Timing slip?

[Pecon]

My A10 plunger has been behaving impeccably up to earlier in the week when while riding in traffic on my way to work I noticed that when the revs got to about 3000 rpm there seemed to be a constant backfire. When I got home I checked the points which seemed to have closed slightly on one side while the other was still OK. I opened them by a couple of thou but when I revved the bike I could see a blue flame shooting out of the mouth of the carb..!!!. Could the timing have slipped somehow or is this perhaps an indication of magneto trouble or something else entirely?. The bike was always inclined towards pinking when under stress even with the timing done as recommended but retarding it slightly made starting difficult. I settled for first kick starting and learned to avoid labouring the engine.

Peter

[muskrat]

G'day Peter.
That sounds odd as retarding should make starting easier. Backfire and flaming carb is too advanced. Is it auto or manual advance? Check the slip ring for carbon build up. This can cause the spark to be erratic.
Cheers

[Pecon]

Thanks Muskrat

 I'm guessing that the only thing that might advance the timing so much would be something to do with the automatic advance and retard mechanism?. I will pull the cover off over the next few days and check it for anything obvious. I cleaned the slip ring not too long ago when I was having starting problems but I'll check it again when the bike is on the ramp.

Peter

[a10 gf]

the automatic advance and retard mechanism

Hello, check that the weights can move freely. And that springs are intact and in good shape, if old\slack etc may be worth replacing with known good ones. And of course, the retaining nut on the mag must be well tightened up.

Good luck.

[chaterlea25]

Hi Peter,
You can check the timing and also wind the auto advance open from the points end of the mag, with out pulling the timing cover off, try turning the points unit in the direction of travel with your fingers, it should move and spring back
Check for up and down play at the points end of the mag which will produce erratic firing
If the timing slips it will retard it, not advance , then the pipes will blue and the engine run very hot 

Have you checked the valve clearances ? they may have closed up since the engine rebuild???
I dont know whether the petrol tank is lined on your bike ? but if the lining is breaking down it will deposit a sticky goo on the valve stems and can cause them to stick 

Regards
John

[Pecon]

Thanks guys, I think that at this stage it will be a couple of days before I can get to it but I'll check the things you have pointed out as possibilities. John, no lining in the tank and when I recently checked the filter in the carb it was clear. As for the tappets, they are in fact very noisy. This was next on my list as I had bought a set of SRM's allan key adjusters but was waiting until I had my other bike sorted. Hopefully the valves are OK but I suppose a compression test would confirm this. The fact that it is running great up to the 3000 rpm mark is a bit odd.

Peter 

[duTch]

 Maybe a bit of a a long shot, but check that the main/needle jets are tight and haven't rattled loose...??

[bsa-bill]

The fact that it is running great up to the 3000 rpm mark is a bit odd.

Not if the problem is carb related

Is it starting first or second kick when both hot and cold, if so that rules out many of the common Magneto ailments and also the A/R unit.
rare possibility would be points spring earthing at higher revs, a longshot and depends on your points type/set up I think  but Magnetos guys here will confirm or shoot down.

3000 revs? do you have a rev-counter fitted?, I'm wondering about this figure, would that be about half throttle, could be an air leak which does not manifest itself until sufficient suck in the manifold (higher revs), quick test - run a finger load of silicon around the carb inlet joint, let it set then go for a run.
Otherwise look at throttle slide, finger in the carb and see how much you can wiggle the slide about, also fuel supply to the carb, do plungers have two taps (sorry bit ignorant on plunger front) if so go for a ride with both taps on.

Just read dutch's comment, yes think that's along the same thinking as mine

[Pecon]

Dutch and Bill, all good points I'll go through each starting with the easy ones  The carb was recently overhauled but worth checking anyway. As for the 3000 rpm Bill, pure guess work, as you say about half throttle and yes it does have two taps and has been a first kick starter even with this problem.

[Pecon]

Just in case anybody missed my entry in the engine section it seems that the "timing" problem has been solved when I found that one of the exhaust pushrods had worn down by 1/4 inch which resulted in one exhaust valve only partly opening.

Peter

[muskrat]

Thanks for posting the result Peter. With so many members/posts/problems it's hard to keep track.
Cheers

[bsa-bill]

thanks for the that Peter, I hadn't related the two posts, didn't see that one cominh though