Mikuni TMX 30 re-build help.

Twangled your powervalve grommit in your woodruff key? ask someone how to fix it here
Post Reply
stevo135+
Light Smoker
Posts: 47
Joined: Tue Sep 01, 2015 1:23 pm

Mikuni TMX 30 re-build help.

Post by stevo135+ » Sun Dec 13, 2020 6:50 pm

Hi evening all.

I just wanted to ask if anyone here knows someone who would be able to re-build a Mikuni flat slide carburetor for me to work on my Kawasaki KMX 125?

I think there's parts missing as it didn't run properly and I can't seem to get the float height set properly either. I need a set up that will work as a baseline on my KMX, but I don't know or trust anyone using this carb to help me settings wise.

I'd also like to know if the float chamber can be modified to allow the main jet and pilot jet to be removable with the carb on the bike?

Has anyone got a carb specialist they can recommend please?

Thanks.

User avatar
500bernie
Avgas Sniffer
Posts: 3351
Joined: Mon Aug 02, 2010 10:22 am
Location: Teesside

Re: Mikuni TMX 30 re-build help.

Post by 500bernie » Sun Dec 13, 2020 10:07 pm

You could ask Allen's Performance they may be able to help.
Firecracker Red and Grey C3 300

User avatar
James P
Oil Injector
Posts: 578
Joined: Mon Mar 11, 2013 10:47 am
Location: Sydney, Australia

Re: Mikuni TMX 30 re-build help.

Post by James P » Thu Dec 17, 2020 11:13 am

Steve,

For 'ballpark' settings, you can look at the jetting specifications for bikes which use the TMX30 as standard. These include WR200, DT230 and TS200R. I think the TS125R might also use the same carb (or at least one of the same pattern). Your own jetting will depend on how freely your air intake system flows, which may differ from some/all of the bikes listed above.

The TMX30 uses 622-series atomisers, which are available in sizes N4 to O6 (see http://www.mikuni-topham.de/DEUTSCHSITE ... eite58.pdf).
If your TMX30 is the standard aftermarket version, it should have a 5EL68 needle. If it is a Yamaha or Suzuki OEM version, it will likely have a different needle (DT230 carb has 6DHY40, TS200R carb started with 5EL68 then moved to 6DHY36). There is a good chance that the 5EL68 or any of the OEM needles could be made to work with your set-up if you fit an atomiser of suitable size. Main jet and pilot jet are just common Mikuni types.

It may be possible to modify the float bowl to fit a large plug to give access for changing jets - there is a boss cast in which looks like it would accept such a plug. However, you would likely have to take the plug from another carb and then bore and tap a hole in the float bowl. This may get expensive unless you can do the work yourself.

Do you have any idea which parts may be missing from your carb? It may help if you post a photo of the carb dismantled with all parts laid out neatly. Unfortunately I couldn't find an 'exploded' view of the TMX30 (or of the similar TMX27) on the internet, but one may exist. Topham in Germany is usually a helpful source of info on the less-common Mikuni carbs and seems to stock a lot of parts which others don't.

Regards,
James

stevo135+
Light Smoker
Posts: 47
Joined: Tue Sep 01, 2015 1:23 pm

Re: Mikuni TMX 30 re-build help.

Post by stevo135+ » Sun Dec 27, 2020 7:52 pm

Hi James.

A huge thanks for such an informative reply. I unfortunately had forgotten to check in here as the last few weeks up to Christmas were manically busy at work.

I think I've got it sorted in that my TMX30 8006 which is the aftermarket carb and purchased from Allen's originally should be ready to fit and hopefully run better than it did before. At one point I was really dissolutioned with it all, as I couldn't find a directly comparible bike to use for reference against mine.

I get the whole need for jetting changes when you have a modified or tuned bike, and if you bolt on different intake or exhaust systems etc. But how people select a different carb from scratch and decide to use it or install it on a different engine with no reference point or baseline is above me. Those who can take X carb and make it work on Y bike must have some magical skills or just a ton of experience or both.

Anyway I found that all the bikes that had a TM30 carb were very different in every way to mine. Ive found out that the TM30SS that's used for example on the Suzuki RG125F has pretty much no compatible parts with the TMX30 8006 that I'm using. Everything from the carb body, slide, jet holder, needle jet fitting and thread is totally different and nothing fits.

