Derestricted/Tune Honda MTX125

[stevo as b4]

The figure's i saw for an MTX125 quoted were, 12.2ps at 8500rpm on the restricted version, and 20.4bhp for the full power engine. This also produced a figure of 16.7bhp ATW, which is about what id expect for a 20bhp engine with the transmission losses added in.

I have also seen the MTX performance test figure's in a French bike mag, and i think the top speed was 75.5mph and standing 1/4m in 16.8sec. Id expect 60/65mph from 12bhp, and the 70-75mph from a full power engine in decent nick, but with much better acceleration. I don't think the gearing would let you get over 75mph as i think that is right on the 9000rpm redline.

(The MTX redlines lower than the MBX engine for some reason, probably due to the manufacturer re-tuning it for less top end power and more midrange for trail riding).

I also agree with keith that i doubt the open ATAC chamber is sapping much power, and id be suprised if it made a 1bhp difference at peak power rpm either way open or closed.

BTW the KMX125 is much faster than the MTX, it's a Kawasaki after all!

Even 12bhp KMX's would do over 80mph on the speedo downhill, and the DTR was never as fast accelerating or could top 70mph on 12bhp like the KMX did in road tests!

[u33db]

Right, attempted to do a compression test but i couldn't get the gauge in as the frame gets in the way by about 2cm...sods law isn't it!

Exhaust doesn't have the marking stated above so i think its an aftermarket item - i can rule that out now at least.

Speaking to my dad and he said he thought it was a bit reluctant to start when i showed him last time he was round so what we've decided is that i should just get a new piston and rings....for the sake of 30quid it should hopefully sort it.

Certianly its nowhere near the performance you guys have said it should be.

I'll post up once i get the piston/rings in but thanks for all the replies - much appreciated!

Will

[u33db]

Hi there,

right i got the engine back together and that seems to have improved it slightly...6th gear is usable again, altho a bit worthless IMO. After having a chat to my dad we think the fact i've come off my 900 amplifies the feeling of it being slow lol

Anyway, now the engines solid i'm wondering if anyone has messed about with different sprokets on these bikes?

Wemoto are listing front sprokets with 1/2 less teeth and a rear with 2 extra...this would obviously bump up the acceleration at the expense of top speed but any ideas what it'd then top out at?

[Kickstart]

Hi

This other thread might help.

https://www.bikechatforums.com/viewtopic.php?t=158189

You are very limited on sprocket choice on the MTX.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[u33db]

Wow thanks for that!

That lad seemed to want more top end but from what was said am i right in thinking for every tooth off the front sproket you loose 5 mph and the same if you take 4 teeth of the rear?

If so i'm planning to go for a 13 tooth front and then if thats ok stick a plus 2 rear on.

Currently (with 15 front/54 rear) i can only use 6th if its readlined in 5th before the change...if i don't the revs just die so hopefully this will cure things and compared to its current state will be no slower top end.

What do you think?

It is very hilly round these parts...

[Kickstart]

Hi

Probably 3~4mph (same revs a bit over 4mph, but chances are it would manage to rev out a bit further) if you go down one on the front. However 2 down would be a hell of a lot, and would leave it screaming at normal cruising speeds. One tooth down on the front would be the same as 3~4 teeth up on the back.

As mentioned you are very limited on the choice of sprockets for the MTX. From memory there are only 2 sizes available as a direct replacement.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[u33db]

Ok, i'll take your word for it - i'll go for the -1 front sproket.

While i'm on here - think it could do with a new air filter and i'm thinking about sticking an open cone on it. Would it need rejetted for this or would i get away with just adjusting the carb mixture?

Thanks in advance once again!

[Kickstart]

Hi

Some people have issues with the cone filters making the flat spot in the mid range worse. Personally I had one on my MBX125 many years ago without any great problems. However you almost certainly would need to rejet (the mixture screw only changes the mixture on a closed or barely open throttle).

I would be tempted to just clean the filter and put it back in.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[u33db]

OK i'll give that a go.

Now, my last question - is there away to lower the seat height slightly or is it fixed?

I had a look at the weekend but there doesn't appear to be anything you can do so i'm guessing use a different rear shock? Is there any that will fit straight on?

[stinkwheel]

Now. I presume this bike has a few miles on it.

You have checked the exhaust isn't full of coke yeah?

