T500 - Early vs late VM32 carbs

[GreenMachine]

Hey guys,

Complex question, I think...

I know the standard main jetting changed from 97.5 to 150 at some point in the T500 model cycle.

I have a T500R with the short rubber intake stubs and factory-type carbs. Have just pulled them down, and they have 97.5 main jets. I suspect they've been swapped on from a later bike. I've only ridden the bike in the driveway before pulling it apart, so I can't comment on how it ran.

Given that I'm running T500R barrels and intakes, I take it that I should convert the carbs to T500R spec. I've also put expansion chambers on and need to go up a size, so I guess I'm actually looking for #152.5 jets...

My questions:

- Assuming these are later carbs, do I need to replace needles/anything else?

- My T500R Keyster kit came with a #150 main jet, but it doesn't thread in to the carb body properly. I take it they changed thread sizes. Do I just need to go to a shop and get a #152.5 jet with the correct thread size?

Additional info: There's two stampings on the carb body:

153
13

on a raised part, then:

35

on the body next to it. Not sure if that helps in identifying these things.

Hopelessly lost on this. Any advice would be appreciated.

NH

[tz375]

There are differences in the motors and intakes and exhausts AFAIK which also make a difference. On top of that, some came with 188 needle jets which use round main jets and some came with 159 series needle jets and they use Large Hex main jets. Round jets are calibrated in mm so double the size = 4 times the flow. Hex jets are flow rated, so double the number=double the flow (more or less)

I'd suggest that if you have the original carbs, use them as Suzuki supplied them because there are changes in the body and that changes the fuel slope. If they are a mixed bag, you may be better changing to new VM34s.

[5twins]

I agree with TZ375 - you can't even begin to tune your carbs until you know exactly which set you have. The big change in the 500 carbs occurred in '73. That's when the mains and needle jets changed. They went from hex head 150s to round head 97.5s. And, of course, the change in needle jet types that accept these.

You could have whole newer carbs swapped on or just some newer parts added to your originals. You need to find this out. Can you post some pics?

I'm really surprised that these carbs are not more well documented than this after all these years. I ran into the same thing when I got into Yamaha 650s a few years back. There was all sorts of erroneous jetting info out there. I had to collaborate with another 650 enthusiast and sort it all out .....

http://xs650temp.proboards.com/index.cg ... thread=731" onclick="window.open(this.href);return false;

The same could be done for these carbs if anyone was interested. From what I've read so far here, nobody knows ....

But anyway, back to your questions GM - I think you're going to have to rethink your "one size up on the mains" idea. From what I've read here, most go 8 to 10 sizes up, lol. Of course, I can't vouch for that being right.From My experience with carb rejetting, it seems a bit extreme. But ..... only actual testing can tell you for sure.

[argo1974]

Mikuni VM tuning manual: http://www.mikuni.com/pdf/vmmanual.pdf" onclick="window.open(this.href);return false;
You can't simply swap different type of jets due to different thread size. I personally used early type carbs with 150 mains and later carbs with 97,5 mains on my 72 T500J without any significant differences.

Argo

[tz375]

Different threads relate to different needle jets. #188 take large round jets which are size rated so 120=1.20mm (smaller numbers relative to large hex) and #159 take large hex jets which are flow rated where #120 = 120cc/minute flow at a certain pressure, and are larger numbers than large round for the same fuel flow.

Look at the needle jets and buy the correct main jets

[GreenMachine]

OK, so I suck at taking advice. Took out the #97.5s on Friday night (when I could've been out having a beer) and put in the #150 large rounds (188 style) that came with my Keyster kit, then set off at 6am this morning for a long-anticipated ride with a mate on his RG500. Epic f#cking fail (can I swear on Sundial?). Bike wouldn't accept anything over 2/3rd throttle and was just generally a pig. Ended up cruising along at 25mph in 2nd while my mate wondered whether I'd left my handlebar basket and spokey-dokes at home. Then it pissed with rain.

So I say *Myth Confirmed* - the #150 jets that early bikes had were NOT large round 150s.

Have ordered 100, 102.5 and 105s this morning and will experiment with those.

Let this be a lesson to everyone - listen to people who are smarter than you. And check the damn weather report.

[5twins]

Well, yes, didn't it seem strange to you that Suzuki would change from 150 to 97.5 on essentially the same motor? It did to me. That tells me the carbs were changed, or at least significant parts within them (and they were). As mentioned in other recent posts, this isn't rocket science (but it might as well be to some). I.D. your carbs 1st, then you can jet and tune them correctly. I may be able to help, send me a P.M. if you're interested.

[GreenMachine]

Definitely interested - they'll be off the bike again next week and I'll get some pics and measurements.

To be honest, it seemed like a big difference in jetting, but the intake path, air filter and porting are all different in the earlier bikes (like mine), so it seemed correct that there would be bigger jets. I know with tuning Holleys on race cars, the jetting at WOT can need to change significantly with filter, manifold and head changes. I also understand the functions of main jets, idle boosters, accelerator pumps, powervalves etc in auto carbs, but I know nothing about bike carbs or jet calibration other than what I have recently read in this thread (and others on here). Cars also appear to be less fuel-critical than bikes. Even if I put 10 sizes too big a jet in the primaries of my Plymouth, worst case scenario it'd blow some fuel smoke and feel a bit doughy. I've just realised today what a difference the right/wrong jets make to a bike.

I also found a comment somewhere on here where someone had toasted a piston running incorrect jetting and had attributed that to the shorter manifold runner. I'm very paranoid about breaking stuff.

[Vintageman]

I still think it is air/filter box change, longer carb to cylinder distance, and extra casting divider inside intake (? I think this is true) on later models. You see this same sort history on first year Yam RD 350 followed through to RD400. Something like 140 mains to 115 mains. Not as much but all they changed was air filter and then box