SQ4 how to advance valve timing

[adrian.hannam]

There are good notes about this subject, downloaded from Drags, written by Bruce L. Also available is Keith's procedure here: http://forum.arielownersmcc.com/viewt ... our+timing

The advice as I understand is to improve max valve lift angle to between 100 and 105 ATDC.

The part in the procedure that is not explicit enough is how to change the timing. There is something implied that I don't understand. I know I need to use a different keyway on the crank sprocket, but how exactly? I have tried all the key ways, rotated the rear crank CW and CCW in relation to the cam. Keep the chain position the same, and also tried varying the datum mark for each key and aligning the cam. Tried a lot of options and drawn many graphs, not getting want I expect. There is a fundamental that I don't grasp. Can anyone just add a couple of dumbed down steps to this part of the procedure please?

Specifically = I want to move my average from 108 deg ATDC to optimum. Thanks.

[paul.jameson]

The answer here is that since you have 3 keyways on the timing sprocket you have 3 options for the valve timing. Set the engine up as per the instructions you have and fit the chain and sprockets. Measure the valve timing as per Bruce's instructions. Bruce's instructions also tell you how many degrees moving the sprocket one link on the chain will change the timing by. Once you are satisfied that you have got the best timing you can on that keyway, mark it (say 1) and repeat the process with the second keyway, then the third. When you have done this, you will have full details of what can be achieved for valve timing with the crankshaft sprocket you have. Note that Bruce's instructions tell you how much leeway is acceptable, and in which direction.

The chances are that you won't get the timing correct to within a degree or so but that you will be able to get it within Bruce's permitted tolerances. The bike will run well enough if it is within Bruce's tolerances. In the mighty unlikely event that you can't get the timing within Bruce's tolerances on any of the keyways you need a different timing sprocket. Do use a new timing chain when setting the valve timing as a worn one will alter the timing slightly.

Be prepared to have to sit down and think hard about the whole process. It does get easier with practice, believe me.

[david.anderson]

Adrian
If you have measured your inlet centreline at 108atdc (standard keyway valve timing) and you want to move it to between 100 and 105atdc then you need to advance the cam by about 5 degrees. If you re-engage the sprocket to the keyway on the left of tdc then you will advance the cam by 5.5 degrees resulting in an inlet centreline of 102.5 degrees.
Now after you have done that for no 1 cylinder and got that perfect, you can also check cylinders 2,3 and 4. They may not give the same result.
The square 4 valve timing is fairly slow and in my experience the difference in performance will barely be noticeable. With your current valve timing the bike will perform better high in the rev range and possibly be faster top speed, but it will become more tractable and will accelerate quicker when advanced.
So I may see you at the annual aar rally covd19 permitting?
David

[adrian.hannam]

Hi David, Yes I hope very much to go to the rally.
Yes that makes sense. I removed the rockers, which has helped. I discovered my crank sprocket datum mark is in a different position to what is shown in the literature. This was very confusing. I was warned, Simon has one too. Using the datum mark results in the most advanced setting. The other two are further retarded. Or another way of saying it - ignoring the datum mark and using the standard keyway, inlet centreline is 114 deg. I expect I will have to use the retarded key; this will add 5 deg, and move the chain to subtract 13 deg, to get 104. That is the plan anyway.

[adrian.hannam]

I have done a survey. Without moving the crank sprocket, and moving the cam by 1 tooth either way, the resulting change is +/- 25deg. According to the notes it should be 13 deg. Why is this?

timing.PNG (5.27 KiB) Viewed 2569 times

I can't achieve satisfactory timing with sprocket and cam adjustments. (I couldn't reproduce 108deg I got before, but what I am doing now is more accurate).

For a sensibility check this is what I am doing:
#1 TDC, set by use of positive stop tool. Timing disc on rear shock absorber. Using a mirror to read it. Rockers removed. Dial indicator on #1 inlet pushrod, second one from the centre. Turning front crank so that timing chain runs clockwise. Reading the angle at equal rise and fall (2mm) from peak push rod lift. Calculate the halfway point between these angles.

I am now thinking when the cam was reground (20 years ago), the keyway ended up at the wrong angle. The notes mention this could be a problem.

My next move I think is to dismantle the engine, inspect the cam.

I didn't take many pics of the cam, this is the best, from the drive side.

The followers have been also been reconditioned.

[david.anderson]

Adrian
If the cam has been reclaimed it is very likely that the grinder did not grind the lobes to cylinders 2, 3 and 4 to precisely 90 180 270 degrees from cylinder 1, especially if he got no 1 out of sync with the keyway. Before going any further I would check the lobes for the other cylinders. It may be a matter of averaging it. I have seen some very large in discrepancies on a couple of non Ariel twin cams, and one of them was a factory cam, not a regrind.
With regard to 13 – 25 degrees are you talking crankshaft degrees in both cases.

It is important to always match the follower to the cam ie no1 lobe to no1 follower, and don’t turn the follower around 180 degrees. That is where the mating surfaces have bedded. With a new cam or a rebuild where order is unknown then the cam should be well coated with a cam running in lube and the normal cam run in procedure should be followed.
David

[john.bebb]

hi Adrian, - please is the pic of your cam followers before or after the recon?

[adrian.hannam]

John: I can't say for sure when they were done, or how much use they have had, except that they have obviously been built up with weld. This motor was serviced by about 20 years ago. It covered 2500 miles in the first 2 years and has stood since.

For David: I find the peak cam lift (in crankshaft degrees) changes by 25 degs by moving the cam sprocket by 1 link. I was expecting 12.8 as per the notes.

[roger.fellows]

Adrian, movement at the crank is twice the movement at the camshaft. I'm never sure which is the half-time sprocket - I try to refer to them as cam or crank.

28 teeth on the cam sprocket - so 360/28 - nearer 12.9 than 12.8 degrees. Therefore nearly 26 degrees - 360/14 - per tooth at the crank.

I'm working through this for my own benefit as I will get to the problem eventually.

I have a crank sprocket - probably from Bruce - which has a fourth keyway. Unfortunately it's not to hand.

The next question is how to assess the positions of the keyways relative to the teeth on the crank sprocket.

I don't possess a timing disc, but that might be used to measure the positions. Given your set of measurements for one keyway it should be possible to work out which of the other keyways would be more accurate. And then because this is a situation where your answer is right or wrong depending on one slip of logic you find it should be the other one.

[john.bebb]

Hi Adrian,
If (as I assume from your pics) you are using a newish reground cam; - you might also consider replacing the followers.
My motor is presently at Hamlins - when I presented it them the cam followers looked remarkably similar to those in your pic.
The engineer at Hamlins, (Andy "BigVee") was alarmed that I was considering using my existing followers...

Cheers, John