The Suzuki TS125R and TS200R have a much more similar carb to mine but lots of fittings and breathers as well as the jetting is different to mine.

I was missing the rubber O-ring and the little flat washer that goes behind it on the pilot air screw. I could have mislaid these parts the first time I took the pilot screw out as they are tiny. I've also gone down to a 190 main jet, and a 25 pilot jet. I think I had a 30 or 35 pilot jet in it before. I've left the power jet as a 70 size. I understand that is some overlap between the main jet and power jet size for wide open throttle operation, so there's probably not one ideal size for both jets, more a case of they over lap each other and just have to add up to right fuel delivery combined together.

Hopefully I'll be able to try it on the bike this week and see if it runs better and if I can get an idle facility too.

I'm still in the back of my head worried about the possible high compression ratio, but I know that the piston doesn't hit the head and I didn't hear anything from the first time I had the bike running. Its just got alot of compression and ideally the head should have had the squish band properly machined out to 56.5mm, which I don't think would actually lower the compression ratio significantly anyway.

I'll report back once I've had it running and I'd like to take it round the local industrial estate to get it to normal running temperature so that I can re-torque the head once it's cooled down.

How long do you need to run the engine with a new plug to get some idea of plug colour? I don't want to jet it to perfection I'd like to see signs of it being very safely rich on the part throttle circuits. My idea is that would allow me to get 1-2hours run time on the engine before it goes off to the dyno.

I just hope that when the corona virus restrictions allow the dyno centres to open that the place I have in mind won't be too busy to fit the bike it for setting up.

I've heard of many two stroke tuning firms having months of backed up work and long waiting lists for customers to either book bikes in or have work done. It's just supply and demand working I suppose.

User avatar
JanBros
Avgas Sniffer
Posts: 3200
Joined: Wed Jul 26, 2006 11:50 pm
Location: the land of Francorchamps
Contact:

Re: Mikuni TMX 30 re-build help.

Post by JanBros » Mon Dec 28, 2020 1:02 am

reading plugs these days with ethanol in the petrol has become pretty useless. even very very slight grey is normal with these pfuels.
and you can't just run arround for an hour and check the plug, as you will only diagnose the last minute or so (or even less) of running. you can only check colour when running long enough with one throttle opening and than check them : then you might learn something from the carb-setting for that throttle-position.

and certainly do not look at pitures from decades ago to compare plug's !
My ultimate goal is to die young as late as possible !

stevo135+
Light Smoker
Posts: 47
Joined: Tue Sep 01, 2015 1:23 pm

Re: Mikuni TMX 30 re-build help.

Post by stevo135+ » Mon Dec 28, 2020 6:25 pm

Thanks Jan.

That's what I've been hearing for a long while now about modern fuel ie unleaded and I don't know if the 5-10% ethanol that's supposed to be in it affects things even more?

It could be even harder to get a meaningful plug colour reading in my case as I'm running 300ml of octane booster with 4.7L of super-unleaded petrol which I'm thinking still gives me a total 40:1 ratio when mixed with 125ml of oil?

I've read somewhere too that AFR sensors are not always accurate on two strokes at lower engine speeds as alot of the fuel goes straight out the end of the exhaust, but when in resonance the air fuel ratio at the tail pipe is very different because alot more of the fuel air mix is pushed back into the engine?

All I want to do is put an hour of running time on the new piston over a couple of sessions or so, but with the engine under some load instead of stationary heat cycles on a stand which probably doesn't help much. If it's rich enough not to foul a plug before it warms up then that's fine as long as it isn't too lean.

The next thing I need is for the snow to melt as I can't even get into the shed let alone ride the bike at the moment. :lol:

stevo135+
Light Smoker
Posts: 47
Joined: Tue Sep 01, 2015 1:23 pm

Re: Mikuni TMX 30 re-build help.

Post by stevo135+ » Mon Dec 28, 2020 6:41 pm

I had a tuned 100cc scooter once, and fitted a bigger carb. (I should have but still didn't learn my lesson after that experience).

