MZ etz 251 -301

[twostroketit]

Has anybody got any advice about obtaining and fitting a 300cc conversion to an MZ ETZ 251?
Can the original head be used? are the gaskets any differant?
Thanks in advance.
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sup up your beer and collect your fags

[smegballs]

I did a "conversion", didn't work well at all.

I later learned that there is a wide variety of small end tolerances that have to be matched between the piston and small end. The chance of just ordering a small end and a piston and it working is very low apparently and therefore there was a huge amount of small end play.

Still have the 300 barrel and head kicking around if you are interested, would need a rebore though as had a seizure as the barrel came poorly machined - not properly honed.

Gaskets are availble from AKF for a couple of euro.

[stinkwheel]

This strikes me as a pointless exercise. I seriously doubt it will go much faster, if at all.

The carburettion, inlet stub length and exhaust are set up for a 251cc bike. Making it bigger will just bugger up the pressure waves. It might even go slower.
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“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.

[temeluchus]

bare in mind any comments regarding the ETZ250 to ETZ300 conversion dont apply to 251-301. The 300 was a conversion by the importer and used a special piston. The 301 was a factory model.

Buy a barrel and piston and head for the 301. The rod, stub, downpipe, silencer are identical. You'll need the gasket set for the 301 barrel.

A slightly larger mainjet will be needed. Cant advise on what that is with the BVF carb as I had a late 301 which had a Bing carb.

May be worth giving F2 motorcycles a buzz as they used to be an MZ dealer. Also check out the MZRC.
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Some shite cruiser. Now with guns and FREEDOM!

[smegballs]

[twostroketit]

Thanks everyone.
I'm not looking for more speed or power,It's an MZ. But i had heard the 300 engine has more midrange and holds it's speed easier on hills and such.I also heard that fuel economy is better.
Perhas i was a bit misleading when i said conversion, what i meant was plonking a 301 top end on 251.
At the moment my bike is needing a new top end because it's worn out, so i figured why not go to 300cc.
____________________
sup up your beer and collect your fags

[smegballs]

Check the gearing, according to an article I read on umgweb - written my the MZRC secretary - he found that lots of MZ's were overgeared with stock sprockets.

I think its MZ's with 18 tooth front sprockets are the overgeared ones, mines an 18. According to this bloke, if you drop a tooth or two off the front sprocket to drop the gearing, acceleration and top speed improve as now the engine can rev out in top - mine never did.


Check your sprocket size, for a few euro it might be worth dropping the gearing to see if there is improvement.

[lingeringstin...]

I only just now saw this and thought I'd post my own findings on the whole 250 vs 300 debate.

The upshot is that yes it makes a difference, BUT there are stipulations.

The way an MZ is built it's pushing it to rev much past 6000rpm really and unless you change the gearing you'll get no better performance, so even if you used a piston the size of a bucket it would only go the same speed as a 250.



I am currently running a bored out 250 barrel using a standard MZ301 piston and a slightly modified 250 head.

When you bore the 250 barrel it loses the little lip that sticks out the top of the liner so the bored barrel is just flat across the top. The old Wilf Green 300 barrels (bored out 250's) had two little dowel pins fitted to the top of the barrel that located into two little holes in the modified head to presumably keep everything located right.

However, there's not a lot of those barrels and matching heads around so you CAN just bore out and use the ETZ250 barrel to take a 300 piston and use the standard 250 head but the combustion chamber needs to be slightly modified to accommodate the bigger piston by having somebody lathe out the combustion chamber area at the edges so it's wider because the piston crown sticks up a couple of mm past the top of the bored barrel.



However, if you don't have access to a lathe (or like me you're just a tight git) another easy mod is to fit a slight spacer under the barrel to raise it by that small amount so the piston doesn't interfere with the head. Plastic or nylon actually works well. I've done this and the extra spacer under the barrel is so minimal it makes absolutely no difference whatsoever in how the thing runs.

I once put a longer ETZ piston in a TS engine and had to make about a 6mm spacer out of a nylon chopping board and even that much difference didn't alter the way the bike ran so don't listen to the doom mongers who predict destruction at the mention of lifting the barrel a bit. An MZ isn't rocket science.

If you go with this method you might end up with slightly lower compression but this can be adjusted by using a longer spark plug to fill up space in the combustion area. Yes I've done this for years and no it does not burn a hole in the piston or cause any other epic failure like everybody said would happen.

I've used the standard 250 head on a bored out barrel without fitting any sort of locating dowels and it still worked fine. The 4 barrel studs clamp the head down pretty well and I didn't have any pressure leaking or displacement issues, and all I use for a top end gasket is automotive coppery flange sealant.

So basically, you CAN just bore out a 250 to a 300 without making any real modifications, BUT it won't go any faster if you use the same gearing. The extra grunt of the 300 piston means you can use higher gearing and there's two easy MZ ways to do this.



I'm running 16" wheels front and back. The ETZ251 used a 16" rear wheel and the standard gearing is a 48t rear sprocket and a 21t front. But the older TS250 sprocket is a 47t, one tooth higher geared, and if you use that it basically amounts to about 5-10mph more at highway speeds for the same RPM, but be warned the TS rear sprocket used slightly different axle gubbins than the ETZ so you will likely run into frustrating complications if trying to mix-n-match TS and ETZ rear wheel parts. Personally I get around this niggling issue by using an axle from a Japanese bike (GN250) and some oddball spacers in key locations.

For even higher gearing use the standard ETZ 48T rear sprocket but go up one tooth size on the front sprocket, in my case with 16" wheels that means going up to a 22t front but if you're using 18" wheels the standard gearing was 48t rear and 19t front so you'd go up to a 20t front.



Basically it all boils down to adding cc's to the engine and upping the gearing, and then yes it will make a nice difference. I can't comment on what to do with the standard MZ carb because I don't use them but common sense says just bung in a slightly bigger main jet or raise the needle. I've found that most common 32-34mm two stroke carbs are a vast improvement over the standard BVF thing they came out with.

So it is doable and doesn't have to cost much. But never, EVER tell an MZ nerd what you've done or they'll shout "MONSTER!" and chase you out of town with pitch forks and burning torches.