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Bonneville Motorcycle Speed Trials => Bville Motorcycle Speed Trials Rules Questions => Topic started by: Gchance on December 09, 2018, 01:58:29 PM



Title: 500 M-CG engine?
Post by: Gchance on December 09, 2018, 01:58:29 PM
HI, I am new here and BMST.  I am planning on building a Yamaha RD 400  to run in 500-M-CG.  I have a few rule questions.  Rules state OEM cylinders and heads,
 Can the The OEM cylinders have port work?
 Can the carbs be up sized?
 Does the stock reed valves have to be used?
 Can the OEM heads be modified for higher CR?
 Any exhaust but not past the rear tire?


Title: Re: 500 M-CG engine?
Post by: nrhs sales on December 21, 2018, 01:22:17 PM
If this is the "modified" class then you can use aftermarket cylinders and you can modify the heads,


Title: Re: 500 M-CG engine?
Post by: Doc B. on December 21, 2018, 09:37:36 PM
Classic engine requires stock style cases, jugs and cylinder heads. Aftermarket is OK if it emulates the original parts, i.e., repro air cooled stuff is OK but water cooled parts would not be allowed even though you could convert to them. You also can't go past an .050" overbore, so you would be at somewhat of a displacement disadvantage in a 500cc class.

You can port, you can use bigger carbs, I would think you could use different reed valves, you can modify for higher compression, and it is correct that the exhaust can't go past the rear tire. But expansion chambers are fine.


Title: Re: 500 M-CG engine?
Post by: donpearsall on December 22, 2018, 01:02:24 AM
The best philosophy when reading the rules is "if it doesn't say you can't do it, then you can." Like the carburetor question. CG rules don't mention carburetor so you can have any carburetion you want.

Don


Title: Re: 500 M-CG engine?
Post by: Gchance on December 27, 2018, 07:32:09 PM
Thanks guys