MZ ETZ250 Tuning: More Torque — Classic Motorcycle Info —

6 Июн 2015 | Author: | Комментарии к записи MZ ETZ250 Tuning: More Torque — Classic Motorcycle Info — отключены
MZ ES 175

MZ ETZ250 Tuning: More

Fitting a 300 top end and an easy porting improve low end power to cure one of the biggest riding problems. two-… specialist Derek explains how he got the right results.

We all the ETZ250. This model MZ is to buy, keeps on going and a good all-round job. It has a couple of problems: typical bad fuel consumption and disappointing low below 3000rpm which is an irritant.

While there’s much which can be done to the bad economy, various people had a few attempts at adding more lower down in the rev range. One mod is to fit a different head and barrel an earlier model 250, and is to convert the engine from 250 to 300 a new top end.

Given that the with the ETZ250 is the near of low end torque, to get a clear understanding on I used my two-… engineering to fully measure exactly is what. This was to identify the and hopefully point the way to a solution.

beach, cafes, palm and an MZ. What more could want in a photo? Actually, my local beach near Melbourne, one autumn evening

apart the top end and measuring the port and capacities quickly revealed was wrong. The stock ETZ250 may a mild inlet port and facility but the exhaust port is big and high. In fact, the exhaust is so it is more like the port found in a mid-1970’s 250 moto-cross layout.

Additionally, the all-important system dimensions not only that high port but at the end a convergent cone should be, MZ has their version of a vertical baffle which effects a rev before 6000rpm. The two combined the powerband is just 3000 to or maybe 6000. Incredibly and frequently frustrating.

Clearly MZ to avoid their engines revved on their ordinary pistons with the then and so they went for silence and wear. But the use of modern oils synthetics) and a more appropriate end to the exhaust with a tapered cone and absorption type would see the engine go on producing to past 6500. However, two-… performance exhausts result in more noise and is certainly not what I wanted.

For me, I was more than willing to a bit of power up top for a torque gain 2500rpm to make the bike user-friendly in the city.

As any first auto engineering design knows, the best way to gain is to add cubes and for my ETZ250 that a conversion to incorporate an ETZ300 top The bigger head and barrel has to a relatively easy additional 20 But that engine’s porting is the as its smaller brother so the fundamental has to remain with sod-all end power.

And as students of the bleeding know, when 20 percent is to sod-all, you’re still at

So the solution had to be to fit a 300 top as well as modifying it for lower end power. Normally, the conversion to correct this exhaust port has to be the following:-

Machine 3mm off the base of the barrel not only lowers all ports but the piston up into the head). touch the exhaust port but the height of the transfer port 3mm (noting that retaining the final directional shape is a job).

— Machine the chamber to both accommodate the height of the piston at TDC and regain the squish clearance as well as the combustion chamber capacity. assuming the head casting has alloy in it to allow this.

Fit a reed valve to the inlet to counter its new low position. (Be warned to lower a barrel that has ANY as the rings will then be to jump the previous wear which was at the top of the …, so causing of seal and losing compression. slightly worn barrels are lowered must be rebored.)

— Slightly lengthen the primary pipe length to the shorter exhaust port

— Compensate for the lower port by fitting a reed of the right flow rate. your reed valve carefully. There are many from all manner of old Yamahas the years, but a good one to use gives a open area when all petals are open of at least 10 more than the open cross section area.

My is for an old Yamaha IT175 or DT250 six valve. Going any bigger and more reed valve is as useless as an over-size carb. the inlet facility has to work as a size-matched unit with all of carb, reed and port all each other.

And as for how such a dollop of metal such as a reed valve assembly is within a nice inlet gives no easy answer. The MZ guys have various of doing it but all involve lots of

— Maybe take of the lower inlet port to enlarge the size of the manifold and window (raise the top by 3mm) to a slightly bigger carb.

The involved in achieving those six is very hard, too hard, so I had to a simpler way. My decision was to do no than to reduce what is as the ‘exhaust lead’ which is the position of the exhaust port top over the transfer ports top Put simply, mild engines have a small amount of lead whereas high engines have much exhaust ports before the of the transfer ports.

As I didn’t to spend hours of work a fine angle grind painstakingly raising the transfers to risk mis-matching the heights or the emitting angles, I opted to use the old of filing notches into the top of the alongside the transfers. The only with this is if too much is off then the squish facility can be and bad combustion could result.

I to remove just under off the top of the piston edges for the transfers dimension. Okay, that no huge reduction in exhaust but at least it was reduced and every bit

The shape of the two small notches into the top of the piston is shown black ink and the worryingly thin 2mm of the liner means be careful not to the modified 300 barrel

When this, be careful to file no more than is strictly for smooth shapes around notches. To make it clear this is done, the piston top in the has had the chamfer outlines highlighted by ink.

