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Went to Sledmere from Hull for a vintage weekend. Bike went great, steady ride. On return, just about 3 miles from Hull on the Beverley bypass, locked up. Managed to free wheel at about 45 miles an hour for what seemed ages then gently release clutch she started again so I took it steady home.
Started it after the weekend and it took a while to start. I know I should have checked it out then not tried to restart. But it started so took it gently up the road, it stopped after spluttering in the next village. I pushed it to my VMCC friend in Thorngumbald and he brought me home in his Recovery van.
Now to the Point.
I have now got around to taking the head off.
Slightly sooty, I did notice this from some smoke from the exhaust before it stopped.
Head is the same.
Now pushed the piston down a little to see what I could in the cylinder bore.
Scored Barrel either side, So decided it needed the barrel off to see exactly what has Happened.
Piston not well.
The other side
Took piston off, can't use that again it seems to have jammed the oil ring in on the worst side and snapped it trying to remove it.
Maybe time to use my spare barrel and new piston.
A job for later in the week me thinks.
Well it doesn't look to be running weak! And assuming your timing is correct and not too retarded, it can only be not enough piston skirt clearance. I would get your engineer to check for a 5 thou skirt clearance. Is it a genuine piston? Some spurious makes have a greater expansion rate Also a good opportunity to re-check your timing with the head off. Ron
email (option): ronpier@talk21.com
Out of interest, how many miles had you done on the piston and liners?
Regards
Pat
email (option): sacombsashtrees@hotmail.com
ps did you stop off at Wetwang chippie?
email (option): sacombsashtrees@hotmail.com
email (option): bsa945@btinternet.com
email (option): bsa945@btinternet.com
email (option): bsa945@btinternet.com
email (option): ronpier@talk21.com
Paul,
I can give you some sound advice on your engine and how to run it, i even may be able to supply a genuine NOS piston depending on your size. I have run and worked on these things for years, also im only over at Huddersfield.
Just send me a direct mail and we can swap phone numbers.
Tim W
email (option): t.j.walker@btinternet.com
But Tim, please post the gist of your advice on this message string as it is really helpful to us all.
Paul,
Looking at you pics again the piston looks to have been running in the linered cylinder for a while, It could be genuine as it looks to have a Welworthy oil ring not like the new ones. The piston also has burnt oil under the oil ring to suggest it had been in there a while. You did not say how long you have had the bike and what miles you have done on it.
So why did you have the seizure, could be a few things main ones as Ron says timing and tight bore but if you have had the bike and ridden it a lot the tight piston would have nipped up before now. Could be running weak on the main jet as you said you were on the Beverley bypass so would have been on the main jet, the blackness you have now could have come from a rich pilot jet as you said you had run in gently after the seizure so this could mask the weak main jet. Dual carriageways and long bypasses don't do out long stroke engines any good holding them at high revs on the main jet for long periods with this new fuel. These engines in their day never came across full throttle for long periods so if you are to use them like this check for weakness at full throttle and just shut off every now and then.
Your cylinder looks like it will clean up with a hone it looks to be only alloy from the piston stuck on the liner you can then check the piston clearance on the thrust face. There looks to be plenty of clean oil about so that was not the problem, also your valves look nice and proud so if it was mine i would use the cylinder fitted find a genuine piston, hone the cylinder and check the clearance and magneto timing, and check the magneto lever has enough friction to keep the cable on full advance when you are riding it, wet sumping is a other issue and nothing to do with your seizure.
Hope it gives you some ideas
Tim W
email (option): t.j.walker@btinternet.com
I think you may be right about the bore, it does look only superficial alloy, just surface laid. seems to be no scoring or tram lines anywhere so anti glazing, a little honing may do the trick.
I have had the bike some time, but only pottered about sorting out basic stuff like Carb, Magneto, replaced mag dyno oil seal, new tyres, repaired broken lug that holds on the rear stand. :-) normal stuff.
Only have done around 140 miles on it then seizure.
Could be slightly out like ron said when I set up timing after Mag refitted.
Will double check this timing.
Apparently came from Ireland and was in service there before I got it.
I do have a spare barrel which I have had refurbed to fit my spare NOS 1/2 mm OS piston. Used an RM11 Dragonfly Oil ring set and managed to source a nice oil ring from Cox and Turner Engineering. Otherwise would have needed to open wider the oil ring slot for the RM11 oil ring.
So now two choices, hone up the old Barrel, fit new spare piston. After checking rings for for correct gaps of course.
Or
Fit valve springs to new Barrel. (Just need to get a spring compressor to fit valve springs.) Then fit new refurbed/painted spare Barrel.
Got my New gasket sealing stuff now.
My old Hylomar tube had gone solid in my tool box.
So now fresh tube.
when I get to put exhaust back hope you guys use the same stuff as this HOLTS firegum.
Decided to put recon spare Barrel onto bike. New springs fitted to new valves all nicely relapped in.
New piston fitted.
Gasket on barrel fitted.
Now to check out the points opening while piston easy to measure.
Paul Crisp
Tims point about weakness at the top end is very relevant...Check the main jet size is correct, needle is straight and that it's clip is in the correct groove for base settings..
Also check you have the correct throttle cutaway...
I'd recommend fitting a solid copper head gasket as well..If that is annealed fully no gasket sealant is needed on the head joint assuming the faces are good...Check the cylinder head gasket face is flat with a straight edge...
Ensure you have an annealed copper gasket under the cylinder head timing access bolt and check that the carb mounting flange is flat..Fit new gaskets to the carb joint as required...
Check the settings of the valve lifter carefully to ensure it is clear of the tappet head with the ex. valve fully closed...
Be ABSOLUTELY sure you have the timing set correctly..It is critical you avoid setting it at all retarded...There is a lengthy piece on the subject in the 'technical section' of the forum....
Retarded timing settings and mixture weakness are the two enemies of these engines, both of which can increase running temperatures to the point where damage is done......Ian
email (option): ian@wright52.plus.com
I ran my M20 for far too long on a very worn carb.
The outcome was a weak mixture that really ran the bike hot.
The carb flange was really badly warped as were the internals.
I bit the bullet and bought a brand new carb.
The bike now starts first kick (not exaggerating).
My extra bit of advice would be to really spend some time on the carb as all your hard work will be undone if it still runs weak.
I am toying with the idea of running the bike without the "bakelite" gasket between the barrel and carb flange. My theory being that the extra gaskets are yet another area for air to get in where it shouldn't.
email (option): sacombsashtrees@hotmail.com
Patrick,
The bakelite gasket is actually a heat sink, they do work and help to keep your carb body cooler and help stop carb body distortion. I think they are a good idea and they seem not to distort, just measure the thickness of it in a few places to make sure it is flat. You are correct about fitting a new carb, fitting one can transform your bike but it all depends on how worn your original carb is.
My point about the advance lever friction material or disc should be checked to make sure it does not retard on its own when ridding the bike, could be one of the main problems.
As Ian says soild copper head gasket and no sealents at all needed either side of the new annealed head gasket.
Tim W
email (option): t.j.walker@btinternet.com