Hi Team, my bike is suffering from lack of compression. BSA WM20. I can actually turn the engine with my hand so something wrong.
slows down on hills.
i have tested compression with tester and got 45 psi which i assume is too low.
i suspected the exhaust valve is not seating, so just taken the cylinder head off and it looks as its ok.it sits tight when closed.I have bought a valve compression tool so can remove it and clean valve seat if needed.
Piston is my next thought, how do i tell if its the issue, how much play should i have,what do i check? There is a carbon build up on the cylinder wall is this a clue something is wrong?
So before i go any further and lift the barrel i would like to hear your thoughts and advice please.
If i do need a rebore, or new piston or rings or all where would people recommed ?
All the best for now
Tony
email (option): tony.g.moore@googlemail.com
Did the engine smoke badly?
Check that the decompressor is working properly and that the head gasket has no signs of leakage.
Do you mean a carbon build up further down the liners i.e. more than just a slight lip at the top?
If so, then your rings and/or liners could be past their best.
I would whip the barrel off and have a check. Any good engineering firm should be able to check if the bore and/or piston is worn badly.
Regards
Pat
email (option): sacombsashtrees@hotmail.com
Hi All, found an issue. The top section of the piston is badly worn above the rings (exhaust valve side)
There is a carbon build up on the cylinder wall which the piston has been touching causing it to wear badly and it must be pushing the piston away from cylinder wall which is the cause of my lack of compression.
Is this just a lack of maintenance, decoke, over the years thats caused this carbon build up? Or am i missing something else.
i have cleaned away the carbon from the barrel, looks like i can get away with new piston and rings.
My piston has the numbers 66-1025 1/2mm OS what does this mean when i look for another piston... if i quote this will it be reconnised?
Has anyone a spare piston and rings as above that they might like to part with? Or wheres the best place to enquire?
just found one on the internet, £80 plus rings, is this the going price?
Its amazing having something so worn the bike still runs... it was just that because i have recently purchased a 1934 BSA that i
noticed the complete lack of compression.
Tony
email (option): tony.g.moore@googlemail.com
Hi Tony
I don't think this is helpful to the alignment of the piston in the bore and it probably shouldn't happen, but quite a few M20s would appear to do this (maybe it is a side valve thing).
Mine has done it in the past, where there is a build up of carbon on the valve side. This carbon can get quite hard and score that side of the piston usually above the rings.
It doesn't appear to affect compression if the rings are ok, but can't be that good for piston, ring or bore wear in the long run.
I have fitted a new piston, new rings and re-bored the liner say, 3,000 miles ago. I had a little look at around 2,000 miles and carbon had built up on that side and had scored the piston a little exactly where you described your wear.
I cleaned that area up and put everything back together, possibly not a good idea. The bike runs fine with good compression. However, if I had another look, I bet it has scored up a bit more.
I came to the conclusion, maybe wrongly that my conrod could be slightly bent, but would be interested in other peoples experiences of this.
Regards
Pat
email (option): sacombsashtrees@hotmail.com
1/2mm OS is 20 thou oversize. Before you buy a piston, it would be best to take your barrel to an engineer first to measure the bore, in case you need to get it rebored and go up a size (40thou OS?). A new piston in a worn bore is a waste of time and money. Ron
email (option): ronpier@talk21.com
I agree with Ron, get the barrel checked.
I recently worked on a bike where you could still see the honing marks on the liner and had no top lip. When it was professionally measured, it was oval beyond wear limits. I would guess that someone in the past gave it a hone with a flexible honer and put some new rings in.
email (option): sacombsashtrees@hotmail.com
Just a thought. Have you checked the tappet clearance?
email (option): j@clogmaker.co.uk
Patrick , sounds like i have got exactly the same as you had. I have cleaned up the cylinder and removed the very hard build up above the rings.
As the bike was actually running fairly well with this worn section at the top i have plumbed for a new piston and rings of the same size for now.
i know all you engineers are gritting thier teeth and frowning, but as i am not expecting to break any speed records i feel this will be good enough for my sunday afternoon rides. Will update you all of my outcome.
maybe a few more regular maintenance services might be a good idea. Had the bike for 23 years and this is the first time the head and barrels been off..couldnt do that with my Aprilia 1200
And thanks again for all the advice
tony
email (option): tony.g.moore@googlemail.com