Valve timing cams

Information relating to the Matchless G3 or AJS Model 16 350cc Heavyweight
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Valve timing cams

Postby dreverl » Mon Mar 12, 2018 8:54 pm

Hi All,

I'm helping a friend with a 1953 16M with a major back firing issue when I go from 1/2 to full throttle and it wont rev out. So.... I've checked a few things and valve timing is one thing I want to check.

After looking at the book, I think the pics attached are right, can anyone confirm?

The inlet cam is 'timing1.jpg' and although the marks aren't quite right, you have to decide if it goes to the left or right of the tooth anyway so it appears to be OK?

The exhaust cam is 'timing2.png' but it doesn't have the marks as per the book so I assume its an aftermarket replacement with a line instead of dots? Again, it looks OK to me, can anyone confirm?

If these look OK, on to the next thing.... Any ideas on that???

Thanks for your help
Andrew
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Re: Valve timing cams

Postby SPRIDDLER » Mon Mar 12, 2018 10:31 pm

Here you are, Andrew. Chapter and verse. Valve timing with or without markings........

http://archives.jampot.dk/technical/Gen ... e_Dots.pdf

dreverl wrote:.....when I go from 1/2 to full throttle and it wont rev out.


Ignition retarded? Timing should be set 7/16" BTDC on the compression stroke with the mag at full advance. Make certain you know which way the handlebar Advance/Retard lever is moved for advance and retard........ If the A/R control cable enters the mag on the right of the mag body (when viewed from the points side) full advance is with the cable under tension - i.e. when pulled.
If it enters on the left - i.e on the frame downtube side, full advance is when the cable is relaxed.

Blocked or undersized main jet?

Wrong carb slide cutaway?

Choke 'On'? .......Choke is 'Off' when the choke cable is pulled tight. To check, put choke handlebar lever at what you believe is 'Off' (i.e. no choke) position then open throttle wide (engine not running!!). Insert finger into carb intake and if the choke is 'Off' all you should be able to feel is the carb needle.

EDIT:
If the bike has been unused for a while the evaporation of petrol leaves a gummy deposit. It may be necessary to give the whole carb a thorough clean. Also check that there is no restriction in the petrol supply - pipes, filter......
Out of interest is it fitted with a Monobloc carb or the original 2 piece unit which has a separate carb body and float chamber?

EDIT edit.....
If the backfiring is on the overrun it's likely to be a blocked pilot jet, an air leak twixt carb and head, or in the zorst system joints.
Failing to rev and mis or back firing could due to a dirty mag slip ring, worn carbon pick-up brush, poor HT lead or a duff spark plug or cap. Modern plugs don't respond well to cleaning.
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Re: Valve timing cams

Postby dreverl » Tue Mar 13, 2018 10:08 pm

Hi Spriddler,

Thanks for the info. Good to bounce some ideas around.

Ignition retarded? Yes, definitely an option. I know I had it the right way in terms of adv/ret but there is the possibility I didnt have enough adv available? IE when I retarded it, it backfired more so maybe I could reset it to allow more advance?

Blocked or undersized main jet? - Jets are correct and Ive cleaned carb out with ultrasonic cleaner and compressed air - I dont think its this one

Wrong carb slide cutaway? - Possibly... I didnt check that. Do you know what its supposed to be?

Choke 'On'? .......Choke is 'Off' - I checked this one, got worse with it in the wrong position

Also check that there is no restriction in the petrol supply - pipes, filter...... - Yes, Ill check this but the backfire is instant (ie one engine revolution) wouldnt petrol issue be a 'slow' dyeing of the engine and then pickup again?

Out of interest is it fitted with a Monobloc carb or the original 2 piece unit which has a separate carb body and float chamber? - Its got the original 2 piece carb. (I have put on a spare Mikuni as a check and the same thing happened)

If the backfiring is on the overrun it's likely to be a blocked pilot jet, an air leak twixt carb and head, or in the zorst system joints. - Its not on the over run, its when the engine is under load, it gets worse going up a very slight hill, I originally thought it was this but after putting the Mikuni on, I dont think so anymore.

Failing to rev and mis or back firing could due to a dirty mag slip ring, worn carbon pick-up brush, poor HT lead or a duff spark plug or cap. Modern plugs don't respond well to cleaning. - Yes true, I need to check each of these as well.

So the 'winning' probable causes...?
Ignition timing
Slide cutaway
Mag/HT/plug/cap issues

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Re: Valve timing cams

Postby SPRIDDLER » Thu Mar 15, 2018 5:18 pm

dreverl wrote:....there is the possibility I didnt have enough adv available? IE when I retarded it, it backfired more so maybe I could reset it to allow more advance?


