WillBrown Internet - Ref:WP10*CT-Main-111216q15_NP_New2012 -  Updated 16/12/11 © 2005-11.

Many FM403 Fantic 300 Professional twin-shocks do not run anywhere near as well as they can do, following re-packing of the front pipe, and modification of the early type port timing to the later specification.

The FM403 front pipe appears to be a pressed steel expansion chamber, in common with the FM450 and FM350. This is in fact not the case, as the pipe on the 249cc bike  includes a passive silencing element , which can be re-packed after the pipe has been cut open.

FM403 front pipe ready for re-packing

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Classictrial modified FM403

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Fantic 300 Professional set up

* Finally please remember that all of the above information is provided on the strict proviso that any modifications or changes not carried out specifically by Classictrial, are not guaranteed or warranteed in any way, and are entirely the responsibility of the individual carrying them out, and that Classictrial are not responsible in any circumstances for damage or injury that may occur as a result of poor workmanship *

Properly re-packing the front pipe on an FM403 means much improved running, with smoother power delivery, and also seems to make bikes grip a little better. The ideal exhaust set up for these bikes is a re-packed stock front pipe, in combination with a passive straight through back box.

The porting on the FM403 was designed to work with the old 5 star leaded fuel, and the position of the exhaust port is such that detonation/knocking will often occur when the motors are highly loaded at low rpm. The later 249cc motors exhaust porting configuration was altered, and this cured the detonation problem and provided increased performance at the same time.

We can modify FM403 cylinders to later spec, but it is quite possible to fit a later mono-shock cylinder (301,303) to the earlier twin-shock bottom end, and this is a very useful modification that makes a noticeable difference to performance. The very last of the 300 Professionals produced, the “Michaud” replica's, were fitted with a cylinder which had the late type porting arrangement, so these don't need to be changed obviously.

Finally  we can assist with repacking FM403 front pipes fitted to machines being modified by us, but cannot help otherwise, as this is a time consuming and dirty job, and most people are unwilling to pay for this type of work to be carried out professionally!

Prototype Classictrial FM450 back box

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