Tweaking a G80...

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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Tweaking a G80...

Postby Pharisee » Mon Oct 23, 2017 3:02 pm

I was talking on the telephone to a well known and well respected engine builder recently and the conversation came round to valve timing. I asked him why the 350cc engine has different inlet cam timing to the 500. His answer was that it was really the 500cc engine that had the different cam timing. Back in the day, AMC assumed (probably quite rightly) that the 500cc bike would most likely end up hauling a sidecar so they wanted the engine to be a bit of a 'slogger'. For that reason, they retarded the inlet cam timing by one tooth (18°). He suggested that if I wanted my G80 'pepped up' a bit, I should re-time the engine using the 350cc timing mark on the inlet cam wheel. I could also go one step further and replace the pushrod rocker arms with a pair from a 'lightweight' rocker box. I'm assuming that these are slightly shorter than the heavyweight arms and would therefore give the valves a little more lift.

Presumably, this is a well known 'secret' so have any of you done that and if so, is there a noticeable difference in performance?

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Re: Tweaking a G80...

Postby Andy51 » Mon Oct 23, 2017 7:14 pm

Hi Pharisee, I have heard of this as well. I was recently speaking to Steve Surbey and he was saying that he has a number of Lightweight rocker boxes in stock with the cam side rocker arms missing, so quite a few must have made this mod over the years! I went the whole hog by putting in SH cams (available from the club a few years ago; don't know if they have any left) and changing the inlet rocker arm as well - the bike goes nicely, cruises at 65 - 70 on less than half throttle, and accelerates briskly. It does,however, make a lot more exhaust noise and a bit more tappet noise (no quietening ramps on the cams) but I enjoy it! Cheers, Andy

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Re: Tweaking a G80...

Postby Tigercub » Tue Oct 24, 2017 6:01 pm

Hmmm ... Interesting.... was wondering why my Matchless is soo much slower than my 54 Bullet...in fact its not much faster than my Tiger Cub...Shock.

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Re: Tweaking a G80...

Postby david pollard » Wed Oct 25, 2017 8:01 am

defiantly retime inlet to 350 timing, increased acceleration , much more ;pep: !! enjoy!

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Re: Tweaking a G80...

Postby StephenG80 » Sun Nov 05, 2017 5:57 pm

Can this be used for tweaking all G80Ss or just certain years?
Also how are the SH cams identified?
Does valve/piston clearance need checking due to the greater lift using the 350 rocker arms or is the clearance still plenty?

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Re: Tweaking a G80...

Postby david pollard » Sun Nov 05, 2017 6:45 pm

if you look at the diagram in haycrafts book you will see 1954-56 engines had different valve timing for 350/500, by timing 500 as per 350 engines the cylinder filling is grater, exhaust to remain the same, it makes acceleration much more zippy and still allows for slow running david

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Re: Tweaking a G80...

Postby raffles » Mon Nov 06, 2017 7:20 pm

SH cams are stamped SH the rocker arms you talk about are Lightweight 250 or 350 not to be confused with
heavyweight.
Tony

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Re: Tweaking a G80...

Postby StephenG80 » Mon Nov 06, 2017 7:29 pm

Hi Tony - so just for clarification, GL3 rocker arms are the ones to try and obtain - for both exhaust and intake? - does the valve/piston clearance need checked?

Thanks for the info re Haycroft David - I'll check that out.

Stephen

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Re: Tweaking a G80...

Postby Pharisee » Mon Nov 06, 2017 10:22 pm

Stephen G80 wrote:Hi Tony - so just for clarification, GL3 rocker arms are the ones to try and obtain - for both exhaust and intake? - does the valve/piston clearance need checked?

Thanks for the info re Haycroft David - I'll check that out.

Stephen


The 'heavyweight' rocker arms are the same for both the 350 and the 500. The 'lightweight' 250/350cc pushrod rocker arms are about 1/8" shorter than their 'heavyweight' equivalents.
From the discussion I had with Ken DeGroome recently, he suggested that changing just the inlet pushrod rocker arm of my M18 for a lightweight arm would be beneficial as the change in ratio would give the inlet valve a greater lift. This, coupled with the advanced inlet valve timing obtained by using the 350cc cam timing marks would give a noticeable increase in performance.
I now have a set of lightweight rocker arms and will, in due course (probably early next year), swap the inlet into my M18. I'll re-time the inlet cam at the same time. I'll let you know what happens... :)

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Re: Tweaking a G80...

Postby Andy51 » Tue Nov 07, 2017 2:11 pm

Stephen, re valve clearance: when I put together my 18S with SH cams, L/Wt rocker arm on inlet cam side only and 8.5 to 1 piston, I did a trial assembly with plasticine on the top of the piston, kicked it over a few times, then measured plasticine thickness (=clearance). I got over 1/16 in, as recommended by no less a person than Phil Irving; fast runs since then have revealed no problems. I recommend you do the same for peace of mind, but there does seem to be enough valve clearance to accommodate any of the options discussed here. Cheers, Andy

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