Not sure what you mean by 'available' advance Andrew, unless you're thinking that the A/R cable is restricting full movement of the mag's face cam.
Ign timing isn't really a 'trial and error' job. It would be sufficient to set it as recommended, (7/16" BTDC on full advance) and then leave it at that except retarding it manually a tad for starting.

Can you clarify 'backfiring'. Are you referring to banging/popping in the silencer/zorst or spitting back from the carb? I suspect you were referring to backfiring in the zorst system, especially as retarding the timing further makes the problem worse. In this case, yes, you need to get the timing set 'to the book' (check/adjust the c/b points gap first) which probably means advancing it. Until you are sure you have the timing right there's not much point in trying anything else. Change one thing at a time!
LATER EDIT. Just one more thing: Does it have good compression? It's possible that exhaust valve lifter is holding valve open a fraction or that the exhaust valve is not sealing properly - perhaps damaged?

The slide and main jet are more than likely correct since the problem occurs 'within one engine revolution' rather than out on the road, but worth checking.
See Page 4 in the Owners Manual (link below) for carb settings etc.

http://archives.jampot.dk/book/Owners_m ... ingles.pdf
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Re: Valve timing cams

Postby dreverl » Sat Mar 17, 2018 1:19 pm

Hi Spriddler,

'available' because it almost felt like I didnt have enough advance, which might mean the original setting wasnt right (ie it wasnt actually 7/16 BTDC).

Ive taken the mag off to have a good look and when I put a drill on it, the spark isnt much to write home about. It also isnt all that difficult to turn by hand and when Ive had mine rebuilt, its a bit more difficult. I dont think the mag is at full strength...

Yes, the backfiring is through the exhaust. Yes, it has very good compression, however, now that Ive triple checked the timing side etc etc Ill have to re check the clearances when it gets hot.

Ill need to get the mag rebuilt and go from there...

Cheers

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Re: Valve timing cams

Postby Duncan » Sat Mar 17, 2018 2:48 pm

Apologies if you have already covered this off, have you discounted the possibility that you have an earlier Matchless mag cam fitted to the exhaust side that would originally have been be on the inlet side?

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Re: Valve timing cams

Postby SPRIDDLER » Sat Mar 17, 2018 2:56 pm

dreverl wrote:Hi Spriddler,

'available' because it almost felt like I didnt have enough advance, which might mean the original setting wasnt right (ie it wasnt actually 7/16 BTDC).

O.K. so just set it at 7/16" BTDC and leave it at that.

Ive taken the mag off to have a good look and when I put a drill on it, the spark isnt much to write home about. It also isnt all that difficult to turn by hand

As you know you should feel a firm resistance to turn at the point of max flux. On a mag in tip top shape you should be able to get a 1/4" blue spark just with a sharp twiddle flick of the fingers on the armature. That's in an ideal world and a weaker spark won't necessarily prevent th'engine from running and yours is evidently firing - but not necessarily at the right time.
One of our members (Groily) is a well respected mag restorer and parts supplier. Have a browse of his website here:

http://www.brightsparkmagnetos.com/

The club also runs a mag exchange service.

http://www.amoc-parts.com/store/comersu ... tegory=122
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Re: Valve timing cams

Postby dreverl » Sun Mar 18, 2018 9:07 pm

Duncan wrote:Apologies if you have already covered this off, have you discounted the possibility that you have an earlier Matchless mag cam fitted to the exhaust side that would originally have been be on the inlet side?



Hi Duncan,

No, I havent, how do I know if its a Mactchless and would it be OK at low/med revs and then cause problems at high revs?

Cheers

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Re: Valve timing cams

Postby dreverl » Sun Mar 18, 2018 9:15 pm

The club also runs a mag exchange service.

http://www.amoc-parts.com/store/comersu ... tegory=122
[/quote]

Yeah, we are going to see what they have.

We can put a reconditioned mag on and move onto the issues, one by one.

Cheers

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Re: Valve timing cams

Postby SPRIDDLER » Sun Mar 18, 2018 9:40 pm

dreverl wrote:I'm helping a friend with a 1953 16M with a major back firing issue


Has it ever run right for your friend or has he always had the backfiring problem?
If it hasn't, then Duncan's comment about a cam from an early Matchless engine (i.e. with the mag at the rear of the cylinder) may be a clue/relevant.

dreverl wrote:The exhaust cam .... doesn't have the marks as per the book so I assume its an aftermarket replacement with a line instead of dots?

To generalise (as thankfully I'm not enough of a nerd to know the exact years ;) ) I believe early cams just had either a line or dots whereas later cams had numbers.

I'm thinking that if your friend has never managed to get it run right then the previous owner may have had the same issue, given up, and flogged the bike.
It does happen :roll:
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