2 Stoke Tuning misery

[smegballs]

As all MZ’s need a good tune to up their street cred, I decided to start with Excel to work out how fast I could rev MZ (theoretically) before ring flutter set it.

The MZ rings are 2mm or 78 thou. I had the following information but no value for 2.0mm so I used Excel to form a calibration curve for 2mm rings.

https://i43.tinypic.com/ang2uo.gif


This gave me the result that 2mm rings are good for up to about 67,000 ft/s/s or thereabouts.


https://i40.tinypic.com/2yybg8w.jpg

Then, using the equation: https://i43.tinypic.com/k4bfrp.gif

I plotted a graph for the MZ’s Gmax at various engine speeds.

https://i44.tinypic.com/30avqth.gif

The purple line is the where the MZ’s powerband ends at the moment. The yellow line is the engine rev limit where ring flutter will set it. The rings start fluttering at around 6750pm.

Haha so much for my ideas of raising peak power come in around 7500 rpm.
Obviously its not worth heavy work to raise peak rpm by just 750 rpm.

So now the hunt is on for a piston with nominal 76mm bore and 18mm gudgeon pin……

Any comments on my workings and wether my ring calibration curve is actually applicable to real life appreciated.


Rich

[MarJay]

I've got the two stroke tuning book by A. Graham Bell, but I've not done any maths from it. Mainly because its too much like School/university/work, and I wanted to do work on bikes for fun. However, it is still generally possible because unless a bike is super brand new, there is always someone else who has tried and succeeded, and a number who have failed.

The main thing that I understand is an issue with MZ's is the low rev limit. Now, from what I gather, the rev limit really comes from the exhaust design. Do you have an expansion chamber? I found a couple of sites from Germany that were selling spannies for 40 euros!! Sometimes tuned two strokes also only run with a top ring, and not a bottom ring and I don't know if that would help.

The A Graham Bell also mentions a Dykes type ring which has an L shaped cross section that resists flutter. It would probably be easier than looking for a different piston and stuff, to get the piston machined to fit a Dykes type ring... but apparently ring sticking can be an issue. The other issue is, you can't just fit a piston from a bike with the same bore and gudgeon pin size, as the ports have to be opened correctly by the shape of the piston. It is possible there is an MX bike out there with a similar bore but I can't honestly say if the piston will work in the bike.

Dunno how realistic your curve is, but I'm normally pretty good at spotting hokey maths, and you certainly aren't a factor of ten out or anything like that so I would guess that your maths is pretty much right.
____________________
British beauty: Triumph Street Triple R; Loony stroker: KR1S; Track fun: GSXR750 L1; Commuter Missile: GSX-S1000F; Cheap project: CBR900RR FireBlade
Remember kids, bikes aren't like lego. You can't easily take a part from one bike and then fit it to another.

[smegballs]

Yeah the zorst was the first plan. Followed by a Mikuni carb to improve fuel metering as the BVF chunk of crap seems rather inaccurate.

The problem I'm having with the whole two stroke tuning seem to be that the zorst matches the porting and the porting matches the zorst. Where do you break the cycle and start modding??

Probs gonna get one of the RZT spannies tho when I have some ready cash.

[iooi]

I would also look at the other parts of the engine. While ring flap Is one thing, can say the big/little end and other parts stand in increased stress on them.
One reason these bikes run so well is the fact they are under stressed/low state of tune.
Take this further and reliability will take a hit.

Good luck though, with all that excell work you must be well tuckered out
____________________
Just because my bike was A DIVVY, does not mean i am......

[MarJay]

[smegballs]

[MarJay]

[chillpill]

pick up a new set of crankcases from ebay to work with , flow the cases so the transfer feeds match the crankcase intake , having a smoother line to the transfer feeds gives less inertia to the mixture , increases crankcase filling and therefore more bhp's

expansion chambers defentley a good idea if it dosent have one as stock , must be a very old bike , if you plan on getting porting done to it you might find it hard to get an aftermarket exhaust to match the barrels powerband so it might be worth investing in a custom made pipe ,are you going to do the porting yourself ?

fitting a single ringed forged piston would also increase revs , so would small things a titanium gudgeon pin

otherways to increase power woul be to start replacing gearbox parts with titanium parts

increasing the compression wouldnt help performance right now untill its highly tuned , unless you plan on running high octane fuel ? all a higher compression will lead too is a poorer burning of the mixture (unless the standard compression is too low? )

you seem to be pretty clued up though (ive got no idea what all that junk youve written at the top is about )

hope it goes well for you

[Robby]

Guess who's been reading tuning threads on scooter forums, and has no idea what an MZ 2 stroke is.

I would go with the marje idea of some general cleaning up and a slight boost in squish. They can handle an increase in compression, likely more so with an expansion chamber.

Seeing as you're only looking for 3 or 4 more HP I wouldn't go looking for different pistons or barrels.

Still, it's an MZ. If I had one of them I'd be doing what stinkwheel mentioned seeing, and fit an SU carb with an adjustable jet just to see what I could get it to run on.

