Cam Timing continued....

On Wednesday I headed back to the shed, unfortunately without my camera which will make this a bit dull for anyone who prefers pictures to sums.....

Anyway I decided to recheck the Inlet cam, this time with a reduced valve clearance. I used 1/2 a turn on the tappet adjuster. By my reckoning if the tappet is 26TPI (Cycle thread) then 1 complete turn is 1/26th of an inch or .0385", if it is UNF it will be 24TPI and 1/24th of an inch or .0417" which for my purposes is close enough to .20" for the half turn to make no difference.

To minimise errors I took three set of readings.

Open       Close
   35          62
   35          65
   35          67

Which I reckon gives cam lobe centre figures of 103.5, 105 and 106.

For thoroughness I repeated for the Timing side getting the following

Open       Close
   37          64
   35          66
   36          65
Giving lobe centres of 103.5, 105.5 and 104.5

So I reckon my original figure of 105 was pretty much right.

I then repeated the process for the exhaust cam, rather than using the piece of angle I used before I used a bit if 1/4" plate clamped to the cylinder head and the magnetic stand for the dial gauge.

These are the figures I got for the drive side;



Open       Close
   50          37
   49          37.5
   50          37

Giving lobe centres of 83.5, 84.25 and 83.5

For the timing side I got

Open       Close
   50          34
   54          32
   55          37


Giving lobe centres of 82,79 and 81

I found a whole load of information on cam timing at http://www.farm.net/~mason/moto/cam-timing.html

From this I calculated the cam lobe centres from the open/close figure quoted.


Inlet Exhaust
btdc abdc  Centre bbdc atdc Centre
650cc 34 55      100.5 55 34 79.5
6T DU.101 on 25 52 103.5 60 17 68.5
TR6 DU.101 to DU.44393 34 55 100.5 48 27 79.5
T120 DU.101 to DU.24874 34 55 100.5 48 27 79.5
T120 DU.24875 to DU.44393 34 55 100.5 55 34 79.5
T100S 34 55 100.5 48 27 79.5
T100T 40 52 96 61 31 75
5TA to H.49833 34 44 95 48 27 79.5
TR5T 34 55 100.5 48 27 79.5
T100R 40 52 96 61 31 75
Triumph 750 short rod 26.5 69.5 111.5 61.5 35.5   77
Triumph 750 intake (T-75) 40 74 107
Jomo 15 43 74 105.5 74 43 74.5

If I take the average of the figures I measured I get the inlet on 105degrees centre line and the exhaust on 82degrees. Which I reckon is as close as I'm going to get without moving the cams on their wheels as 1 tooth on the cam wheel equals 15 degrees (more or less).

So I reckon the engine is timed up now.

Hopefully the next job I do will involve less maths....