Hitchcocks Forum

Just found my 2008 500 trial has a restrictor fitted in the front pipe where the silencer joins on. My question is if I remove it will I need to re-jet the carb?

If it is the same as my 4speed classic it is the catalyst. If so you can bin it, along with the pav. I increased the all the jets by one size, a very simple 5 minute job, and it made a huge difference, much bigger than changing the horrible long silencer ever did.

Don't bin the PAV - keep it. You'll have to pay a lot of money to get another should current legislation change and require us to re-fit them.

Tom

Getting rid of that is a bad idea. I did it with my friend's AVL 350 and he lost a lot of back pressure. It is complicated it seems... so we bought another one and opened it up by 1.5mm to 2mm (Learnt from BulletWhisperer) and that really helped a lot. In my friend's case he was running kind of lean so we had to richen the mixture.

Looks like I could remove it as one piece, not open it up. Might leave it for now if I have to mess about with carb jet set up . The PAV has already been removed at some point.

Sorry for using the term "Opening". I meant sanding or porting. I did with another such type of similar ring which was in my bike. Took sand paper and made the inner surface smooth and increaded inner dia by 1.5mm. Did that just by sanding. I think it's better if you don't do that with your stock part. Instead, buy another one and do it on that. I do it this way.. what ever performance mods I do.. I keep the stock parts intact. So in future... I can sell my bike if I want to.. with stock parts and sell the performance modding parts as a "Kit".

Dampking, have a read here; http://www.uucmotorwerks.com/html_produ ... uemyth.htm I don't think back pressure is a issue here.

nd here (not that I believe everything I read in Wiki) http://en.wikipedia.org/wiki/Back_pressure

I recall from my engineering training days that "back pressure" was only required in the exhausts of basic 2-stroke engines where, crudely put, it acted in a similar way to the exhaust valve in a 4-stroke in that the pressure wave travelling from back to front of the exhaust prevented the fresh petrol/oil/air mix from venting into the exhaust before combustion.
In our "modern" (sic) Enfields what the restrictor in the exhaust down-pipe does is prevent the burnt exhaust gases from leaving the exhaust and therefore literally choking the engine by preventing it breathing out.
Its a con, like the PAV which does nothing to actually reduce emissions, but which merely dilutes the exhaust with fresh air to make the emissions appear less. There is just the same pollutants coming out, but with fresh air mixed in.

Thank you Pete F - I have learnt my lesson