Hitchcocks Forum

I have a 1936 500cc Model J Royal Enfield. I have been told by a fairly reliable source that swapping the 2 cams around improves the performance quite markedly. ie run the exhause cam as the inlet and vice versa, Has anyone tried that in these bikes?
If so how do you get the right timing as I imagine that the standard method of matching the cog markings will not apply. Would love to hear from someone about this. Cheers

I spoke to my friend Chris who runs 2 1938 model Js one sporting and one standard and he says he's never come across this. He also says the cams seem to have the same profile which would render the swapping process pointless! He speculated that the 1930s 4 valve models (LF) might have had different cams but he hasn't seen any. Also, getting the timing right with no marks would be difficult. He further says if you must mess with cam timing why not try the alternative timing pinion offered by our Hosts (in Accessory Catalogue,) which has 3 keyways offering the possibility of standard advanced and retarded timing. Hope this helps, REgards, Mark

I can't speak for these machines in particular, but I have done it on some 'Bullet' models with some success. As for timing, you have to start from scratch and first of all check what you have to begin with for a 'base setting'. Be very careful and set it all up to favourable numbers using a degree disc on the crank and making sure no contact between the valves and the piston crown occurs. Numbers falling within and close to 40/70 inlet and 65/35 exhaust [degrees] after the swap should go quite well, these figures are just an example and assume the swapped over exhaust cam has a longer duration than the inlet and perhaps slightly higher lift, too, these being the usual reasons for doing the swap in the first place. As can be seen in my example, you might get a longer inlet timing, in the above, it has been increased by 5 degrees at either end. The consequential loss for the exhaust timing is not so great as the gain on the inlet side, but if your exhaust cam is of a longer duration and lift than the inlet, what about leaving it where it is and get / fit another exhaust cam for the inlet - I have also done this trick before now, with good results.
Regards, Paul.