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Mikuni TMX 30 re-build help.

Posted: Sun Dec 13, 2020 6:50 pm
by stevo135+
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.

Re: Mikuni TMX 30 re-build help.

Posted: Sun Dec 13, 2020 10:07 pm
by 500bernie
You could ask Allen's Performance they may be able to help.

Re: Mikuni TMX 30 re-build help.

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

Re: Mikuni TMX 30 re-build help.

Posted: Sun Dec 27, 2020 7:52 pm
by stevo135+
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.

Re: Mikuni TMX 30 re-build help.

Posted: Mon Dec 28, 2020 1:02 am
by JanBros
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 !

Re: Mikuni TMX 30 re-build help.

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

Re: Mikuni TMX 30 re-build help.

Posted: Mon Dec 28, 2020 6:41 pm
by stevo135+
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.

Re: Mikuni TMX 30 re-build help.

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

Re: Mikuni TMX 30 re-build help.

Posted: Fri Feb 12, 2021 4:16 pm
by stevo135+
Afternoon all.

Thanks for the very useful informative replies, and sorry for the long time replying.
Right well first off I sat down with the carb once I had all the parts I needed to re-build it with. I did the following:

1, float height measured and set to the factory 13mm.
2, cleaned and blew through every jet and orifice, then I re-assembled the carb and fitted the missing O-ring to the pilot screw, which I set at 1.5turns this time instead of 2turns out.

3, I've fitted a 25 pilot jet, 190 main jet and a 75 power jet. Before I had a 30 pilot jet, 200 main jet, and 80 power jet installed.

Then I re-fitted the carb and I washed out the foam air filter in pre-mix fuel and squeezed it to get it fairly dry and oil free. I've heard that you don't need to run road bike filters with thick oil sprayed all over them.

After all that and some 40:1 pre mix of 98RON and 3% toluene based octane booster (my trying to do a band aid fix for high compression) I filled the portable test fuel tank and tried to start it.

2-kicks later it's running and after 30sec or so it's sat ticking over at 1500rpm sounding quite nice. I let it warm up for 10min while checking all over the engine, carb, and cooling system for leaks. It seemed fairly smooth and it picks up pretty instantly when the throttle is blipped.

I tried adjusting the pilot screw and found that I have to turn it in so it's just under 3/4 turn out before it stutters and stalls from being too rich. I've backed it out to just over 1turn out so it's still set a bit rich but won't stall at idle or if you open the throttle.

I can't really do much more now without either riding it and getting it Mot'd or just waiting until the dyno is available again for setting it up and installing my ignitec CDI.

I was surprised at how much better it was at my second try and once I'd re-built the carb. I'll try and get to ride it locally when the covid situation allows and then pull out the plug just to see if it's got any colour to it.

I'm interested in people's thoughts about the power jet function on these carbs. I've heard of some people blanking off the power jet and just setting up to use the main jet instead. I can't understand why you would want to do this on a carb that's designed with a power jet as standard or what the advantages would be?

I guess the dyno centre might have some thoughts on the best way to set it up, but really I don't like the idea of not using the power jet as it works on other bikes with these kinds of carbs.

Lastly I've bought a selection of mikuni jets too. I've got main jets from 175-205, 25/27.5/30 pilot jets, and 60/70/75/80/85 power jets all labelled up in a screw organiser case ready to take to the dyno. I did also buy 2 NGK BR10ES spark plugs, but I'm using a BR9ES at the moment so I might try it like this first.

I'm not expecting it to last forever between top end re-builds but if I could reliably get 2000-2500 miles between top end re-builds once it's set up I'd be happy at that. I might look to have a spare head machined properly to get the compression ratio back closer to standard in the future too.

A tuner has told me that with super unleaded fuel in a water cooled 2-stroke that 8.0:1 is really the absolute highest you'd comfortably run before detonation. I would say that mine is closer to 8.4:1 with a 12.6cc head volume not allowing for the piston deck height and dome height.

If they can set it up to run ok like it is, I might just leave it and use octane booster in the fuel if it runs well enough?

Thanks for all the advice and help, and I'll update this when I've been to the dyno to get the bike set up.

Re: Mikuni TMX 30 re-build help.

