Carb 30mm from 24mm in a 125 2T

[MattWadz]

Okay, looking through my spares earlier found a what i believe to be 30mm Flatside Mikuni carb from possibly a KMX/KX.

Took the float bowl off to find a 210 Main jet :S, the rest of it seems okay, now i want to put this carb onto my MTX, MTX currently has a 24mm carb and a 120 main jet, so i put the 120 main jet in place of the 210 on the flatside and stuck it on the bike, no start situation for some reason, is the 120 too small for that carb to function? would the MTX run with a 210 in it? i have a selection of jets in my other house from about 130-200 which will fit the flatside.

So would the MTX run with the 30mm carb and 210 main jet (temporarily)

also the idle screw seems to be completely missing from the flatside, would this cause it not to run or would it just make it impossible to idle off of carb settings?

the MTX does have the snorkel removed from the airbox allowing a bit more air and it does have a performance expansion chamber and silencer.

Any ideas?

Thanks


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Pitbike x2, Mini Moto x4, China Quad, 02' Suzuki AP50 > 03' MBK Nitro > 86' Suzuki GP100 > 02' Yamaha Jog R > 57' Honda Lead 100 > 95' Piaggio ZIP Rst 50 > 51' Suzuki TS50x > 83' Honda CB125TDE Twin > 51' Yamaha Aerox > 98' Kawasaki KX60 > 89' Yamaha DT50MX > 80's Honda QR50 > 05' CG clone > 125cc China Pitbike > 91' Honda MTX125 RW-L > 09' Peugeot Speedfight 2 LC LR > Lucky MX 140cc > Apache Tomahawk 50 Quad (current = 2010 Yamaha WR125R

[MattWadz]

https://www.1977mopeds.com/media/catalog/product/5/0/50095-0.jpg

Carb looks like this
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Pitbike x2, Mini Moto x4, China Quad, 02' Suzuki AP50 > 03' MBK Nitro > 86' Suzuki GP100 > 02' Yamaha Jog R > 57' Honda Lead 100 > 95' Piaggio ZIP Rst 50 > 51' Suzuki TS50x > 83' Honda CB125TDE Twin > 51' Yamaha Aerox > 98' Kawasaki KX60 > 89' Yamaha DT50MX > 80's Honda QR50 > 05' CG clone > 125cc China Pitbike > 91' Honda MTX125 RW-L > 09' Peugeot Speedfight 2 LC LR > Lucky MX 140cc > Apache Tomahawk 50 Quad (current = 2010 Yamaha WR125R

[smegballs]

[Kickstart]

Hi

Wouldn't expect the main jet to make any difference to the idling (but a major difference once you open the throttle).

I thought the MTX used a Keihin carb and not sure that the main jet size markings they use are comparable to Mikuni ones.

Is the missing screw for the idle speed or idle fueling? If the first it won't idle and will probably be a bit lean at idle. If the 2nd then probably no chance it will idle without it

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[MattWadz]

i see! so keihin jets are on a different size scale to mikuni ones?

yeah i understand, im not expecting it to idle due to there being no idle adjustment screw in it, just wondering why it wouldnt start, perhaps if i stick the 210 jet back in the flatside il try and start it with that in?

Il give that a go and report back!

thanks guys!


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Pitbike x2, Mini Moto x4, China Quad, 02' Suzuki AP50 > 03' MBK Nitro > 86' Suzuki GP100 > 02' Yamaha Jog R > 57' Honda Lead 100 > 95' Piaggio ZIP Rst 50 > 51' Suzuki TS50x > 83' Honda CB125TDE Twin > 51' Yamaha Aerox > 98' Kawasaki KX60 > 89' Yamaha DT50MX > 80's Honda QR50 > 05' CG clone > 125cc China Pitbike > 91' Honda MTX125 RW-L > 09' Peugeot Speedfight 2 LC LR > Lucky MX 140cc > Apache Tomahawk 50 Quad (current = 2010 Yamaha WR125R

[Kickstart]

Hi

Not sure. Think the standard sizing for jets is based on size, but think Mikuni have also used that system and also one based on flow rate (this is from memory). But think the flow is not 100% dependent on the size, so a slightly different design will flow differently (although I suspect with a reasonable range unless you are trying to fine tune the settings).

Missing idle speed screw is possibly allowing air in bypassing the idle circuit - hence lean. Try blocking the hole off (tape it up for now)

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[bladerunner]

I would deffo start with the original jet fitted to the 30mm carb...at least you know your at a right starting point. Idle screw hole needs blocking off or a proper screw finding for reasons you already know. Also no idle screw means you'll need to open the throttle a couple of mm to start/ tick over. 30mm should be about right on a derestricted/ tuned 125 mtx....I'm running a 38mm flat slide mx carb on a big bored rd125 lc with no real issues a bit fluffy below 1/4 throttle but great after that.
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Current bikes...cbr929, KDX200's, Rd125lc mk2, RGV250's

[-]

Also the airbox can make a huge difference to jetting. Just removing the internal spout on my TZR needs the 230 mains replacing with much bigger 320's.

With this bigger carb, will you be running the standard airbox? If so, you may need to increase airflow somehow. Just drilling one or two pound coin sized holes makes a huge difference.

I know a few people who, rather than changing jetting to suit the weather, cover or uncover holes on the airbox to lean or richen the mixture.
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TZR250 2MA road, TZR250 1KT road, TZR250 2MA race, TDR250, YZF-750R Boost colours.
Jaguar S Type 3.0 V6 Sport R, VW Transporter T5 GP LWB Shuttle 140ps DSG.

[stevo as b4]

Have you measured the bore size of this Mikuni?

I think what you have there is a TM28 or TM32 flat slide series. There is no TM30, but a TMX30 that looks slightly different is more compact and has a power jet system fitted.

If you had a TMX30 I would ask if you wanted to sell it?

Also how are you going to mount the TM28/32 onto your MTX reedblock? The std Keihin 24mm will have a smaller spigot size and I don't think there's any chance of the TM28 fitting which has a 35mm OD spigot.

You will need to find a bigger rubber intake manifold to fit the MTX125 reedblock. Maybe an early CR125 part or look at other bikes that use the same size reedblock.

Otherwise you will need to machine up either an adapter plate for a bolt on spigot or one that you can slide the new carb into with an O-ring seal inside for sealing etc.

[temeluchus]

[dangerousdave]

[Kickstart]

Hi

Looking through a book, with Mikuni main jets the hex head jets are flow rated in cc per minute while round head jets are aperture sized.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[bladerunner]

Spot on kickstart that is how they are measured! And when you get picky....a flow bench is the best option .......and a real necessity if using cheap aftermarket jets....I've seen them 4 sizes out of stamped value!
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Current bikes...cbr929, KDX200's, Rd125lc mk2, RGV250's

[MattWadz]

well i got it too run without the idle screw, and it just revved its bollocks off! il take it all apart and find the right idle screw for it and check the slide goes all the way down before i re install it!
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Pitbike x2, Mini Moto x4, China Quad, 02' Suzuki AP50 > 03' MBK Nitro > 86' Suzuki GP100 > 02' Yamaha Jog R > 57' Honda Lead 100 > 95' Piaggio ZIP Rst 50 > 51' Suzuki TS50x > 83' Honda CB125TDE Twin > 51' Yamaha Aerox > 98' Kawasaki KX60 > 89' Yamaha DT50MX > 80's Honda QR50 > 05' CG clone > 125cc China Pitbike > 91' Honda MTX125 RW-L > 09' Peugeot Speedfight 2 LC LR > Lucky MX 140cc > Apache Tomahawk 50 Quad (current = 2010 Yamaha WR125R