Ajs 16 Trials Project

[tyrefryer]

Hi everyone, I've just bought my first AMC bike which is an AJS 16 that's been built as a trials (style) bike. A fair bit has been done to it but there is also a lot I need to sort out and improve upon. The plan is to use it for some road trials like the Arbuthnot, some rides out with my local VMCC sections and a bit of playing around at my trials club's local practice ground. I'm not looking to use it for serious pre-65 competition use.

From what I can tell so far, it's quite a bit of a bitsa. I believe the frame is a 1959 but the engine may be earlier as it has a pressed steel primary chaincase and magneto with manual retard control. I was also told that the crankcases are from a G80. I'd be happy if anyone can shed further light on what I have.

I've started sorting through all the immediate problems I can, I've adjusted the primary chain tension, re-assembled the gearbox adjuster the right way around, corrected the timing and adjusted the carburetor so that it will run. However there is one major problem I need to sort out before I can start using the bike.

The rear chain is way too loose and the rear wheel is adjusted as far back as it will go but before I shorten the chain I need to sort out why the rear chain is rubbing on top of the swing arm where it pivots in the frame. This has obviously been happening for some time as there are shallow grooves worn in the swingarm by the chain. The cause for this amy be due to any of the following or a combination of more than one:

1. The gearbox sprocket is 16 teeth.

2. The rear wheel sprocket is 42 teeth.

3. The frame and swingarm are from a road bike as far as I'm aware.

4. Girling gas shocks have been fitted.

5. Aluminium engine plates have been made up and fitted.

The pivot point of the swingarm seems too high compared to the gearbox and rear sprocket. I've looked at pictures of other AMC trials bikes and the angle of the swingarm and length of the rear shocks look the same as mine? Also the mounting of the gearbox on my bike doesn't look any lower? Also I understand that the gearbox and rear wheel sprocket sizes I have are the same as was standard for trials bikes?

If anyone can advise me on what could be wrong I'd be really grateful because at the moment I'm stumped!

Thanks for reading, Martin.

[totalshell]

blimy apart from jampots and the oil tank i thought it was my 16.. the chain rub on the swing arm is simply because the rear sprocket front sprocket sizes, shock length means the chain is having to go up and over the swinging arm,, get a piece of nirte and fix iit to the top as a 'slipper' my jampots are very short compared to your modern shocks..i believe my bike is a 53 off the top of my head.

[tyrefryer]

Thanks for that, I did wonder if something like what I've got on my Sherco swingarm might be the answer. Is "nirte" what it's really called or is that a typo and where can I pick some up?

Cheers, Martin.

[tyrefryer]

Just found these on Ebay, are they the sort of thing you mean?

http://www.ebay.co.uk/itm/UNIVERSAL-CHAIN-SLIPPER-PRE-65-TRIALS-PROJECT-BIKE-TIGER-CUB-ARIEL-HT-C15-b-/400584932464?pt=UK_Motorcycle_Parts&hash=item5d44b8fc70

http://www.ebay.co.uk/itm/UNIVERSAL-CHAIN-SLIPPER-PRE-65-TRIALS-PROJECT-BIKE-TIGER-CUB-ARIEL-HT-C15-/161194768070?pt=UK_Motorcycle_Parts&hash=item2587f4f6c6

Cheers.

[totalshell]

ys looks like they ll do the job. remember that the original trials bikes had 19 inch rear wheels and the road bikes even bigger so with the 18 inch rim you ve lowered the height of the top edge of the rear sprocket.. a bigger sprokect would not only make the bike more usable but give more clearance.

.nitrite is the plastics proper name.. not a typo sadly.. following a little 'episode' words can look right to me but to you they are plainly bonkers!

[tyrefryer]

Thanks totalshell and Derek that's all interesting stuff, I'm beginning to get a much better understanding now. I initially thought that the problem was due to the swingarm angle being pushed down by the shocks being too long but I have a copy of Classic Bike from January 1982 featuring the Gordon Jackson AJS. In the pictures that has what I thought were the same rear shocks and the swingarm angle looked the same as mine (parallel with the bolted on tube under the seat) but there isn't a picture of the drive side of the bike. However it's now apparent (thanks to a member on the AJS and Matchless forum) that the Gordon Jackson shocks are 12 coils and mine are longer at 14. And possibly the rear wheel was a 19" in 1982?

I'll start with a slipper for the swingarm and then consider a larger rear sprocket and maybe shorter rear shocks. I have to check but I thought the rear sprocket was part of the brake drum and not separate? (A member on the AJS forum was running a 50 tooth rear sprocket).

Cheers.

Edited September 17, 2014 by tyrefryer

[jon v8]

"You cant buy ability" - Truest statement ever made in reference to trials ! I would go for the shorter rear shocks,never mind about ground clearance,lift the front wheel if necessary.I've ridden a few jacked up pre 65 bikes that are horrible,they are massively top heavy and just want to fall over as soon as they get a couple of degrees off vertical.Then you are fighting the weight and not concentrating on steering/where you are going.Plus when it starts to get out of control in a sticky situation you have much less chance of getting it back.

You simply cant get past the fact that all the pre unit engines are very tall and very heavy - with little chance to reduce either.I'd rather keep the weight down low and spend time,effort and if necessary money on getting the engine to run sweetly and reliably.Then attend to gearing,clutch action and making the brakes work.

With practice its suprising where you can get heavy old bikes to go,and its VERY satisfying to get lower scores than riders of lightweights - its a good challenge.Whatever you do with it,dont give up,it will be another old bike out being used .

[tyrefryer]

Thanks Jon, I've ordered the slipper and am now making a list of the cables I need to replace and sorting out the controls while I'm waiting for it to arrive. The choke is missing, the valve lifter was in the wrong place and the cable was broken and I've moved the mag retard control to the left instead of the right where the choke lever is going.

[tyrefryer]

Thanks again for the info and advice Deryk, I'll have a search for some shorter shocks.

Is it best to stick with Girling gas shocks and how should I determine the best length?

Thanks, Martin.

[tyrefryer]

Thanks Deryk that's excellent info, I'll have a measure up tomorrow and give Norman a call.

[scramblebike]

hi tyrefryer when all said and done you have a nice looking bike. enjoy.

[tyrefryer]

Thanks scramblebike, I'm enjoying learning about the marque and tinkering and I can't wait to start using it.

[jon v8]

Everyone has their own favourites with shocks,any of them will do the job,the only ones I'd avoid are Betor. NJB shocks are here; http://www.njbshocks.co.uk/

I have Falcons on my HT5 Ariel which I really like,easy people to deal with and will rebuild any of their shocks from parts in stock - all the time.They will also supply different length ends to modify overall length,which may help you. http://www.falconshockabsorbers.co.uk/

The other good choice would be John Bull's Rockshocks ; http://www.rockshocks.co.uk/

Far more important than the decision of the make of shocks is sorting a decent carb, Surrey cycles will do a Mk1 Concentric which will help no end,and they have good advice on jetting etc.I ran standard slides in my AJS until I was happy with the cutaway,then they supplied a brass replacement which will last a lot longer;http://surreycycles.com/

[tyrefryer]

Thanks for the info on the shocks Jon, shouldn't I stick to a Monobloc because of the age of my bike though?

[tyrefryer]

Hi Deryk, so do you mean remove the choke, It was and I've just been replacing it!? I quite like the Monobloc as (I thought) it's period and I have a bike with a pre-Monobloc and also one with a Concentric but I've never had a Monobloc before.