AJS Very poor starting advice needed

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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AJS Very poor starting advice needed

Postby Diana3899 » Thu Aug 15, 2019 2:33 pm

I have a 500 cc single cylinder AJS overhead valve motorcycle and she is a pig to start. First a little history: she was starting reasonably well but the plug was sooting up very quickly indicating a rich mixture so much so that after a journey I had to wait until the engine cooled down so that i could clean the spark plug before returning home. Needless to say this did not fill me with confidence so i was reluctant to use the bike on runs. I could not just change the carb as it is a 289 horizontal slide type which are like hens teeth. So I had Martyn Bratby a very well respected man for his work on carburettors overhaul the carburettor. Apart from it being worn he found that someone in the past had drilled the mixing block oversize which was clearly the reason for the rich running.

I have since fitted the overhauled carburettor and got the bike to start and run and the good news is that the rich running was cured and the plug did not soot up. I now find that she is extremely difficult to start but when she does she runs well but now I am reluctant to use her in case she won't start when I wish to return home.

The bike has good compression so in my view I either have a user problem in not getting everything lined up correctly, a carburettor problem or an ignition problem.

The book recommended starting procedure is to turn on fuel, close air lever and position advance lever. Flood the carb then 3 or 4 kicks and drop the valve lever. I know I have the position of the advance and retard lever correct as I have marked the position on the lever when she has decided to start.
Most of the time when I kick her over she won't start but sometimes she kicks back like a mule and nearly throws me off the pedal. And of course now and then she will decides to start. I have tried different positions of the air lever and sometimes not flooding the carb but I can't find that elusive combination which will get her to start on a regular basis.

Information on the carb is as follows ; Type 289/148, Main jet 180, pilot 30t, slide 29/4, needle jet 106, needle position 2.

When I removed the plug after a kicking session to no avail I have found the plug tip to be wet with fuel which indicates that petrol is finding it's way to the plug. So could it be that the spark is weak under compression and only firing from time to time. I have tried different plugs of the same type and the results are the same so I don't believe it is a plug problem. Even though the carb has been overhauled I don't want to assume that it is a carburettor problem so any advice and ideas would be appreciated as I very much want to ride this bike. For information she is a pre-war model 37/8 but I am using this forum rather than the pre-war forum as I believe that there will be many more users using it. Thanks in anticipation.

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Re: AJS Very poor starting advice needed

Postby Mick D » Thu Aug 15, 2019 2:52 pm


I think my first port of call would be the magneto - check out the Brightsparks web site for diagnostic tests:


Regards Mick

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Re: AJS Very poor starting advice needed

Postby Rob Harknett » Thu Aug 15, 2019 4:30 pm

Have the different plugs you have tried all been old and have been cleaned.
Its been decades since plugs had the central electrode glazed, making them no longer good for cleaning.
I have not gone through all the history, but if you change, plug cleaned to plug damaged. That would dismiss poor spark like several little sparks that will not fire the engine under compression. If you have used a new unused plug. That should be OK. You do now need to check other areas for a cure. If you have not tried a new unused plug, that may be your problem.

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Re: AJS Very poor starting advice needed

Postby spookefoote1956 » Thu Aug 15, 2019 4:34 pm

Hello Diana. I never use choke on my 500. The spitting and kicking back is probably too advanced on the ignition. Is the timing set to fully advanced half an inch before top dead centre? I went through a very steep (vertical) learning curve with this. In my case the magneto was also knackered so I fitted a Thorspark unit which, once fettled, is great ................. but that's another thing! Are the points in good order and set right?

So my money's on the mag and timing.

As far as starting technique this in itself can be a dark art. I too have had these problems and various helpful suggestions have been made. I think Clive suggested taking it over dead centre with the valve lifter up and then another third or so to increase the swing factor. Another suggestion was to flood the carb and keep the valve lifter open whilst kicking then release as the piston's getting back to top dead centre. Me, being 3 stone dripping wet, take it just over top dead centre with the valve lifter (bike on centre stand) then apply my vast weight in a positive shove not kick.

Anyway, let us know how you get on.

Cheers, Chris
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Hail Joe Lucas ............ Prince of Darkness!
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Re: AJS Very poor starting advice needed

Postby clanger9 » Thu Aug 15, 2019 6:21 pm

Did you fit a new plug? You can't clean modern plugs, they don't work properly after cleaning.
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Re: AJS Very poor starting advice needed

Postby willowbedone » Thu Aug 15, 2019 8:24 pm

clanger9 wrote:Did you fit a new plug? You can't clean modern plugs, they don't work properly after cleaning.

Hi have you tried following what the original owners book says Bring it to compression pulling in de compression lever release lever kick it and the weight f the fly wheels will spin the engine so it should fire up Just a thought

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Re: AJS Very poor starting advice needed

Postby Pharisee » Thu Aug 15, 2019 8:35 pm

Ask half a dozen owners what their starting technique is and I guarantee you'll get half a dozen different variations.
These are big old engines with very heavy flywheels and fairly low kickstart ratios so you're not actually spinning the crankshaft very much, or very fast, for a single prod on the kickstart. You need to get the engine spinning with enough kinetic energy in the crankshaft to force the piston over TDC with the mixture having fired just before TDC. If there is not enough energy in the crankshaft when the mixture fires, the piston will be halted and forced back down the cylinder rather than going over the top... and you end up in the air at best or with a damaged ankle at worst.
I'm 73, weigh in at around 11 stone and I can start my '55 AJS 500 reliably and without too much stress... I certainly don't worry that I won't be able to start the machine for a return journey home.
If I can, then I prefer to start the bike on the centre stand (against the advice of some). I'll "tickle" the carb until it drips, hold the exhaust valve open and turn the engine over a couple of times slowly to prime the cylinder. If it's cold, I'll half close the choke. If it's not, then it will be fully open. My engine has an automatic advance so I don't have that to worry about.
Then I'll hold the lifter open, give the kickstarter three full swings in quick succession to get the engine really spinning and drop the lifter halfway down the third swing. It's rare that Bess doesn't fire and run on that third swing. Just my 2p's worth :)

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Re: AJS Very poor starting advice needed

Postby SPRIDDLER » Thu Aug 15, 2019 8:41 pm

Long swinging kick Final. (640x468).jpg
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I poke badgers with spoons.

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Re: AJS Very poor starting advice needed

Postby BRobbo » Fri Aug 16, 2019 5:29 am

I have a 1948 500 single. I have found from hard experience if the plug is wet you are wasting your time. But it does need a little tickle to catch, the art is knowing what is enough and what is too much. I never touch the choke and keep the throttle closed until it's running and be very gentle with it initially. Use the lifter to take it over tdc and give it a longing swinging push right down to the footrest and hold it there. Never take your foot off until the swing is complete. I've got it to start first time from cold after years of trial and error and occasionally it will catch me out but compared to earlier times it's a sweetie. Fitted with pre mono block. Also make sure the float is operating correctly, another trial and error episode!

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Re: AJS Very poor starting advice needed

Postby Diana3899 » Fri Aug 16, 2019 12:22 pm

Thank you all for your feedback. It seems that no one thinks I have a carb problem. Given the responses it looks like the problem could be a poor spark either due to the spark plug or a tired magneto. I will also take the advice regarding the starting procedure. Plan of action is to give her some fresh petrol and a new plug and then try the suggested starting techniques. Failing that I will get the magneto overhauled. Thanks again for the advice.


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