ET: Engine (Twin) Mk3 cams - timing figures

[fogrider]

I have club copies of the original works information sheets, the detailed figures for setting cam timing with Mk3's is very different to those recorded in the "red" book.
I've not been able to find any reference to why there is a difference , or which is correct.
ie, inlet at BDC, works gives 0.100" lift, closing, but the red book gives 0.190". That's a lot of crank degrees, I fitted brand new Mk3's and am not impressed although my first twin was on Mk 2's, so no real comparison......................
I would love to read some comments about this, anyone ?
Regards, Terry, East Yorks.

 

[timetraveller]

The problem is that Mk IIIs are supposed to have quieting ramps which make it difficult to find exactly where the valves have been lifted to, say. five or ten thou. It was suggested originally that the best way to time them was to get a lift at some particular rotation of the engine, say, TDC. I have never been able to find any evidence of quieting ramps on any Mk III I have measured. My own method, which is tedious but gives a lot of information, is to measure the lift of both inlet and exhaust cams every ten degrees of engine rotation. One can then see if there are any anomalies with the lift curves and arrange the timing so that the centre of the overlap occurs in the four to six degrees BTDC range. It is also worth checking that the base circles really are circles. I have seen new cams with a 30 - 40 thou dip in the middle of the base circle and if the tappets had been set to zero there then there would have been a lot of blow by, lack of compression and premature failure of the valve seats.

 

[davidd]

Terry,

I don't seem to have checked a Mk3. I will look and see if I have one loose.

If you are worried about the numbers varying, you might start by timing the cams by the equal lift method. This is using the 4 degree/equal lift position Norman refers to above as a way to time the cam. I won't describe it here unless you are not familiar with it.

Essentially, there is only one place during all four strokes of the engine when both valves are open and are exactly at the same height. This is at 4 degrees BTD as the intake is closing and the exhaust is opening. This is also where all of the timing marks can be lined up. If your degree wheel is set correctly at TDC then you can turn the engine until both valves are open equally and the degree wheel should be right near 4 degrees BTC. If it is way off, then you have set the cam either advanced or retarded. If it is close, it is probably fine.

David

 

[Robert Watson]

Actually David it is where the exhaust is closing and the intake opening, right between the blow and the suck stoke! Right after the intake closes you get squish, then bang, followed by blow, and back to suck again......

 

[davidd]

Thank you! That is the second time I have done that. It is not that I am losing my memory, I just remember the things that never happened.

David

 

[Robert Watson]

I only commented because I had to think about it a few days ago timing this

 

[vibrac]

Actually David it is where the exhaust is closing and the intake opening, right between the blow and the suck stoke! Right after the intake closes you get squish, then bang, followed by blow, and back to suck again......

Of course if your engine ran backwards...
I was on a Scott once in gear when that happened very disconcerting it actually gave me bad dreams for a long time afterwards

 

[fogrider]

Thanks gents for all the above, I ended up setting them at full lift (inlet) at 96 deg ATDC, I also tried 101deg with little difference( my previous twin on Mk2's was set at that and it was blistering fast), this year I intend to take the UFM off and put the dial gauges on , I didn't have the adaptors when I built the engine. The question still remains though, why are the figures in the works sheet and the red book ( and Original Vincent) different ? Which is correct ? That was my query, rather than how to do it.
Regards, Terry.

 

[davidd]

Terry,

Sorry, I looked at both sources and never found what you were referring to.

I have mentioned this before with some puzzlement that I would never set any cam at maximum lift because that event does not occur at a specific place, unlike equal lift. The dwell at max lift can take place over many degrees. There is no dwell at equal lift.

David

 

[JackB]

Nice picture of Robert Watson"s twin engine, But what made me smile was all the lightning holes drilled in the two cam wheels.....Many years ago Phil Irvine was sat in my lounge at my home in Dursley Gloucestershire UK together with Marty Dickerson from Los Angeles, when Marty produced a set of camshafts with both wheels neatly drilled with a ring of lightning holes. What do you think of these Phil he said. Phil took one quick look and flung them to the floor. You people make me cross he said, I personally made these cam wheels heavy to help prevent valve bounce at high rpm.

Both Marty and I were shocked and surprised Phil's reaction. But after thinking about it for while I came to the conclusion that he was quite correct. What PEI didn't know about 4 stroke motorcycle engines wasn't knowing. I once stayed with him at his home in Warrendyke Victorand in his work room he had literally hundreds of motorcycle engine blueprints an drawings. In fact he gave me two of his early drawings of the early HRD units.

I was still in Australia staying with my cousin in Perth WA, when I received call from
Dr Fuller in Adelaide saying that PEi h`d just died. So I returned to