Also might be worth having a quick gander at the reed valves, make sure they are in good nick. I'm not sure if replacing normal ones with carbon/plastic ones makes much of a difference but if it's suffered some abuse they could be in a bit of a state (particularly if it's been backfiring after ham-fisted tuning, which face it, is more a probability than a possability with a 15 year old learner bike) .
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Kickstart]

[u33db]

Hi there,

just an update - 14 tooth front sproket arrived today and have just got it fitted.

What a difference!

Main improvements seems to be
- noticably better acceleration
- more comfortable crusiing round town
- more stable handling (IMO)...back wheel has been pushed back to compensate for the slack chain? Edge of rear wheel is now right under the numberplate.
- better top end...pulls to 70 flat out in 6th now!

I was toying with getting rid of the bike but with fitting this today i'll defo be keeping it as its a laugh and a half now.

Just need to tart it up a little now lol

[stevo as b4]

Wow, You wouldn't belive that a 1t smaller rear sprocket has had such a big effect on the performance of your bike, and that it has also indirectly improved the handling too! Bit of a result you had there mate! Hope you keep it for the laughs even when your 900 is back on the road.

[u33db]

Aye really loving the thing!

Think its the riding position and lightness TBH - much more chuckable than the 900! I'm almost tempted to trade the Ninja in for a supermoto next year its that much more fun.

Anyway in the meantime i'm thinking of splashing some cash on this thing now its running right.

Has anyone had any work done by stan stephens on their barrels?

Theres been a few spare mtx barrels on ebay lately so i was thinking of getting one and sending it down to them for porting (its only 150 for the work IIRC).

What would the difference be like if it did get this done?

I've heard that porting can take the power up to about 25bhp or so...is that crap or not and would there be massive difference in power (for a 125)?

May also get them to block the ATAC chamber off too...

[stevo as b4]

Personally i wouldn't go down that route with Your MTX now you have re-built the engine, any by your own admission got the bike running really well again.

Porting the barrel could add a fair bit more power, but it would need alot of other bits to make the most of it, probably a bigger carb, and aftermarket pipe etc.

The best way to get another 5-6bhp from the MTX is with a 200cc lump fitted. More cc makes more difference IMO than anything else you could do, and so does tuning work. The gains from say fitting a bigger carb or ported cylinder on the 200 engine would probably be much better and more worthwhile. The big downside to this plan is actually finding a secondhand 200cc lump. I bet if you scour the big bike breakers that one might show up eventually. But then it would probably a full re-build to make it work and be worthwhile.

I'd keep riding the bike as it is for now and look out for a 200cc engine as a re-build and transplant option for later on when you have more time.

I assume that you have cleaned out the expansion chamber and silencer on the bike already? If your just after a little bit more poke, then i might be tempted to get an aftermarket pipe and silencer, and maybe fit a slightly bigger carb if you can throw one on from another bike fairly easily. The std 125 has a 24mm Keihin, but the TM28 from a DT125 etc, might go on with some fiddling around. I don't know how much of a difference it would make though, so thats why id put the cash toward a 200cc unit and an overhaul, as you know you will gain around 6bhp minimum this way, and quite alot more torque too.

[u33db]

Already been scouring the breaker for a couple of months now - no 200s anywhere!

That only leaves the porting really - i agree i'd really need the carb done to get the best but even without i should see a reasonable gain surely?

On the exhaust, do the aftermarket ones (e.g. big one) make that much difference...I think mine is a derestricted standard item so how would they compare?

Not cleaned out the existing pipes yet (how do i do this) but surely they don't get clogged that much?

[stevo as b4]

If the bike is run in again and has good compression, as well as running well and starting easily, then i think it would be a shame to disturb the engine again, and possibly make it less reliable.

The porting work on the cylinder would probably make a 2-3bhp increase in peak power, but i would say it's likely that you will lose some midrange, and the power will probably drop like a stone after peaking, with the std carb and exhaust system. I really don't think it's worth spending £150 on tuning a secondhand cylinder on a 125 TBH.

Changing the carb for a slightly bigger item might be cheaper and give a bit of a gain, but it's probably going to be a load of hassle, with jetting, connecting up the cables and getting it to fit onto the reed block and air filter hoses.
I don't think you'd gain a massive amount more power with an aftermarket exhaust, and you don't get something for nothing. But if the std pipe was all gunged up and full of carbon/oil deposits, you could maybe change to a performance one and that might give it a bit more zing. The pipe and silencer would be probably less than £100 for an MTX, so it's cheaper than having the porting work carried out.

