 (permalink) Old 07-28-2014, 12:19 PM
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Quote:
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Girl level is correct too
Edit: make that the fuel level. Autocorrect on my phone..

And I doubt the old jets are too eroded after the miles on them, but it's one more variable I could definitely eliminate.
To be honest, if the fuel level IS correct, it is probably not gas. It is probably time to follow the old drag racers ditty-90% of all carburetor problems are ignition.

Gas should never 'erode' jets. Not in one lifetime, anyway. If you have the correct series, genuine Kiehin jets, they should be fine.

I will be busy today, but will try to follow this.

edit-if you can tell the difference, taking spark away from one cyl at a time, you could swap ign coils.

Last edited by wmsonta; 07-28-2014 at 12:29 PM.
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