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Hello,
I'm new here so have mercy and don't crucify me for stupid questions, also I would really appreciate detailed answers because I'm not too familiar with all the terminology.
two weeks ago I bought Ducati Darmah 1978, for several days there were no problems and everything was working neatly, until one fine day I couldn't start it. It backfired vigorously( I mean really really loud much louder then a normal backfire, with long flames coming out of the exhaust). I unscrewed the spark plugs and saw that one of them was dark brown and the other was completely chary-black. So I replaced the plugs hoping that this will solve the problem.
However the problem came back the next day. The upper cylinder refused to work until I cleaned the plug.
I was advised to adjust the carb. to lower the needle in order to lower the fuel ratio. Because the screw was fully screwed in I opened the carb and moved the needle to a lower position on the Omega clamp. ( I cant lower it anymore). I also changed the air filters and cleaned the plugs once again.
I started the engine and it seemed to work fine for a while, I drove about 50 meters and once again the upper cylinder died on me.
I checked for spark and I do get a steady nice blue spark on both plugs.
Any suggestions to what can I do?

P.S. There is some faint however present smoke coming out of the exhaust.

It sounds like the carb has a stuck float needle causing the fuel level to be too high and it is fouling out the plug. You need to take the carb apart and clean all the passages and inspect the float needle and seat for dirt or damage.Put the needle back the way you found it or you will have running problems at higher speeds and could possibly do some harm.To start you should get a manual and read it and understand it so you don't get in over your head and cause more harm than good.There can be a few other causes for a dead cylinder and the carb is one of the first places to start. Also make sure the battery is charged as they won't run when the voltage is under approx. 11 volts. The battery needs to be checked with a load on it as it can show 12v or more and take a dump with the ign. switched on.

Read this post http://www.bevelheaven.com/~BB/phpBB2/v ... php?t=2251
and I found it quite helpful, I took off my carb and found out that the choke cable was stuck in its tunnel, got everything clean and no problems since then

Hello....I have just gone through the same issue with my Darmah however on the front cylinder. My engine has only 3500Km since engine rebuild and carby rebuild.

Front cylinder running very rich and blowing a bit of smoke. Plug will last a little while before it needed replacing or cleaning.

- Checked all jets (leaned the main jet on advice from expert) and float heights....all Ok.
- checked needle and seal...ok
- changed choke pistons.....they can leak if old.
- check if choke is permanently on due to cable slack. You should be able to hear if the choke is on.
- swapped front carby with rear (just carby not slide and needle makes job quicker) to see if problem followed the carby. Problem stayed with front cylinder.
- (problem 1) Found valve stem seals floating and letting oil into the cylinder (front cylinder worse because oil pools around inlet valve). After cheking all seals on both cylinders only 1 was ok. Fix this problem....still had the issue.
- (problem 2) I found out the Wiseco piston rings wear very very quickly. Checked and found front rings were stuffed (4 mm gap)...replaced with after market (not Wiseco rings which last longer I am told) rings.....still have an issue!!!!!!
- (problem 3) Last thing I tried was to replace the needle and needle jet. Both looked in perfect condition however after replacing the problem was fixed.

Only a Ducati could have 3 problems on the same cylinder causing the same symptoms.....it gives you something to do in your spare time.

Hope it gives you a couple of ideas.

One other thing. When I finally found the last issue I attempted to go for a test ride and the sprag bearing ate itself.

cheers

Having just read your post I have experienced your frustration but this only makes the good times so much better.