18CS Replica Pushrod & Jetting

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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AMacD_LAPSED
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18CS Replica Pushrod & Jetting

Post by AMacD_LAPSED »

Hi All,

A couple of basic questions.

Please can you confirm the correct pushrod tappet adjustment method for the alloy competition engine (circa 1955 long stroke). I read that they're set to zero clearance (unloaded - just free to rotate at TDC comp) cold on engine with a iron head and the same but warm for those with an alloy head - but wasn't sure whether that is also true for the alloy barrel.

Out of interest why are they set so tight - I guess they open a little as everything warms?

Secondly, my bike is fitted with a 932 Wassell carb with a 220 main jet, 9/3.0 slide. Is that a sensible fitment & jetting. I know there's mixed feelings on these carbs. It has been running bit rich and I've dropped the needle and opened the air screw from 1 to 1.5 turns as a first attempt to improve. Haven't tested yet due to other issues. Thought I'd enlist your help in the interim to confirm whether there's a reasonable chance of the bike running well on this carb or whether I'd be better going original?

Many thanks
Alex
Alex MacDonald
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dave16mct
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Re: 18CS Replica Pushrod & Jetting

Post by dave16mct »

I suppose the reason for the setting is to have a quiet engine. Maybe someone has another idea? On my short stroke CS the inlet is nil, free to turn and the exhaust is 0,005" , set to nil then back off one flat.
I don't know anything about Wassell carbs but you've already broken the golden rule : 'only make one change at a time'! Difficult to help with carb problems without seeing the bike. Is there an air filter fitted? 220 seems a bit small for the main jet but try it out first. The main jet only come into play at wide throttle openings so do a plug chop and see what colour the plug is. How does it run?
Dave.
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Re: 18CS Replica Pushrod & Jetting

Post by 56G80S »

+1 for 'only make one change at a time'!

Johnny B
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Re: 18CS Replica Pushrod & Jetting

Post by AMacD_LAPSED »

Thank you both. Point taken on the experimental methods.

The key thing I want to confirm is whether this is a suitable carb at all. I think the correct Amal carb is a 389/1 with 1 5/32" (~29.6mm) bore and 260 main jet. My carb is 32mm with a 220 main. Not sure whether the bore difference is significant. Not sure whether the jet sizes read across. It's running without an air filter although I may chose to add one.

Unfortunately when I had the carb apart I didn't think to check the size of the needle jet.

The bike is running fine but the plug is rather dark. I've only been using it gently hence the main is unlikely to be the culprit.
Alex MacDonald
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Pharisee
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Re: 18CS Replica Pushrod & Jetting

Post by Pharisee »

AMacD wrote:Thank you both. Point taken on the experimental methods.

The key thing I want to confirm is whether this is a suitable carb at all. I think the correct Amal carb is a 389/1 with 1 5/32" (~29.6mm) bore and 260 main jet. My carb is 32mm with a 220 main. Not sure whether the bore difference is significant. Not sure whether the jet sizes read across. It's running without an air filter although I may chose to add one.

Unfortunately when I had the carb apart I didn't think to check the size of the needle jet.

The bike is running fine but the plug is rather dark. I've only been using it gently hence the main is unlikely to be the culprit.
My '56 short stroke G80CS runs a Monobloc 389/12. It has 1.3/16" bore, a 440 main jet, a 30 pilot jet, a number 3 slide, a .106 needle jet and the needle is set in the middle notch.
Hope that helps.
John
I'm from the Fens.... Gimme six.
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dave16mct
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Re: 18CS Replica Pushrod & Jetting

Post by dave16mct »

That's the same as mine except I went for a 400 main (440 is for an open pipe). Alex's bike is completely different though. It would have had an Amal TT originally! The Wassell carb may be fine. I'd say stay with it and see how it goes.
Dave.
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Re: 18CS Replica Pushrod & Jetting

Post by AMacD_LAPSED »

Just a brief update. Since resolving my sliding seal issues I've been out for a few runs on the bike. The plug colour looks better since the changes and the bike seems to be riding nicely. I'm without frame of reference but for the moment all seems fine and I'll keep gently extending the radius of my sorties and confidence in the bikes reliability.

Thanks
Alex
Alex MacDonald
jmbradford
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Re: 18CS Replica Pushrod & Jetting

Post by jmbradford »

Alex
If you have CS cams then the 400-440 main jet is correct , 63CS 1inch 5/32 monobloc carb. A concentric would normally run 10 to 20 less jet size.
With no CS cams then something closer to 190 - 220 main jet is a good start , the 32mm carb maybe a litte on the large size and could make starting a little harder (less air flow to pull fuel at low speed) a 36mm MK2 Amal certainly had that problem for me.
John B.
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