Jawa 350 638 Cylinder porting & results

[Flying_fish]

Hello.

Around a year ago I made a post asking what could be done to improve the motor in a JAWA 350, the answers proved to give some inspiration, but I decided to ultimately read a 2 stroke tuning handbook and look into it with a bit more fines.

A thing that caught my eye was porting via use of specific port-area-times.

It is essentially a means of quantifying how big does a port need to be for optimal performance at a certain RPM value. And after some calculations, I had come up with some changes that could in theory bump up the maximum RPM of the engine from 5800 to 6500. However the book also mentions that ring flutter is a thing, and that the 2mm thick rings will flutter at around 6400RPM for the specific engine.

Never minding that "small" problem, I changed the ports. Exhaust port was made wider by ~1.5mm on each side, and the intake was made bigger both height and width wise by 1mm, as well as the piston skirts being shaved down by 3mm increasing the intake port duration.

Ignorantly I assembled everything back and fired it up. Ran OK, but not up to snuff. Also after 1 run a bad ticking noise developed, and upon inspection the lower 2 piston rings had decided to separate into bits. Issue? - The ring pins were in positions where the rings now popped out of the exhaust port a bit, making them break off.

After acquiring new pistons and changing the pin locations it runs with no ticking.

Performance gains? - Currently not much. A healthier idle sound, not as much struggle, and up top it does go to 6k in 4th gear if you give it time, but this isn't much more than the stock 5.5k it managed to squeeze. Perhaps a modified exhaust and intake is the next step.

Perhaps I had done it wrong, that's why I'm making the post. The port job has been done like this - I took a pneumatic grinder and a self made guide, colored in the meat I would grind away and went to work. But I only made the port openings larger in the bore itself and a bit into the port, but not the whole port. Thinking about it logically this maybe wrong, since in the end the gases flowing would end up in the same size hole than before, just now they have a bigger door.

Any porting tips would be greatly appreciated. At the time of making this post I don't have any pictures of the actual port jobs, In the upcoming days I shall upload them.

[stinkwheel]

I referr you to my previous answer. Porting is something you do after everything else is bang-on. Do the existing port holes in the liner and barrel even line up? None of the ones I owned did by a significant margin and I don't see the point in trying to fiddle with port timing when the gas is flowing through something akin to an elevator stopped between two floors.

As I said, getting everything how it should be is a good starting point. So ensuring the bores are round, concentric and paralell (which they are not as standard). Ensuring the squish bands are at least equal (which they are not as standard). Ensuring there are no air leaks at both the inlet manifold and crank seals (including the centre mechanical seal). Ensuring the bearings are round.

A properly designed expansion chamber exhaust would probably do a lot for them. The maths is outrageously complex for this but finding one for an engine with roughly equivalent bore and stroke would be a good starting point. Or looking at pictures of exhausts off some of the Jawa/CZ race bikes and working out roughly where the cones start and end and how long the stinger is could help.

If I was doing it on the cheap, you can get MZ125 race exhausts very cheaply (as such things go) online. You'd obviously need two of them. When they arrive, they have a "silencer" fitted which is just a can which has a spring up the middle with exhaust packing wrapped round it. This needs replacing with a stinger pipe of an appropriate length. I recently fitted one to my Minsk 125 (same bore, stroke and port timing as the MZ), I used a solid pipe going the full length of the end-can with holes drilled in the last 2/3 of it. This gave a significantly improved performance, it revs up quickly with a real crack to it and only fires occasionally on the overrun which shows it's scavenging well.

The pressure wave timing wont be correct for a Jawa, but it'll probably be at least as good as the standard exhausts, you might get lucky and it works. It'll probably SOUND faster.
https://lh3.googleusercontent.com/pw/AP1GczNIiRA-iGWPHn-DOD9Q9Yj_uepNvycnH3gfQbb7gaKYPtFs-weqT2vQrLIg7ODvmFbBCZvTIjfiVNvBPqxOHekpD8QK4l8aw2CfwmFzR_qEWYodQy0vTCWXZv96p1C2C2QRIRiuSy1eSfu8U9NhpZHz=w1669-h939-s-no

Your Jawa will blow up. As I said before, tuning is what you do once everything else is running optimally. Round, paralell, concentric bores, equal squish bands, round bearings, perfectly set ignition timing, clean gas-flow, a properly sealed engine, dialled in carburettion. THEN tuning. I did all these things to my Minsk before fitting the expansion chamber, then had another go at the carburettion and ignition timing to suit. It's still dog slow but runs really well.
____________________
“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.

[that_impulse_guy]

if you watch this video, which is trying to explain expansion chambers
https://www.youtube.com/watch?v=cDCeOjHQZsM
you'll also inadvertently realise the main thing about 2 stroke is actually the port opening and closing timings as the pressure pulses are where gains are at.

I can 100% guarantee you that a 1mm or 2mm bigger port will not need the entire port widened as you propose. The timing of when the port opens is much more critical.