If you want to clean out the existing exhaust, then i would use a caustic soda solution, and block up the expansion chamber at the engine end, and then pour the caustic solution through a funnel into the other end of the pipe. Hang the pipe up for 24hours and then remove the bung and drain out the liquid. you can then swill out the pipe with paraffin or white spirit etc, and mabe throw a few bolts etc in and shake them around to dislodge any stubbourn carbon deposits. Id clean out the silencer the same way, unless it's the re-packable type, in which case you can just buy some new exhaust wool for it, after cleaning out the baffle.

Shame there are no signs of a 200cc engine around, even if it was seized and worn out. They were 26.5ps and had about 2.6kgm of torque. I'd expect the performance to be similar to a KMX200 which is certainely enough to be fun. Maybe DK or one of the big bike breakers might turn up an engine? The 200 was always a rare beast though and was only sold in the uk for about 2years.

[sumpgard]

the only thing restricting you mtx is the exaust front and tail pipe my mtx 125 will do 80 mph on standard gearing does rev off the clock. all i have done to this is cut the front pipe into sections removed restrictor pipe welded into the part that meets tail pipe this is a thin tube about a foot long also ground out part that meets the barrel and removed sound deading this is incased in a mesh then fitted a cr 85 tail pipe jetted it to run properly .

[zene]

Hello every one, quite interested on this thread as i swapped mi Yamaha sr 125 for a Honda MTX 125 93" last week,
but mine does 70 max speed also it spits oil through the exhaust.
is that normal on all two strokes??
also did u33db get more power on the bike in the end and how if so????
sorry to bring an old thread back but mi bike seems allot slower than the 80mph sumpgard one does
thank you for any help

[stirlinggaz]

[zene]

Gaz thank you for the tip!!!, just waiting to get mi manual so i can get mi hands on the engine as the kick-start spring failed it seems that i will have to split the engine open to get too it but hopefully i can get it done this week, while at it i was looking for a mtx 200 barrel but its nearly impossible to get it pity that you sold urs!!
do you know if there are any modifications to do on the 125 engine to fit the 200 barrel?? or is it a good option to bore mine up to 200cc????
regards
juan

[stirlinggaz]

hi,
you dont need to split crankcases to remove kick start mechanism & spring.
i gave away the haynes & the big honda wsm (which is far better than haynes) but it will become clear how when you follow the pics.
the mtx 200cc cylinder DOES NOT fit straight onto the 125cc bottom end, you need the full engine.
only other diffeernce is extra spring in clutch & atac, so the full engine just slots in.
well worth sending off your cylinder to get "tuned", expect another 5bhp or so, combined with a decent exhaust & you will get gains.(with the powerband changed to not being so narrow)
whether its financially worth it or not, i dont think so.
will probably cost you the same as a 2nd hand 200cc engine. (if you can find one)
the combination of a 200cc engine, the bigger carb & a decent exhaust gives you over 30bhp & with correct gearing makes it a decent bike to ride on the road, with speeds of over 95mph..
the 125, apart from having very little low down power (like most 125 2t's) is still perfectly useable & capable of 75mph (indicated) with a decent exhaust & correct gearing.
which is more than enough for a 125 trails style bike.
if you do get the cylinder tuned drastically, the expansion chamber probably wont cope with it & 200cc exhausts are hard to get.
good luck,
cheers,
GAZ

[zene]

Gaz I don't see any other way to get the kick start mechanism done , got the manual today was trying to do it but miss is sick so have to take care of the little one!
do you have by any chance that tool to remove the clutch basket??
its some kinda castle nut
regards

[gwernybwch]

[stirlinggaz]

as above, never had to split crancases with any of the problems i had with kick starter, but will bow to the superior knowlege above.
i was sure i got the spring out before i split it...............
& i made the mistake of taking the water pump impellor off - after a fashion,lol
getting a 200cc engine is a big enough pain, & expense, then you need the pe29 carb, easy enough.
but try finding a decent exhaust!
dont try & run it with the 125 exhaust, as you lose a helluva lot & you will burn your leg.. (i tried)
as above, no-one really made aftermarket ones so you need an oem expansion chamber (pretty rare these days, though i have 1 in my shed ) & you can use any muffler.(doesnt do anything to performance)
costs far to much to do.
& i either cut up an old socket, or get lazy & ht it with a lump hammer & cold chisel to get castle nut off, though it can get damaged if you hit it too hard.
decoking makes a HUGE difference, especially if you fit new rings at same time.
cheers,
GAZ