JAWA 350 tuning(?)

[Flying_fish]

Hello.

I have a JAWA 350 638, and I was wondering maybe I could squeeze a bit more out of it? Perhaps make a twin carb setup, and so on.

From what ive heard, in 1986 JAWA produced the 638 with 34hp, but then in 1987 they redesigned the pistons and exhausts to reduce power. Would anyone know how to tell if a piston is from 1986? This seems like a good starting point.

Any info on this would be nice!

[stinkwheel]

They are a hair off exploding as it is. I have blown up several.

Twin carb is probably a non-starter. For one, they don't run in a balanced fashion with one carb. For another, the inlet ports are really close together and the manifold is held on with an assymetric bolt pattern of 4 studs. So you'd have to make up a manifold that angles out from the engine to have room for two carbs then you'd need to find a way of bolting it on.

Some ways you could tune it:

One is an overbore. They make up to a +2.5 piston.

Spanny exhausts. But you'd need to work out the maths of it all. I always had an idea you could use a pair of MZ150 exhausts.

Increase compression by skimming some off the spigot and running it without a gasket by lapping the heads on. You'd need to keep an eye on the squish band thickness, best way would probably be to skim too far then set the squish with gaskets.

Polish the casting burrs and texture off the inside of the ports and inlet manifold (and even the inside of the crankcases) to improve gas-flow. You may have your work cut out here because on a lot of them, the port in the liner and the port in the alloy barrel don't line up properly leaving a distict step.

Chamfer the piston crown and top of the pistong ports to advance the port timing.

See if you can fit a reed valve block in the inlet (you'd probably need to fabricate this).

Then you'll need to dial in the carb to suit all of that.

Then it'll blow up (as I say, I've blown up a few, at least three).

Some pistons have a long skirt with a rectangular window shaped port in them. Some are shorter with a semi-circular cutaway in the skirt. I always assuned the latter were the higher performance ones.

If I was going to bother my arse with a Jawa again. Here's what I'd do:

Pull the whole engine apart. Throw all the bearings in the bin and replace them with good quality branded ones. Replace the crank seals with branded viton ones. Make absolutely certain the centre mechanical seal/bearing holder is a perfect, airtight fit in the cases. Machine/file/burr the cylinder ports so the liner at least lines up with the barrel. Polish and knife-edge the inlet manifold. Meticulously ensure the manifold and carb fitting is leak-free. Lap the heads in, measure the squish and space them so they are equal. Polish the piston crown and put a slight chamfer on the inlet edge and piston ports. Fit a VAPE powerdynamo. I'd consider fitting a different carb like a dellorto and dialling that in. If it's an auto-oil remove that and premix it. Have the brake shoes relined with modern linings. Consider fitting a pair of MZ exhausts.

Most of the above is aimed at making it run properly rather then tuning for performance. They do no run properly out of the box. No point in tuning a guitar with a broken neck.
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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.

[A100man]

[Flying_fish]

[Flying_fish]

I already have fitted the Powerdinamo VAPE ignition, so that front is semi ok now. I guess what I can gather from all this is that, the build should focus on reliability instead of performance....

Well, ill still try and make SOME improvements - "Polish the casting burrs and texture off the inside of the ports and inlet manifold (and even the inside of the crankcases) to improve gas-flow."

The reed valves also could be an interesting option, I will look into that as well. Thanks for the ideas.

[stinkwheel]

In fairness, making sure the cylinder bore is the correct size and both circular and parallel wouldn't be a bad idea too.

But yes, I'd probably focus on getting it running as well as it was designed to. Once you have that, then you can think about improving on it.

For what it's worth, the older ones (pre model 634) are much better made bikes so if you can come up with a 50's or 60's engine, it might be worth a look. They seemed to loose interest after the mid 1970's and just contented themselves with churning out mass production junk. The 638 was an improvement on the "oilmaster" though.

Seriously beware of auto-oiler systems on them. They kept reducing the oil ratio to comply with emissions regulations. I've heard of some putting out 70:1 or even less which is not good for long term lack of exploding.

If I could go back in time and talk to the teenage me who wanted a quirky Eastern European motorcycle, I'd tell him to buy an MZ TS or ES 250... They are better built, more reliable and the performance tuning of them is well understood and documented due to there having been a single marque race series for them.
____________________
“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.

[Flying_fish]

[Pete.]

Damn things were lucky if you got them running reliably let alone tuned.

BTW they did a 250 too, I know because when I was a lad I bought a 350 which someone had fitted with 250 barrels but had also dropped a load of little end needle rollers in the engine which promptly wrecked the whole top end.
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[to v or not to v]

you can still buy a new 350 in the UK. 6 grand though
https://www.jawamotorcycles.co.uk/350Classic.html
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current bike Yamaha Thunderace.
Moto Guzzi V7.

[stinkwheel]

[blurredman]

[PotatoHead202...]

There is a guy on one of the UK Facebook groups who could tell you what you needed to know parts wise. He'll also tell you if the bolt heads are the wrong size (yes, really). Anal doesn't even begin to describe it.....

Finding parts that actually fit and do what they're supposed to do is a fucking nightmare for these bikes. They really are absolute shite. It was rather amusing seeing roughly only 10% of owners actually turn up to the owners rally on one rather than drive or riding a Japanese bike....

Never again.
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Suzuki V Strom 1050XT, Honda CBF500 ABS,

[stinkwheel]

Parts are easy. There's a place called MZ B who keep loads in stock. Also various Turkish places who sell parts for buttons.
____________________
“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.

[PotatoHead202...]

[blurredman]

Some things are not well made and not well finished, others are perfectly fine. Some items that have been reproduced have broken fairly quickly and I've just decided to buy original 2nd hand, but other items (such as cables) for about 1 or 2 pounds each, providing the length is right have been completely satisfactory!
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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 - 50k.