Jawa 350 638 general tech chat

Hi Smokiejoe

I have never owned or worked on Jawa twins. As it was a wet Saturday afternoon I spent a couple of hours reading up on the Jawa 638, 639 and 640 twins.

When petrol / oil lubricated two strokes smoke excessively it is usually one of the following reasons:

1. Too much oil in the petrol. The Workshop Manual states that the later 638, 639 and 640 twins should be run in on a 50 to 1 mix and this can be reduced to 60 to 1 when they are fully run in. This sounds like a recipe for disaster on an older design like the Jawa. There are comments online that 32 to 1 is a sensible oil ratio if you want the engine to be reliable and have a long life.

2. The engine is drinking its gearbox or primary chaincase oil because the crankshaft seals are worn.

3. The engine has been used for a lot of short journeys and oil has accumulated in the silencers. A good long run to get the silencers hot will burn off the accumulated oil and the excessive smoke will disappear.

4. The carburation is too rich. This is normally caused by a worn carburettor needle jet. It causes the engine to four and eight stroke which does not combust the fuel and oil properly.

5. The ignition system is faulty which is causing the engine to four and eight stroke.

6. The silencer baffles are blocked with carbon which is restricting the exit of exhaust gasses and preventing proper combustion from taking place. 

The Pinking and vibration issues could be one of the following:

1. The engine needs de-coking.

2. According to the Workshop Manual the compression ratio is 9.8 to 1 but it gives a tolerance of +0.7 to -0.3. So it could be 10.5 to 1 maximum and 9.5 to 1 minimum. There are mentions online that the engines do not like E10 petrol which causes them to pink. Jawa spares specialists sell 2.0mm thick copper head gaskets to reduce the compression ratio which apparently solves the pinking problem.

3. The ignition timing could be incorrect. The Workshop Manual gives the ignition timing as 2.5mm BTDC. There is a relatively large section in the manual about fault finding and testing the VAPE electronic ignition systems, presumably they are not very reliable. One of the problems with electronic ignition systems on twin cylinder two strokes is that there is only one ignition timing adjustment. The two firing points are fixed at 180° apart. In theory this should be fine as the crankshaft should be a 180° crank. However if the crank has not been built accurately it may not be 180°. On the old points systems you could adjust one set of points relative to the other to correct any discrepancy. Some owners of Villiers 2T, 3T and 4T twins have found this problem when they fitted electronic ignition and the engine ran rough. Check the ignition timing on both cylinders to see if there is a timing error. In theory it is possible to make a special electronic ignition rotor to overcome any timing problem, which is probably cheaper and easier than rebuilding the crankshaft.

3. Check that the engine mounting bolts are tight and that the holes are not worn. As the Jawa uses metric bolts you should not have the problem that I find on some BSA Bantams, where the imperial bolts have been replaced with slightly smaller diameter metric bolts.

I hope that the above is of some help to you. If you want a copy of the Workshop Manual it is available as a free download on www.jawamotorcycles.co.uk . The only problem with the manual is that some of the translation is not very good so it can be confusing.

Regards
Butty Bach

Thanks for all your help here Butty,

Some of these things I am addressing, I am running the bike on E5 and 40-1 mix. I am planning on replacing the whole exhaust system as it's pretty curded up and the chrome is peeling places, that and it has some damage (but I will keep and clean the old pipes)

I have done a few long runs and it still smokes and oils up, but I have the engine out now and will replace the head gaskets with the 5mm ones and I have a set of crank case seals to put in. I will also over hall the carb or replace for new.

The big problem with the vibrations apparently comes from a clutch fault which I haven't got to the bottom of yet, the clutch looks fine, almost as if it's been replaced, but there is some play in the drive through to the gearbox, so maybe some new bearings are in order?

Many thanks for giving this as much thought as you have.

Joe 

I had the quire chair too, but with the little bouncy wheel and the screen never use to last long....not many pictures of it though

Hi Smokiejoe

It's very sad when you find that kind of damaged in so called 'rebuilt engines'. I have come across quite a few engines that have been totally wrecked by people attempting to re-build them. I would rather buy an unrestored wreck than a bike that has been re-built.

I have watched most of the 'Tweeds Garage' videos on 'YouTube' and was particularly interested in some of the later ones where he is attempting to restore a D14/4 BSA Bantam. When he dismantled the engine I noticed that various shims, spacers and washers were missing and the crankshaft was wrecked. However you do come across some factory cock-ups. I re-built a D14/4 engine for a friend and when I checked the gearbox found that it had two 21 tooth sliding gears. There should be one 21 tooth layshaft 3rd gear and one 22 tooth mainshaft 2nd gear. The two gears look identical except for the different number of teeth. The gearbox worked but the 2nd gear pinions were not meshing correctly.

Regards
Butty Bach