Poor behavior at low RPMs - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 12 (permalink) Old 05-12-2019, 07:36 AM Thread Starter
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Question Poor behavior at low RPMs

So a recent introduction thread got me wondering about whether I had bad driving habits since I don't spend much time in the low RPM range, so I paid careful attention to my tach over the past couple days.

I can idle fine at 1k while warming up, and have plenty of smooth power above 3k, but if I spend any time cruising (not actively climbing gears or engine braking) in the 1k to 2.8k, my engine feels like it's anemic and stumbling. There's no way I could do a slow comfortable ride down a country lane at 2.5k.

Are my pilot jets possibly too rich and I'm choking out the engine when the throttle is closed? I would have thought that would cause idling issues, which I don't seem to have.

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post #2 of 12 (permalink) Old 05-12-2019, 08:38 AM
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You could be right, if the pilot gets clogged, it shuts the cylinder down below 2500 rpm. When one of mine clogged I had to crank the idle adjustment way up, since it ran on one cylinder below 2500..

What size pilots are you running?

If it's rich on the pilots, the plugs should be fairly black after a low speed run or just a bit of idling.

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post #3 of 12 (permalink) Old 05-12-2019, 09:52 AM Thread Starter
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I got the bike second-hand, with a fair amount of work done on it (tux mod), so it's possible the jets were changed. Never disassembled them, and hope not to ever have to!

I did increase the idle screw shortly after I got it to keep it from dropping below 1k and stalling when idling. Do the 750s have a separate idle circuit from the pilot? That would explain a lot.

I'll try to do a low RPM circuit around the neighborhood when the weather clears, then check the plugs.

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post #4 of 12 (permalink) Old 05-12-2019, 10:35 AM
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You don't need to dis-assemble the carbs to change the jets, though. That can be done with them in the bike.

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post #5 of 12 (permalink) Old 05-12-2019, 11:12 AM
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The pilot jet feeds the idle circuit, so 1000 to about 2800 rpm. The air screw also affects (only) the idle circuit, but I doubt that's the root of the problem.

Also check the petcock vacuum line to make sure it's dry.

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post #6 of 12 (permalink) Old 05-13-2019, 11:38 AM
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My bike runs well, but cruising below 3k RPM is out of the question for me. 4k-6k are ideal cruising RPM's. This is a high-revving engine that doesn't start making good power until about 4k RPM's. It's just the way the motor was designed. Whoever said you ought to spend time in the low rev range may have misinformed you, since this engine lugs like crazy at low revs.

Barring any issues with running, but if it idles solidly and revs to the top of the rev range smoothly, you are probably lugging it. I'd advise revving it out. I certainly do, almost never letting the revs go below 3k.
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post #7 of 12 (permalink) Old 05-13-2019, 11:49 AM
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My low speed shift point is 2300 rpm, it pulls away just fine from there. Plenty of low-end torque.

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post #8 of 12 (permalink) Old 05-14-2019, 01:38 PM
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On my spirited ride to work, I was shifting at 6K as I pulled away from a stop, settling into a 70 mph cruise at 5k rpm. Once I got into the city, I was shifting around 3.5k. I'm not afraid to dip down to 2300 through a school zone, if I know it only goes up to 30 mph on the other side.

My personal rule of thumb is keep the throttle at or below 1/4 until I'm past 3k. If I need more power than that, I should be downshifting anyways.

Low RPM itself won't destroy the engine or it'd blow up every time it idles. But putting a lot of load on the engine with a low RPM can cause issues because the oil pressure is also low. Higher RPM gives higher oil pressure, and more of a cushion against metal-on-metal contact.
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post #9 of 12 (permalink) Old 05-14-2019, 10:31 PM
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I tend to really wring it out if I have room. These engines like to spin. They are a very short stroke design, and don't have the torque of, say, a Harley Sportster. I have hit nearly 100 mph down a freeway on ramp, shifting at around 6000-6500 at full throttle. Remember this engine has a 9500 redline compared to about 5500 for a Sportster. The short stroke accounts for that. It has to spin to make power. It has more power at high RPM than at low RPM. Cruising down the highway at 75 mph in top gear at 6500 feels just about right. This is NOT a typical cruiser engine. Don't try to compare it to a Honda 750 Shadow.

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post #10 of 12 (permalink) Old 05-15-2019, 10:53 AM
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Quote:
Originally Posted by VN750Rider/Jerry View Post
Remember this engine has a 9500 redline compared to about 5500 for a Sportster.
I've seen you mention 9500 redline twice now. Another guy mentioned doing 9500 rpm in 5th gear. Mine's clearly at 8500. Are there differences over different years? Or is that why you have a parts bike VN750 and just bought another one?

And...how did someone get theirs up to 140 mph without a hill, tailwind and tow vehicle?


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Completed modifications:
Removed mufflers, straight pipes out of GB
110/90 Front Tire
Pick-up coil re-gapped to .020"
Compufire 55402 r/r series regulator
USB Outlet + Voltmeter
Repainted Metallic Blue w/ new tank bages
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