Author Topic: Jetting guru needed  (Read 2245 times)

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Offline Graeme M

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Jetting guru needed
« on: August 25, 2008, 08:14:03 PM »
Took the TM125 out on the weekend to run it in and have a go at setting up the jetting. I've gone from the standard 26mm carb to a 34mm. The RM125S has basically a hopped up TM engine in it, and it runs a 34mm carby, so I started out with the new carb jetted pretty much the same as one of those.

Riding it tho left me confused. It started, idled sort of, ran through the rev range and revved out. No obvious nasties. But, it wasn't very... good. So just wondering if anyone has any suggestions based on the following observations.

Idle was a little unsteady, and screwing up the idle a bit resulted in a big increase in revs. Best setting on the air screw was at 1/2 to 1 turn out.

Response from low to mid seemed fine if uninspiring (I guess it IS a TM125), but the transition through mid was marked by a slight miss and hesitation. Not a bog as such, more a slight lag/hesitation.

Mid to top was... flat. It revved out, but skowly and without any real hit and felt dull. Didn't seem to crackle right at the top.

I didn't have room to really wind it out, and also didn't do a proper plug chop. But on checking the plug after the last few laps of full throttle in 2nd and 3rd, it is chocolate brown and dry.

My guess. Lean down low and rich up high. But could easily be convinced it's lean up top too.

The bike is running a Circle F pipe, Uni Filter sock type filter and no airbox, and it is ported. Would identical jetting to a stock RM125S with stock pipe and airbox be close enough?

Anyone got any thoughts? I have no chance of any more tuning runs before the DT Champs, so any changes now will have to be tested in a couple of laps of practice on the day...

Offline Freakshow

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Re: Jetting guru needed
« Reply #1 on: August 25, 2008, 08:29:59 PM »
Put the standard carby back on.  From my short experiance with these TM, it seems they just like everything normal, you screw with normal and they can be the biggest pain in the arse to run right.

I struggled with mine for 4 race meeting, before i discovered it was not the original carby, i put the right one on it and the jets from the book and ringa ding off it went.

Although i still think its slow, but at least it doest die in the arse or do that surging thing it was prior to being back to Stock.  Just a thought.

You can always play with it after the NAtls.

Sorry just read it again and that s what my TM did with the wrong size carby, it would also run then die off like it was flooded when you had it cracked open, i have about 50 jets and near through ever one and pilot at it and it sent me spastic and about a week from selling it.  Somehere sent me the specs which is where i realised it was only one carb up and about 10 jets out, these TM like lean for ex my 250 MX yam is on 270-300 MJ the TM only wanted 180-200 - its not even in the same ballpark.

IF your on one last try on that carb maybe jet back to stock MJ an PJ and see if you get some kinda response, but id be looking to Richen up the old stock 26mm and maybe in a few weeks test a 28 or 30 if your able to borrow one.  But i agee with Doc on this one thats a BIG move on a carby no matter how big its ported out to its still only really 125cc of volume able to pass in one charge
« Last Edit: August 25, 2008, 08:48:14 PM by Freakshow »
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Doc

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Re: Jetting guru needed
« Reply #2 on: August 25, 2008, 08:31:14 PM »
34mm on a TM125 sounds' to me' a little large Graeme. The S motor is a TM bottom end but the top end is where the difference is, it flows. The extra transfers and shape of the ports make all the difference. I've never tried a 34/32 on a TM but I believe Don Thornton runs a similar setup to you on his sickman. Might pay to drop him a question or 2. The best I've had my stock TM125 (matched ports etc. but no porting) running was at Coffs where I ran a 'brand new' RM125M 28mm carb (essentially a 26mm TM carb reamed out and rejetted) It went like a cut snake (for a TM that is) and surprisingly this was running on ol' crappy 91 octane. I believe if you really had the TM ported to the hilt a 34 would/could suit but I just don't feel the TM125 can handle such a big carb with it's limited flow. Admittedly you'll be running flat out most of the time but I still kinda think it's a little big, doesn't help your problem I know as you wish to sort what you have but I feel a jump from TM's 26mm to 34mm without the extra ports of the M or S is going to be very very difficult to dial in. You may have to totally forsake bottom end to get the performance up top which again, is essentially how an S is..as you'd know, they don't do diddly squat till they get to 10000rpm and by 11000 it's all over ;)
« Last Edit: August 25, 2008, 08:41:11 PM by Doc »

Offline Freakshow

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Re: Jetting guru needed
« Reply #3 on: August 25, 2008, 08:37:32 PM »
HEy doc how hard is it to put those transfer into a TM barrel ? or cant be done ?  i cleaned the barrel out, smoothed it and rounded flow etc... it  whet cleaner but not really what i would call arm ripping stuff for a 250.
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74 Yamaha  flattracker's
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For sale -  PRE 75 Yamaha MX stuff, frame, motors and parts also some YAM DT1,2,A and Suzi TS bikes and stuff

