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89 vmax breaks up at 4500rpm
Old 10-11-2018, 01:36 PM is offline
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Join Date: Oct 2018
Location: rockland maine
Posts: 2
My VMax: 1989 grey
Default 89 vmax breaks up at 4500rpm

hi guys my name is Will and I am new to the Vmax for him and new to the v max motorcycle in general. I bought this bike probably 2 1/2 months ago and it wasn't running had been sitting in a storage unit for two years had some dust on it but I have years of experience with motorcycles so knew when I was getting into (or so I thought). Anyhow I was opening a new business in July and didn't have the time to deal so I passed it off to a close friend and really highly trained mechanic in my town who now has a Dyno. after many hours of research and many hours of labor on their part we are all stumped. The carbs have been pulled cleaned properly and reinstalled, all of the coils have been checked for primary and secondary resistance, all spark plugs have been hooked up to an electro tester to test the strength of the spark on all four coil's, The fuel pump has been checked as well as the bike was run with an external 12v pump to rule out the possibility of a sending unit issue, the vboost functions perfectly(side note: when you rev the bike from idle in neutral it whails up to 9000 rpm and the vboost opens smooth as butter but under load it breaks up horrible) we have replaced the CDI with a brand new Dynatech CDI that is for a first GEN Vmax, and my mechanic Bernie has even gone to lengths such as drilling holes in each of the exhaust pipes to measure the independent oxygen levels of each pipe to ensure that each piston is firing the same and it appears they all are… After all of this the Dyno still says that the bike is lacking fuel in the mid range. The bike idles perfectly and runs up to 4500 perfectly but once it gets there it breaks up and does exactly the same thing that it did when I first bought the bike. Even after everything being cleaned and checked and some things replaced it still does exactly the same thing. At this Point we are all a bit lost and starting to take a shot in the dark such as maybe there is something broken within the carburetor that we didnt see while it was apart. if anybody has had an issue like this or has any insight at all it is greatly appreciated. Also I would like to post photos of the Dyno graph so that anyone who knows how to read one can see whats happening.
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Re: 89 vmax breaks up at 4500rpm
Old 10-11-2018, 02:33 PM
D-Max2012's Avatar
D-Max2012 D-Max2012 is offline
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Join Date: Oct 2012
Location: NJ
Posts: 1,177
My VMax: 91 tan
Default Re: 89 vmax breaks up at 4500rpm

Welcome Will

Seems like you're on that long haul up stumble road, like I was..
This may seem long winded, but I figured I'd let you know what happened in my case, just to see if this sounds familiar in yours.

I picked up a used 91 about 6 years ago.
Found that it was fine until around 4-5k in all gears.
It would stumble bad from 4 to 5K then pull hard after that.
I did multiple leak-down tests on all rubber parts and found some age cracks around the v-boost boots. figuring I've located the problem, I replaced ALL rubber boots, yet problem still persisted.
Replaced plugs, wires, even changed a couple of coils to see if it was an issue.. Same thing..
Did everything that was suggested from guys on the forum and on-line.
Broke down each carb for cleaning, at least 10 times.
Even went as far as picking up an Ultra-Sonic cleaner from Harbor Freight. (Used some Simple Green and distilled water mix - 50/50).
I have to admit, it actually removed WAY more crap than I ever did by manually cleaning all parts,, yet problem still persisted.

I did the shotgun cleaning multiple times (found on this forum)- no change.

I wondered why so many people complained about rust buildup in their tanks, so I did extensive research on it. Found that ethanol (E10 gas), draws in water from the air, which collects on the steel walls of the tank, and causes rust particles for form. For this reason, I decided to coat my tank.

Now, back to the stumbling:
Not gaining any better results, no matter what I tried, I decided to try some Seafoam in my tank.. I put 1/2 bottle to a full tank of fuel. Did this 3 times at each tank fill-up.
I don't know what changed, but by the time I used 2 1/2 bottle of Seafoam, I noticed that the transition from 4K-5K was smooth as butter in all gears. It's been that way ever since.
It must have dislodge some particles that even the Ultra-Sonic cleaner couldn't.

