T500 with vm34 Mikuni

[tz375]

Geoff, you are right, lack of data is annoying especially when adapting modern carbs with fixed needle jets and no data on the available needles - in fact not a lot of data on what needles are available say as OEM Polaris or stock Mikuni or for other OEMs.

The old versions of the VM manuals hard some simple charts but no dimensions. SUDCO's site lists a lot of needle dimensions. Stitch them all together and you end up with a more comprehensive list but it's a lot of work so most of us get close enough and learn to ride through the flat spots. If you want to go down a real rabbit hole, try converting a 28mm Keihin to run on methanol - there is no data to work from, so I started with AMAL Z needle dimensions and jet sizes and worked from there. I got close but ended up with some very unusual jets and a 6.0 slide, modified on an angle grinder between races. Gotta do what you gotta do..

I suspect I would end up with a different set up today but it ran pretty well.

[Zunspec4]

Hi tz,

I do feel envious of the modern ability to re-programme the ECU with a new fueling map at the touch of a laptop mouse click, with the dyno able to almost instantly show the effects. One of the pains of carbs is all the stripping down and re-assembly just to change a jet or needle. If it was easy, it would'nt be half as much fun though

Cheers Geoff

[tz375]

On a TZ or a TR, it's really not much of an issue. Dirty and smelly from the fuel, but no big deal and guessing what combo to try next is always fun. On a street triple with a gang of carbs stuffed tightly between two immovable objects, not so much

Of course EFI on a 2 stroke is a whole other can of worms that Rotax and Bimota and others spent a truckload of cash trying to get right. Latest DFI seems to work pretty well on sleds at least and until someone comes up with a good 2 injector design for DFI, we won't be plugging in laptops anytime soon. Except for data logging and ignition timing


Richard

[Vintageman]

You guys are motivating me to put 34 mms on my 1975 T500, I'll use early air filter (have the later one now).

http://ivansperformanceproducts.com/t500.htm" onclick="window.open(this.href);return false; If it works as he states, worth the coin to me and he gives you extra jets to tweak as needed. I have chambers but from what I recall I only had to up the mains a step or two when I put them on and all else is stock (early cylinders).

I sure Zunspec4 gave a good starting point and tuned for pods

Any idea what Mic 34s to start with that fit a close to stock as possible

Thanks.

[GT-Karl]

This may help a bit too!


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[GT-Karl]

Or this, with some more common used needles for the VM series

https://imageshack.com/a/img924/5112/PfUjgc.gif" onclick="window.open(this.href);return false;

[Vintageman]

The latter part (tip) of the needle can be compensated for with main jet size. However the early part (1/8 to 1/2 throttle) of the needle not too many option as shown. What I find frustrating is you can't find round slide with different cuts which would help this area. Suz carbs don't seam to be standard (length for example). Am I wrong? Of course you can a take a round slide and make it a steeper cut but, can't put material back on too easily. Somewhere I saw "stuffers" that would help reduce turbulence by filling in cut area. They were sized per cut angle as well but can't find them anymore. Ideas?

The Cobra Carb has some weird jetting values IMO. Some of that has to do with Homo pressure maybe, but still. Getting a standard 34mm carb to work street looks like a challenge. I do plan to call Ivan to see what he thinks... the Cobra may be unique versus today's 34mm Mikuni so Ivan's jets may not fit.

The 32mm works pretty well, not sure what I would gain. Need to check cylinder window size to see if it has sufficient area (I am sure someone knows). But so far a little more dia has helped and good mod.

Fun Stuff

[tz375]

I believe you are thinking of these: https://thunderproducts.com/shop/u-f-o- ... optimizer/" onclick="window.open(this.href);return false;

[Vintageman]

Those are even fancier than the ones I recall.

Pages says
""**When installing UFOs, the pilot jet must be reduced substantially (usually about 50%, sometimes even more). ""

Closing up that space must provide much stronger pilot signal as the slide lifts and you get to the second pilot hole. And, then as the needle jet is initially exposed.


Thanks Tz... some more Fun Stuff to check out

Check this out -
http://www.farmersracer.com/?p=1149" onclick="window.open(this.href);return false;

and it references this
http://www.siue.edu/~rsutton/SR/carbmod.htm" onclick="window.open(this.href);return false;

which kind of explains why pods (air box mod) is sometimes more than tuning just main jet for enjoyable street riding.
It explains a "trick" to make a lean cut slide richer... hmm

[ja-moo]

Closing up that space must provide much stronger pilot signal as the slide lifts and you get to the second pilot hole. And, then as the needle jet is initially exposed.

Actually i think the actual effect is smoothing the turbulence caused by the double lips on the bottom of the slide. Lips make for nasty turbulence.

[ConnerVT]

Things you find on the Internet: A Graham Bell - Two Stroke Performance Tuning.pdf

[Vintageman]

https://www.google.com/patents/US6082711" onclick="window.open(this.href);return false;
http://www.xs650.org.au/UFO%20Installation%20Guide.pdf" onclick="window.open(this.href);return false;

[A.Holley]

After much mucking around, I have actually ended up with pretty much the standard TR500 settings.
280 main, Q0 needle jet, 6FJ6 needle, 35 pilot.
I think buying a new pair of needle jets helped with things, as I believe the original ones had worn bigger over the years.
As for air filters, I have ditched them. I never go near dirt, so am now just using mesh domes over the intakes. ( House brick and pigeon strainers)
It now seems a much happier engine.
I guess it's pretty logical that this setup works, as my porting is retry close to what the TR ran.

Andrew Holley

[dgoodsy]

Good carb info in this thread, taught me a bit more about how much I don't know. Glad you got things figured out A.Holley.