G80S Loss of Power with Throttle Opening

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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G80S Loss of Power with Throttle Opening

Postby BobGoodwill » Tue Oct 01, 2019 5:41 pm

Hello all,

This is my first post having been gifted a 1956 G80S by my wife for my 30th Birthday back in May.

The bike generally runs well and starts fine. It has had a bottom end rebuild fairly recently and I am slowly running it in.

I have one issue that i would appreciate some guidance with on as i haven't worked much on this bike and most of my experience is with two stroke trials bikes.

If I am cruising at around 60 mph in top gear, all is well and can be comfortably sustained, but if I open the throttle beyond a point (around 1/3 throttle) it seems that the engine loses all power almost instantly. It felt like the engine was seizing or perhaps overheating the first time I experienced it, but closing the throttle progressively back to around 1/3 throttle almost immediately restores normal operation again so I suspect carburation.

My first thought was a fuel flow issue but I am surprised, if this is the case, that i have not experienced it when climbing sustained gradients or from similar throttle demands in lower gears and the issue is not progressive like when the fuel needs switching to reserve. I suspect that carburation may be a bit rich as friends who follow comment on a more strong smell of petrol than is normal even for this era of bike.

Any advice for setting the carb up properly would be appreciated before I start, or more importantly, if there are any other culprits to check as i am surprised that a carburation issue would not be more progressive with misfiring rather than a total loss of power.

Is there a recommended way of setting up the float height and getting the main needle height correct?

Advice gratefully received.

Kind regards,


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Re: G80S Loss of Power with Throttle Opening

Postby Rob Harknett » Tue Oct 01, 2019 9:44 pm

Have you the correct carb?
Needle possition has to be correct, also other carb. parts need to be correct, Throttle slide jets etc.
Do you have correct SR1 mag. ? That needs the AR cleaning. Mag chain tension correct ?
The bike should run OK up 60. It should go up to 80, but if you want a bike to constantly ride at over 60, Then a bike over 60 needs a little more TLC than it did when it was younger. So not the bike for you.

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Re: G80S Loss of Power with Throttle Opening

Postby Mick D » Wed Oct 02, 2019 7:03 am


The details of the original carb fitted to the G80 are given on page 6 of the Owner's Manual which you can read / download from here:

http://archives.jampot.dk/book/Owners_m ... ingles.pdf

Agood guide to setting up the Monobloc carb is available here:

http://amalcarb.co.uk/downloadfiles/ama ... d_Tips.pdf

To view the fuel level, (which should be co-incident with the cast dot on the float chamber cover), fit a short length of clear tubing over the pilot jet, having removed it's cap, bend it in a 'U' shape in front of the float chamber cover and turn on the fuel.

If you're running rich the spark plug should be sooty - easy to check.

Another favourite - check the vent hole in the fuel cap is clear, (blow through it or see if you can induce the lack of power with the cap off).

Regards Mick

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Re: G80S Loss of Power with Throttle Opening

Postby SPRIDDLER » Wed Oct 02, 2019 7:04 am

Welcome to the Forums, Bob. Congrats to your wife (she's a 'Keeper' ;) )
As you've discovered, these old thumpers are a totally different experience from two-strokes. I'm sure you'll love it.
Which carb do you have, Amal Monobloc?
Several things to check, but first, at 1/3rd throttle you're only just moving onto the needle position and Main Jet so check needle position and remove the Main Jet to check that it's clear.
Here's a couple of guides:

http://amalcarb.co.uk/downloadfiles/ama ... _guide.pdf

http://amalcarb.co.uk/downloadfiles/ama ... d_Tips.pdf

You can find original manuals for your bike, and much more here:

I poke badgers with spoons.

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Re: G80S Loss of Power with Throttle Opening

Postby BobGoodwill » Thu Oct 03, 2019 11:17 am

Thank you so much for the really helpful hints - They should save me a lot of time when I do get round to tackling this, which is the one thing that I am really lacking at the moment.

Rob, you're absolutely right - I don't expect the bike to be running over 60 mph for sustained periods, but on the short length of dual carriageway that I sometimes need to go along near home, it is very disconcerting to lose power and drop down to 55 or less when approaching a slow moving tractor with high speed traffic in the outside lane! My other hobbies (read burdens on my time and wallet) are steam engines and veteran cars, so top speeds as low as 6 mph are quite typical for me!

Good tip to clean the contact breakers/points, is this what you mean by AR? The previous owner (who restored the bike) was very active in the AMOC so I doubt that there are any non-compatible parts fitted although they may be 'well used'. I'll check the model of the magneto. Will also check the magneto chain tension if carb inspection and setting does not reveal anything suspicious. Generally at higher engine speeds in lower gears, there are no issues with misfiring or knocking so I suspect that the ignition system is OK, but difficult to say without really testing the engine, which I don't intend to do for a few hundred miles more.

Spriddler and Mick, yes I do have the monobloc carb. Excellent tip with the clear tubing on the pilot jet and thank you for the setup procedures for the carb - I hadn't tracked these down.

It may be a month or more before G80 maintenance makes its way to the top of the to-do list again as I am only at home three days a week, but I will report back once fettled or if in crisis.

Thanks again. Your time is greatly appreciated.


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Re: G80S Loss of Power with Throttle Opening

Postby alanengineer » Fri Oct 04, 2019 10:48 am

not all float chamber covers have a raised dot. The dot should be in the middle, so have a good guess

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