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  #1  
Old 24/05/2017, 08:09 PM
Ken Wilson Ken Wilson is offline
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Default 2T Centre seal

Returning from spectating at a classic race meeting in Holland at the weekend my usually totally reliable 25DC started to play up.
Steady state running on the flat was OK at about 45mph but when the engine had to work it started missing with popping in the silencer and "made smoke" on a grand scale.
When we stopped I checked out the ignition, points, carb, plugs etc but could find nothing wrong.
After a slow ride up to home from Hull I had a closer look at ignition etc. But could not bottom it.
Over the years I have replaced big ends, bearings and seals so now have started to worry about the centre seal.
Remembering something I read a while ago, I have removed the barrels and filled one crankcase with petrol and it looks very much as though the level is going down. I will leave it until morning to check properly but I guess this is an indication that the seal is leaking.
If it is the centre seal is it a job for VS or can an M/C engineer replace it?
Any advice as to how to proceed would be appreciated ?
Regards Ken
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  #2  
Old 24/05/2017, 09:22 PM
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John Wakefield John Wakefield is offline
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I am not convinced that filling one crank case with petrol is a good way to test the centre seal, unless it is virtually running through very quickly, there is bound to be a bit of seepage over a period. The symptom you have of popping and banging in exhaust suggests a wiskered plug. The smoke is caused by unburnt fuel & oil burning off in the siamese exhaust. Centre seals rarely fail that quickly. From what I have been told by others who have had centre seals actually go (apparently the early 2T's were prone to this before Villiers fitted a different seal) is that the engine is difficult to start and requires a push start & then keeping revs up. So before you start pulling it all apart worth checking plugs, and also the connections in the multi pin plug & socket. Does your bike start ok & run at slow revs? If so its not the seal.
If in the end you need to get the crank rebuilt then you will have to have it done by a specialist, VS or Simon Bateman or other engineer with the appropriate alignment jig & a press. There used to be a number of people back in the day including Mike Palmer of Bury St Edmunds who reconditioned cranks. Most have long packed up, sadly Mike, last time I heard was in a residential care home with dementure.

Last edited by John Wakefield; 24/05/2017 at 09:27 PM.
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  #3  
Old 25/05/2017, 07:36 AM
Ken Wilson Ken Wilson is offline
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Default 2T Centre seal

Thanks for that John
Overnight the petrol in the crank has equalled out between the 2 sides, your comment about seepage seems logical.
The bike still starts perfectly OK and is fine under no load so that seems to rule out the centre seal thank goodness.
The muti-pin connector has been a problem in the past but checked out OK with the multimeter, plugs and points are OK too.
So later today it is back to the garage to go over things again with a closer look at the multi-pin connector and a look at the ignition coils. Some years ago I replaced the Villiers coils with some Japanese ones.
I'll post my findings!
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Old 25/05/2017, 08:09 AM
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John Wakefield John Wakefield is offline
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I have an occasional problem on a 4T engine, sometimes starts on 1 cylinder but comes back in on both after its run a few seconds. I have checked pugs, points, connections etc but can find no obvious fault. Think it may be one plug oiling till it warms up. I use extended tip NGK BP6HS plugs.
I also sometimes get a whiskered plug when the engine is working hard up a hill or incline. Just one of the 'joys' of riding a two stroke.
Looks like it was whiskering on your bike, the whisker may have burnt off by the time you took the plugs out.
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  #5  
Old 09/06/2017, 10:34 AM
Ken Wilson Ken Wilson is offline
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Default 2T Centre seal

Well I have finally cracked it!
A dicky condenser on the RH cylinder.
A good healthy spark when the engine is cold and no problems starting.
After a run to heat the engine up it was back in the garage, quickly whip out the plugs to find that the RH plug was sparking but not as well as the LH one.
Swopping the condensers over moved the problem to the other side.
As one condenser lead is longer than the other I swopped them back but as the engine cooled the spark returned to its "normal" strength.
I replaced the condenser and the problem has gone away.
I now have an ignition system that has been checked and tested to an inch of its life and a twin that is running perfectly.
This Sunday is TYMC's 100 mile "Durham Dales Run" so for me it will be 150 miles hopefully in good weather.
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  #6  
Old 09/06/2017, 11:00 AM
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John Wakefield John Wakefield is offline
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Glad you have sorted out the problem Ken & at very little cost, much cheaper than an engine strip & rebuild. Good to here the bike is being used as intended.
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