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 appear 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 correct 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.

[bsa- bill]

Todays fuel seems to burn richer, I had this and changed to a hotter plug and it kept the plugs cleaner.
This would be the easiest to try first if you have plugs else drop the carb needle one notch and go for a short run

All the best - Bill

[A10Boy]

This sounds too dramatic to be plug values, cold plugs normally soot up only after some miles of use.

Its unlikely to be valve seating problems, some missfire maybe would be evident but you would notice low compression / power before the valve burns out [if its exhaust]. Is the compression ok ? Is the choke slide lifting ok?

If you have checked and double checked carburation, ignition timing etc and all is ok I would look to valve timing, these are classic symptoms of incorrect valve timing, make sure the timing marks are lined up correctly and check that someone hasn't added different marks which has thrown the builder - it happened to me once!

BTW incorrect valve timing can cause low compression.

You don't say if it ran OK when your father bought it.

[beezalex]

How did it run BEFORE you rebuilt the carb?  Did you check the needle jet for correct size?  Did the new needle look like the old one?  If it's sooting up in regular riding conditions, it sounds like something to do with needle or needle jet, float level or other.

If it runs well on fresh plugs, I wouldn't bother looking at anything other than carburetion.

What you really need to do is a proper plug chop.  With multiple rebuilds, different silencers, different fuels, etc., mixture conditions are going to change, and you have to figure out what they are now.

[Brian]

What year is the bike, does it have a air filter fitted.

If it does have a air filter then try it without it.

[Triton Thrasher]

There is a carburettor tuning procedure, as Beezalux hints and it's the only really valid way to get it right.

Having said that, if you want a quick confirmation that you are suffering from richness, drop the jet needle in its clip, by a notch and see what happens.

[A10Boy]

I think we need a bit more info on how the bike ran before and exactly what work you've done to it.

[rio169]

When the bike was bought it would only run for about 1 to 2 minutes and then would would konk out. After alot of playing about we decided to take the mag off and took it to dave linsley. on checking it out it was in a very sorry state. the brushes where cheap graphite which made a mess. the mag was original from 1959 so it was probably due some work. Because the bike would only run for maybe a minute or so it is hard to judge what it was like. the carb looks in good condition it appears to have had on overhaul kit bought for it, new neddle jets etc. Everything it set correct for this bike. It is not running with an air filter just a simple bell mouth. From the replies I am leaning towards maybe incorrect valve timing. I have discounted the mag carb settings. plugs valve clearances. If the inlet opens a little early would that let too much mixture in and conversly the exhaust would close a little early resulting in not letting all the combustion material out leading to a build up. A more high performance cam obviously opens early but the exhaust also closes later so in balance so to speak. As I said originally when the throttle is opened it is not smooth. There is a ddddddd sought of resinence that obviousy increases with revs. I does this with clean plugs. As the plugs start to foul up then it starts to misfire. Going to Stafford tomorrow

[A10Boy]

It still sounds like valve timing to me. see you at stafford, I will be the fat bloke in the black BSA hat...

[BSA_54A10]

Very soot plugs & bike that misses worse the further open the throttle is could very well be the main jet  loose in the jet holder.
Ols carbs often need some sealing compound to seal them off properly.

Go out and spend £ 30.00 on a Gunsens Colour Tune.
Read the instructions.
Read the instructions
Read the instructions.
Screw it in and look in the mirror.
If the maggy is firing you will be able to clearly see the spark so that elimimates the maggy.

First principles of science ( & engineering) OBSERVE TO PROBLEM.

[rio169]

I should of got back earlier but the problem is now cured. I went over some hints and tips which I downloaded off the Amal site. On looking through these I noticed there are three different needles B C & D. Basically the 376 carb on the bike had been fitted with the needle from a 375. I purchased the correct needle. This was about 3/8" longer and a different taper shape. This has cured the richness. On the old needle as soon as the throttle was opened and the needle raised an excesive amount of fuel was allowed through causing the problem.
On looking at manuals such as Haynes they tell jet sizes, throttle valve size and cutaway and clip position on the needle etc but no mention is made of there being different needles. After much tearing out of hair it is now running great.
If others have a similar situation where everything seems correct it is worth checking the needle. In this case the carb had seemed to have had an overhaul. The needle was shiny. It was all clean. Take nothing for granted.
Thanks for all the feedback