sooty plugs

[rio169]

My father has recently purchased a BSA A7 which has undergone a complete rebuild including engine rebore. He got it off a guy who got it off the guy who did all the work.
The mag was shot so it has been rebuilt by Dave Lindsley. The valve clearances where quite big so these have been reset. New NGK plugs B6HS which are the right value. Mag installed with pretty much bang on timing. The carb has been stripped and seems to have had an overhaul new needle jets screws gaskets etc, all to right value Now the problem. when the throttle is openend you apear to get resinence similar to a high speed misfire. I took the bike for an MOT no problem on the way back it started to misfire badly and cut a couple of times. At home the plugs where examined. they were very sooty. Once cleaned the bike started ok but after a mile or so run it starts to misfire and plugs are very sootty again.
Any suggestions please. Obviously a rich mixture but what cause, Carb good. Have installed a new brass needle and float ok no flooding there. All carb settinge correct for the bike. Timing corrct both plugs firing. I can only change the plugs now but I did have one spare and this one behaves the same. Could I try a hoter plug B5HS. but that doesn't seem right. It could be badly seating valves. But before going to that much trouble I am looking for other suggestions.

[Goldy]

The plugs should not be sooty but should be a sand colour. It sounds as though the mixture is too rich. The pilot air jet screw needs to be screwed out to allow more air. This should also increase the speed and smoothness of the tickover. If this cannot be achieved then it may be necessary to lower the needle by one notch. All the best

[rio169]

I have played with the pilot air mixture leaning it to the point where the carb is spitting so too lean. Have tried lowering the needle and it runs worse. exhaust poping abit

[Goldie]

Try dropping the main jet a couple of sizes, to see if it improves the mixture or even try another carb.

[Rex]

With respect, that's wrong.
Altering the main jet size only affects the top end of the rev range, whereas this bike sounds like the richness is across the range. Altering settings on a whim is no way to fault-find either.
Best thing is to start with the original carb jet sizes (freely available on the Net) and then check float height, air filter, etc for any possible reasons for such apparent richness.

Don't discount other causes for the sooty plugs though. Rebuilt mags may have those soft brushes which proved so problematic a couple of years back, and the timing etc could still be slightly off, leading to a less-than-perfect combustion.

If you have a manual I suggest going through all the settings making sure they're right.

Nice bike, by the way.

[Justin_Faithfull]

What you are describing could be a classic magneto problem, works fine when cold but developes a misfire when hot. I know you have just had the mag. rebuilt, & I know Dave Lindsley is very good. However I would still advise you to check this, although very unlikely, things can go wrong even with a rebuilt mag. Also check mag pick-ups, HT leads & plug caps while your at it (note: - do not use resistor caps).
Although I would not have expected the plugs to soot up, which does point to carb. trouble.

[harry]

Your problem seems similar to one I had about fifty years ago I was convinced  it was a carb problem;the engine would tick over sweetly but as soon as the throttle was opened it would splutter and baulk the problem was the condenser in the mag.By the way the bike was a 3T Triumph.Good luck Harry.