Join Date: Dec 2008
Location: Wanaka, Central Otago, New Zealand
Checking for a spark by earthing the plug against the motor may give a spark, but under load in a compression chamber is another matter all together.... If there is a short or weakness anywhere in the electrical system it will show up under load when the motor is running. Plug caps can fail, but still present a spark to the plug under a test situation as mentioned above, so a few bucks on some new caps might be money well spent. You could "borrow" some from another motor to check the theory. Also, look for any "arching" where the leads may be shorting out under load. This will present as distinctive marks on the engine casing where the plug leads sit.
I am sure many others way more experienced and knowledgeable than myself will offer their advice also....
2002 K1200RS Ride #18. Owned from new. Pacific Blue of course. Suspended on Ohlins. 120,000+km
1951 AJS 500 single - my Dads
1916 Triumph Type H, Western front, France WW1 - my Grandads
The cheapest is often the most expensive form of economy