Loss of Power at High RPM? - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 24 (permalink) Old 05-29-2011, 09:22 PM Thread Starter
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Loss of Power at High RPM?

Earlier today I was out riding, and I decided to see how my '89 VN750 ran at high (6000-ish) RPM's.

I noticed that as I approached this, the bike seemed to falter a bit and lose power. Problem went away as I switched to next gear.

Is this normal? If not, what would be the issue, most likely?

.
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-Az
http://www.asbrand.com

1989 Kawasaki Vulcan 750

Upgrades so Far:
Coastered
R/R relocated
Analog Voltmeter
PFIII Windshield
RAM GPS Mount w/Garmin GPS
Cigarette Lighter Power Adapter
Saddlemen's Classic Explorer Seat
Pro-Tac Driver's Backrest
LED Tail Light
LED Rear Turn Signals
Luggage Rack
Storage Trunk
Engine Guard w/Highway Pegs
Radiator Grill Cover
Turn Signal Relocation Bracket
Saddlebags
Floorboards
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post #2 of 24 (permalink) Old 05-29-2011, 09:28 PM
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well mine a bit newer then yours (2005) but mine has a pretty steady pull all the way up to the readline
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post #3 of 24 (permalink) Old 05-29-2011, 09:52 PM
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Need bigger main jets??

Gordon

1991 VN 750 -"Cosmic Lady" or "Bad Girl"?
Purchased May 16, 2008
Approx.19,300km (12,000 miles)

H-D windshield
Relocated R/R
MF-AGM battery
Fiamm Freeway Blaster horns
F&S luggage rack and engine guard
Kury Offset Hiway pegs
July 13, 2016, Riding on the DARKSIDE now, Classic Radial 165/80-15


TOP TEN THINGS A NEW RIDER/OWNER SHOULD DO. Click on link.
https://www.vn750.com/forum/11-vn750-general-discussion/9127-top-ten-items-you-would-suggest-new-owner-do-his-new-ride.html
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post #4 of 24 (permalink) Old 05-31-2011, 05:34 AM
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I know that my bike is EXTREMELY sluggish when I'm running on one cylinder. This normally happens because one of my jets is clogged up and after a few ounces of Seafoam and 30 or 40 miles in the 6-8RPM zone I'm able to blow the **** out.
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post #5 of 24 (permalink) Old 05-31-2011, 08:10 AM
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Mine used to not be able to get to redline.......All I had to do was turn the air/fuel mix screws out 3.5 turns from seated......and that fixed the problem for my bike......it used to get starved for fuel at 7000 rpm.
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post #6 of 24 (permalink) Old 05-31-2011, 06:45 PM Thread Starter
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Quote:
Originally Posted by BRET H View Post
Mine used to not be able to get to redline.......All I had to do was turn the air/fuel mix screws out 3.5 turns from seated......and that fixed the problem for my bike......it used to get starved for fuel at 7000 rpm.
That just might be it.

Because it runs perfectly the rest of the time.

So...where are those things? I'm a complete idjit when it comes to mechanic stuff.

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.
.
-Az
http://www.asbrand.com

1989 Kawasaki Vulcan 750

Upgrades so Far:
Coastered
R/R relocated
Analog Voltmeter
PFIII Windshield
RAM GPS Mount w/Garmin GPS
Cigarette Lighter Power Adapter
Saddlemen's Classic Explorer Seat
Pro-Tac Driver's Backrest
LED Tail Light
LED Rear Turn Signals
Luggage Rack
Storage Trunk
Engine Guard w/Highway Pegs
Radiator Grill Cover
Turn Signal Relocation Bracket
Saddlebags
Floorboards
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post #7 of 24 (permalink) Old 05-31-2011, 07:16 PM
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Quote:
Originally Posted by asbrand View Post
Earlier today I was out riding, and I decided to see how my '89 VN750 ran at high (6000-ish) RPM's.

