MZ/MuZ Riders

I have an ES150, which my lad is not able to ride due to not having passed his full test and thus being restricted to 125cc.

I also have a complete 125cc top end and carb which would (with a bit of paperwork effort) solve this problem. However, I'm aware that the pistons will be different masses, and the crank could potentially then be out of balance, so also changing the crank would be the ideal solution. This'd mean buying a crank and that pushes it beyond budget.

Were the 150 and 125cc cranks actually balanced differently at the factory?

Has anyone done just a top end swap? Does it make that much difference to vibrations and bearing life?

Is it a definite 'don't do it' or just a 'not advisable' or hopefully a JFDI?

There are no different crankshafts for 125 and 150 ccm. So changing to 125 ccm is quite easy. You need cylinder plus piston, carb.plus intake, and Cylinderhead. That´s all...

Did this swap the other way though. But before I changed the rebored barrel and new piston to my Ts 125. I had the only used the carb.

The swop only took 30 minutes as the carb bolts onto the barrel is not much bigger, the exhaust port/ downpipe and silencers the same

After I fitted the barrel I didn't notice much difference apart from more torque two up. So since it's only 25cc I never changed the log book size.
P.s. I also used the 150's grearbox drive sprocket as it had one more tooth. The whole bottom end is the same as the 125 too.
Same crank same internal.

Thanks for the replies, which confirm what I was thinking.

JFDI it is...

Gerryman Ts125 wrote:So since it's only 25cc I never changed the log book size.
.

I think this will be the biggest part of the job for me, as it will need to show 125cc on the logbook for it to be legal for my sons to ride.

I'll give the DVLA (UK vehicle licensing authority) a call. If I do that right now, I might get to the front of the queue sometime next week.

Nothing wrong with being legal. The police in Glasgow stopped me in September 1986, and they where convinced that my Ts 125 was a super 5 (As they shared a similar front end) on the m74 motorway.
As I passed their Unmarked Autumn Gold 3litre Opel Monza, off the clock at 65-80 mph (Rear speedometer drive problems) they pulled me over into the hard shoulder and checked the bike all over.

Then once it came back as my Ts 125, they kicked me back out into the hard lane, but before I left they told me ' some speed son, you know the brakes on that couldn't stop it above fifty' so take care now.

And left.

Swapping to a 125cc top end on your ES150 is feasible, but keep in mind the potential balance issues with the crank due to different piston masses. While some have done a top end swap without changing the crank, it's advisable to consider the impact on vibrations and bearing life. If budget constraints are a concern, you might want to weigh the pros and cons. It's not a definite 'don't do it,' but a careful evaluation would be wise.

I think you are over thinking the balance issue to be honest. As far as I remember they both use the same cranks, the only difference being the piston and the size of the hole it sits in!
Look at the number of big bore kits for high revving four stroke that are on the market (Including the 250 to 300cc kits for my own Fantic) that use the same crank and don`t have subsequent balance/vibration issues.
Your biggest problem might be getting it verified as a 125 with the authorities.

breakwellmz wrote:I think you are over thinking the balance issue to be honest.

I have a tendency...

Yes, the paperwork will be the worst bit, although the route to approval is clearly defined, I can foresee some goalpost shifting by the DVLA once the process gets started.

Won`t you have to prove to them that it is actually 125cc? You could tell them anything, and not actually change it from its current 150cc!

breakwellmz wrote:Won`t you have to prove to them that it is actually 125cc? You could tell them anything, and not actually change it from its current 150cc!

Yes, the requirements are published thus...

Change of engine number or cylinder capacity (cc)

You need to provide one of the following:
- a receipt for the replacement engine that includes the engine number and cylinder capacity
- written evidence from the manufacturer
- an inspection report provided for insurance purposes
- written confirmation on headed paper from a garage (if the change took place before you bought the vehicle)

The ES barrels and manifolds are clearly marked with the capacity, so an inspection shouldn't be too difficult a hurdle to jump.

It might be a better prospect for my sons to just get their collective arses in gear and get round to taking their part 2 tests!

Hi.

You have that covered one way or another then.

I suppose making it a 125 will make it more sellable in the future if you choose to, especially with a ready to fit big bore kit!

Kids eh!

Cheers.

Very good point!

But I do seem to have a problem actually selling bikes. Buying them - no issues; selling them - can't seem to manage that bit.

I'm with you there! I can buy, but cannot sell!

Well, that was a quick turnround!

The V5c arrived today, with the CC down as 125, no inspection required. 125 top end and carb are on and working well. I didn't expect to have that all sorted this time of Christmas, top marks to DVLA.

In case anyone's interested, there's a marginal difference in acceleration and low-down torque, but the 150 has more of a power band than the 125. In real terms, I'd say the 150 has the edge, but only by a tiny amount, not enough to warrant changing unless you have to.