Doc

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Re: Jetting guru needed
« Reply #4 on: August 25, 2008, 08:47:26 PM »
That's out of my scope Freaky, I dunno. Big Lozza would be able to tell you if possible but from memory, no it can't really be done but yes there is an alternative port method that could/maybe?? be run. I'm neither brave enough or rich enough to try it but for mine with a little tuning the stock TM doesn't go that bad. I'd like to give it another run at dirt track with a my little hindsight cause I reckon it could have gone better. Even with my lightweight the smallest possible rear tyre is the go, TM125's hate being overtyred, obvously it saps power if the motor can't spin out as freely as possible. It's weird freaky, I rode Dodgee's stock TM250 on it's maiden voyage and this bike picked me up my first ever DT race win. It out grunted Tony Cullens Bultaco 250 which is a pretty mean feat and I would not have thought possible. It rode exceptionally nice and again with a little hindsight and experience or a quicker rider it could go a lot harder. Another guy rode this bike a Connondale and loved it also..it's stock as a rock but she definatley doesn't lack any neddies ;)
« Last Edit: August 25, 2008, 08:58:13 PM by Doc »

Offline Graeme M

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Re: Jetting guru needed
« Reply #5 on: August 25, 2008, 08:59:12 PM »
Yes, it could be that the 34 just isn't going to cut it. I could I guess throw the 28 off the RM onto it. But I would like to see how it will run if I can get the jetting on the 34 right.

So I come back to my real question, which is, does it sound rich, or lean? And yes, I know that I really should do proper plug chops n stuff...

Offline Freakshow

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Re: Jetting guru needed
« Reply #6 on: August 25, 2008, 09:10:36 PM »
both. its got too much jet and loaded up on the way, but then its got to much volume of air so its lean, but the ratios are up the shit so youll never get the balance that it wants of air to fuel. IMHO  unless you stumble across the right balance of all 5 parts of the carby you could be there for months. you will need to find someone who has got this set up to actually work and steal from them, or go to plan b, till you can find the time to do all the variations.  HAve you tried the smallest cutaway slide you have and noticed any changes ?

from my little experiance on these ponies these piston port thingos seem prity unforgiving on ratio's it needs to be spot on and im no little jap with a slide ruler, plain ol stock seems to be right everytime ?  Reeds are not so, they seem to balance out my 2 thumbs
« Last Edit: August 25, 2008, 09:18:11 PM by Freakshow »
74 Yamaha YZ's - 75 Yamaha YZ's
74 Yamaha  flattracker's
70  Jawa 2 valve speedway's

For sale -  PRE 75 Yamaha MX stuff, frame, motors and parts also some YAM DT1,2,A and Suzi TS bikes and stuff

Offline holeshot buddy

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Re: Jetting guru needed
« Reply #7 on: August 25, 2008, 09:14:49 PM »
pilot jet to lean, mixture screw should be one and a half turns out
go up 5
needle sounds to lean richen clip up and try
richen up main jet about 10
bigger carbs on smaller engines do work but need a lot of time spent getting right
go up 10 on main up 5 on pilot
set needle clip in middle and try
if it feels a bit flat in midrange drop clip
if you drop clip all the way to bottom and it feels better
you will need a richer needle
and visa versa for a lean needle
slide cutaway is another story
but you would not want too much on a 125 ;D
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Offline Graeme M

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Re: Jetting guru needed
« Reply #8 on: August 26, 2008, 06:23:06 AM »
Thanks guys. HBuddy, sounds about right to me, I'll give that a go. We'll see if it'll run any better than with the stock carb once I get the jetting pretty right. But I do take the point about the possibility it's a bit too much. Maybe should have just bought a 30mm. I guess I can do more experimentation after the Nats and get bit sorted one way or the other. Or just buy a Honda...

Offline Noel

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Re: Jetting guru needed
« Reply #9 on: August 26, 2008, 08:14:24 AM »
Hi Graeme,
have a read of this, may help

http://www.eric-gorr.com/carb%20cleaning%20and%20jetting.pdf
tuning starts at the bottom of page 5


( by the way my piston port Honda won't idle ( to rich) but goes like stink on pipe )
cheers
Noel
« Last Edit: August 26, 2008, 08:18:34 AM by Noel »

Offline Graeme M

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Re: Jetting guru needed
« Reply #10 on: August 26, 2008, 09:24:30 AM »
I don't think my TM will ever go like stink, but we'll see how it shapes up in a couple of weeks. I'd be happy if I could beat a couple of those CR125s!

I see Lex is selling his Maico and gonna buy a 77 CR125. I might have to do the same. Buy a CR that is, I love the look of those things and they seem to go pretty good too.

Offline Lozza

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Re: Jetting guru needed
« Reply #11 on: August 26, 2008, 09:40:23 AM »
34 will be fine on a 125. If it revs but doesn't pull hard it's lean, if it won't rev out and blubbers along it's rich. Above all else the plug knows the answers.
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Offline Graeme M

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Re: Jetting guru needed
« Reply #12 on: August 26, 2008, 12:06:20 PM »
Thanks Lozza, that's exactly what I wanted to know. I hate being so hard pressed for time that I can't get to and test things properly. Same as what happened last year. You'd think I'd learn. But I definitely felt it was running lean up through the midrange, I just wasn't sure about the top end.

So I'll jet her up a bit, probably two sizes on the mains, maybe one clip on the needle, and a couple sizes on the pilot and see how that feels on the day.