TDC Timing Valve

roy the mechanic

Well Known and Active Forum User
VOC Member
cam timing

As a general rule , inlet valve lift at tdc should be greater than exhaust. this is how Aston Martin's instructions go, if this is good enough for them it should also suit you. Numbers like inlet open .110"+ exhaust open.095" should be good .Have spent days on the dyno adjusting cam timing on race cars (mostly Jaguars) to acheive "the racers edge". By working this way you will avoid the vagaries of quietening ramps and dodgy '50's cam grinding!- If still in doubt, pay a proffessional -that's what we are there for! Be lucky, Roy.
 

Tom Gaynor

Well Known and Active Forum User
VOC Member
Valve timing

My current Manx Norton, my previous Honda K4 racer, my Rudge Ulster, my Vincent, all have equal lift between 5 (the median value) and 0 btdc. Only one of these was set that way by me. The Honda (works camshaft) came that way.
I asked the guy who takes care of my 86 bore Manx Norton (145 mph at Chimay last year), and who also builds all of Andy Molnar's 92 bore Manx engines (£27,000 a pop), if this was just coincidence. Nope. Any engine timed for equal lift within a few degrees of TDC will fly. Ignition timing is far more critical, and has to be right. (The Manx is 13.8:1...)
When I timed the Rudge, with a presumed hot cam for which I had no timing figures, I didn't bother with opening and closing figures, but set it for equal lift at TDC. I got about 3 degrees btdc. Before being detuned (down from maybe 7.3 to 7) it would touch 100 mph, now barely does 90. But cruises easily at 80. One swallow does not a summer make, but I think it indicative.
(As a sidenote, my experience is that the combination of a "race" cam with a low compression piston results in a motor that will go up the side of a house at zero rpm, not at all "buzzy".)
Even if you choose not to time your motor that way 1) it's a useful check to make sure you aren't a mile out and 2) if you should ever (heaven forfend) have to time a bike by the roadside, you now have an easy way to do it.
 
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