Question about timing sc cam in 86 930
 

Sorry for the x post. Thought this might be a better forum.

Its been 24 years since I've done this so bear with me. Timing LHS cam, Dougherty SC cams on '86 930. Yes, I have the LHS cam in the left cam tower :D

Crank at Z1 (the second of the two close together pulley marks when rotating clockwise to pass the TDC notch). Set the cam with dot and Woodruff key at the top, "930" at the bottom. Pin in sprocket, idler arm and chain tight, intake #1 in and adjusted. Verified nothing moved. Checked at dozen times, I'm at Z1, cam aligned as described above, rocker foot adjusted right, dial gauge on the valve retainer lip.

Now, Wayne's book says valve s/b starting to open not long after I start turning the crank, but I have to go almost 360 degrees before there is any movement. Valve doesn't start to move until about 5 BTDC or 355 degrees of rotation. At TDC I'm at 0.0737mm or 0.0029" Is this right?

Then, I reach the target of 1.8mm or 0.0709" of lift at about 25 ATDC. That is, almost exactly the same distance the BTDC mark is before the Z1, except after Z1. Way, way before I'm at TDC again.

My maximum lift is 11.303mm or 0.445" at about Z1 + 125 or 25 ATDC cylinder 6/3.

I'm very confused. What am I not understanding or doing correctly.

Cam timing..........
 

Flightlead,

I am assuming you are aware how to identify a left-side cam from the right-side cam. And you have installed the correct cam on the left cam tower. What mechanical device are you using to make the timing chain taut or compressed? Are you doing the cam timing with the ignition distributor installed and rotor aligned to spark plug cable #1? Have you tried to move the pin to increase the value? Keep us posted.

Tony

On the crank pulley. Is the pin to the crank in place. TDC the pin is at 9 o’clock
On the dial you should be getting valve moving a about 35-40.degrees BTDC
OR. about. 320 degrees from TDC
Bruce

Thanks guys yes all those basic things are done. I was just confused about the wording in Wayne's description and thinking too much into it.

Chris Carrol of Turbokraft actually picked up the phone and called me, on a Saturday night, and spent a few minutes clarifying my thinking. Basically, the cam was more advanced than I anticipated so I had to make a bigger change, but got it down.

Which leads to my next question, how much is difference is appropriate or acceptable difference left to right?

These cams are supposed to be timed at 1.8mm. I can get the LHS to 1.75 and change. I can get the RHS to 1.68 or about the same diff the other way. That is, just less than 1mm diff. That seems like a lot.

Thoughts?

Watch the. Videos on stompski racing for setting cam timing
There is some things that work across the board as general info
Bruce

Consecutive readings........
 

Take 3 to 5 consecutive readings and get the average. Do this on both sides. Post the numbers. Do not watch the dial indicator and keep turning the crankshaft pulley until Z1 line up with crankcase seam and look at the dial. Record this value and repeat several times. Like to see the values of your measurement. Thanks.

Tony

Target setting is 1.8mm

I got the LHS to 1.831mm and the RHS to 1.869mm

Not sure what is an acceptable divergence from target and between sides.

I forget what the math says based on the number of holes in the sprocket and gear, but I use .05mm delta as my target and if the first number after the decimal is the same as the spec, side to side I'm usually happy. If not, you can pull the gear and move where it lands in the chain differently and then you can usually get within the .1. Mainly the challenge is to check that your measurement is repeatable 3-4 times each side.
As for a tolerance side to side... many people over the last 50 years of Mezger motors set them "dots up" and the motors ran.
How tight the tensioners are side to side will definitely change that third decimal...