RPM surging/missing on acceleration - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 6 (permalink) Old 10-09-2018, 03:59 PM Thread Starter
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RPM surging/missing on acceleration

Today as I was merging onto the freeway, the bike started surging/having major RPM fluctuations. There was also a fair bit of backfiring. Seems to happen whenever I give the throttle a good twist to accelerate, in any gear.

It may have been my imagination, but it seemed like the bike was vibrating more than usual while crusing. Also, it seems like the dash lights and headlight are dipping more than usual when I increase throttle, but that also might just be my imagination.

The problem continued after several stops to inspect; no leaking fluids or obvious loose wires. Bike starts up fine each time and idles cleanly.

My first guess is a fueling issue, but not sure where to start troubleshooting. Second guess is a stator failure?

Of note: there is moderate rust in the tank, and the previous owner installed two fuel filters to compensate.
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post #2 of 6 (permalink) Old 10-09-2018, 05:03 PM
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drain the gas from the float bowls to see how it looks, check the battery connections, check the battery voltage (resting, running, and at 4k rpm)

That should be a good starting point to eliminate the most obvious things that could be wrong. (rusty tank is gonna be an issue no matter what, though. couple gallons of EvapoRust could take care of that, my stepdad fixed his tank which was rustier than anything I'd ever seen in just a few days, easy procedure)

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post #3 of 6 (permalink) Old 10-09-2018, 07:45 PM
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Rust is a good suspect. Sounds like the main jets are clogged or partially clogged.

Two fuel filters, I'm surprised it runs actually. Some manage to run with one filter, but others have had trouble using any additional filters beyond the screen in the tank.

Seafoam story (but seafoam won't eat rust):

My bike sat a lot this summer, but I put seafoam in the tank back in June.

Got out for a ride last week. 10 miles in, it started cutting out at half-full throttle. I figured it was the jets.

I held it at full throttle until I caught the bike in front of me, then backed off. It took about 1/4 mile to catch him each time. After about 15 miles of this, the jets cleaned out and it ran great again. Rider behind me complained about the smell. The whole time it was bogging, then surging. After it cleaned out, it ran great for the next 150 miles to finish the ride.

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post #4 of 6 (permalink) Old 10-09-2018, 08:08 PM
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Originally Posted by timgill View Post
Today as I was merging onto the freeway, the bike started surging/having major RPM fluctuations. .
If the tach was jumping then you want to check your grounds and make sure your battery terminal connections aren't loose too. Easy thing to check.

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post #5 of 6 (permalink) Old 10-09-2018, 08:12 PM Thread Starter
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Thanks for the advice guys. The fuel filters are in line on either leg of the two lines exiting the petcock; so, I guess just ahead of each carb. Honestly if I end up going so far as to pull the tank, I'd rather try a de-rust treatment and take the filters off if I can.

The more I ponder it, the more I think some kind of rust clog is the issue. Is there a way to flush the carbs/jets without pulling the tank and carbs? Or should I just plan to have a full evening of wrenching ahead?
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post #6 of 6 (permalink) Old 10-09-2018, 08:38 PM
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No direct experience here, but others have said that extra filters can affect fuel supply. Eliminating them (and the need for the them) might clear up a lot of your trouble--my opinion, anyway?!?

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