MX125 and YZ125A

[firko]

I've had a bit of a score today, two separate mates are cleaning out their mancaves and have loaded a bunch of their crap onto me. From one mate I've been given 6 crates of mostly Yamaha MX125 engines and parts. I've just spent a lazy three hours going through the stuff and it seems that I've got three good bottom ends + another two dismantled bottom ends. There are seven barrels and six heads + ignitions, clutches, carbs, reed blocks and a shitload of other bits and pieces. After looking at the barrels I've found three are chrome bore and the other four have iron liners. I'm aware that the YZ had a chrome bore and the MX iron but are there any other differences between the two engines? Are the gear ratios the same? Head/comp ratio?  What are the engine prefixes for both? The reason I ask is that if they are different, I'd like to match up the correct top end to the correct bottom.

Before anyone asks, two of the engines are earmarked for our Trackmaster mx frame and the rest have already been spoken for. Sorry .

[Freakshow]

YZ start prefix 453 and the mx will be 401 but there not that much differance to worry about. and all parts should interchange, pity there already spoken for.

[gt96]

Could be mistaken, but seem to remember that the YZ had a close ratio gearbox. Also, CDI vs flywheel magneto. Different pipe. YZ was so much prettier, but was about $200-300 extra but not that much better.

[Davey Crocket]

Arent the g/box ratios diferent Freaky?, ign aswell, also the MX barrels port very well.....our YZ125A's up here with the JTR treatment smoke them CR125's in a straight line....who needs a 6 speed. ...if you want to go fast Firko, ring me. You need the combo works burger.

[gt96]

Yep, MX was good for porting. No good on the MX though cause the woodruff key for the magneto flywheel would always shear and turn it into a one piece motor.....
YZ was also premix while the MX had the oil pump.

[firko]

...if you want to go fast Firko, ring me. You need the combo works burger.

moi? on a 125? Not bloody likely my friend. I might be losing weight but I'll never be small enough for a 125. One of the barrels has some interesting ports and FP etched into it in a couple of places so we're hoping it might be some of 'Frankie the Files' (Frank Pons) work from the seventies. Then again FP might relate to the fat prick who owns it.

Do the YZ's have CDI? We'll probably fit later ignition anyway possibly an X model ignition I've got in the cupboard. I'm curious about the gear ratio's though....anyone know for sure if they're different between the models? Ditto the compression. Without doing a test, two of the heads look different to the rest....they appear to have a larger squish area.

For anyone interested one of the other things I scored is a complete and unsiezed little Yamaha GT80. It doesn't run but I don't think it'll take much. The ignition is all rusted and gnarly so what CDI will fit it? I might paint it up like a DT1, fit some flash looking Chinese suspension and use it as a pitbike.

[albrid-3]

YZ A has CDI, chrome bore, different size carbie, close gearbox., chrome bore cost $550 to get it rechrome.

[Nathan S]

YZ gear ratios are definitely closer than MX ratios. I think the primary drive ratios are different too.
YZ-A CDI ignitions are like YZ-C, and different to YZ-X/D/E.

Heads are funny - best I can guess, Yamaha made running updates. I had four or five 537 (125C) heads here a while ago - there were three different combustion chamber shapes. None were modified.

[head]

definately closer ratio gears on YZ125A

[firko]

Thanks Chaps. The different combustion chambers had me buggered. I had another look after dinner and I now think I've got three different heads .

Anybody got any ideas on if a later alloy barrel will fit on the GT80 engine to replace the cast iron original?

[vandy010]

YZ gear ratios are definitely closer than MX ratios. I think the primary drive ratios are different too.

to the best of my knowledge, the primary drive is the same but gear box's ratio's are definately different.
they share 1st gear only with the YZ's ratio's being closer than the mighty MX box.
my little MX may as well be a 4 speed as i never use 1st gear.
i believe the YZ is the other way round as they never seem to get the full use of top gear without making 1st too low.
a good box/ratio combo would be the DT box fitted with the MX 2nd gear.
once again Firko, that was too much beer, a calculator and a lot of head scratching.
for my money, the YZ and MX are the same bike except for the gearbox/tank and ignition as most 125's seem to get modified somewhat.
sounds like you picked up some good stuff.

[Slakewell]

I really like this project Firko with the reed valve engine I go for 34 Bing and I will set it up for you.

[Nathan S]

to the best of my knowledge, the primary drive is the same but gear box's ratio's are definately different.
they share 1st gear only with the YZ's ratio's being closer than the mighty MX box.
my little MX may as well be a 4 speed as i never use 1st gear.
i believe the YZ is the other way round as they never seem to get the full use of top gear without making 1st too low.
a good box/ratio combo would be the DT box fitted with the MX 2nd gear.
once again Firko, that was too much beer, a calculator and a lot of head scratching.
for my money, the YZ and MX are the same bike except for the gearbox/tank and ignition as most 125's seem to get modified somewhat.
sounds like you picked up some good stuff.

Ah yes - you're jogging my memory here Mr Vandy.

DT and MX share 3rd gear.
MX and YZ share 1st gear.
DT has different (lower) primary drive ratio than MX/YZ.

What this means, is that if you combine a DT primary drive with YZ gearset, the overall gearing is VERY low - I ran 15/47 sprockets on my DT-based bike, and its still too low (pinned in top, well before the big table at Clarence, for anyone who's ridden there). When it finally re-emerges from the garage, it will have 15/44.

My YZ has standard sprockets. I've only ridden it once, but it was geared too low for Buladelah. It can definitely stand taller gearing there, but I think it would be OK on most tracks.

Honestly, with a decent sized carb and non-Vandy porting , I think the wider spread of MX ratios are better.

[Davey Crocket]

Your pretty close Nathan, we run 15/45 and it suits every type of track, you wont run out of top speed and its a good spread of power so your not tap dancing on the gearlever every 2 seconds, good for flat track too as I have found out.

[firko]

I really like this project Firko with the reed valve engine I go for 34 Bing and I will set it up for you.

Which project Mick? The Trackmaster 125 thing? It's to be a Klub Kevlar community project, everyone's pooling their resources and parts to get it together for next season.
Thanks for all of the info boys.....I'll look further into just what I've got and hopefully can get a YZ bottom end matched up to a good MX iron bore top end, the X model CDI, a decent 32mm carby, the NOS Vintage Iron YZ125A pipe I've got and pass it all along to Jonesy to finish off. If the bike looks as if it's going to be competitive we'll look into either whacking in some ports or fitting the 'FP' barrel.
Besides Webco, were there any other aftermarket heads made for the MX/YZ?
             
Below: the Trackmaster frame that'll be used for the MX/YZ125 engine, YZ125X tank seat,and wheels and a pair of DC translucent 'guards. The frame is absolutely featherweight so it'll be an interesting project.