Author Topic: Rough carb setup for 34-35mm carb's on R model ?  (Read 583 times)

muz_j

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Rough carb setup for 34-35mm carb's on R model ?
« on: February 26, 2018, 10:46:02 AM »
Hi All,

I'm looking for some guidance from others running larger sized carb's on their 3XV's.
I'm now at the point with a set of 35mm Mikuni TMX carb's where I'm wanting to get their jet and needle settings correct.
As is, they are on default settings from Mikuni - which clearly are not ideal.

I have just been reading the default carb settings from this site (http://pure2strokespirit.net/forums/index.php?topic=329.0) and it's clear that typically the front cylinder is jetted quite differently from the rear cylinder.

I appreciate all of our bikes are setup different, but what would be a good starting setup - or does anyone have any suggestions on good initial settings?
The bike does run as is, but spluttered after a minute or two of warming up and at that point I thought I will pull both carb's off and go through all their settings and make sure things are "normal"

>>> Also how can I confirm the size of a Mikuni main jet (and pilot for that matter) ? - is the number printed on the top of the jet ?
(I have a spare set of different sized main jets and it looks like they do have a number printed into them, but it's very hard to read - pretty sure they're size 320).

I'm guessing I should probably start with jet sizes the same as the stock carb's - as the porting on my bike is stock, or would I want to reduce the jet size given the larger carbs?
Default main jets are 190 rear and 260 front. I have not checked my old ones for a fact, but I'm pretty sure they will be the stock (original) jets that came with the bike.

Will the jets from the Mikuni TM28SS carb's fit my TMX35's ?
I have a feeling the Mikuni jets are pretty standardised, which leads me to think I could swap the old jets over...?

...and I do realise larger carb's with default porting isn't ideal - I'm just kind of working my way through de-restriction of my 3XV R as my budget allows.
Ideally down the track I'll fit better chambers, alter the porting to SP spec' remove the analogue speedo and re-jet the carb's to suit.

Any helpful suggestions appreciated :)
« Last Edit: February 26, 2018, 10:48:58 AM by muz_j »

mellorp

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Re: Rough carb setup for 34-35mm carb's on R model ?
« Reply #1 on: February 26, 2018, 02:22:18 PM »
Think of the V twin as two 125cc engines bolted together. You have to set up each side independently. It's not like jetting a parallel twin

Any engine will only create a certain amount of negative pressure to suck air and fuel thru the carb. Put the same carb on a bigger engine and the negative pressure will be higher, and a smaller jet is required. Put a bigger carb on a smaller engine and you get less negative pressure so the jet needs to be bigger

I'm not sure about the maths but if the carb is 20% bigger then without the proper applied maths you will need at least a 20% bigger main jet.

You also need to appreciate which bit of the carb does what, pilot circuit, cut away, needle profile, emulsion tube flow rate before you even get to the main jet and power jet

The bigger carb will be great for long flat out WOT runs or very fast circuits like the IOM, but on the road you wont get enough air flowing for them to be of any benefit

All jets have the size stamped on them. Some jets are numbered based on the size of the hole, others the flow rate

Std. jetting is here
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Scotty4321

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Re: Rough carb setup for 34-35mm carb's on R model ?
« Reply #2 on: February 26, 2018, 09:21:41 PM »
Hi Muz,
I think your last paragraph answers your own question. There really is not much point in putting larger carbs on a std R.
You'll just make the bike worse and run the risk of siezing it up for no gain. If you need std mains you need std carbs.
Have a good read about the threads on here and you will see that 28mm std carbs are good for pretty decent hp on bikes that are fully derestricted in every other way. 4LO on here has an R to SP spec bike making 60 on a dynopro with 30.5mm bored out std R carbs.
Save your time/effort and money up, get the porting sorted, buy some second hand pipes, pods and a zeeltronic. Get it to a dyno with a tidy 2T operator/tuner (Dave Reynolds is one). Try the carbs then and probably only if you have full race pipes and its a track bike.
I've tried jetting doing plug chops and can say it is not at all conclusive and I probably should have siezed my bike.I was lucky as I didn't have a feel for a lean bike. When I got it to a dyno I went up so many jet sizes I could not believe it.
No one will risk giving you jet sizes as they will not want any responsibility when it goes pear shaped. Start rich and on a dyno that has a A/F ratio reading for 2Ts. Some will not use the sensor. Cheers Scotty.

