T500 with vm34 Mikuni

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A.Holley
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T500 with vm34 Mikuni

Post by A.Holley »

I am thinking of putting back on my set of aftermarket type vm34's.
I ran them for years when racing so was never really concerned about fine tuning in the mid range, just made sure it would handle a big fist full of throttle.
Would like to get some input from you guys on what sort of jetting you have found works well for street use.
Current set up is short intakes, mild port job , chambers and Uni pods.

Thanks for any info.

Andrew Holley
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Re: T500 with vm34 Mikuni

Post by Vintageman »

if you would run the stock air box (at least to get that tuned correct first) I would contact Ivan Performance. He spent many months tuning his Cobra which has the 34 mm. He also, if I recall turned them into "normal" bowl venting style. So if your 34 mms has the same slide cut angle, you could use his custon needle jet, custome jet needle, main jet and pilot. He includes bowl gasket but that is unique to Cobra I think. Sometimes for me chambers won't affect things if jetted well in the first place. And if it needs a change usually's it just the mains. I would think his jet needle and needle jet would give you good midrange. Getting jetting correct takes time and patients when doing it via a butt dyno. Starting close sure helps
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tz375
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Re: T500 with vm34 Mikuni

Post by tz375 »

That raises an interesting question. OEM 34mm carbs on a cobra are very different than aftermarket VM34s, but I wonder if Ivan's kit might work on aftermarket carbs with chambers and no air box with minor jetting changes.

I doubt that the parts in the kit would be idea for such a different set up, but only Ivan is likely to know the answer and it's a question worth asking.
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Re: T500 with vm34 Mikuni

Post by johnu »

Andrew,

I have the same question as you with pretty much the same setup on my GT500. Would be nice to get at least a ball park setting for the 34's. Somebody must be running the same setup successfully.

John.
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Re: T500 with vm34 Mikuni

Post by Vintageman »

johnu wrote:successfully
That's the key to jetting especially street where you need all riding scenarios. I hope someone offers what you guys want. Over the years and various bikes and sleds not common and even though people complain about factory jetting they get you close everywhere any weather.

I would contact Ivan' Performance and say Suzuki2strokes.com sent you. He used to check here now and again.
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Re: T500 with vm34 Mikuni

Post by Zunspec4 »

I am running VM34's on my T500 race engine #1 (very mildly ported). Centre plug heads + Tony Green pipes + 50:50 blend of 100LL AvGas/Premium unleaded + Ign@2.9mm BTDC

Main Jet - 230 to 260
Pilot Jet - 30 to 50
Needle Jet - Q2
Needle - 6FJ6
Needle clip position - Start in the middle
Slide Cutaway - 2.0 or 2.5

As ever start on the rich side and work towards leaner, preferably on a dyno that provides Air/Fuel ratio data as you run it. No magic solution I fear, I now have a bunch of cash invested in Mikuni jets/slides/needles etc. so I can adjust settings as circumstances (Air Pressure & temperature) dictate.

Cheers Geoff
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Re: T500 with vm34 Mikuni

Post by tz375 »

Those numbers make sense and are right in the ball park. I have seen over the years in other people's carbs -

Main Jets: 230 - 300
Slide: 2.0 or 2.5
needle: 6FJ6 (most common) or 6DP4
Slow jet: 30-60
Needle jets: P-8 to Q-6

I like Geoff's numbers as a good starting point.
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Re: T500 with vm34 Mikuni

Post by johnu »

Thankyou Geoff and Richard! That is exactly what I was looking for, a good starting point! I've been around two strokes long enough to know there is no cut and dried setting.

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Re: T500 with vm34 Mikuni

Post by Zunspec4 »

With all the many variations in jets/needles/slides etc. you can have on a Mikuni, a reference chart of their various differences (leaner - richer) would be handy. It just so happens that they are in a book on 2-st tuning I have, not seen anything else as useful on Mikuni carb web sites. I will scan in the relevant charts and post them up.

Cheers Geoff
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Re: T500 with vm34 Mikuni

Post by Zunspec4 »

I loaded the relevant pages to flickr and set up an album.

https://flic.kr/s/aHskEYkL2s

Sample page below
Image


Cheers Geoff
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Re: T500 with vm34 Mikuni

Post by johnu »

Thanks Geoff, that's great!
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Re: T500 with vm34 Mikuni

Post by Zunspec4 »

Hi John,

I nicked it from a book titled "Two-Stoke Performance Tuning" writtem by A. Graham Bell ( ISBN 978 1 85960 619 3) . It is easier to get your head around it if you have the full chapter on carbs. Worth getting hold of a copy as it will give you a sound understanding of 2-st basics.

Cheers Geoff
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Re: T500 with vm34 Mikuni

Post by johnu »

Zunspec4 wrote:Hi John,

I nicked it from a book titled "Two-Stoke Performance Tuning" writtem by A. Graham Bell ( ISBN 978 1 85960 619 3) . It is easier to get your head around it if you have the full chapter on carbs. Worth getting hold of a copy as it will give you a sound understanding of 2-st basics.

Cheers Geoff
Will do :up:
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Re: T500 with vm34 Mikuni

Post by tz375 »

Good recommendation Geoff. A.G. Bell is easy to read an understand.

Thanks for posting that needle and NJ chart, as letters and numbers rise, the orifice increase, so a P6 is richer than P-2 and Q-0 is richer than P-8 and so on.

I always had a problem with needles though because if you don't know the starting diameter or where the taper started, it's not possible to compare one needle to another. A lot of needles are listed in the MIKUNI VM handbook and it's possible to measure every needle that comes your way and build a more comprehensive listing. Of course Mikuni must know the dimensions of every needle but they don't publish that data AFAIK. They used to publish a series of hand drawn charts of needle diameters but haven't seen those for a long time.

With the diameters at fixed points along the length of a needle, it is possible to compare most needles at different points along their length and needle and Needle jet dimensions can be plugged into a spreadsheet to work out percentage changes in fuel/air mix flow. An optical comparator is a cool tool for measuring needles but there are simpler and cheaper tools available.
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Re: T500 with vm34 Mikuni

Post by Zunspec4 »

Hi tz,

When you really need to understand your carb settings because you are chasing a good set-up the lack of Mikuni published data is somewhat of a hinderence. Even the main UK Mikuni agents could not come up with definitive Needle comparisons, AG Bells book is the first time I discovered anything which has these tables.

Cheers Geoff
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