Derestricted/Tune Honda MTX125

[u33db]

Hi there,

Last week i became the proud owner of a 93 Honda MTX125 to use over winter so my 900 gets kept nice and clean.

Its a good bike but i'm finding it a little slow compared to the full power KMX i had years ago when i was learning...or at least from what i remember.

Are these bikes restricted and in what way?

6th gear seem practically useless at the moment so i'm thinking there must be something holding it back.

Anyone have experience of these bikes?

Thanks in advance!

[Kickstart]

Hi

Restricted in the exhaust, plus in leaving the ATAC chamber open all the time. First can be dealt with pretty easily, 2nd isn't financially viable nor does it make a massive difference.

All the best

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

[u33db]

Thanks for the quick reply!

Is the exhaust restriction easy to get to so it can be ground out?

Failing that what'd be a good aftermarket exhaust to go for?

On a similar note, would it be worth sending the head of to a tuner (e.g. stan stephens) for porting? I had a look at the thing tonight and i'm not sure as the inlet seems rather small to cope with any mods to the head.

Knew about the missing ATAC - been scouring ebay for a 200 head/engine but to no avail...think due to the age of these things they must be pretty rare...

[Kickstart]

Hi

Never had an exhaust off one but think they are not meant to be that difficult to remove the washer from.

You might gain something with some barrel and head work, but not sure it would be worthwhile if you bought it as a winter commuter.

The 200 was never common, and haven't been around for 20 years unfortunatly.

All the best

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

[u33db]

Thanks very much! 8)

Will have a look at the exhaust tomorrow, or may get an aftermarket one...big one are claiming at 15% power increase and it looks damn nice! lol

[iooi]

[Kickstart]

[salty21]

does the mtx share the same engine as the nsr? if so would'nt the same restrictions apply?
____________________
04 NSR 125(sold) ---- 03 CBR 600rr(sold) ----90 pan euro ST1100 '' ----02 CG 125
94 CB400 Super Four ---- 2000 VTR SP1 (sold) ---- 08 ninja p8f(sold, meh) ----05 CBR600rr

[Kickstart]

[gwernybwch]

[u33db]

Going to be doing the exhaust tonight hopefully but i've been doing a bit of reading into the ATAC system;

As i understand it theres a shaft that goes into the side of the head and at high rpm moves inward to close off a chamber that sits above it. This then directs all exhaust gases out into the exhaust for full power instead of a mix of gas going to the exhaust and some going into the large chamber therby sapping power.

I found the following pic on this net and it got me thinking;

Is the "chamber" the large hole coming out of the top of the barrel which then gets covered by a copper looking cap off the head?

If so this cap only seems to be held on by a small screw...would it not be possible just to take the cap off and drop a small bolt/dowl down into the hole and then reift the cap? This would obviously reduce the amount of area for the gases to squeeze into and hopefully simulate the effect of a close power value...i.e potentially giving full power.

What are your opinions?

I appreciate i'll sacrifice bottom end power but its a bike that does very little town work so would be worth doing...bouncing off the red line isn't really a problem after all



https://i129.photobucket.com/albums/p206/u33db/mtx_125/e1dc_1.jpg

[u33db]

Heres a pic of what i mean (excuse my crude paintbrush skills);

Red dowl would get gropped down from the gold cap right to the well of the shaft therby covering the port.

I think it could work...i read elsewhere someone cap fell of their head and it made a right racket...this must have been becuase the chamber became exposed so that means i can definately get access for a dowl/bolt...

https://i129.photobucket.com/albums/p206/u33db/mtx_125/mtx125.jpg

[Kickstart]

Hi

Close

The ATAC chamber works to increase exhaust port volume. More volume is better at low revs. The Yamaha system is far better (which changes the exhaust port timing) but suspect it was difficult to do something similar without trampling Yamahas patents.

The chamber goes from the side of the exhaust port and then does a 90 degree turn and goes up. On the full power versions, from memory, it is driven mechanically by the clutch through a centrifugal drive. When the revs got high enough it moved the linkage. The linkage went into the side of the barrel (you can see the blanked off hole on the side of the barrel) and pushed a piece out that blocked the chamber off from the exhaust at high revs.

