Sav0r said:
You can do the seals without splitting the cases. Definitely worth it.
It is definitely worth it, but while possible to replace the seals without splitting the cases, please do not do it. It will be far more difficult to remove the seals (as they have a very pronounced ridge along their perimeter specifically intended to prevent the seal from moving with the cases together), than it will be to take the engine apart. It is absolutely NOT possible to install the correct seals without removing this ridge first, which is a VERY bad idea as this could allow the seal to move possibly creating the very problem you are wishing to solve. I believe (?) that earlier versions of this engine (the R-5 and DS-7) had different seals which Yamaha changed in the '73 350 which is the first year of the reed valve design which may be what the very knowledgable mr. Sav0r is thinking of. Regardless, your 350 is exceptionally easy to take apart, so if you wish to change out the seals - and I recommend that you do - split the cases. You can perform a crankcase pressure test, but personally I would not consider that a long term solution as your bike is old, and presuming it seals now, that is not a reason to expect things will be fine down the road. The very nature of 2-cycle engines make them very susceptible to catastrophic damage (compared to 4-cycle) with either too much spark advance or too lean a fuel mixture when you run them hard. This can happen VERY quickly and even the very experienced may not suspect that trouble is afoot until it is too late. For that reason, it is important to ensure your ignition timing is correct and you don't suffer a lean fuel condition which the extra air leaking into the crankcase past old crusty seals does. Fortunately, these things are very easy to maintain (seals very rarely need to be replaced so long as the engine is used regularly).
Your RD has famously weak ignition coils, and presuming yours are original, age has very likely made them worse. There are aftermarket options, including replicas that are much better than the oem units. The points and condenser ignition is fine and works great with the normal tune ups. It is very possibly that you will have a lot of trouble keeping the plugs clean enough to run reliably with the oem coils, which will make getting it to run right pretty difficult.
You will absolutely have to take your carbs apart and clean them. This is not hard or complex, but you have to be super careful to use exact fitting tools (precise fitting flat blade driver for the pilot jets for example) plus great care and patience to avoid damaging some of the very soft brass parts. Heating the carb body with a hair dryer (hot as you can - hair dryers don't get that hot) may help a lot if you have trouble. Ryan is very correct about the importance of cleaning the tiny passages 100%. As a rule, I never need anything except spray carb cleaner and compressed air (and I have "rebuilt" many hundreds of carbs) to achieve flawless operation, so resist the urge to poke anything into the tiny passages. You may do more harm than good. It does happen that passages get clogged with some pretty hard goo, but it is rare.
Don't worry at all about your pipes. They can and do collect an astonishing amount of oil, and this can often drip out once the pipes warm up and even smoke enormously when you take it out on very fast hard rides and get the pipes really hot and burn off accumulated oil. There is virtually no chance that the baffles have carboned up enough to be significant at this point, but usually they are not to hard to remove once you get the little screw out which holds them in. There is a hole at the very back on the bottom of the pipe that lets you place a screwdriver through it to remove the screw. Once that is out the baffle simply slides out the back, though if it is crusty it may be a pain. If you have drips at the muffler-header connection try tightening the big nut that screws the muffler to the header. You will have to get an appropriate spanner wrench. There is a gasket in there which gets compressed when you tighten this nut. If it still leaks, you will need a new gasket, which are long discontinued, but you can find them if you look. Looks like your pipes are pretty nice. Really nice original pipes are very rare so if yours are take care of them! There are lots of aftermarket pipes available if you want more power. It is extremely unlikely the pipes are causing you any problems at all.
Cleaning your plugs with a wire brush is commonly a last ditch. Commonly (mostly?) plugs don't foul, they short. This happens because it is too hard to make an arc across the electrodes due to too much fuel and/or oil in the combustion chamber. Your likely very anemic coils probably make this a real problem. Although there may not be enough energy to bridge the too saturated gap, it is still a lot of energy, and the result is that it finds its way to "ground" by traveling along the insulator back to the base of the plug and the rest of the electrical system. If this happens a lot, a sort of carbon track results on the insulator which is a far easier path for the electricity to follow than jumping even even super clean electrodes in clean air. So the plug is trash and only a new plug is the solution. Not saying you can't clean plugs at all, but once the insulator shorts they are pretty much impossible to revive. And weak ignitions on 2 strokes do this A LOT! Once you get everything sorted out, plugs have a fair lifespan - even on an RD!
That looks like a really nice complete bike. If you decide to alter it, take care of all the parts you take off. There's lots of people out there looking for them! I have several of these things, but I ride my totally stock one most of the time.