I noticed that as I approached this, the bike seemed to falter a bit and lose power. Problem went away as I switched to next gear.

Is this normal? If not, what would be the issue, most likely?
No, this is not normal, your bike should pull steady all the way to redline.

How did you approach 6k rpm - WFO or just sneaking up on it slowly?

Done any mods to your bike from stock?

As to the pilot screw...let me suggest that one turn of the pilot screw has about enough effect on your system at 6k rpm as removing 3 lbs of weight from your bike makes it faster. Technically, yes it's true, but it's so inconsequential that you'd never notice it - and neither will your engine at that speed.

The pilot screw is part of the pilot circuit - it has little to no effect on your engine after about 1/8th throttle.

If at a steady state cruise, your bike is hiccuping or missing a little, it could probably use the needles being raised by a washer or two.

If at WFO it misses noticably in the upper rpms, you likely need a bigger main jet.

If you haven't modified your bike at all and it's doing this, I'd check the condition of your air filters to make sure they aren't deteriorating and letting little bits of foam get sucked into your carbs and plugging up the air jet passages at the mouth of the carb inlet...and then I'd pull the ducts coming from your airbox and inspect those three little passages at the bottom of the carb mouth to visually verify that they're clean & clear, and maybe shoot some carb cleaner down each one for good measure.

Hope this gives you a little more direction as well as explanation.

'98 VN750, Mustang Seat, F&S Engine Guards, Highway pegs, Pro-Tac backrest, Harley 58121-85B windshield lowers, Autometer 3391 voltmeter, ACCT to MCCT, pickup coils at 0.020", Coasters, National Cycle N2220, Vista Cruise Universal, Single Filter Ear Shave, Givi Monokey Sidebag Mounts, Givi E21 side bags, Knifemaker style turn signal relocation, helmet locks relocated.

Last edited by abyssdncr; 05-31-2011 at 07:19 PM. Reason: Grammar...
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post #8 of 24 (permalink) Old 05-31-2011, 09:10 PM Thread Starter
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Quote:
Originally Posted by abyssdncr View Post
How did you approach 6k rpm - WFO or just sneaking up on it slowly?

Done any mods to your bike from stock?
If I am understanding what "WFO" is, then indeed, that was what I was trying.

The only mods were done by Ron (Lance) where he coastered it, and relocated the R/R. A previous owner removed the goat's belly...dunno when though.

Ron put in new air filters a year and a half ago...so I doubt that's it.

Might just be the main jet you refer to.

Thanks!

.
.
.
-Az
http://www.asbrand.com

1989 Kawasaki Vulcan 750

Upgrades so Far:
Coastered
R/R relocated
Analog Voltmeter
PFIII Windshield
RAM GPS Mount w/Garmin GPS
Cigarette Lighter Power Adapter
Saddlemen's Classic Explorer Seat
Pro-Tac Driver's Backrest
LED Tail Light
LED Rear Turn Signals
Luggage Rack
Storage Trunk
Engine Guard w/Highway Pegs
Radiator Grill Cover
Turn Signal Relocation Bracket
Saddlebags
Floorboards
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post #9 of 24 (permalink) Old 05-31-2011, 09:28 PM
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Well.......all I can say is that after I rebuilt my carbs with 40 / 140 jets....I had the air/fuel mix screws turned out 2 turns from seated. I took it out for a test drive. I could not get it to rev in any gear past 6500 - 7000 rpms.......and the backfiring on decel was horrible. I went back to the garage and turned both air/fuel mix screws out 3.5 turns from seated......all backfiring completely stopped and I could then easily pull in any gear to 9500 rpm! That WAS the ONLY change I made.
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post #10 of 24 (permalink) Old 05-31-2011, 10:18 PM
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I had those same jets, but I had 2 shims under each needle. As per Ron.
You can put the shims in just by removing the fuel tank. You are shaven right?

02 honda sabre 1100
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