Author Topic: My TS125B jetting specs, eh?  (Read 1505 times)

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Offline John Orchard

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My TS125B jetting specs, eh?
« on: April 11, 2023, 08:13:29 PM »
*EDITED* I just thought I'd bounce my jetting specs off you guys, they seem all over the place to me, after comparing to other 125's.

SETUP

TS125B
RM125A port timing
RM125A pipe
VM32 carb
External flywheel
TS185 electronic ignition, standard TS125B ignition timing.
TS125B head with 1.2mm off the face, head gasket 1.2mm thinner, .8mm squish ....... It's got high comp, maybe too high?

Main jet: 195
Needle: 6DP17 #4 groove
Needle jet: P-8
Pilot jet: 55
Slide: 2.5

It seems to go ok, plug is chocolate brown safe, I just expected a much larger main jet (about 230?) and much smaller pilot jet (about a 35?). I guess I just need to stop second-guessing myself?
« Last Edit: April 12, 2023, 08:49:55 PM by John Orchard »
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Offline PeterC

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Re: My TS125B jetting specs, eh?
« Reply #1 on: April 12, 2023, 08:49:43 AM »
What size and type of carb?. The main jet does seem small. However if it runs good then you are on the money
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Offline pokey

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Re: My TS125B jetting specs, eh?
« Reply #2 on: April 12, 2023, 03:26:13 PM »
Its the pipe, its for a 125

Its not only the amount of air the engine can pump its the velocity of that air in and out. With faster air you can run larger jets for often better performance and that can come from going to a smaller carb. Then you have the pipe that also dictates how much air that engine needs. If you had a size smaller carb  like a VM29 from an ER it may be what you need for that pipe ,Worth a try if you can lay your hands on an ER carb.  Your on the right track though perhaps over carbed" for now" and underpiped for optimum efficiency.  Overall id stick with the 32 carb (I run one on my very modified 199cc PP engine) and build a pipe that suits the engine as thats where its choking and it cant use any more fuel.
Best of luck with it.

Offline John Orchard

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Re: My TS125B jetting specs, eh?
« Reply #3 on: April 12, 2023, 04:42:06 PM »
Thanks Guy's.

The pipe, carb, port timing, and even the piston-port/crankcase reed engine is all as per the RM125A, so I was not expecting the carb or pipe to be mismatched. It is running standard TS125 ignition timing, not sure if advancing or retarding might change things; who's experienced with ignition timing parameters? TIA

Out of interest, the RM125M with a 28mm carb runs a 180 main jet, the RM125S with its 34 mil carb runs a 280 main, and the RM125A that runs a 32mm carb has a 230 main, I was expecting to have to run a 230 but plug was black, hmmm??
« Last Edit: April 12, 2023, 04:56:02 PM by John Orchard »
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Offline John Orchard

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Re: My TS125B jetting specs, eh?
« Reply #4 on: April 12, 2023, 04:52:54 PM »

With faster air you can run larger jets for often better performance and that can come from going to a smaller carb.


I was thinking that "faster air" creates a greater drop in venturi pressure, causing more fuel to flow from the jet?
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Offline pokey

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Re: My TS125B jetting specs, eh?
« Reply #5 on: April 12, 2023, 06:13:34 PM »
Quote
I was thinking that "faster air" creates a greater drop in venturi pressure, causing more fuel to flow from the jet?

Correct John,  with  greater available lift comes more potential volume. It also effects droplet size for better atomisation. (this is why sometimes a smaller carb is better as the air is faster)The key is keeping the charge volume up while keeping the velocity up and thats where the pipes harmonics starts doing it thing so its a matched set and maximising the engines efficiency.

Other things the bike will enjoy is dropping a fair bit of weight off the flywheel as its a heavy sucker and it honestly doesnt need it all. The original was designed that weight to smooth acceleration on road and have a bit of roll on in super tight stuff and i dont think you will be doing much of that.

Offline John Orchard

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Re: My TS125B jetting specs, eh?
« Reply #6 on: April 12, 2023, 08:46:33 PM »
Thanks, food for thought :-)

I have found many times over the years, that with jetting, where you add & remove the power is where you have to add & remove fuel. I'm guessing that because the ports are a little on the small side (unbridged ex port), and not conducive for good top-end power, that the fuel is not in such demand at that rpm?

