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Discussion Starter · #1 ·
Looking for some thoughts on a stalling issue. Bike is a 2002 Vulcan 750. She idle and revs fine while cold. Once bike warms and I start riding, if I give her gas in 4th gear she starts sputtering and sometimes stalls.

Seems to be more of an isdue with front cylinder.

My current thoughts are possible bad coil or air/fuel mix is off.

I have custom k&s air filters on it so I had to adjust the air/fuel mix screws.
 

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Discussion Starter · #2 ·
Just a follow up. I marked the original position of the mix screws, then turned them all the way in and then 2 1/2 turns out as a starting point.

Also recently replaced all 4 plugs.

When I first start her, it seems she starts on back cylinder, then front kicks in when she warms slightly.
 

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I would take a compression reading on the cylinders as step 1. Compare them to the specs and make sure they are not significantly different across each of the cylinders

Carbureted bike, right? When was the last time you rebuilt the carburetor(s)? If it has been awhile, I would do this and I would put all high/low speed adjustments back to OEM if that applies.
 

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Probably wrong but I have heard of 'vapor lock? Maybe the intake manifold is getting warmer than it should? Seems there is to be a special intake manifold gasket between engine and manifold, maybe also manifold and carb?
 

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Discussion Starter · #5 ·
I have previously cleaned all the carbs thouroghly and installed new jets, so I would not suspect any dirty jets.

Also compression on both cylinders is 120 psi.

When I stopped for gas there did not seem to be any pressure build up either.

Still questioning front coil at this point. Going to test that next.
 

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The air/fuel screw only controls slight throttle (idle and low rpms). So if its idling ok and you are getting the sputtering in 4th gear with more than 1/4 or so throttle , I would look somewhere besides the air fuel screw. Bad gas, dirty jets? What did plugs look like when u replaced them? Whens last time you cleaned carb. If youre thinking a bad coil, check your plug wires and see if they look ok, then you can test the coils. Also, like mentioned compression test to check cylinder(s).Has this been going on a while. If you made changes to the air box then you might need to change jets. Could be a loose battery connection. but, most likely fuel related.
 

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stalls out when warm

Since the carbs have been all cleaned out w/ new jets and nice clean air filters, + the fact she goes fast enough to shift her into 4 gear. . .

It CAN be a bad coil. Having worked on the larger size gas engines for roto tillers and generators , I have found that a magneto coil can go bad- sort of. At an idle and about halfway up, the engine ran fine. The carb was brand new so I put in a new magneto coil in and the engine worked as it should.

Sometimes with a weak coil, the insulation is not what it should be and voltage jumps around. A hi voltage magneto coil is not much different than a battery ignition coil, both have a primary and secondary winding. The magneto coil is mounted or wound on a laminated iron core that allows a flywheel magnet to pass over or under it to make the current for the primary which gets opened an closed through points with a condenser across the points to absorb the back voltage when the points open.

ignition coils are very similar to a magneto coil, having a laminated core or a core of heavy iron wires and has both ends of the low voltage or
primary winding available. Bat+ goes into one side of the coil, other side of coil goes to the 'points an condenser' and of course bat- is grounded. Sometimes if both sides of a magneto coil's primary is available, I have wired the magneto coil up like an ignition coil because flywheel magnets were weak and the engine ran great. SO yes it can be a bad ignition coil at higher engine speeds. If one does not know a coil can go bad, ( because the engine does run ) the engine when run at higher speeds will start to **** out as if from lack of fuel, when in fact it is starving for a spark.
 

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I just replaced the coils to be amazed at the lack (none) of spurting and sputtering and it had lots before. I vote check coils too and wires and caps. Maybe try trimming the wire to caps also?
So true Aphrodite, ignition magneto's, step up transformers that sometimes work with low voltage mags and ignition coils, anything to make a spark.


Sometimes the insulation goes bad and voltage arcs inside the Hi voltage coil, might run good at lower speeds but higher speeds, and higher voltages, these coils just won't work well, most of the time leading a person to think it is a 'fuel' issue not knowing coil will **** out at higher engine speeds. Sometimes putting a coil in a low temp oven for a few hours WILL DRY it OUT. in a high voltage or ignition magneto, either the hi-voltage secondary goes bad, dampness is one reason, or maybe magnets are weak. a bad condenser will cause a mag to either have no spark or a weak spark or a good spark at lower engine speeds. I know this through experience.

Also there are companies that can re-strengthen magnets for magneto's.
 

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Discussion Starter · #11 ·
So I had some time to test out the coils. Both coils read at 1ohm on primary and 0 (nothing) on secondary. Also checked for spark and get very good spark from both cylinders.

Put in some sea foam to eliminate moisture from tank.

Took her for a spin, ran better, no stalling.

However still a bit of sputtering in 3rd and 4th gear at lower rpm range from 2 to 3 rpms. So I shifted from 4 to 6 rpm. Even a little sputtering, but not as bad in that range.

She has aftermarket air filters on her, so wondering if she is possibly catching air at hiring speeds, possibly leaning out the mixture.

Now I am thinking I may have a air/fuel mixture issue. I had to fiddle with the mixture screws after I put on the new air filters.
 

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So I had some time to test out the coils. Both coils read at 1ohm on primary and 0 (nothing) on secondary. Also checked for spark and get very good spark from both cylinders.

so wondering if she is possibly catching air at hiring speeds, possibly leaning out the mixture.
.

Did you check for air leak around intake manifolds, spray carb cleaner where manifold bolts to engine an carb/s if idle goes up you have a leak.
 
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