[chillpill]

im geussing that was aimed at me and the mz is an old 2stroke bike



so is the theory wrong ? he mentioned 30 bhp mz's so i got the vibe that he wanted to go all out with it , wich will include more than just fitting a pipe and getting a thiner base gasket

[Smooth]

What in particular makes an MZ 250 less able to produce maximum power than a Japanese 2 stroke of similar size?

Surely correct porting, a suitable expansion chamber, reed valves, lighter piston, carb jetting etc. could reduce in more than 30bhp, even if it is very peaky?

Even if it doesn't last very long
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TDM850

[Kickstart]

Hi

It is a single, and with a stroke getting on for 50% longer than the sport 250 twins. Longer stroke means less revs. As power is torque x rpm, less revs means less power unless you can make a hell of a lot more torque.

All the best

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

[Smooth]

[stevo as b4]

I'd definately go for the increasing of the torque route, instead of trying to get it to run a higher peak speed, which would entail serious port timing changes, and if you wanted to maximise the power spread, then you would need to match the pipe design to the new tune, and exhaust time area, as well as increasing the carb size and inlet port timing to match. You will have mechanical limitations in terms of piston speed etc, and also your likely to need a way of altering the ignition advance curve if your trying to radically increase the peak rpm and make power at higher revs. This is likely to be impossible with the std CDI, unless it runs a fixed timing setting where you can set the timing for peak power rpm, and live with it elsewhere, like some moped engines for example.

Id definately have the barrel off and clean up all the ports carefully, polishing the exhaust port with a rotary sanding drum in a dremel etc. Don't try and do much work on the transfer port windows as you will find it hard to match them accurately. Port matching the base gasket and transfer passages is worthwhile.

The compression is probably quite low on an MZ lump, to tolerate crap fuel. You can probably have a fair bit skimmed off the head to raise the CR and reduce the squish clearance, especially if you run on decent 97RON fuel.

Id try and get a bigger carb say 3-4mm bigger bore and set it up with a large cone filter. It would be worthwhile finding an expansion chamber for it too, as you will reduce weight and improve ground clearance probably too.

[stinkwheel]

MZ tuning involves doing all manner of horrible things to the engine. It's all weird and wonderful crap which has more to do with black magic than science.

I seem to recall a favourite is to put a 45 degree chamfer on the back half of the piston crown almost all the way down to the first ring groove. They usually do this with a file. Does something to the inlet port timing apparently.

They also do a lot of fannying with the inlet manifold length.

Don't take my word for it though. I got this info off a guy called Rex who sometimes posts on www.ratbike.org/bbs and he doesn't spend all the time on the same planet as the rest of us.

He does however have the quickest revving off the throttle MZ I have ever seen. Doesn't even go "bung-bung-bung" on the overrun.
____________________
“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.

[sickpup]

[Fnatic]

https://edj.net/2stroke/jennings/

Link for Gordon Jenning's Two-Stroke Handbook. Really interesting read but I can't work out half of the maths.

Really interesting about the expansion chambers.

[iooi]

[stinkwheel]

[MarJay]

You've got me thinking about MZ tuning now. Bastard!

I've had an idea. Measure the squish gap, then take a guess as to how low you can go before the compression ratio goes above about 7.5:1...

Then, remove the head gasket, and send it to these guys.https://www.coppergaskets.us/

I've just ordered a thinner RD gasket from them to improve my squish gap. They'll make you a head gasket using your old one as a pattern and make it a bit thinner than the original. As long as this won't decrease your squish gap too much, and take the compression too far it should improve torque. You also have the added bonus of the copper gasket being reusable, and probably better than the original. Mine leaks like a bastard!
____________________
British beauty: Triumph Street Triple R; Loony stroker: KR1S; Track fun: GSXR750 L1; Commuter Missile: GSX-S1000F; Cheap project: CBR900RR FireBlade
Remember kids, bikes aren't like lego. You can't easily take a part from one bike and then fit it to another.

[Vincent]

Interesting fact - In the late 70s/early 80s, the MZ250 was the fastest 250 off the lights, (only up to about 25mph tho), and would temporarily leave the RDs, GTs and KHs behind

[Smooth]

[stevo as b4]

I'd have liked to have seen this happen, but i can believe it as the Classic motorcycle mechanics mag i've just read, was testing some 1982 Jap 250's back to back, and they stated that the RD250LC really doesn't even begin to feel interested below 5500rpm. Still it was never designed to be ridden like that, being the 1/4litre top speed king!

[stinkwheel]

Clutch is on the end of the crank on an MZ and you should feel how heavy it is! It acts as a flywheel which contributes to the massive torque at low revs.
____________________
“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.

[Robby]

Well, you've now got me wanting to buy an MZ, as well as some kind of 2 stroke dirtbike and a really unreliable noisy harley (1000cc ironhead should do).

This is from someone who had more or less given up on riding, and hasn't been on two wheels in over a year.

Fuck you all. I was meant to be saving for a deposit.