Firstly, is the firing stroke, on any cyl, when piston is at TDC with both valves closed ??

I have never done this before, but want to learn.

GR ground my cam RE 13 so what do you mean it will be 106 deg

Hi GT,
The cam I have put in is an RE 13 and if this has a cam angle of 106 deg, does this mean that the inlet valve will be fully open (max lift ) with a reading of 106 deg on the degree wheel ?? If this is so, in my case, I have just checked it and I have full lift of the inlet valve at 135 deg.So, I need to remove timing chain and move the cam around clockwise, fit timing chain and check again if full lift is at 106deg. I think Simon said he had heard that you said one tooth on cam sprocket is 18 deg, is that correct ?? If you cannot get 106 deg exactly is it a question of getting as near to it as you can ??

cheers

Jethro

OK, your interpretation of what it needs to be timed to is correct.

And yes, 1 tooth on the Cam Sprocket = 18 degrees.

All sprocket sets and cams are not indexed accurately, not knocking the guys that make them, it is just a fact of life. This is why we need to check them and then correct them when we fit a cam and / or new sprockets.

What to do about it? There are several options;-

1) Off-set Woodruff Keys for the camshaft. These are generall available in 3, 5, 7 & 9 degree off-sets. About $25 ea. IMO, very rudely priced but certainly do the job quite well.

2) You can try a few different sprockets with each other. There might be enough difference between them to get you where you want to be.

3) There is a Sprocket set available and the one that fits to the nose of the crank has 7 (from memory) different slots each producing 2 degrees difference. Fiddley to set up, but not too badly priced for what they are and also allows you to 'fiddle' with cam timing with reasonable ease. I have used these and like them.

4) Vernier Sprockets. These allow an infinite, precise setting and generally have a range of 20 degrees. Not cheap though. These are what I use most of the time.

5) Vernier Belt Drive. Similar to the above but don't suffer 'chain stretch' (read below). Again, very expensive (think around $200 +). I have one of these that I use on the dyno as it allows for FAST and accurate cam timing changes. Down side is you need to be vigilant that oil doesn't get on the belt as it will rot and fail. There is a risk here is if the valves do open a long way, they could kiss the pistons

6) You can file the key way (if you patient!) and then shim up the key way to give that little bit of rotation you are looking for. Practice on an old set first!

Now How accurate?

Ideally spot on however, when I run a chain drive I advance them by about 1 1/2 to 2 degrees to allow for chain stretch. To adjust it to an advanced postion, if the actual specified setting figure is 106 degrees, setting it at 104 degrees is 2 degrees advanced.

How close should I be?

Generally within 2 degrees and note that 2 degrees can make a surprising difference.

OK, you can also play around a little with Cam Timing depending on what you looking for.

Advancing Cam Timing brings it on the cam sooner, improved lower RPM torque, but sacrifices top end.

Retarding Cam Timing brings it on the cam high in the RPMs, looses some lower torque but goes like a bat out of hell higher in the RPM range.

If you want to play around with your timing like this ONLY make 2 degree changes and DO NOT advance it to more than about 98 degrees or the valves may kiss the pistons. Note too that you will need to adjust your dissy timing too because the dissy is driven off the cam.

Hope that is some help.