Big 'ed RB250

[Lamaddict]

I didn't know you could get them precut, this is the way I got it off Rayspeed last Easter.

Any tips on cutting it would be good. I'm thinking about a grinder?

[sydduckett]

Ron, told me himself to use the 7 so that's what I'm trying. Maybe you know something he doesn't?

[Darrell Taylor]

a 7 will get extremely hot and do a good job of burning the 2 stroke oil deposits off the plug ,its generally a plug grade used on very low power / low heat motors as a means of keeping the plug clean , the rb 20 is a performance kit and air cooled with very high running temps ,a 7 running within these conditions located in such a close proximity to the piston crown will quickly take on the role of a glow plug and cause issues on a properly jetted motor with a high specific power ,an overjetted motor will quench this situation and u may find running a 7 may clean up a plug fouling problem but is generally masking another issue
on any performance motor i,d recommend the highest plug grade you can get away with running without regular fouling,if running a 10 be aware that starting from cold with choke left on when the oil has settled after being laid up for 2 weeks and you pop in the house to grab ur helmet and gloves do expect it to have stalled and fouled the plug that didnt get hot enough to burn the deposits off,once aware of this start it b4 ready to set off and youll find a 10 will last years

[Adam_Winstone]

All comes down to the kit and the compression ratio. Ron's Avanti kit does not need to rev as hard as the RB and the compression is probably not as high as the RB's, these are both factors that mean the B7 will be better suited to the Avanti than it will be the RB.

My thoughts only and ready to be corrected.

Adam

PS - I'd have thought an 8 would suit the RB around town, however, I'd keep an eye on it when riding out of town on long fast roads, and probably look to change to a 9 for these conditions. I doubt that you'll need to look towards a 10 though, not unless you are running way too much compression for a road motor.

[Adam_Winstone]

^... PS - That was not meant to read like I was questioning what Darrell had written! I put this only because I once tried to ride to a Euro with someone that was told to run nothing less that a B10 on his high compression top tune Monza, only to find that we were forever stopping to swap plugs out because it wasn't getting hot enough to fire correctly. Swapping to a 9 sorted that out and resulted in his bike performing SO MUCH better than it had done. 9s... now 9s I like

[Darrell Taylor]

forgot to mention compression ,good point adam ,the rb 20 has close to 11-1 a very good reason to run a 9 minimum

[Lamaddict]

Started cutting with a grinder, the alloy seems tough.

[bsso78]

Whats the score with cutting the head? Ive never used one so i dont know the exact science. Can someone explain the theory behind it?

[jonashford]

The head is very wide and has a flat bottom which over laps the rb barrel so the air from the flywheel will just hit a flat surface, cutting slots in the fins allows the cool air from the flywheel to get over the avanti head. The avanti head is excellent at cooling I use a n3 plug in my rb260 with avanti head, I can't remember wat the comp ratio is but there is lots of compression on the kick start, I'm more of a ride it and see than all the facts and figures. But I'm always open for advice from experienced people.

[bsso78]

So instead of grinding lumps out out of the head wouldnt it be easier to cut a scoop into the cowling?