I had it jetted very rich at idle and up to 1/4 throttle but it wasn't far off on the main jet I remember the dyno place telling me.

But it was so rich low down that it would splutter really badly and struggle to rev. I assumed that I needed a smaller main jet, so I went down 2-3sizes and then fitted a new plug. I took it for a 3-4mile ride and it was only when I went down a slight hill that the engine was able to climb into the power band. I kept it pinned as I thought that would give me a better plug reading.

When I stopped and got off I could feel the heat coming off the exhaust and all the clear coat had burnt off the exhaust and it had gone black. It really stank of a burning smell too. When I got the plug out it was a white Ash colour, where up until then it had been oily and black.

I only knew what was going on when the tuning centre explained it that I was probably running so rich low down it was affecting the fueling at high rpm and stopping the bike revving before it could make any power. But by fitting the smaller main jet I had gone lean on the top end and it was just enough to allow the engine to get into the power band where it was then running really lean and very hot.

I kind of gave up reading plugs after that, but I suppose its experience you need to sense a lean or rich engine condition and what's going on from things like throttle response, sound and feel.

User avatar
James P
Oil Injector
Posts: 578
Joined: Mon Mar 11, 2013 10:47 am
Location: Sydney, Australia

Re: Mikuni TMX 30 re-build help.

Post by James P » Fri Jan 01, 2021 11:19 am

Steve,

It should be easy enough to set the jetting 'safely rich' just so you can ride the bike around to do the running-in. As you probably already know, the various 'setting parts' in the carb work in different throttle opening ranges. There is of course some overlap, but you still should be able to get something workable:

Up to quarter throttle - Pilot jet, air screw setting, throttle slide cutaway
Quarter to three-quarter throttle - Needle and clip position, needle jet (also called atomiser or emulsion tube) size
Three-quarter throttle upwards - Main jet and power jet

If you mark your throttle grip with five positions (closed, quarter, half, three-quarter & full), you'll be able to ascertain which parts to change to make it richer (or leaner, if required) at the various openings. You can probably get the jetting close enough to ride the bike just by changing the pilot jet and needle position. There is probably little point using full throttle during running-in...just make sure the main jet is plenty large enough (I'd suggest drilling out an old main jet to about 3mm - that way the engine should immediately start to bog down when you get to about three-quarter throttle, so you will know it isn't lean!).

However, setting up the carb properly will require some investment in jets (unless you know someone who can lend them to you). I'd recommend about half-a-dozen pilot jets (in increments of 2.5), a dozen or so 'large-hex-type' main jets and three or four needle jets. Since throttle slides are expensive, you can probably avoid buying any other sizes unless you find that you can't correct the fuelling by changing the pilot jet and/or air screw setting. If you have the TM30-8006, I presume your needle is a 5EL68 - this will probably work OK with a suitable size needle jet and a suitable clip position.
Luckily the TM30-8006 uses the most common types of Mikuni pilot and main jets - I'd recommend buying genuine Mikuni to achieve some consistency. The 622-series needle jets are much less common, but I'm sure Topham can sort you out with these if you can't find a local dealer which stocks them.

Your comments concerning high compression could be of some concern. I'd recommend ascertaining (with some accuracy) your compression ratio and comparing it to the standard figure. There may be some leeway for higher compression, but you could also end up with a melted piston crown! It may be possible to retard the ignition timing slightly to offset the effects of the high compression, but it could take a lot of effort to find the most suitable setting, so you may be better off just 'blue-printing' the engine to standard specifications.
I'd also recommend using an EGT & CHT monitor with appropriately-placed thermocouples. Not only can this be used to help detect weak air/fuel mixtures (particularly using the EGT), but can also be used to avoid engine damage due to dodgy fuel or air leaks later on.

Before going too far, it would be a good idea to 'fix' your missing O-ring and flat washer from the air screw - these should be fairly common Mikuni parts (at least common to all versions of the TMX30...and likely lots of other Mikuni carbs too).
Concerning the power jet; I personally don't use power jets in custom applications, preferring to fit a larger main jet instead. I made some comments about power jets under another topic (viewtopic.php?f=1&t=46872), which may be of interest to you.

Regards,
James

Post Reply