You can prep an iron MAC so that it goes pretty hard, 100mph not out of the question. M17/8 cam, higher compression ratio, bit of porting, decent carb, zorst, simple mods to the lubrication system, EG 2-start oil pump worm, later type timing cover to give better lubrication to the camshaft. But it leaves the gearbox mounting with it's single adjustable mount on top vulnerable to twisting and potentially breaking the shell. On mine I added (welded on) a second top mount on the gearbox shell and used x 2 left hand gearbox adjuster plates, one on each side. I'll see if I can find my notes on how I did mine.
Note (I was told) that the iron MAC motor is a better proposition for tuning than the alloy MAC. Can't remember the reason behind this though.
Thanks Joolstacho, that sounds like some good advice. If you could find your notes that would be amazing.
I wonder why you have heard of better results with the iron MAC?
I now have a spare engine so I can start stripping it and seeing where improvements can be made. I have an M17-8 cam and have researched using a Triumph piston to increase the CR. I imagine it might be a fine balance between timing, CR and valve to valve and valve to piston clearance.
The second of my build videos is online now if you are interested.
Iron heads heat up too quickly, but if say the iron head has better breathing than the alloy head then for short high speed runs that should not matter or if you use methanol (different class ?). I would want to know why the iron head is allegedly better before deciding what to do, first mention of an Iron head being better than alloy I have seen but odd things can happen.
Iron head has 15 degree intake tract inclination, alloy head is only 7 degree. Iron head has pressure oil feed to the rocker shafts, alloy head is drip feed but can be modified. In Australia and New Zealand where lots of iron MACs are raced they usually run them on methanol so cooling is not a problem. I knew a pretty successful tuner (AJ Lewis} who preferred iron heads on any motor he built. He was given a Triumph GP by the factory and gave them back the head and barrel. One year his bike finished top three at Daytona when the Norton Manxs where running (that was after his rider crashed and restarted).
Don't ignore the fact that the iron head has no rockerbox cover, the valvesprings are pretty much exposed, which means that cooling air can get MUCH deeper through the head.
But you've got a couple of alloy Mac donks there so you'll probably want to work with them. (It may be the better downdraft angle with the iron head that is good). I was advised by a couple of ol' blokes (when I wasn't an ol' bloke!) One was Eric Bushell If I remember rightly. He is (was) a Mac racing guru. The other feller was a Tasmanian who had very good results in Tassie. I'm still searching for Eric's letter, but my above post explains the important stuff. Yes a Triumph piston -can't remember which, fits and ups the compression ratio, I think I got around 9.?:1. My carb was 1 1/16" same as Viper - and I'd reckon viper settings would be a good starting point. I think I used a Viper exhaust valve (Nimonic 80) - It might have been turned down). I timed at 38 deg with no manual adjustment - fixed timing using a Criterion alloy mag pinion. W & S Valvesprings. Roller (converted) clutch release bearing. I think I fitted a 7 plate clutch (KSS) but not sure about that.
That exhaust... (well you asked for opinions!) - Rip off the kickstart assy (they're pretty useless anyway due to low gearing and big overlap of the M17/8 camshaft). Put on a banking plate and bump-start it (or make a roller paddock starter) -Then you can use a KTT bend or maybe a Manx norton bend, both of which should tuck in much better and give good clearance. (And look dead sexy into the bargain!) I can't quite see what gearbox you're using, if it's iron Mac it will have the selector lever on the front face. Early MSS / KSS will have the selector on the rear face. The MAC gears are narrower, but if they're in good condition I don't see why you couldn't use them... the 'box would be a bit lighter eh? - and bear in mind my earlier point about adding a second top gearbox mount if it's the iron MAC shell.
You'll need (at least one) good brake. The 7" SLS Velo brake can be very effective, but I'd certainly machine relined shoes to ensure complete drum contact.
If I think of anything else I let you know. Reminder - I am no expert, and I'm relying on unreliable memory!!! I think it's good to see a younger guy doing a Velo project rather than 'bobbing' CX Hondas!
Last edited by Joolstacho; 01/08/192:27 am. Reason: Extra detail added
There is an art to starting a single which you have picked up well but there are a couple of wrinkles that can help further
The Velocette kickstart is very low geared compared to other bikes, so unlike other singles you need to move the piston further on than just over compression so the energy you push into the flywheel is enough to get the piston over the next TDC, only practise will tell you how far to go, too far and the next TDC will be too near and you will get a kickback, so experiment carefully with strong boots.
On the carb if you use the throttle stop screw to set a fast idle when the bike is cold and then you can leave the throttle untouched. That saves the guesswork in exactly how much throttle to give the bike and reduce the risk of you altering the opening during the kick, too much throttle risks a kickback. As the bike warms you can back off the throttle stop screw until its at its warm setting, for cold starts screw it in 1/2 to a full turn, experimentation will tell you the best setting. If you have manual advance then 1/2 way advanced is good for starting.
Every engine is different so what works for one Velo may not work for another, singles of all makes seem to vary more than engines with 2 or more pistons.
Goodonya Chris... I like the amazed look on your face when it did go. And we might have got only 3 seconds of motor run, but I'm sure that was because Chris had to quickly check the oil return in the oil tank :-) (Weren't you mate).
Yes, and maybe run it a little faster at tickover initially to warm it up and get that oil pump pumping black stuff around, and check the oil return in the oil tank. (It doesn't need to be a continuous high flow, it tends to spurt and dribble a bit). Sorry to state the obvious, I'm sure you don't need gratuitous advice... but it's always worth re-iterating. Crack the oil feed up top at the rockers to check that it's pumping up there too. (Then re-nip it). Good stuff Chris.
I thought I should get some practice starting the bike. Oil flow to the tank seems good. Heres a short clip for everyone that wanted to see more running. P.S. if you haven't already subscribed to my channel, please do.