199b Knock And Chatter

[sherpa]

My 199b runs great when first started but as time goes on performance degrades substantially. By the 3rd loop she is knocking and chattering as if running out of fuel. Let the bike sit for a while and the bike gets better. Carb is original rebuilt and is clean, new points and condenser last year. Any suggestions?

[1oldbanjo]

Try loosening the fuel cap when it is running badly and see if it improves. It could be that your tank vent is not working and you are getting a vacuum in your tank as you ride.

Edited July 14, 2010 by 1oldbanjo

[sherpa]

Gas cap is clean and venting properly, tank is full and petcock is also clean. I was starting to think about ignition and that the problem is temp related. I heard about some people relocating the condenser away from the engine like on the frame next to the secondary coil. Thoughts?

[martinm]

Reckon it might be the low tension ignition coil on your stator plate.

I have had exactly the same problem with a few SWM's recently (same points & condenser style ignition as Bultaco). Symptoms are, that the bike will run fine from cold and start first kick, by second or third lap of event, starting becomes difficult and performance is noticeably down, even had them stop altogether. We eventually traced this to the LT coil, which have broken down over the years, and as they get hot they expand and it weakens the spark.

In all 3 cases, rewinding or replacing the coil sorted the problem.

May be worth checking with Dave at Bultaco UK, as I would think he has had the same problem before.

[sherpa]

Martin: It has been suggested that I look at the mag side crank seals. I will have to remove the stator in order to do that, so if I see no evidence of a leak I will replace the stator while I have it appart. Thus far the bike has not had any starting problems associated with the loss of power. Thanks for the info, I will keep you posted.

[sherpa]

Mag seals looked ok, I replaced the stator (I did notice a nick in the windings and a slight blue tint as opposed to the normal copper color). My manual recommends .016 breaker gap and .110-.118 btc on the timing, it does not specify the 340 sherpa only the 350. Are there any other numbers that would work better?

Thanks for the help

[sherpa]

Replaced stator, mag seals, no change at all. Replaced carb with a mikuni I had laying around and it made a huge difference, much better but not perfect. Can't seem to find anything wrong with the old carb except that it has always been pron to overflow. Even after replacement of float and needle and seat. Thinking maybe its time for a new carb. Any suggestions for MFG and model would be appreciated. Preferrably something available in the sates.

Thanks

[sparks2]

Hi

By 'knocking and chattering' do you mean the engine is pre-igniting?

If so check the exhaust is internally clean and unobstructed.

What happens when the exhaust is not clear and the engine is warm, the hot exhaust gases

do not clear the cylinder sufficiently and the incoming charge is pre-maturely ignited.

I suspect that the 340cc M199b is particularly prone to the problem as it uses the same exhaust

as the 250cc M198b.

I may be barking up the completely wrong tree of course, but worth a thought I think.

Regards

Sparks

[bultacosteve]

I also have the same problem with my M199B. 1st loop it runs great 2nd or 3rd loop it gets really hot and knocks horribly. I have the timing and point gap spot on. The crank seals and gaskets are new. I have a flatslide mikuni on it with a 20 pilot and 185 main. I tried an extra base gasket to lower compression and it still gets way too noisy. I took the top end off yesterday for some regular maintenance and there is no indication of preignition or detonation. No seizure marks on the piston. I quite honestly am at a loss as to why it knocks so bad. I actually think I'll put the bing carb back on and see if that helps.

Steve

[sherpa]

Placing a 30 mm mikuni round slide on mine seems to have helped, but as the bike was stolen, I never got to try it out in a trial. I now have the bike back (great job Schenectady Police dept) the top end is off and the piston is severely scored as a result of the thieves running the bike on raw gas. When I put it back together I guess I will be at step 1 again. I have a spare exhaust system, so I was thinking of trying that as the bike is apart already. If she runs smooth like a baby I will go back to the original carb to see if the problem comes back.

[sherpa]

Just an update, after the bike was retrieved I had to rebuild the engine due to being run without oil in the gas. Following completion the test run was still not good. I went back to the original Bing carb and reinspected all the jets and found that the pilot jet although not plugged was restricted. After cleaning out the jet with an equal sized drill bit the bike now runs perfectly. The mikuni did help the problem but it still never ran like I remembered it should, now it does.

moral of the story, just because fuel passes through a jet does not mean it is clean, make sure it is the proper opening.

[nigel dabster]

Just an update, after the bike was retrieved I had to rebuild the engine due to being run without oil in the gas. Following completion the test run was still not good. I went back to the original Bing carb and reinspected all the jets and found that the pilot jet although not plugged was restricted. After cleaning out the jet with an equal sized drill bit the bike now runs perfectly. The mikuni did help the problem but it still never ran like I remembered it should, now it does.

moral of the story, just because fuel passes through a jet does not mean it is clean, make sure it is the proper opening.

...or there is nothing wrong with the bing on a bulto!

[sherpa]

As usual dabster you are correct!