Saturday, 22 June 2013

Cam Timing.

My bike has a 750 inlet cam, 650 exhaust cam (but I have no idea which 650) a crank from a unit 650 and timing wheels from god knows what. This has made me wonder if timing the cams using the marks on the cam wheels is a good idea so I decided to do the job properly and get out the degree wheel.

Stuart had arranged to come over to help on Sunday which made things easier but also meant I couldn't make up an excuse to put it off. So after reading up about the various techniques on the Jockey Journal and Brit Bike forums I decided to go with the lobe centre method.

First job was to sort out a pointer. In the past I've used bits of wire and all sorts so I know the pain of getting half way through the job only to knock the pointer and have to re-time the degree wheel. So I dug out some scrap ally and after a bit of turning, sawing and filing I had a way of securely holding a scribe in position.

Once I'd found TDC the first thing  to do was to determine the opening and closing points on the Inlet cam.

The instructions I found on the Brit Bike Forum suggested setting the tappet clearance to 2 1/2 turns out, this wasn't possible on one set of tappets which would only go 2 1/4 turns out before reaching their limit so I set all the tappets to 2 1/4 turns. If my understanding of the theory is correct the actual tappet clearance isn't critical with this method as by definition the cam lobe centre is half way between opening and closing so if the tappets are tighter they will open earlier but also close later.

The instructions I had recommended using a feeler gauge to judge when the valve stated to open but I don't trust my judgement with feeler gauges as I don't use them often enough to get a... err... feel for them. So I decided to rig up a Dial Gauge instead.

The figures I got were 31 BTDC and 33 ATDC, giving a total opening time of 31+180+33=244degrees. Cam lobe centre occurs half way through opening =122degrees.

As cam lobe centre occurs 122 degrees after the valve opens the valve opening position needs to be subtracted giving a cam lobe centre of 91 degrees ATDC.

This didn't tally with any of the figure I had for inlet timing so it would seem that I was right not to trust my interpretation of the timing marks. Only problem was I had forgotten to make a note of the timing marks I had used so I decided the best bet was to start again, taking photos this time.

On Stuart's recommendation I tried to get a Tippex pen to mark the timing wheels but all I could find was a own brand one in the Range which was too runny. It did let me highlight the dots and dashes by applying and wiping off though.

Anyway with the timing set as above I took a new set of valve opening and closing times, did the calculations and got a lobe centre figure of 91 degrees again.

So I moved the inlet cam onto the other mark (unfortunately not too clear in the picture below)

Measuring the valve open and closing gave me an opening figure of 15 degrees BTDC and 45 ABDC. Doing the maths gave me a cam lobe centre of 105degrees. Which I think is in the right ball-park.

I found the figure below which makes things slightly clearer, My idler wheel has two lines but by aligning with and if anything they are reversed but if I take the first attempt to be the 6T timing and 2nd the T120 timing it makes sense.
The next thing to do was repeat the process for the exhaust. I couldn't find a way of securing the DTI stand in a way that would let me use the gauge so I improvised with a piece of angle and a clamp.

With this set up I got one set of figures, 26degrees BBDC and 10 ATDC. This works out as 

82 degrees ABDC, at this point my head was spinning so I decided to call it a day.

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