MPG class question

[andychaos]

I am new to this so please be gentle.
We are planning to have a go at the speed trials next year and are almost ready for our first test run of a BSA B50 to run in the
500cc MPG class. At present we have a modified B50 frame with a standard B50MX motor while the race engine is away having a short stroke crank made. What I cant seem to find any info on is how far are we allowed to go with the race motor in the MPG class. Is there a limit on the size of the carb and Dia of the exhaust. Also we race CCMs and B50s over here in Scrambles (UK for old mans Motocross)  and all our bikes run Castrol R what are your thoughts on castor oil in the high temps expected, is it a definite no or do some of you lads run it.

Cheers Andy

 

[firemanjim]

No restrictions on motor work, exhaust, carbs etc, frame and swingarm length  are whtt will put  you into APS

[andychaos]

Thanks for the reply.
When I say a modified B50 frame, It has only been raked and the seat height lowered with a 2" longer swing arm apart from that its standard so hoping for the MPG class.
That's great news re-the carb size and exhaust diameter, we can go wild then.
Can we use any make of carb? or do we have to stick with the Amal.


Andy

[Dakin Engineering]

Section 7F applies.....

....rider seating position shall be above the top of the rear tire...unless OEM.

Sam
#6062

[Koncretekid]

Andy,
You can download the rulebook from the Bonneville Motorcycle Speed Trials site here: http://bonnevillespeedtrials.com/rulesclasses/
Click on the "2015 AMA Bonneville Supplemental Regulations" to get both the rules and the current records.  However, you posted this question on the SCTA rule questions site and I don't have an SCTA rulebook.

From the photos you posted on the B50 site, your bike looks legal to me except BMST require a front fender. You can't have anything in front of the rider that has the perceptible purpose of reducing drag, but I don't think your tachometer would disqualify you.  Make sure you have a BSA B50 operators manual or something similar showing the standard wheelbase length because you cannot exceed that by more than 10% and they will ask for proof. Also, be prepared to pull the cylinder head if you set a record, cause BMST tech doesn't have any other way to measure bore and stroke and the limit is 500cc with no allowance for extra.

I think that SCTA actually do allow 1st overbore if the original motor size is within a certain %age of the class limit, which would apply to the B50. Someone with an SCTA rulebook will have to verify that.

Tom

[Nortonist 592]

The SCTA overbore rule is .020" over the stock bore.  And 10% max. increase on the wheelbase.  No limit on carb or exhaust pipe size.  R will be no problem.  I'm running a Weslake speedway engine in my sidecar and I've been running R for years without a problem.  You won't have any trouble running R.

[andychaos]

Thanks for all the reply's
I already  have the rule book, and the front fender is now fitted but I just wanted to clarify what mods were allowed to the motor in MPG.
Good news re- running R as every bike I have runs on it already so I will be able to swap motors into a road legal frame for testing. we have reached the time to tear down the bike for painting and a final rinse out of the frame so a decision had to be made as to which oil to run. Engine oversize shouldn't be a problem as we are running a short stroke crank to suit our chrome bore and piston and we are well in the 10% on wheel base.
All looking good so far, just need to start making more power.
Andy