The 125 flywheel is considerably smaller & lighter than a 185 item, I'm pretty impressed with how quick the revs build, one thing I do have on order is a UFO slide optimizer, they are great, I expect the bike to explode off the bottom-end once that is fitted, I fitted one to the RM125B that I sold to Ted, it won a Aussie Champs, it was a rocket. The slide optimizer will balance-out the larger carb.

But if an RM125M runs a 180 main jet, I'll be stoked if this thing goes as good as one of those :-)

« Last Edit: April 12, 2023, 08:51:42 PM by John Orchard »
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Offline pokey

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Re: My TS125B jetting specs, eh?
« Reply #7 on: April 12, 2023, 10:30:12 PM »
 Seems the only thing your missing is a decent pipe  to start surprising more exotic bikes. 
Blair and Frits Overmars both have good theory to base a pipe on that will out perform any of the old after market or stock chambers and a good place to start.
 You can download a demo software that works for calculations free or you can buy it. ( Direct link so save as and run as admin) 
http://ypvsbox.free.fr/software/PipeDesignerDemoEngSetup.exe
 And you can use either Blair or Overmars calculations.


Offline sleepy

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Re: My TS125B jetting specs, eh?
« Reply #8 on: April 13, 2023, 09:57:02 AM »
Thanks, food for thought :-)

I have found many times over the years, that with jetting, where you add & remove the power is where you have to add & remove fuel. I'm guessing that because the ports are a little on the small side (unbridged ex port), and not conducive for good top-end power, that the fuel is not in such demand at that rpm?

The 125 flywheel is considerably smaller & lighter than a 185 item, I'm pretty impressed with how quick the revs build, one thing I do have on order is a UFO slide optimizer, they are great, I expect the bike to explode off the bottom-end once that is fitted, I fitted one to the RM125B that I sold to Ted, it won a Aussie Champs, it was a rocket. The slide optimizer will balance-out the larger carb.

But if an RM125M runs a 180 main jet, I'll be stoked if this thing goes as good as one of those :-)
What size is the air correcter jet in the mouth of the carb. A small jet in there will bring down the main jet size at the top end and with your rather large pilot it seems it does need a good amount of fuel when of the main circuit. The most important thing is that the plug colour is good and it goes hard.

Offline John Orchard

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Re: My TS125B jetting specs, eh?
« Reply #9 on: April 14, 2023, 01:30:17 PM »
Thanks heaps sleepy, man I love this forum, the 'main air jet' is .4mm, bloody tiny! I'm glad I brought this matter here, I chased my tail with regards 'main air jets' about 25 years ago with my VTR1000 racebike, I shoulda remembered :-)

Not sure how main air bleeds are measured, does a 2.0 run a 2mm hole? On the VM32/34 that I ran on my RM125B, I took the air bleed out completely, and it was awesome!

I might drill out to 2mm, and back with a 230 main jet and see how things look :-) Be back soon.

Ok stand by
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Offline sleepy

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Re: My TS125B jetting specs, eh?
« Reply #10 on: April 15, 2023, 01:24:56 AM »
Thanks heaps sleepy, man I love this forum, the 'main air jet' is .4mm, bloody tiny! I'm glad I brought this matter here, I chased my tail with regards 'main air jets' about 25 years ago with my VTR1000 racebike, I shoulda remembered :-)

Not sure how main air bleeds are measured, does a 2.0 run a 2mm hole? On the VM32/34 that I ran on my RM125B, I took the air bleed out completely, and it was awesome!

I might drill out to 2mm, and back with a 230 main jet and see how things look :-) Be back soon.

Ok stand by

They are measured in mm's. .4 is tiny, that would certainly give it a rich top end. I wonder what that carb was from? The air jet is probably the most ignored jet, especially when new carbs are thrown on during a resto.

Offline PeterC

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Re: My TS125B jetting specs, eh?
« Reply #11 on: April 15, 2023, 08:25:40 AM »
Interesting. Bultacos with vm mikunis are recommended to have the air jet removed as part of the jetting
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Offline John Orchard

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Re: My TS125B jetting specs, eh?
« Reply #12 on: April 16, 2023, 06:47:14 PM »
Well, I've taken the main air bleed out completely, still sticking with 195 main jet, lovely tan plug, revs out with more power now.

Running a 55 pilot, I reckon it needs a 65 (revs high on cold start up, like a split inlet manifold), I guess it must be somewhere near correct if an RM125M runs a 50 pilot and 180 main, even though that is only a 28mm carb.
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