New problem

[Pecon]

It seems that a new problem is beginning to surface with my GF plunger. I have about 600 miles on a rebuilt engine which was running well up to about two weeks ago when it started to develop a very slight hesitation mid-range. Now I notice that it is pinking ( pre-ignition ) under load and is getting harder to start from cold. If I don't get it to fire first kick I get a real workout..!!. When the timing was initially set up it started fine, I have checked the points and they also seem fine on both sides of the ring, although I have never replaced them. Changing plugs can help but only for a short time. The problem seems to come and go but is getting more frequent. My main worry is that the timing may have become too advanced and may damage the new pistons.

 I am more familiar with coil ignition systems so the magneto is a bit of a mystery to me. I am making the assumption (always dangerous) that it is an ignition problem. If anybody can point me in the right direction I would be very grateful as with my limited knowledge of the magneto I don’t want to make things worse.

TIA

Peter

[renos-a10]

Hi Peter ,

 I don't think that if you have too advanced make damage to the new piston.
My opinion is that your problem is the magneto.The same problem with you i have and i send the magneto for full restoration in England and i solved my problem. I change plugs every time i want to go ride and every time in first kick !!

 Best regards
Renos

[Topdad]

Hi Peter, Renos could be right but worth checking a couple of things first. 1st what colour are the plugs if white /grey pos weak mixture that could cause both the pinking and starting prob. If ok take the points out and check condition of the contact points , if any burning take 'em apart and dress them level and shiney then regap 'em . Then recheck timing 5/16 btc with the points just letting go of the proverbal fag paper ! if spot on great if not retime and hopefully that'll sort it if not and everything else like fuel flow from tank to carb and float height  is in order perhaps get the mag checked form a reputable company who can be sorced via this forum. Best wishes BobH.

[KenF]

I'd echo what Topdad says. If that doesn't sort it out, I've put a list on our web site of basic checks you can do on the magneto yourself with nothing more specialised than a multimeter, here:
http://brightsparkmagnetos.com/DIY/index.htm.

Best of luck.

Ken.

[Pecon]

Thanks for the suggestions guys. The plugs are in fact a bit on the black but I think that this is down to not quite having the mixture right as I have been trying different jets. The points look OK but they are the ones which were in the bike for the past few years. Bob would the float height be a cause for pinking?, my assumption was that it would be a timing issue. I have heard of low octaine fuel giving similar symptoms years ago but thought that this was a thing of the past with modern fuels. Thanks for the link Ken, magnetos for dummies  . I'll go through all the suggestions and eliminate them one by one. As I mentioned the pinking was the main thing which was worrying me as I had read of holed pistons and I do not want that..!!

Thanks again,

Peter

[Topdad]

Hi Peter, my understanding re the float height is if set to low it'd be fine on small throttle openings but get past that you could get fuel starvation/weak mixture which could I think contribute to pinking , much wiser heads than mine will confirm or denigh but really my suggestions are simply that of elimination , remember anything can be sorted ! best of luck bobH

[muskrat]

 G'day Peter, pinging is 90% of the time ignition too advanced, 10% lean fuel mix (slide-needle-float height). It wasn't doing it with nice clean pistons but after 600 miles a bit of carbon build up got it going. Spark plugs don't like being wet with the modern crap, I mean fuel, hence your starting prob.
 Close the points down a thou or two to retard the ign a bit and try. If it helps re-time her to 11/32" btdc with the points back at 12thou.
Pinging CAN destroy a motor in a very short time.
Cheers

[Pecon]

Muskrat, you’re probably right in your assessment of modern "fuel". I doubt if the mixture is lean as the plugs are constantly black, in fact I was about to drop the needle down a notch as fitting a smaller jet has not sorted out the richness I am experiencing. I guess that I will have to recheck the timing as this seems the logical place to start. I would be surprised if the timing has changed unless there is a problem with the automatic advance and retard mechanism, are these known to give trouble?.

Peter

[Pecon]

Well I dug a bit deeper and tried adjusting the points which were just under 12 thou and now it will not fire no matter what gap I put in them. I removed the points which look as though they have been rubbed down a couple of times so a new pair is in order there. I also noticed that the brushes are different lengths, one is just over 9mm and the other is about 2mm shorter. I had a look at the slip ring and gave it a clean up but while turning it over I could see that about 1/3 of the ring is bright shiny brass and the rest dull. Any ideas what might be the cause, could it have something to do with the brushes being different lengths?. I'm hoping that the ring does not have to be replaced but if it does will it be a job for a specialist or is it something that the (very) average home mechanic could tackle. Finally any recommendations as to where to buy points and brushes. Having heard the horror stories I want to avoid eBay for such important parts.

TIA

Peter

[bsa-bill]

the bright bit of the slip ring is brass the rest is Bakelite, the brushes contact the brass bit as it comes around picking up the charge to the plugs.
Difference in the brushes is not important at 2mm
 
No spark after you've moved the points -  suggests there might be something amiss in that area

[morris]

Peter,
For your convenience, I just measured mine. They are both 13 mm, and are in for quite a while as I never changed them.
As you have one that's 9 mm and the other 7, it wouldn't do bad to have a couple of spares at hand

[muskrat]

 I don't know that much about magies but I remember a prob I had once with the spring that works the points was "just" rubbing on the cam ring earthing out the points. Yours may have moved when adjusting the points, stopping the spark.
Cheers

[bsa-bill]

Good call Mushy,  I've had similar, I put a smear of grease on the cam ring and unbeknown a blob on the spring which resulted in a complete lack of spark, a cotton bud later all was well

[Pecon]

Thanks again for all the help guys, if I had thought about the brass insert on the slip ring it should have been obvious, I guess that I will have to learn to engage the brain before asking  . I have made contact with Bright Spark Magnetos and have ordered a new set of points and brushes. If that does not work it looks like it may be time for a recon job.

Thanks again,

Peter

[Triton Thrasher]

Buy what you like, but points being old does not make the spark disappear. Nor do brushes of different lengths. If you disturbed the points and had no more spark, you may have made the spring short out, or got grease between the points. You may have let an HT brush fall out of the pickup. Is the earth brush (or brushes) in place?

It's hard to think of something that would cause a good running bike to start pinking. Why would the timing advance itself? Timing slips, but usually goes retarded. Pinking can be caused by weak mixture, as said above. A mixture which becomes weak in service can be caused by a manifold air leak. Test by spraying WD40 at the joints in the manifold while it's idling.

As someone mentioned, too much coke in the combustion chamber can cause pinking. Not a common problem nowadays. 

Sounds like something is fouling your plugs, if new plugs help for a while. Blocked air filter causing richness and sooting of the plugs?  Did the bike start smoking?