Now, once a year, I'll throw a 1/2 bottle in the tank, just to keep things running smooth.

Hope this helps..

Good luck
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Re: 89 vmax breaks up at 4500rpm
Old 10-15-2018, 03:55 PM
caseyjones955's Avatar
caseyjones955 caseyjones955 is offline
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Join Date: Jun 2015
Location: Black Hills, South Dakota
Posts: 2,148
My VMax: 99 Blue
My other bikes: 89 Venture Royale
Default Re: 89 vmax breaks up at 4500rpm

I love Seafoam!

Mine had quite a stumble from 4k but usually cleared up as I got close to 5k. Turned out to have main jets from a Dyanjet kit and they were way to fat! 4k is about where the slides begin to lift the needles (less for a bike with drilled slides), so if the main jets are to big then you might see that stumble around 4k.

There is a very common Dynajet jetting kit out there for VMX and the inlay will direct you to install jets that are way to rich, I'm speculating that there are lots of Vmax on the road drowning in their own fuel for this very reason. Take my speculation with a gran of NA, I think I have the only gen1 in South Dakota haha. They like to be a little on the lean side. At least mine do but I live at 4000' and ride up past 11000', so both my Yamaha V4s are setup a tad lean, and seem very happy that way. Factory main jets are 152.5 IIRC, and I'm running 147.5 with a full Kerker and free flowing Morley intake setup.

If you throw out the modifications done to your bike and your elevation perhaps you can get some other ideas as well as they great one above. As mentioned above various things can cause this, including faulty jetting, which had mine on it's knees.

Did your tech friend mention what size jets and needles were in the carbs now?
99 Yamaha Vmax
89 Yamaha Venture Royale 1300
06 KTM XC-W 250 2 stroke, plated.
06 Audi A4 2.0T 6-speed quattro
96 Lexus LS400
88 Suzuki Samurai crawler (for sale/trade)
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Re: 89 vmax breaks up at 4500rpm
Old 10-15-2018, 05:10 PM is offline
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Join Date: Oct 2018
Location: rockland maine
Posts: 2
My VMax: 1989 grey
Default Re: 89 vmax breaks up at 4500rpm

Hey guys thabks for the info. Casey, makes alot of sense what youre saying but the dyno says its lean in the mid range, not rich although i have been to sturgis twice and both times leaned up my carns while there (on a honda shadow).... the status update is as follows.... bike is back together and i dumped some seafoam in and ran it today for about 25 miles and noticed a difference in that shirt period. as if one of the carbs, or two, came out of it... it can smoothly ascend to 5 or 6 before a full stumble... i am going to see tomorroe if i can get it to improve some more... after a talk w my mechanic... we both said screw it lets try to sea foam and I have seen it work miracle before - seems to be helpig here. must just be one little port in eah carb that gets clogged when the bike sits - probably on account of the carbs being sideways as well. Aaaaaannnyhow once i can get some more improvement (hopefully) I will still have the carbs ultra soniced and new diaphragms, and gaskets and o rings all over. no reason not to if Im haveing the carbs soniced. Ill let u guys know whats happens from here on out but thanks for all the info
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Re: 89 vmax breaks up at 4500rpm
Old 10-15-2018, 07:53 PM
Fire-medic's Avatar
Fire-medic Fire-medic is offline
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Location: Miami Florida
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My VMax: 1992 Red-E Sparkle
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Default Re: 89 vmax breaks up at 4500rpm

In my experience it's been the smaller of the two jet block jets which becomes plugged. At a minimum, carb removal and splitting the four into two pair, left and right, and removing the float bowls, and then the four jet blocks to access the 37.5 pilot jet (#42, carburetor fiche)

to be cleaned.

The jet block gasket sometimes rips when removing the jet block.

Don't forget installing the o-ring (#33) on the main nozzle (#34)



I'm no expert in these carbs like Sean Morley, dannymax, or Captainkyle, but I can share what I've experienced.

With care in disassembly, you can re-use the rubbers throughout, unless something breaks or tears.
"This old thing? Totally-stock! And not runnin' so-well, either!"
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4500rpm, 89, breaks, vmax

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