When lapping the taper I move the gear around as you would when lapping in a valve...I don't think the method is critical though as both ways produce the desired result...I don't use any form of Loctite or similar as a taper is a mechanical lock and doesn't require it when functioning correctly..The thread on the armature is 3/8"BSF...Always check the setting after you have completed the timing procedure as movement of the relative positions is common when tightening everything up...
Personally, I like to time the bike with the head removed, using a depth micrometer to determine the correct piston position and a 'buzz box' for determining the point at which the contact breakers open...With practice this method ensures accurate timing every time...Using the 'stick down the timing hole' method will produce very variable results...I once timed the engine 5 times on the bench and got 5 different results...Of course the bike would have run but the timing wasn't 'accurate' to the degree I prefer...
Regarding the piston failure, weak mixture setting, mixture weakness caused by air leaks, retarded timing, incorrect bore clearance or driving too hard in the initial stages of running in are the most likely causes...Ian
email (option): ian@wright52.plus.com
Thanks for your reply and advice Ian. Very much appreciated.
The initial timing was set up with the head off with a dial gauge and buzz box. I used the 4mm rod just to verify while running the bike it had not shifted appreciably.
I checked it with the buzz box and dial gauge after I found the damage and it was correct.
I drained the oil after 300 miles and found metallic fragments in the oil tank. This prompted the removal of the head. Disappointing as the bike was running quite well.
Probable Cause – combination of incorrect bore fit and lean mixture
Incorrect bore fit
The piston and barrel were bought as a second hand refurbished set. Not sure if they were matched correctly.
Lean mixture
The bike was run initially for a few very short gentle runs on its original carb. The head was removed after this and the bore was fine. The carb was worn out and leaked so I had to solve this for a roadworthy. The 626 was introduced settings –
Pilot = 19 (Premier)
Needle = 106
Cut Away = No 4
Needle was in initially in position 1 (lowest position)
Gave the bike a test run and using the air lever found this to be to lean. The bike did seem to get quite hot.
Needle position 2 (Middle)
On testing this was an improvement, use of the air level indicated a lean mixture as the bike would run much better when you close it. The bike seems to get quite hot.
Needle position 3 (Highest position)
Bike ran very well. Not getting hot. Introducing the air lever resulted in an indication of rich mixture now.
I did remove the exhaust between the carb set up and getting the roadworthy. I do remember seeing some white flecks in the exhaust port. This is probably when the damage has occurred.
Learnings
If you buy a piston and bore that as supposed to be matched – get them checked.
Probably not a good idea tuning a new carb an a fresh top end rebuild. Use a proven set up.
I hope this may help someone else avoid this situation down the track.
Cheers
Brian
email (option): zola.industries@bigpond.com
I'm pleased to see this thread resurface.
The damage to my engine wasn't as great as feared. The bore cleaned up well, what looked like scoring was piston fragments adhering to the cylinder.
So a washout, new piston & careful running in & it's done several hundred miles now & feels OK. But I'm not running it hard.
I tried a #180 main jet which the engine didn't like at all. Sluggish with a hunting sensation at low throttle openings. I haven't moved the needle from its #2 position.
Re reading my first posts, the information I put up there was double checked by others, nothing seemed so seriously out as to cause the failure so I still don't know what went wrong.
BW
Do you mind telling me what part of the world you're in?
G'Day Bob.... Australia mate...
Ian has posted some settings for his 626. Will be using the same when I get going again.
Hopefully will yield a different outcome.
Cheers
Brian
email (option): zola.industries@bigpond.com
Thanks for the quick reply, Brian.
I live out the back of Adelaide.
At one stage I had a Concentric on my bike. It was a lot less hassle. I can't remember the size, it was fitted as part of an unsuccessful big bore experiment. The carb was about the only good part of the whole thing.
Bob
Know how you feel Bob.
The carb is probably the only good thing on mine....
email (option): zola.industries@bigpond.com
In the old days we used to time the old field bikes T D C on full retard and advance as needed until the engine ran sweet
Yes it's quite easy to adjust the timing on the road with a distributer. Not so with an M20 though:thinking_face: Ron
email (option): ronpier@talk21.com
My WM20 doesn't have the timing cover access plug for the magneto pinion, which is probably correct. In practice, how easy do these make "in the field" timing adjustments?
Far better to get it correct in the shed before you enter the field Bob. Ron
email (option): ronpier@talk21.com
Quite so Ron. Attention in the workshop saves many a problem on the road. I may've been lucky but I've never had slippage of the pinion on the road. Or at all, come to that.
Without experience of the plugs, I've always thought they looked like both a good idea & an inconvenience. It saves having to remove & replace the timing cover but looks fiddly & it would be hard to keep oil off the taper.
I am right in thinking that the spring washer fitted into the mag pinion is a non standard thickness? Could some of the pinion slipage mentioned in this thread perhaps be attributed to an incorrect spring washer being used? Just a thought.
email (option): fuzzyempire@hotmail.com
I never fit a spring washer and have never experienced 'slippage'...
Regarding the timing cover with the plug I find them very fiddly to use for resetting timing and would rather take the cover off...I do carry a timing cover gasket in my 'touring spares' package but have never actually used it...As long as the tapers are good the timing setting is very unlikely to move...Well it hasn't for the last 45 years!:laughing: ....Ian
email (option): ian@wright52.plus.com
Same story as Ian.
Because there is no advance mechanism with bob weights hammering ( well tapping any way )on the taper as you get with the auto advance models there is little to cause the pinion to walk on the tapper
I carry a pinion puller inmy kit as well and like Ian have never used it roadside.
I have been known to cheat on occasions on the side of the road and alter the points gap when I suspected the mag on some one elses bike was off time a bit.
The only time I get s slip is tightening the pinion bolt but a LIGHT tap with a lead or leather mallet will usually lock the taper before you tighten the bolt
Note the use of the word LIGHT amd it has to be a dead blow, rubber mallets just don't do the job