Well, there is some good news and some bad news. A word of advice to anyone looking to sprint - make sure you have nyloc nuts !

We need a new sprinter motor and Ben beat me.

Having lightened the flywheel, re-aligned the crank and reprofiled the head, the sprinter managed 14.00 in practice, which equalled the record. The difference being that it did a 64 foot time of 2.08 seconds - the third fastest of all entries in practice, and a terminal of 88mph. This means the starts are probably as fast as they are going to get (can it get faster?), and now its time for more power.

The problem was that there was too much compression and pinking was an issue - so I just got slower, ending up with a timed run of 15.3 going over the line at about 50mph as I could not keep it on the throttle.

Ben on the other hand just got faster. We added a boost port, played around with some of the other ports, and I made a new head. The exhaust was now a JL4 with a Mikuni 35 TMX. It ran great, so well done Ben with 14.71 - there is plenty more from this motor, next step will be to reduce the flywheel weight, and depending on what we use the motor for in the future, maybe increase exhaust timing as its still quite low.

Back to the sprinter - having adjusted a few things to try to keep the pinking in check, I listened to the motor in the pits revving it - and it made some horrible noise and stopped....

It turned over but rattled a bit - Oh dear, then some guy came over to let me know I had dropped a bolt and washer on the ground.

Anyway after stripping off the head, it was clear that it had gone bang and I assumed it had thrown the rod, the crank was out of alignment and it would not go past TDC. I took the piston out and inside, under the crown - guess what - a washer. It seems the fixing holding the seat unit on just above the carb had rattled loose and two washers werer sucked into the carb.

Piston and head are mashed, cylinder is fine, although the washer stopped the piston going past tdc and twisted the crank on the pin (even though its welded!), and presumably the rod is bent, but we will check.

We were going to retire the cylinder anyway as I feel its reached the end of its development, so the decision was either to put the Rapido 250 into the sprinter and develop it as a dedicated sprint motor, or do another standard cast iron cylinder from scratch. Over several bottles of wine at Nicks I think we decided that it would be far better to carry on with the iron standard barrel route, simply because no-one else seems to be doing this (and lets face it Keith Terry didn't have a problem).

Keep watching the site, as the motor develops I will post pictures. I already have the cylinder complete with cobwebs, a piston, manifold and head. Do I need a new crank though? we shall see.

Click here for details and pictures

Cheers
Jez

 

Exodus motor: The motor is a TS cylinder with the inlet considerably enlarged to take a YZ125 reed cage and a handmade manifold.  The transfers have been blended throughout and made significantly larger and re-angled and now there are seven transfers.Read more here...