Warwick

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Re: Rough carb setup for 34-35mm carb's on R model ?
« Reply #3 on: February 26, 2018, 09:59:27 PM »
As others have said, if the porting is stock, chuck the bigger carbs back in the box for later. Once you've sorted the porting, pipes and ignition and are getting over 60bhp on the stock 28mm carbs, it might (and that depends on the use really) be worth trying the bigger carbs.

Probably not what you wanted to hear. But it is the sensible way to proceed.
« Last Edit: February 27, 2018, 01:30:08 AM by Warwick »
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muz_j

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Re: Rough carb setup for 34-35mm carb's on R model ?
« Reply #4 on: February 27, 2018, 05:38:04 AM »
Thanks all for comments - I heard them loud and clear.
I'm still plugging forwards with my 35's - as given the theory - if they are setup properly they should work just fine. The original carb's on my bike would need a major overhaul before I used them again. I prefer to move onwards with the new bigger ones.
It's likely the low down response won't quite match the 28's, but they should be able to be jetted safely for road use without the risk of seizure.
I tend to run my bikes a little rich too - for that exact reason.

I'll start experimenting with the jetting :)


mellorp

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Re: Rough carb setup for 34-35mm carb's on R model ?
« Reply #5 on: February 27, 2018, 08:38:18 AM »
It's likely the low down response won't quite match the 28's, but they should be able to be jetted safely for road use without the risk of seizure.
I tend to run my bikes a little rich too - for that exact reason.

I'll start experimenting with the jetting :)

The main jet has no effect on 1/4 to 1/2 throttle openings. That's all down to the pilot jet air screw, slide cutaway, needle and emulsion tube and the TPS setting on your ignition map for smaller throttle openings

When you say moving forwards, the way forwards is SP porting, aftermarket pipes and programmable ignition.

Good luck with jetting the big carbs.

What's actually wrong with your std carbs ?
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maccas

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Re: Rough carb setup for 34-35mm carb's on R model ?
« Reply #6 on: February 27, 2018, 11:27:00 PM »
Murray,

Jetting is a tough one as i'm not sure anyone has used those carbs on a 3xv before.

An added difficulty is that the 35's you have selected use primary air correction instead of secondary air correction on the usual big 3xv carbs, or no air correction at all with the sugo kit fitted. So comparing to 3xv big carbs won't work so well perhaps?

TZ carbs work the same way as the 35's so if you can find someone running those with airboxes fitted you could look at their jetting for inspiration. I'd have thought somewhere in the 350-400 region as a safe start as you aren't running powerjets will be fine.

Pilots probably 22.5-25

All guesses so please don't hold me to that!

Dan


muz_j

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Re: Rough carb setup for 34-35mm carb's on R model ?
« Reply #7 on: February 28, 2018, 06:11:01 AM »
The main jet has no effect on 1/4 to 1/2 throttle openings. That's all down to the pilot jet air screw, slide cutaway, needle and emulsion tube and the TPS setting on your ignition map for smaller throttle openings
When you say moving forwards, the way forwards is SP porting, aftermarket pipes and programmable ignition.
Good luck with jetting the big carbs.
What's actually wrong with your std carbs ?

Yep - thanks - I have the Mikuni carb documentation and other reference info which details which parts of the carb operate at which throttle opening ranges.
In an ideal world I agree with you. I actually have a programmable Ignitech ignition unit (already programmed with a working map for a 3XV), which I'll fit soon'ish, but I'd like to get things running vaguely normally before complicating the scenario.

My original carb's are filthy, which is admittedly minor - but more relevant is the fact that the shop who made my custom carb cables for the TMX35's cut up my original cables into pieces to measure them.
I did politely explain to them how they had f#%ked me by doing that, as it prevents me easily rolling back to using my stock carb's, but it's too late to cry over spilt milk.
I could probably make the new cables work on the old carb's with spacers of some kind, but I had them made a little longer on purpose, to fit the new carb's better.

So I'm not totally screwed if I had to go back to using the 28's, but it would need some work and I'd have to tweak the cables to suit.
I think from my reading on carb setup, that using the 35's is the easier proposition.


mellorp

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Warwick

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Re: Rough carb setup for 34-35mm carb's on R model ?
« Reply #9 on: February 28, 2018, 07:38:51 PM »
I run early TZ carbs on the track bike, Murray, as you probably remember. As Dan says, they are broadly the same carb as the TM35, but I doubt my settings would work very well on your set-up at all to be honest.

