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-   Carbs and Fuel System (https://www.vn750.com/forum/22-carbs-fuel-system/)
-   -   Clogged main jet? (https://www.vn750.com/forum/22-carbs-fuel-system/104615-clogged-main-jet.html)

Jason Pittenger 09-10-2019 09:22 PM

Clogged main jet?
 
I haven't been on the Forum all summer, frankly because everything has been running so well! 5500 miles so far this year, with fuel economy in the high forties!

I noticed on my ride home from work, that at Full Throttle, it starts cutting out a lot above 7K, and it's like I hit a rev limiter at 8 k! If I back off the throttle to about half, it runs right on up to Red Line. Occasionally, I would have problems as low as 5K, but most of the time it didn't start cutting out until 6. 5K or 7K. Always at Full Throttle. Every once in awhile, it would clear up and run strong right up to Red Line, only for the problem to return in the next gear.

It runs strong up to half throttle, and up to 6 k. I'm not exactly sure when the problems started, as my normal riding doesn't typically take me above 6K or at Full Throttle.

My best theory, is that the main Jets are partially clogged. I wanted to double-check before I tear into the carburetors again. I hate hate hate working on the carbs on this bike! But after six times, I've kind of got the hang of it.

Jason Pittenger 09-12-2019 08:33 AM

Seafoam in a fresh tank of gas cleaned it right up!

Knifemaker 09-12-2019 02:43 PM

Another glowing testimonial for Sea Foam;)

Jason Pittenger 09-16-2019 08:35 PM

Well, it came back! It seems to go away when I put a fresh tank of gas in comes back before the gauge moves off of Full. That might just be coincidental. I thought maybe p o o g s, I tried turning off the petcock and that had no effect good or bad.

It runs great at all throttle settings below 6K. And it runs great at all RPMs below half throttle. But the combo, above 6K at more than half throttle, it runs like crap.

Knifemaker 09-16-2019 09:17 PM

Perhaps you need to drain and clean your tank. It's possible there's just some crap in there that's floating around.

Spockster 09-16-2019 09:18 PM

Does it run better if you just open the gas cap? The petcock shouldn't have any effect on POOGS.

Could still have crud floating around. Try seafoam again? The more you ride on the main jet, the more it will get cleaned.

Clean the carbs?

Keep it under 6k :q

Jason Pittenger 09-17-2019 05:02 PM

Quote:

Originally Posted by Spockster (Post 1279795)
Does it run better if you just open the gas cap? The petcock shouldn't have any effect on POOGS.

Could still have crud floating around. Try seafoam again? The more you ride on the main jet, the more it will get cleaned.

Clean the carbs?

POOGS is caused by the tank not venting and forming a vacuum, correct? Wouldn't closing the petcock result in a similar lack of fuel flow?

I haven't tried running with an open cap. I'll try that tonight.

Jet's are new as of spring. It hasn't gone a week without running. I've put over 5k miles on it since April. I normally fill up at the local Shell with 87 (unfortunately 10% ethanol). I suspect the main jets, but I thought those kicked in before 6k. It seems odd that both would have a problem. Most of my riding is outside of the problem area.

My coworker (general mechanic, not familiar with the VN750) suggested a weak ignition system. He said: "High throttle requires more voltage, and high rpm requires more current. The combo could cause misfiring? Stopping for fuel allows the coils to cool off, making them run well until the coils get hot again." Seems plausible...
I'll check battery voltage next time it happens.

Quote:

Originally Posted by Spockster (Post 1279795)
Keep it under 6k

I've been doing that...it's just so hard sometimes! >:)

Quote:

Originally Posted by Spockster (Post 1279795)
The more you ride on the main jet, the more it will get cleaned.

So...ride faster? >:)

Spockster 09-17-2019 06:21 PM

Quote:

Originally Posted by Jason Pittenger (Post 1279825)
POOGS is caused by the tank not venting and forming a vacuum, correct? Wouldn't closing the petcock result in a similar lack of fuel flow?

I haven't tried running with an open cap. I'll try that tonight.

....

Yes, but if the fuel has already stopped flowing due to POOGS, the petcock won't change that, or relieve the vacuum.

High throttle/rpm also takes more air/fuel.

I think if the electrical was suffering, it would show up in the lower rpm ranges also.

Have you seen the slide diaphragms? In good shape and properly seated?

Earshaved? Got the vent lines protected from wind?

Jason Pittenger 09-17-2019 08:44 PM

On the ride home, I checked my battery voltage. It stayed between 13.4 and 13.6 volts, at anything above 2.5 k. I tried a pull once the temp gauge past the second line. It ran just fine. I tried again when the temp gauge reached the bottom of the thermometer symbol, and it sputtered a bit. I tried a third time when the temp gauge reach the middle of the thermometer symbol (red lights/city traffic) and it sputtered a lot.

I pulled over and open the fuel cap and tried another pull. That had no effect on it. Close the fuel cap and went on a long cruise at 60 miles an hour. The temp fell to halfway between the second line and the thermometer picture. I tried another pull and it ran flawlessly. It is definitely temperature related.

Before anyone's suggest incorrect jet size, it ran better with 143 main jet on a hotter day in the middle of the summer, than it runs now with a 140 on a cooler evening. It stumbled a bit on the high-end on the hot days, so I went down in jet size and it ran fine all the way up to redline, at all temperatures. I still have the 140 jet in today. It runs worse with heat, which would imply rich conditions. Choke has no effect.

Spockster 09-17-2019 08:58 PM

What color are the plugs?

If it's rich enough to cut out, you should be able to see black smoke (or someone following you could), dark sooty plugs, and probably find black soot at the tailpipes.

So, when it's cutting out, the voltage hasn't dropped any large amount? That would rule out a brownout condition in the charging system.

Wouldn't hurt to run it (fully warmed up) in pitch black lighting conditions, see if there's any arcing going on, From the spark plugs to the coil bodies. I found one of my caps arcing to the throttle cable.


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