Altering the valve timing on my Model 50

Hi Mike,

   If the compression is low, it is likely there is an issue with the condition of the bore/piston/rings, valve seating/tappet clearance, head joint or tight valve lifter cable. You need to check all these out before you try changing the valve timing. As a general test, you should be able to stand on the kick start for a couple of seconds before over-coming the compression, and the bike should have a top speed of at least 70mph in normal conditions.

   Your photo looks like you are on TDC with both valves on equal over-lap, which is correct. To check, set the timing disc pointer at zero and then turn the crankshaft backwards 30 degrees and the inlet tappet should just be on the point of lifting when you turn the crankshaft forward. Remember the cam-wheels turn the opposite rotation to the crankshaft. If you advance the cam-wheel by one tooth, this equals 9 degrees and may advance the opening too much, so anything more than 25 degrees should be left alone. If you can find an earlier half-time pinnion they have 3 unequal key slots for more precise timing. The ideal timing for your engine will be  inlet open 30 BTDC, close 70 ABDC, exhaust open 70 BBDC, close 30 ATDC. The later cams have a gradual pre and post open period profile for less mechanical noise, so you will have to allow for this when determining the effective opening point.

P.S.

Re - Robert's post.    If you can find a copy, "Tuning for Speed" by Phil Irving is an invaluable book to understand the fundamental dynamics of valve timing.

Valve timing is complex.  The inlet should start to open at around 30 degrees before tdc on the inlet stroke  , the timing is sometimes found to be well out on these motors so a bit of experimenting can wake the engine up. Do some more reading up on the subject  .

Setting the cam by when it starts to open is too hard to be precise. The crank moves a long way because of the gradual ramp on the cam. I've always gone by valves rocking (on a single cam engine) or full lift.

Full lift inlet cam, on your engine  - from the figures given above - is 110 degrees after TDC on the non compression stroke. People say you can't find the top of the cam but of course you can - if you can use a dial gauge - in fact some performance camshaft suppliers recommend using the full lift figure as the most accurate way of dialling a cam in. 

Try it !

do a leakdown test to locate your leakage. Cam timing is a wild goose chase.

You have it, from TDC, ( non compression stroke), engine rotated the normal firing direction of course, 110 degrees later ,the inlet lobe should be fully open, and, as you say, should then be at the 70 mark. Full open checked by rotating back and forth, preferably with the dial gauge on, but for a rough check, it's an easy visual check anyway.

110 is half way between inlet opening at 30 BTDC and closing at 70 after ABDC.  It simply avoids the slow lift movements at beginning and end. Easy to work out from any cam spec figures.

I take it that the figures Richard Cornish quoted are correct. ( I don't have any information about the singles ).

Have a go Mike !   it costs nothing and it's all part of the fun. 

Just one thing when checking compression, fully open the throttle, if it can’t get air in, it won’t compress.  Makes a big difference on mine. 

Hi Mike,

   I think your problem here is the quietening ramps as when the engine is cold, these extend the timing period to an artificial amount. When the engine gets hot the barrel and head expand much more than the push-rods and this corrects the situation. Set the push-rods with the engine cold on TDC of the firing stroke the turn the top cup hexagon back a 1/2 turn on both push-rods, which is .019" and should eliminate the ramps issue for setting purposes. Try Terry's method and see how the opening and closing points look like. The numbers might read lower, for instance 25 - 65, but as long as you have the same difference either side it should be O.K. When you re-set the tappet clearance, you will still have the same issue with low compression until the engine gets up to full temperature. When the bike is run in , it should be good for 75mph, but rember it's a 350 in a 600 sized bike.

Mike,

You have written that following a top end rebuild your bike has very low compression, but when you loosened the tappet adjuster, it had good compression.

If this is correct,  then you have not adjusted the tappets correctly. One of the valves is slightly open because the tappet clearance is too tight.

On our singles there is no actual tappet clearance gap, the tappet clearance is adjusted correctly when you can turn the pushrod with your fingers but not have any up and down movement.

Maybe your fingers are too strong! 

Try and back the tappet adjuster off until there is just discernable up and down movement in the pushrods AFTER YOU HAVE TIGHENED THE LOCKNUTS.

It takes ages for me to get mine 'just right' and often I feel you need 3 hands to do it properly!

Don Anson

Melbourne 

Er, sort of - the first paragraph would be better stated :  set the piston at TDC on the non compression stroke and the timing disc also at TDC. Then, rotate the engine forwards a total of 110 deg. At that, the inlet valve cam lobe should be at max. You can then work the crank back and forth a few degrees to see if it is at the peak, or use a DTI.

The rest , your experiments to check it all out , fine, do it !, experiment and satisfy yourself all good ( or otherwise ). It will be something either eliminated or sorted.

You're welcome to PM me if you need to, but I think you have it sussed out already, tell us what you find .Are those engines separate cams for in and ex ?  If so, work out the peak lift for the ex and check that too. Same principle.

Hi Mike,

  This is getting a bit confused, the total cam opening period is 280 degrees of CRANKSHAFT rotation. Set the piston at TDC on the induction stroke and then turn the engine FORWARD 110 degrees, which is just over half-way down the stroke, then fit the inlet cam at it's maximum lift. Turn the engine forward again so the piston goes down to BDC and then comes up a third of the stoke where it should be just closed at 70 after BDC. To set the exhaust cam, go to the same TDC and turn the engine BACKWARDS 110 which is just over halfway down the stroke. Fit the exhaust cam at maximum lift. Turn the engine backwards 140 down past BDC and a third of the way up the other side to 70 before BDC when the exhaust should just be ready to open when the engine goes forward. If you find your 90 degree disc confusing mark top and bottom centres and then mark the complete opening ark of each valve in 2 different colours. All this should result in both valves being open an equal amount at TDC which is the valve overlap. 

for the equal lift point is what I read in articles about cam timing.

... to see if the valve timing is correct (not 1 tooth out!):  At TDC on the non-ignition stroke, the inlet and exhaust valves should be nearly equally open. If one is much more open than the other, there is a problem.

Equal lift point on Mikes' M50 is TDC. 

Inlet opens 30 BTDC, exhaust closes 30 ATDC. 

Overlap centre, ie "valves rocking ", thus  has to be TDC, but it's true many engines do have the overlap point a few degrees before.

Valves rocking is the old way of setting a camshaft in, without knowledge of the timing figures.

This was all explained to me in 1967 by an old hand lorry mechanic , I was very dubious and wanted to get a degree disc," nah, it'll be reet lad ".   

And it was.