7 replies to this topic

[malc_west]

Hi guys need a bit of help. I'm slowly working out setting the timing on my mini using some yoh tube vids and a guys blog on it. So I've got a depth index gauge and the degree disc from mini spares. I've set the 1st piston (rad end) to top dead centre. I've then put the push rod in the first inlet valve (2nd hole) and taken the degree measurement when the push rod is at its lowest using the depth index gauge. My reading was 94 degrees. Than again when the push rods at its highest my reading was 104 degrees. I've then added the two togeather and and divided by two to give 98.5 degrees. If my understanding is correct and the methods correct that means I have to advance the timing by 9 degrees? Does this sound right
Thanks

[nicklouse]

Sounds like you have confused two things.

 

you describe setting up the cam shaft.

 

you state a ignition timing info.

 

so what are you trying to set?

 

ps. Please get or read your Haynes manual.

Edited by nicklouse, 20 December 2020 - 11:32 AM.

[malc_west]

Sounds like you have confused two things.

you describe setting up the cam shaft.

you state a ignition timing info.

so what are you trying to set?

ps. Please get or read your Haynes manual.

Hi nick, so it's cam timing I'm setting, the advance I thought I needed would be done by using an offset woodruff key to change the sprockets position. Which part of what I'm doing suggests ignition timing? Cheers

[Ethel]

The pushrod will be constantly at its lowest for 3/4 of the time. I assume you mean it's at its highest - the process for the cam is the same as for the crank (piston tdc). Find two points, with equal lift, either side of maximum lift, and divide the angle between them.

 

 

Maximum inlet valve lift happens after top dead centre, the 104^o sounds around the right ball park, so turn the engine to there. Then back a little bit, so it's showing obvious movement on your dial gauge. Give it a tiny nudge clockwise (so it's going in the right direction and takes up any slack), zero your gauge and note the angle. Then slowly nudge (turn) the engine forwards until it's back at zero on the dial gauge, note the new angle.

 

Add your 2 measured angles together & halve to find your cam timing (e.g. 102 +108/2 = 105. Compare the answer to the figure specified for the cam to get the offset key size you need. If it's a new chain & sprockets you might want to allow a degree or two for stretch - i.e. that 105 would become 107.

[malc_west]

The pushrod will be constantly at its lowest for 3/4 of the time. I assume you mean it's at its highest - the process for the cam is the same as for the crank (piston tdc). Find two points, with equal lift, either side of maximum lift, and divide the angle between them.


Maximum inlet valve lift happens after top dead centre, the 104^o sounds around the right ball park, so turn the engine to there. Then back a little bit, so it's showing obvious movement on your dial gauge. Give it a tiny nudge clockwise (so it's going in the right direction and takes up any slack), zero your gauge and note the angle. Then slowly nudge (turn) the engine forwards until it's back at zero on the dial gauge, note the new angle.

Add your 2 measured angles together & halve to find your cam timing (e.g. 102 +108/2 = 105. Compare the answer to the figure specified for the cam to get the offset key size you need. If it's a new chain & sprockets you might want to allow a degree or two for stretch - i.e. that 105 would become 107.

Ahh the pennies dropped cheers for that very useful response. So my maths now gives me 101.5 degrees so that means I need to advance by 7 degrees or do I advance by 5 degrees and allow chain stretch to do the final bit?

[Ethel]

If you got 101.5 degrees on the crank pulley as where the pushrod was at its highest by the method in my 2nd paragraph and your cam specifies 108 degrees atdc, then yes.

 

Advance & retard are confusing as it's not clear what's being referenced from what. The cam sprocket (and crank) needs to turn further clockwise relative to the cam.

[Spider]

I've just edited the Title to make the subject a little clearer.

[Spider]

I've snapped off some photos from a recent build and will do an FAQ on this shortly.

 

Ethel has covered it well, but is I can jump in here, how have you found TDC ? 

 

I have 2 ways I use to find TDC, but if you have a dial gauge set it up and position it over the gudgon pin to take any piston rock out of the equation.

Fit up your degree wheel and pointer. Turn the engine over so the Piston is rising and measure off the Angle when the Piston is 0.005" from being at the top.

The continue winding it over until the Piston is about 1/4 the way down the bore, then reverse the direction so the Piston is again rising, stop at 0.005" before it gets to the top. Average the 2 angles and that will be smack on TDC.

I suggest here measuring both these angle with the piston in a rising direction rather than one rising and the other falling as there can easily be 2 or 3 thou difference here, due to take up of all the clearances.

 

When measuring the Cam Angle though, be sure to ONLY turn the engine over in the direction that it runs in order that the natural slack in the valve train is taken up.

The way I find full lift is similar to finding TDC only, as noted above, keep turning in the direction the engine runs, ie, when looking from the water pump end that's clockwise.

I'll wind it over and make a mental note of the full lift figure, then keep going over again and stop at 0.005" short of full lift, read off the angle. Keep going again until you get to 0.005" down from peak lift, note the angle. Average these 2 angles and that's your full lift angle.

I feel it's necessary to measure either side of TDC and Full Lift to get accurate numbers as there is some dwell at the top of these and it's next to impossible to get accurate numbers. In regards to the Cam though, this only works if it's a Symmetrical Grind, which 99% you'll come across are. If it's Asymmetrical, then usually these are set at 0.050" lift figures, but the Grinder will (should !) include instructions how to set them.