Yes, gains are to be found in every little detail, but if its "no real better than stock" then spending half a day ensuring the port is exactly equal diameter all the way to the pipe fitting isnt going to fix that.
____________________
Gone: Yamaha DT50lc, Suzuki DR500, Suzuki A100, Kawasaki z250ltd, RD350YPVS, Suzuki DR Big, Kawasaki AR125, Kawasaki KMX200, Suzuki GS1000S, Katana 1100, GS550M, Suzuki RGV250
Now: Suzuki GSX400X, Suzuki RF900R, NS400R

[stinkwheel]

I also seem to remember some of the "special edition" model 640 Jawas they came out with just as the brand was dying off had factory expansion chamber exhausts. I think one was the 640 "sport" and another was the 640 "tramp".

I'd imagine the silencer is very restrictive and could do with being modified with a stinger in it but they are a proper expansion chamber and are designed to fit that engine.
https://www.bikechatforums.com/files/200904022331441.jpg
____________________
“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.

[A100man]

[A100man]

and yet..

https://www.motorcyclespecs.co.za/model/Jawa/jawa_350%20TS%2087.htm

27hp @ 5500 rpm
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Now: A100, GT250A, XJ598, FZ750

Then: Fizz, RS200, KL250, XJ550, Laverda Alpina, XJ600, FZS600

[Flying_fish]

[Flying_fish]

[Flying_fish]

[A100man]

Now you've 'opened it up' perhaps it's worth tryimg a decent pair of carbs - something off a JAP 250 or 350. Got to be better than the standard single set-up, no?
____________________
Now: A100, GT250A, XJ598, FZ750

Then: Fizz, RS200, KL250, XJ550, Laverda Alpina, XJ600, FZS600

[stinkwheel]

[A100man]

[Flying_fish]

I have a work-in-progress 2nd engine that will use Yamaha pistons and will have custom reed valve adapters machined for each cylinder in the somewhat near future (hopefully).

The cylinders for that engine too has been ported so it should be quite OK at the very least.

I will make a post once it the build has progressed.

[that_impulse_guy]

when I was a youngster and had access to a welder and a 50cc two stroke I took upon myself to make some pipes up. I had no books, or maths to fall back on...just "well, this looks ok".

I made 3 pipes. They were all fitted to the same japanese 50cc rated at 7.2ps with standard exhaust.

I had no dyno.

This isnt going to surprise anyone with the results as two of the pipes did nothing much. The third one moved the powerband down from a peak at 10k down to around 7.5k. It wasnt only that it wouldnt rev higher..it was actually torquier lower down. Not by much. But it did move things down the rev range. I went from a 12front sprocket to a 15 tooth.

Ok, why the chat.
Well, with same port timings, the exhaust itself can make a massive difference.

And this conversations seems to have moved slightly towards "why isnt the jawa making 200hp/litre like an RG500" which I'm actually quite interested in.
____________________
Gone: Yamaha DT50lc, Suzuki DR500, Suzuki A100, Kawasaki z250ltd, RD350YPVS, Suzuki DR Big, Kawasaki AR125, Kawasaki KMX200, Suzuki GS1000S, Katana 1100, GS550M, Suzuki RGV250
Now: Suzuki GSX400X, Suzuki RF900R, NS400R

[blurredman]

I don't think an expansion will help you.
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CBT: 12/06/10, Theory: 22/09/10, Module 1: 09/11/10, Module 2: 19/01/11
Past: 1991 Honda CG125BR-J, 1992 (1980) Honda XL125S, 1996 Kawasaki GPZ500S, 1979 MZ TS150.
Current: 1973 MZ ES250/2 - 18k, 1979 Suzuki TS185ER - 10k, 1981 Honda CX500B - 91k, 1987 MZ ETZ250 (295cc) - 40k, 1989 MZ ETZ251 - 51k.

[that_impulse_guy]

also regarding this:
"Perhaps I had done it wrong, that's why I'm making the post. The port job has been done like this - I took a pneumatic grinder and a self made guide, colored in the meat I would grind away and went to work. But I only made the port openings larger in the bore itself and a bit into the port, but not the whole port. Thinking about it logically this maybe wrong, since in the end the gases flowing would end up in the same size hole than before, just now they have a bigger door. "

using your terminology, the door ideally should not be larger than 75% of the port size. So to reiterate, its really doubtful that enlarging the inside diameter of the port area to match your new port shape is going to do anything.

here's another nice video, but youll probably need to add subtitles.
https://www.youtube.com/watch?v=_9DteiVQNl8
(its worth watching as much as you can of this youtuber...as he is going through possibly the same things as you)
____________________
Gone: Yamaha DT50lc, Suzuki DR500, Suzuki A100, Kawasaki z250ltd, RD350YPVS, Suzuki DR Big, Kawasaki AR125, Kawasaki KMX200, Suzuki GS1000S, Katana 1100, GS550M, Suzuki RGV250
Now: Suzuki GSX400X, Suzuki RF900R, NS400R

[stinkwheel]