Posted: Fri Feb 12, 2021 11:29 pm
by JanBros
normaly a powerjet is used to lean out the mixture after peak power by closing the powerjet. it makes the exhaust gases hotter which again make the waves in the expansion travel faster which has the same effect as a shorter pipe : it shifts the useable rpm-range of the pipe upwards giving better overrev.

and about the compression ratio :
is that the standard or correced compression ratio ?
if corrected : that is actualy a useless number that should never be used.

Re: Mikuni TMX 30 re-build help.

Posted: Sat Feb 13, 2021 7:14 pm
by stevo135+
Hi JanBros.

Thanks for the explanation of how the power jet system works and why it's used to alter the high rpm fuelling. I don't know if it makes any difference but my carbs power jet is just a tube fed type from the float bowl and it sprays into the carb just before the throttle slide on the airbox side.

I hear that some power jet systems are electronic and controlled by a solonoid driven by the ECU. I think this is used on the Suzuki RGV250 and h***a NSR250 for example. I don't know if this type of power jet works the same way as mine as they can be switched on and off at different rpm, where as mine I presume will spray fuel whenever you use full throttle?

As for the compression ratio, you were right in that I was quoting it after I'd worked it out from when the exhaust port closes and not the full swept volume of the cylinder. I didn't know that the corrected compression ratio was useless to quote for tuning. I just assumed it was the one to use as it's often what the Japanese manufacturers quote in their specs.

Anyway I went in the shed and found the paper I'd worked it out on to get the compression ratio and I've worked it out to the non corrected figure now:

The head volume including the gasket was 12.6cc, and the piston crown volume protruding into the combustion chamber was 3.4cc. The swept volume is 134cc.

When I work it out it gives me 15.56:1 compression ratio. Is there a figure for the non corrected compression ratio that's regarded as safe to use with 98RON unleaded or does it not really work like that as such?

Re: Mikuni TMX 30 re-build help.

Posted: Sat Feb 13, 2021 9:32 pm
by JanBros
the powerjet I explained is with electronic controled solenoid.

yep, jap's used to mention corrected compression ratio, as that is supposed to be the "real compression", but that's only real in a non-running engine. as in a running engine the compression ratio depends on how much mixture the expansion-chamber has "supercharged" into the cylinder.

when using the normal compression ratio, anyone can calculate the combustion chamber's volume, when using the corrected, you can't without knowing the timing. Maybe the jap's wanted to keep that a secret that way :?: :mrgreen:

15.56 is VERY high, GP-level high. I would say it definitly is too high for road use.
also : with higher compression , more energy is used to push the piston down (good no ?) so there is less energy left in the exhaust gases which again leads to a a pipe that's not working as good as it might be.

total sum might be that you lose power instead of gaining.

Re: Mikuni TMX 30 re-build help.

Posted: Sun Feb 14, 2021 1:41 pm
by stevo135+
It's funny as they quote all the port timings in the service manual so I don't think they be too secret about it, but then it was a lowly road/trail bike not a racer.
What I do like about the KMX and some of the other road going or off road Japanese 125's is that their engines are either influenced by or directly based on the MX bike of that era. I guess it's easier to design a road bike engine by casting new crank cases with provision for oil pumps, rev counters and balance shafts and then just use very de-tuned Motocross style internals and top ends etc. The KR1 to me looks like a Motocross influenced design, with a road race engine bottom end and gearbox too.

Anyway I did think all along my compression is too high, even if it's only 0.5-0.6:1 more than standard. The thing is that the engine seems to run ok at least when stationary, but it's starting it that's the issue. If I set it at TDC and kick it hard it starts easily, but it's not a bike you could stab at the Kickstarter rapidly, and definitely you couldn't hand start it like say a DT125. It had a fair bit of compression before I re-built it and still liked to be at TDC to start, but not like it is now. I fear it could damage the Kickstarter pinion as much as anything else.

I'll take it to the dyno guy when he is open and also I'll bring the spare head so he can have a look at it to see if it needs the dome machining deeper and the squish band re-cutting. I'll give him the spec of the bike and tell him what fuel I'm using. I guess it's up to him then if he wants to run it and tune it at 15.56:1 compression or if he says it's too high and there's no point even trying until it's modified. I could try buying race fuel and mixing it 50/50 with unleaded even, but that won't be much good if I go far enough to need to re-fuel.

I'm just trying to avoid keep stripping down a freshly painted engine in a freshly painted frame I guess more than anything else. Working on this bike stresses me out as I'm always worrying about scratching paint off or damaging stuff, and that's a bit silly I know on a trail bike of all bikes. :lol: