SQ4 how to advance valve timing

Singles, twins and fours.
allan.walker
Holder of a Waxed Cotton Anorak
Holder of a Waxed Cotton Anorak
Posts: 338
Joined: Fri Sep 28, 2018 8:35 am
Contact:

Re: SQ4 how to advance valve timing

Post by allan.walker »

adrian.hannam wrote: Thu Mar 19, 2020 5:17 am 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?
I realise that I am following-up on this rather late, but I am having exactly the same difficulties. I see that no-one posted a reply to your question...

Bruce Longman's otherwise excellent and comprehensive instructions seem to contain what seems like a red herring to this particular greenhorn, in that the movement by one tooth of the cam sprocket is quoted in terms of cam rotation, rather than crankshaft rotation.
In the instructions, all the fundamental measurements about setting/checking the valve timing are based upon crankshaft degrees.

I had arrived at the same conclusion as yourself; this is what I found:
Setting the valve timing initially using the the line on the crank sprocket and the two holes in the cam sprocket resulted in max. valve lift at 112 deg. ATDC.
I moved the chain on the cam sprocket by 1 tooth and max. lift occurred at 87 deg. (when I was expecting it to be about 99 deg. I was hoping to be able to retard it a little using one of the other keyways on the crank sprocket.)

So, I am stuck with valve timing considerably adrift from 105 degrees, with no way that it is clear to me to rectify it.

I have a new cam (from Iron Horse) with new followers. The cam chain is used (but Bruce says a worn one only affects timing by 2 degrees or so, so not really a major contributory factor. I have a new one on order from Drags, anyway.) It would seem that making a stepped key will only yield a couple of degrees difference.

Did you manage to sort-out your valve timing?

Allan.
User avatar
chris.shearwood
Holder of a Silver Anorak
Holder of a Silver Anorak
Posts: 646
Joined: Mon Feb 28, 2011 5:28 pm
Location: Sutton, Quebec, Canada
Contact:

Re: SQ4 how to advance valve timing

Post by chris.shearwood »

Allan,
Pleas excuse me if I am not understanding your above message. If you want to alter the timing from 112 deg. towards 105 deg. can't you use the crankshaft sprocket keyway Number 111 on the below diagram instead of keyway Number 1?
Attachments
Sq4 Valve Timing20220903.jpg
1946 4G and 1951 VH
allan.walker
Holder of a Waxed Cotton Anorak
Holder of a Waxed Cotton Anorak
Posts: 338
Joined: Fri Sep 28, 2018 8:35 am
Contact:

Re: SQ4 how to advance valve timing

Post by allan.walker »

Thanks for the diagram, Chris. I haven't noticed the Roman numerals on my sprocket - I'll have a good look tomorrow!
I did try using the other keyways and the effect was approx. 5 degrees improvement, but still leaves things outside the range of 100 - 105 degrees which Bruce Longman recommends.
I've since spoken to Bruce and he suggested filing the key for the cam sprocket (where it enters the camshaft) and then shimming it to move it slightly, thereby rotating the sprocket relative to the camshaft. He said to remove approx. 3 thou for each degree of adjustment needed.
I'll try this tomorrow and see if I can get somewhere in the 100-105 degree range.
User avatar
paul.jameson
Holder of a Golden Anorak
Holder of a Golden Anorak
Posts: 2934
Joined: Wed Oct 12, 2011 3:04 pm
Location: Herefordshire
Contact:

Re: SQ4 how to advance valve timing

Post by paul.jameson »

Research undertaken by the late Ralph Hawkins in the late 1980s or early 1990s revealed that the relationship between keyways and sprocket teeth was inconsistent. So it may be that you have an odd sprocket. IF it is simplex, I have several spare ones here which you could borrow and try. If it is duplex, I don't have many to try.
Paul Jameson
35 LG (project), 37 RH500, 52 ex ISDT KHA, 54 KH(A), 75 Healey 1000/4.
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist (but not Machine Registrar), Gauges and Clocks Spares Organiser.
allan.walker
Holder of a Waxed Cotton Anorak
Holder of a Waxed Cotton Anorak
Posts: 338
Joined: Fri Sep 28, 2018 8:35 am
Contact:

Re: SQ4 how to advance valve timing

Post by allan.walker »

That's interesting, Paul and thank you for your offer. Were you referring to the crank sprocket or cam sprocket possibly being a problem?
I'll see how I get on with filing and shimming the key. If it all goes wrong, a new key is cheap enough!

I'll let you know how I get on..... :?
User avatar
paul.jameson
Holder of a Golden Anorak
Holder of a Golden Anorak
Posts: 2934
Joined: Wed Oct 12, 2011 3:04 pm
Location: Herefordshire
Contact:

Re: SQ4 how to advance valve timing

Post by paul.jameson »

The crank sprocket. I suspect the variability might arise from which way up the sprocket was when the keyways were broached.
Paul Jameson
35 LG (project), 37 RH500, 52 ex ISDT KHA, 54 KH(A), 75 Healey 1000/4.
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist (but not Machine Registrar), Gauges and Clocks Spares Organiser.
User avatar
paul.jameson
Holder of a Golden Anorak
Holder of a Golden Anorak
Posts: 2934
Joined: Wed Oct 12, 2011 3:04 pm
Location: Herefordshire
Contact:

Re: SQ4 how to advance valve timing

Post by paul.jameson »

Ralph's initial article in Cheval on the subject was on Page 16 of the February 1988 edition which you can see on the website. He did do quite a survey of the sprockets, the results of which are in the archives, but the archives are in the shed and a severe storm is underway currently so I am not going out there till it stops! He was talking simplex sprockets though.
Paul Jameson
35 LG (project), 37 RH500, 52 ex ISDT KHA, 54 KH(A), 75 Healey 1000/4.
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist (but not Machine Registrar), Gauges and Clocks Spares Organiser.
allan.walker
Holder of a Waxed Cotton Anorak
Holder of a Waxed Cotton Anorak
Posts: 338
Joined: Fri Sep 28, 2018 8:35 am
Contact:

Re: SQ4 how to advance valve timing

Post by allan.walker »

Thanks for the pointer to Ralph's article. I have the simplex set-up on this engine, so it is very relevant.

I had a brief spell in the workshop yesterday, trying different things. I was able to file the camshaft sprocket key sufficiently to allow the amount of adjustment needed (equivalent to about 7 degrees of advance) and shim it using a section cut from an old feeler gauge.

Unfortunately, time ran out for me just at the point where I hit a new source of confusion.......I'll have another go today and report back. :?
Post Reply

Who is online

Users browsing this forum: No registered users and 4 guests