Holed piston! Could this be the cause?

[nelson pk]

As Rosscla said....pilot does not stop delivering fuel.
When you are reading how to jet and you read " pilot effects 0 to 1/4 throttle with airscrew and needle/atomiser effects 1/4 to3/4 and main effects 3/4 to full" this is where they have the LARGEST INFLUENCE and gives you a guide on what cicuit to change if you have a problem in a circuit area. The only circuit that shuts off completely is the choke!

For example your main jet has the biggest influence at 3/4 to full but if you drop your main by say 8 sizes it will have its biggest effect there but it will substantially effect fueling all the way down to 1/4 throttle......if you go up 8 sizes you will notice that the whole throttle range will get spluttery as the fuel delivery gets richer.

The reason the pilot has the biggest influence at 0 to 1/4 is that the vacuum is not strong enough at idle/part throttle to draw enough fuel from the main circuit to keep the bike running evenly, so at idle it draws the fuel from the first small hole in the carb venturi (the one that the airscrew/mixture screw controls),then as you open the throttle a bit more (say 1/8) the vacuum increases enough to take extra fuel from the idle hole (the next hole along in the venturi) and then as you open the throttle to say 1/4, the vacuum increase even more so it can take more fuel from the main jet/needle atomiser circuit but no circuit STOPS delivering fuel!

Take your carb off and look down the venturi from the side that fits closest to the engine and you will see a tiny hole first (airscrew/mixture screw delivery hole) and then a second slightly larger hole (pilot delivery hole) and then the hole with the brass part sticking out (main jet/atomiser delivery).

Hope this helps you.

[nelson pk]

In answer to your question.....dropping from 55 pilot to 53 is unlikely to cause a major lean out........but.......if you leaned off the airscrew as well and had a small head leak for example, it could be a combination of factors like that causing a heat build up on the top of your piston.

[alanscottj]

In answer to your question.....dropping from 55 pilot to 53 is unlikely to cause a major lean out........but.......if you leaned off the airscrew as well and had a small head leak for example, it could be a combination of factors like that causing a heat build up on the top of your piston.

Thanks for taking the time to write this up for me Nelson pk, it has given me some food for thought. I have got it all back together, blown out with an air line so hopefully the seals will survive, if not, it'll be rebuilt. Have torqued everything up to 20 psi, gonna do the leak test tonight before I stick the engine back intop the frame. Fingers crossed it'll be alright. Will keep it steady for the run in period and keep an eye on my temps. I am gutted more because I have a EGT on the scoot but didn't have it switched on when the thing holed! Won't be doing that again Will hopefully get it to a dyno in the next month or so

[bristolmod]

"........Will keep it steady for the run in period........ "

a bit of welly now and again will do more good than being too careful.

Chris

[alanscottj]

"........Will keep it steady for the run in period........ "

a bit of welly now and again will do more good than being too careful.

Chris

Yeah, I will give it a little every now and then. Got a nice ride out planned.tomorrow, plenty of opportunity to roll on and of throttle