(I learnt the big effects small port changes in the late 1970s when I was as a test rider for an American magazine and I had criticised the Yamaha models for being too peaky. the maker released the F model the barrel was no different than a lower exhaust port and the bike definitely had a slightly flexible powerband. A definite from so little change how two-… ports can be fussy

Warning. DON’T even of trying to reduce the exhaust by lowering the height of the exhaust top with smart build-up of across its width. All you’ll do is melt the near alloy in the which will leave a that will reduce transfer which results in hot spots and probably wear or

All fairly easy so far. But another big job ahead. You see, the 250 have a barrel aperture of 80mm which is too small for base spigot of the 300 barrel at

So that bottom outside of the larger liner has to be reduced. people will say this can be in a lathe but I think clamping the top of the up against stock jaws in a and just machining into the diameter all of 145mm away is too much of conventional jaws.

MZ 250s on’t eBay Now.

I sez it’s too risky and attempt to do it that way. I had a invert the barrel and clamp it top to the bed of a mill and then the cutting could spin around the with complete safety. way the O.D. was reduced to 79.8mm for a stock bore 300 means the bottom of the liner is now only 2mm wall thickness.

Be careful.

And else can stay the same: exhaust, inlet, ignition and

This is one of the frequently misunderstood with two-… development. Big can mean more top end power if aspects of the engine are altered to but they almost never more low range torque. or not to fit a bigger carburettor all comes to if you will use your 300 mostly at

If so, then go a 32 carb or even a and enable either to work by enlarging the inlet manifold its length and the port window rectangular hole at the bore) by no more than the dimension of the lowering.

MZ ES 175

Leave the width and don’t remove the unusual that hangs down the top of the port window as it aids skirt/ring life. And no matter anyone tells you, 34mm in carb diameter for a 300 bike exhaust limited to is wrong.

Obviously, the squish must be checked on rebuild and I sez go a tighter and use 95 octane petrol. because compression like delivers torque.

About the changes remaining are then to the and that could mean up a size or two on the main jet. I my bike from 2500 to or 5000rpm max so the original size carb was retained. Right now I№m the Amal 2930 Concentric Mk2 and up one size in the main jet from I was using when it fuelled the

And that about sums up all I did.

The one-eyed East couldn’t even keep politics out of their motorcycles. The MZ doesn’t have a red line at the max red meant go in the old communist doctrine. So tacho has a broad red line 3000 to 6000 which is the engine works at its most

Which all leads us to the result and the of was it worth the money and effort. The answer is yes and no, depending how the results are

The power characteristic was not totally to take some power from 6000 and put it closer to There was only a small so yes, some gain And that improvement along the overall effect of 20 percent cubes has made my MZ a bit better to not as much as hoped but definitely the lines of what I was aiming at.

To put into the equation, whereas the 250 engine had no useable power 3000, now it comes on a bit stronger at a bit stronger again at 2500 and by the increase can definitely be felt. this does is provide enough increase to take a turn by leaving the bike in gear and roll the throttle off and on, of shifting down to force the up past 3000 in a clumsy action. Pull-away from is also slightly better.

A is the way the slightly wider torque that enables me to run the engine has allows me to switch to a softer spark plug for around running. That’s gotta something.

The big surprise was the motor’s new reduction in vibes. Clearly the and so slightly heavier piston the balance factor set into the layout and this manifests as removing the irritating tingling-type on an ETZ250 when cruising at 60 or Fitting the bigger piston smoothed out my engine.

An unexpected and welcome improvement.

The other line of money is a more one. You see, by the time I a 300 top end in UK, had it mailed to Australia, paid for ultra cleaning, bought new had the barrel honed, rebadged the covers, paid for the precision and threw the whole thing with new gaskets, the result was a upgrade which cost enough what I paid for my originally. Crazy.

But I did get to increase the capacity to the biggest size and the bike also now 300 badges.

Interesting Numbers

from 250 to 300 cubes is a 20 percent which, all things being could give a 20 percent in power output. But the factory only a 15 percent improvement is probably down to the fact the stock 30mm carb is presumably for production economy Increasing the carb size to should realise more of the mid to top end potential but it would also do to the lower end torque and certainly away some fuel

My fuel economy number is at 45mpg because I can shift up as the bigger engine pulls a gear sooner during

A Few Final Tips

Do not go hacking ports/piston unless you are prepared to be and clean out all filings. Stay from a trick extra port at the rear of the cylinder as layout is mainly for mid to top end power. the exhaust stock unless you what you№re doing and forget that noise and are not the same thing.

And always the top mechanical condition and fits of a piston, bore and rings as are the heart of a good two-… at any

MZ ES 175
MZ ES 175
MZ ES 175
MZ ES 175
MZ ES 175
MZ ES 175
MZ ES 175
MZ ES 175

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