right piston not firing when hot

[Martin]

Hi, I am sorry for having again some question but BSA is still for me a motorbike which I need to study more. No experience in the past with any other 4 stroke engine.
I have some troubles with firing of the right piston after few miles of riding. So as soon as the engine is hot the right piston lose the power it runs "randomly" a little bit better with open throttle.
I have went through of some previous forums topic and I assume I need to look at valves and their guides.
what I did:
1) new spark
2) new spark plug (no resistance)
3) new high voltage cables
4) measuring a gap between points in magneto .012"


I know it is not enough and I should spent some time with checking of magneto timing but because the problem appear when the engine starts to be hot I think I should start with checking of the compression and after that with engine head/valves/guides.

Am I correct?
Thanks a lot.
Martin

[muskrat]

G'day Martin.
A cold motor likes a rich mixture and leaner as it gets hotter. When the problem starts, stop and check both plugs. If the right is markedly darker than the left you could have induction bias. Carry with you a hotter plug (NGK B5ES) and put it into the right side and carry on. If this helps it's a bias problem.
Other causes:
Could be ignition timing not being the same on each side. Checking with a strobe light on a timing disc on both sides will show this up.
A loose valve guide allowing a bit of oil in and fowling the plug.
Start with the easy checks first.
Cheers

[JulianS]

Check the mag pickup brushes and holders. Old holders can breakdown. 

[beezermacc]

Possibly poor quality pickup brushes, dirty slip ring, oily earth track. All easy to clean / fix.

[Martin]

Thank for your comments. I have ordered new sparks NGK B5ES and also pickup brushes. But it will take some time to get the pickup brushes from UK.
Therefore I also checked valve clearance and I have some doubts if the previous owner ride the bike in recommended values.
For my engine there should be 0.010" when cold. I opened left exhaust valve and measure the clearance of the right exhaust valve and there is not any gap (I used 0,25mm plate which should be the correct one but neither 0,1mm was able to put in).

I can set them correctly but I have some doubt about the condition of the valves therefore I would like to disassemble cylider head to investigate how the valves look like.   

Do I need the be aware of something triky? Push rods,...

thanks
Martin

[Swarfcut]

Martin. If your bike has a standard cast iron head, the valve seats are part of the casting, they are not inserts. The engines were designed to run on leaded fuel, and without the lead in modern fuels there is a tendency for the seats to be eaten away. The result is that the valve moves further back into the seat, and the valve clearance closes up. Worst case is that valves will need replacing and valve seat inserts fitted to restore things to how they should be. You have some doubt about previous owner, so good chance it has been run on standard unleaded fuel.

 Try the simple things first. Set the tappets, give the bike a run, then do a compression test. You may be lucky. Always run the bike with a lead replacement additive in the fuel to prevent further valve seat problems.

 Swarfy

[Martin]

Swarfy, i did the setting of the clearance and even with cold engine I achieved significant encrease of compresion. Before 90psi. Now 120psi. But now i have to wait for new pickup brushes. One is destroyed.

Thanks
Martin

[berger]

martin hopefully you're on a winner there