The cap at the top can pop off. Do lots of "high" speed riding and the cap wears around the screw hole, with the screw hole elongating until it eventually allows the cap to blow off.

You probably could get someone with a lathe to knock of a spacer to go in at the top and fill the chamber, and bolt it on at the top where the cap has its mounting. Difficult bit would be sealing it.

Personally I am not sure it is that worthwhile. Don't think the chamber being open costs you that much power. Although we did consider such a mod many times when I had my MBX125. Realistically I think the MBX managed about 18hp at the back wheel given how quick it went compared to full power bikes (best on the flat was 92mph, ie, just off the end of the speedo, but it would cruise at 80mph fairly happily, and did so regularly).

All the best

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

[u33db]

Thanks Keith - think i'll give it a go!

Shouldn't need to worry someone with a lathe...recon i can drop some crude dowl of the right length and width in and then bolt the cap back on top which retain the standard seal design.

Worst case it doesn't work i can just pull the thing out i suppose.

Will let you know how i get on...

[Kickstart]

Hi

Word of warning, the cap is a bugger to get off sometimes.

Suppose a piece of wood dowelling of the right length would do the job for a while. Not sure how long it would put up with the heat sitting about 1"~2" from the exhaust port.

The screw that holds the cap on could probably locate the dowelling in place.

Would say to up the jetting a bit as well when you do the exhaust.

All the best

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

[u33db]

Aye thought about the heat!

May opt for cutting the head off a long bolt instead so its all metal - don't fancy risking a wooden one bursting into flames...my balls are only a mere 20cm away after all

[Kickstart]

Hi

Would suspect if it caught fire then it would be pretty well limited to how far any flames could travel.

From memory it would need to be a 15mm~20mm diameter bolt / dowelling to fill the hole.

All the best

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

[u33db]

Well...

Took the exhaust off last night and could see no restrictor in the front pipe. There was kinda a small "step down" after the neck (by a few mm or so) into the rest of the pipe but this was part of the casting so can't be removed.

So looks like either

a) the front pipe is std item with restictors inside the expansion chamber
or
b) the front pipe is a std item with restrictors removed but my engine is down on power/fooked

There are no markings on the front pipe so it looks to be a standard item and rather heavy...i assume its possible they changed the design/where the restrictors are??

[Kickstart]

Hi

Could also be an imported bike rather than a UK model (but then it would have all the gubbins in place for the ATAC).

What speed is it managing at the moment?

All the best

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

[u33db]

On average 60...maybe 65 down a hill.

The lad i bought it off said the engine was rebuilt a few years back then it stood unused in a garage until he bought it this year to help with his DAS training.

How long do these things last - maybe it needs the rings swapped?

I know its been about 2 years since i was on a 125 but i remember my KMX being a LOT more sprightly that this...even with a chipped barrel it'd pull to over 70 despite belching oil out the back wheras this one just seems breathless.

[Kickstart]

Hi

From an old copy of PB with a 125 road test in it the restricted MTX managed 65.5mph with the rider prone and 60mph with the rider upright (the KMX was 1mph slower on both). These are true speeds rather than indicated speeds. So sounds like you are down on performance.

Worth checking over the air filter to check it isn't too dirty. Also check someone hasn't put in a massive main jet when it was derestricted.

However could be a case that when it was rebuilt someone made a mess of it. First step would be a compression check.

All the best

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

[u33db]

yes i think compress test makes sense.

What do i need to do this on a bike as i'll be going near a halfrauds at lunch?

I assume haynes will have the benchmark figures to compare against.

[Kickstart]

Hi

Cheap Gunsons compression tester should have the right fitting.

Not 100% sure on the compression figure. The Honda MBX125 manual lists it as "Cylinder compression 178psi" which seems pretty high to me.

All the best

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

[u33db]

Right, got a Draper compression tester at lunch...14mm fitment which hopefully work.

Will let you know how i get...

[gwernybwch]