If I was trying to set the 35s up on a motor like yours, I'd probably start with a relatively small main jet (280 or so maybe?) and try to get the tickover, take off, and low and part throttle sorted first by experimentation with pilot jets, air screw settings and needle clip position just using the needle and needle jet that came with the carbs. If you can't get it running well from the pick-up to about 3/4 throttle that way then you'lll need to think about richer or leaner needle and needle jet combos as required - and possibly different slides too depending on how it goes.

Once I had it running nicely through part throttle, I'd whack some mains in it that wouldn't allow it to rev out at WOT (400s maybe? - given the state of tune of the motor you are unlikley to blow it up to be honest if you get it a bit lean as it simply won't be working hard enough at the top end) and work backwards from there. You might need to tweak the part throttle jetting at this point too depending on what mains you end up with as main size can sometimes impact at part throttle too,

If you are running the oil pump, you may be fine running relatively small pilots - 15s or 17.5s maybe? You didn't say what jets the 35s came with, but they should be somewhere near to the ball-park at least in most regards really.

Good luck. It will be a bit of work. Satisfying once you get it right though. And you will learn loads about carburation in the process  (-P).

I have to reiterate though, that if it was my own bike, I'd definitley be favouring simply finding some cables and bolting the 28s back on until the motor might actually be able to make some effective use of the 35s  :-\

« Last Edit: March 01, 2018, 12:48:50 AM by Warwick »
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James P

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Re: Rough carb setup for 34-35mm carb's on R model ?
« Reply #10 on: March 10, 2018, 04:47:11 PM »

Will the jets from the Mikuni TM28SS carb's fit my TMX35's ?
I have a feeling the Mikuni jets are pretty standardised, which leads me to think I could swap the old jets over...?


Murray,

I assume you have the "aftermarket" type TMX35 carbs (Part No.TM35-1) bought from a Mikuni agent - not one of the types that was fitted as standard (OEM) to some YZ125s (and perhaps others). As you have likely already discovered, the main jet (4/042) and pilot jet (VM22/210) are the same types as those used in the TM28SS.

I have some experience of the TMX35, but not on a TZR. The throttle slide has a very large cutaway (6.0) and the TMX generally needs a much smaller pilot jet than comparable set-ups using other Mikuni carbs (I think the TMX35 comes with a 15 or a 20 pilot jet...?).
As you probably know, the TMX35 has no atomiser (or at least not one that can be removed or changed) and all adjustments in the quarter to three-quarter throttle range are done by changing the needle and/or clip position. TMX35 needles are all of the 6EN11 series - the only difference between them is the diameter (the taper angles and taper starting points are all identical). I think the smallest/richest needle is 6EN11-52 (diameter 2.52mm) and the largest/leanest is 6EN11-58 (diameter 2.58mm). To make fine adjustments in the quarter to three-quarter throttle range, I have found it necessary to change both the needle itself and the clip position at the same time - changing just the clip position or just the needle is too coarse an adjustment. I thus found it necessary to have several different needles to hand.

Years ago when I was trying to get a TMX35 set up, I made up a table showing the needle diameters at different reference points (and clip positions) as a quick means of determining the most suitable needle and clip position to try next, based on whether I needed to go richer or leaner at any particular throttle opening. If you are having any difficulty setting up the carbs and want a copy of the table, let me have your e-mail address by PM.

Regards,
James

Hoatso

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Re: Rough carb setup for 34-35mm carb's on R model ?
« Reply #11 on: April 01, 2018, 12:28:21 AM »
This book is old, but good. Start with good pistons, rings, plugs, air filter, reeds, no leaks, etc. before tuning. Change one thing at a time. Better fuel, like 100 octane av gas or 108 octane VP C12 will give you more safety against detonation. The philosophy is to make an acceleration run, get data with a stop watch, and log everything - timing, jet sizes, etc. in a book. Make a jet change up or down, make a run. Continue. Learn how rich and lean sound. Note that different jets affect different THROTTLE OPENINGS not RPM'S.

Air density (temperature, barometric pressure, altitude, humidity) will affect jetting. It seems like the C12 fuel is affected less by weather than pump gas.

Most guys adjust the main, needle clip, and idle screw. I found that trying different needle tapers also gives you big improvements.

If you have a programmable ignition, you can adjust power valves, timing, and power jets or variable air jets

Watch your oil mixture, as increasing oil in premix will lean your mixture. And, too much oil can make you run hot.