Twin Valve Timing

[Cyborg]

I had everything set up to time my Mk2 cams at equal lift at 4 degrees BTDC and then discovered the cams had that .009 divot ground into them. I purchased a new set of Terry's cams that just arrived. They are his .380 lift, 105 degree lobe angle Mk2's with quieting ramps. The instructions say to set them with .170 lift inlet opening at TDC. I asked him about setting them with equal lift at 4 BTDC and his reply was: "you will not get a correct reading of the ramps better to do it with the.170 lift". I also inquired about measuring the lift at the follows and subtracting the rocker ratio from the .170. He said it would be roughly .010 to .012 less, but tends to vary a lot, so again, best to check for the .170 at the valve. I would really rather set them using a dial gauge on the rocker, so I guess I'll try that and check them at the valve later when I have the backlash sorted and the correct 1/2 time pinion selected. I'm assuming that as long as I do both cylinders the same... if they do end up being out a bit, at least they will be out the same amount and I can play with the keyway on the 1/2 time pinion to get them closer. Are there any landmines hidden in this path I'm taking?

 

[davidd]

"you will not get a correct reading of the ramps better to do it with the.170 lift".

I am not sure what is being said about the 0.170" lift. Just guessing, but the intake might be open to that amount as it crosses TDC. At equal lift it would be lower, as stated, but nothing wrong with that.

I would set it at equal lift. If you want to know what the ramps are doing, as Terry suggests, take readings every 10 degrees after you have set it at equal lift. Then you will know the profile of the cam. With a new cam it is best to do it the first time so you can verify that there are no dips. Put it in your notes, and if the results of setting it up at equal lift seems sensible, you can do it there after without worry or further measurement.

I am reminded that ABCD corrected me on the 105 cam years ago as I believed 105 was the lobe separation angle (LSA). I had seen a graph in Terry's notes of his 105 LSA cams and assumed he had done an Ian Hamilton style cam. That was incorrect. The 105 cam has max lift at 105°, not a lobe separation angle of 105°. I think the LSA on the Hamilton 105 cams is a near stock Mk2 LSA of 97°.

David

 

[Cyborg]

Yes he was saying to set it so the intake is open .170 as it crosses TDC. I also took it to mean 105 degrees of lobe separation. In his instructions it says quote " All my cams have 105 degrees lobe angle".
So would you set it at equal lift at 4 degrees BTDC or at TDC or does it really matter?

 

[davidd]

I think Terry is telling you to take more measurements of the cam just to be safe. I would set it at 4° and run it around and see that everything is good according to Terry's specs. I think that if you set max lift at .170" instead, you will find that at equal lift it will read 4°! I would do what you prefer and then just check that it all works by checking the other.

David

 

[timetraveller]

Sorry if this seems pedantic but the only way to do this properly is with the engine assembled, with the heads on, the tappets nipped up ten or twenty thou and the dial gauges on the ends of the valves. Take a measure every ten degrees of engine rotation and plot the graphs. Do the whole of the base circle as well. I have seen new cams with a ten thou plus dip in the middle of the base circle lasting for about 50 degrees. Imagine setting your tappets on that part of the cam after which there would be a ten thou gap between the valve and the valve seat for much of the time and both performance would be down and a very good chance that something would burn out. You only have to do this one as once you know what shape your cams are you can resort to quicker method to set the timing in future. It would also get round confusion as to where the 105 degree is.

 

[Cyborg]

Doesn't seem pedantic at all. The reason why I'm still farting around with this, is because the NOS cams that I got with the bike had that dip you are referring to. I ordered new ones from Terry. On a side note I wish they didn't have quite so much lift. Anyway, once assembled I will do as you suggest. I agree it it the only way to measure it accurately. One reason being it seems like rocker ratios are a little random. I have a sufficient supply of dial gauges.. I just need to cobble together one more holder. Keeping in mind that I haven't been inside one of these engines for decades, my approach or some of my questions may seem a little odd. I still have to deal with installing the cam pinion on the front cam, setting the backlash, and selecting the 1/2 time pinion. I'm thinking this would be way easier to accomplish without dealing with spring pressure. I don't know for sure, but I believe the steady plate should be fastened in place when checking lift. I'm operating under the assumption that without the plate, the front spindle will deflect under spring pressure and have an effect on the readings. As previously mentioned, I hope to set both cams with dial gauges running off the rockers. If they end up being out, at least in theory, they will both be out the same amount and I can make an adjustment at the 1/2 time pinion. I sure that as I perform this task, I will answer some of my own questions and come up with some new ones.

 

[bmetcalf]

I vote for the steady plate in place and light test springs.

 

[Cyborg]

I vote for the steady plate in place and light test springs.

Yes, I realize that now, but in my exuberance, I assembled the heads and installed them, so don't want to go that route at this point.

On spindle deflection... I clamped an old front spindle in a vice, rigged up a dial gauge and measured defection with just thumb pressure. Yikes!

 

[timetraveller]

Spindle deflection; note that the cams which take the pressure are right at the back next to the crank case. Not much mechanical advantage there for bending the spindles.

 

[Cyborg]

Spindle deflection; note that the cams which take the pressure are right at the back next to the crank case. Not much mechanical advantage there for bending the spindles.

True and I took that into account when measuring it in the vice, but I did notice some deflection in the front spindle that was in the case (and the spindle is tight in the case). That's what took me down the path to measure it. It probably doesn't amount to much movement at the lobes because they are next to the case as you state and I'm not sure that any change in backlash will matter. I just don't really like the idea of putting the valve spring pressure W/high lift cams on an unsupported spindle. Perhaps I worry for naught. I've been accused of that before.