Howdy, I've got a problem with my bike running rough under load. It was running sweet until I removed the leads and plugs to get serial number off the fly wheel. Realised I must of cracked an old lead replaced leads and plugs and fixed the issue so went for a 60km ride happy days, until I next started it and now it drops its guts under load with and back fires. I've reduced the plug gap which has eliminated most all of the backfiring but still no power under load. I'm thinking coil breakdown, does anyone know how to check for coil breakdown under load? The seem to be producing plenty of spark at idle. I'm staying away from the carby at this stage as all was well before hand. Thoughts or insight? 

 

 

 

What year bike do you have.

Funny you should ask. Just bought a bitsa. Chopper. 1999 crank 2001 heads. It's a S&S supposedly a 117 but have just sent all the serial numbers (hence checking the flywheel) to S&S as they can give me all details. So let's say 2001 until further notice.

this may sound totally stupid BUT...........

when you push the leads on, is the air inside escaping enough to allow them to stay all the way in???

you know what I mean, the preasure pushing the lead off, possably pushing more when hot

 

 

the rest of you's can piss off and stop laughing, I am trying to help

The old bad lead contact due to thermal expansion causing a silconised rubber sheath to telescope along the spark plug trick!..... Nah not funny!!!  Totally plausible.

I've already closed up the terminals so they're a tight connection once fitted and I twist them on to get them seated properly.
I might get a spare set of plugs and try them just in case. I've replaced the leads twice just in case.
Cheers.

Where would one start to track that down? Compression test?

Just a guess: crank position sensor. Can you check codes?

How does one check codes?

Hard call with out seeing the engine but Choppa might be your best bet at this stage with a manifold leak. Spray WD40 around the manifold and card and listen for the revs to jump but with all honesty does it miss down low or up in the rev range but where all shooting ducks in a pond with out all the details.
Sorry

How does it run from cold? If OK, perhaps coil breaking down as IT gets hot. If so, try cold start having heated coil with hairdryer before starting. Does it now run badly from the start? If so, swap coil out.

When its cold and you start it, dont have the choke on. See how it goes and lightly idle for a couple of minutes.

Ok everyone stop looking!!
Thanks for everyones input and help. I've found the problem.
After removing the tank and starting to check for manifold leaks I saw some sparks through the loom hole in the main tube frame, whilst the ignition was turned off I might add. I disconnected the battery feed the wires out to find a wire that had some insulation scuffed off.
Bloody thing had been shorting out periodically.
Complete fluke finding it.
Oh the joys of motoring hey. Anyways happy days.
Once again thanks for everyone's insight into helping me sort this problem let alone the hundreds saved by not taking it to the shop. I owe you all a beer or two.
Cheers Dave.