Bike surging on acceleration at low RPMs - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 12 (permalink) Old 04-14-2013, 01:02 PM Thread Starter
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Bike surging on acceleration at low RPMs

Recently my bike starting surging on acceleration at lower RPMs. It doesn't happen all the time, but it usually happens after I've been riding a while. It will sometimes even backfire. It feels like the engine is suddenly starved for fuel. After I get above 3k RPMs it will smooth out. The tachometer will jump when I rev it in neutral. I took it by a guy I know who works on bikes and of course it wouldn't do it for him when he rode it. Anyone experience anything similar.

95 Vulcan 750 w/10k miles.
No mods.
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post #2 of 12 (permalink) Old 04-14-2013, 01:35 PM
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You may have what we call POOGs. In other words, the gas tank is not venting properly, causing a negative pressure which causes the fuel to quit feeding through the petcock. Symptoms are only apparent when the tank reaches the half empty point. Short term resolve is popping the gas cap for a few and the bike will run normally until neg. pressure builds again.

Some things to check:
1.) Blow thru the vent hole under the gas cap to check for obstructions
2.) Disassemble the gas cap and clean. Any blockage here can cause ventilation issues.

Look HERE for a complete thread on this issue.
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post #3 of 12 (permalink) Old 04-14-2013, 03:23 PM Thread Starter
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Originally Posted by blazblu82 View Post
You may have what we call POOGs. In other words, the gas tank is not venting properly, causing a negative pressure which causes the fuel to quit feeding through the petcock. Symptoms are only apparent when the tank reaches the half empty point. Short term resolve is popping the gas cap for a few and the bike will run normally until neg. pressure builds again.

Some things to check:
1.) Blow thru the vent hole under the gas cap to check for obstructions
2.) Disassemble the gas cap and clean. Any blockage here can cause ventilation issues.
I read that, but it happens when the tank is full too. I tried adding SeaFoam to the tank. We'll see if that helps. but I'm not to optimistic.

Last edited by BigB; 04-14-2013 at 03:27 PM.
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post #4 of 12 (permalink) Old 04-14-2013, 05:20 PM
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replace the vaccum line from petcock to carb also...it could have a small leak and could cause that....also check the carb to head boots...

Surging however, often involves the floats...try doing a backblow, stickied in the carb section.

Could also go with a non vacuum petcock...Yamaha TW200 is a bolt on, just need to get a wye or a tee as it only has one outlet. Cost me $14...



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post #5 of 12 (permalink) Old 04-14-2013, 10:17 PM
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carb vent hose going to the right ear will cause it too. between your carb should be two hoses that go to a t, then from the t will route up and into the back of the right ear. pull the hose from the ear and snip it at a 45 degree angle and insert back into ear.

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post #6 of 12 (permalink) Old 04-15-2013, 12:20 AM Thread Starter
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Quote:
Originally Posted by wib714 View Post
carb vent hose going to the right ear will cause it too. between your carb should be two hoses that go to a t, then from the t will route up and into the back of the right ear. pull the hose from the ear and snip it at a 45 degree angle and insert back into ear.
What does cutting the vent hose at an angle do?
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post #7 of 12 (permalink) Old 04-15-2013, 06:18 AM
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It ensures that it doesn't bottom out and block the air flow completely.

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Leave it to me to NOT be fond of something then end up banging it.
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post #8 of 12 (permalink) Old 04-15-2013, 09:44 AM Thread Starter
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It ensures that it doesn't bottom out and block the air flow completely.
I'm having a hard time picturing that. I can see the T from the side, but I guess I'll have to get the seat and tank off later.
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post #9 of 12 (permalink) Old 04-15-2013, 10:27 AM
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In some cases, like in mine, this line connects to a box under the fuel tank.

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post #10 of 12 (permalink) Old 04-16-2013, 09:54 AM Thread Starter
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Originally Posted by Wolfie View Post
replace the vaccum line from petcock to carb also...it could have a small leak and could cause that....also check the carb to head boots...

Surging however, often involves the floats...try doing a backblow, stickied in the carb section.

Could also go with a non vacuum petcock...Yamaha TW200 is a bolt on, just need to get a wye or a tee as it only has one outlet. Cost me $14...

You're the man. There was a tiny crack in the vacuum line on the rear carb. Replaced the line and the bike runs better than it ever has. At some point in my bike's life it probably sat for a long period of time. Most of the hoses and boots need to be replaced.
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