Timing belt change

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MotorizedYak
Posts: 2
Joined: 19 Oct 2021 17:59
Location: Salisbury England

Timing belt change

Post by MotorizedYak »

Hi All. My first timing belt change having acquired my '77 3½ last year as a non-runner. I've watched Paul Compton's video (https://www.youtube.com/watch?v=rvqPVx3ZlBw) a dozen times and, of course, mine wasn't quite so swift! Getting the old belt off took an hour and I wondered if I might just have well cut it off instead. Sadly, the cam pully has now moved and, although I'm pretty happy about lining the top and bottom marks up, I'm a little anxious about the result of getting it wrong. The cam pully has two potential marks which are 180° apart, but am working on the basis that the one that is uppermost when the "A" mark on the pulley is the right way up (towards the front of the bike) is the right one. What really throws me is that, with the mark lined up with the mark on the crankcase, the pulley won't stay in position and keeps wanted to rotate back (anticlockwise) by about 30°. I'm guessing this is a valve spring acting against a cam. Is this to be expected?
On the crank, there is a hint of a mark in line with the woodruff key on the crank and a good mark on the case, so I think that's good.
Given that the "A" pulley ought to be on the smaller side, I'm also concerned that my new belt doesn't exactly go on easily. I've seen mention of talc/french chalk to ease the process.
If anyone has any advice to help a newbie, I'd be most grateful.
Cheers, Harry
70sbikes
Posts: 96
Joined: 13 Nov 2014 11:44
Location: Melbourne Australia

Re: Timing belt change

Post by 70sbikes »

The "A" should be upside down and on the left.
3potjohn
Posts: 1243
Joined: 02 Jun 2007 13:58
Location: Devon

Re: Timing belt change

Post by 3potjohn »

Is there not not a punch mark on the periphery of the crankshaft pulley which lines up with a mark on the crankcase, around the 2 o’ clock position and then a single punch dot mark on the cam pulley which lines up at the top with the mark on the case? As per diagram 117b on P68 in the blue book?
If it has not turned all the way it should work. If in doubt you could check valve movement in relation to piston movement I guess.
John
norbert
Posts: 750
Joined: 15 May 2007 15:15
Location: Lübeck/Germany

Re: Timing belt change

Post by norbert »

The A, B or C mark has nothing to do withe the timing.
It´s the punch mark that counts.
MotorizedYak
Posts: 2
Joined: 19 Oct 2021 17:59
Location: Salisbury England

Re: Timing belt change

Post by MotorizedYak »

Thanks to all for the swift responses, especially 70sbikes who, I reckon, has cracked it. I HAVE got a (faint) punch mark on the crank that lines up with a mark on the case at approx 2 o'clock. I've also now confirmed (by removing the plug) that this corresponds to TDC on the front piston. The problem with the cam pully is that there are TWO marks on opposite sides and I think it has somehow got 180° out (which might explain the difficulty getting it to stay in position).
70sbikes
Posts: 96
Joined: 13 Nov 2014 11:44
Location: Melbourne Australia

Re: Timing belt change

Post by 70sbikes »

Image
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Daddy Dom
Posts: 512
Joined: 16 Nov 2006 19:48
Location: Auckland, New Zealand

Re: Timing belt change

Post by Daddy Dom »

I'm sorry I missed this question - next time, follow these four simple steps:

1. Get a scalpel/Stanley knife.
2. Carefully slice the old belt lengthways in two and cut the front half off.
3. Slide on your new belt halfway and then carefully cut off the remaining old half.
4. Slide new belt fully home.

Works a treat.
Cheers,
DD
MRC 3082½
ivanca
Posts: 23
Joined: 27 Jul 2019 18:34
Location: York

Re: Timing belt change

Post by ivanca »

Brilliant ...like all ideas so simple and makes me think why didn't I think of that ! It's things like this that make MRC such a great riders club. Thanks
Canuck750
Posts: 111
Joined: 29 Jan 2018 22:14
Location: Edmonton, Alberta, C

Re: Timing belt change

Post by Canuck750 »

Another picture of the marks aligned

Image
3potjohn
Posts: 1243
Joined: 02 Jun 2007 13:58
Location: Devon

Re: Timing belt change

Post by 3potjohn »

That’s a very clean looking set of pulleys.
John
Canuck750
Posts: 111
Joined: 29 Jan 2018 22:14
Location: Edmonton, Alberta, C

Re: Timing belt change

Post by Canuck750 »

vapour blasting and zinc plating, makes old things look new
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