AJS & Matchless Owners Club

Hi all,
Finally sorted my front mudguard, and got my petrol tank back from the painter, so got to ride my girl for the second time in 2.5 months of ownership. After a false start due to points set coming loose 10 mins from home, I headed for the hills, and became very aware that she vibrates the bejeesus out of my bum at highway (motorway) speed! So much so it felt like a swarm of bees on speed - woody wood peckering my backside!!!

Just wondering, is it normal for the '56 onwards compy 500's to vibrate like that? Funny though, vibration only through the seat to my bum, my feet, and hands are under normal conditions - ie, no abnormal vibration.

I only ask, as my '55 G80CS does not do this, nor, does my '49 G80S!

Cheers for any comments.

Hi Marty

May be worth checking the engine and frame mounting bolts. Also the head steady and steering head bearings.
Could also be tight chains especially if the rear chain has pulled the gearbox back and tightened the primary after giving it some beans up those hills!

Jim

Will check... thanks

G80CS, 18CS engines usually run very smooth. Loose engine mountings or heavy piston can make them shake. Also, standard gearing is for off-road & if run at highway speeds can also shake. The crankshafts are balanced for original weight of the original BMB pistons. I have an original factory piston in +0.040 size that weighs 16.6oz, (469.4gr.) bare & 21.3oz, (602.6grl) with rings & original taper bore pin. Hepolite & Robbins pistons were similar weight but most others are very heavy & had straight bore pins. 22t engine sprockets work for most streets & 23t is better for open road. Cheers! Don

Marty,
I second Don's comments about the short stroke CS motor not being as bad for vibration as you report. Allied to a comment you made in another thread that the bike won't travel at 60mph comfortably, this suggests (as Don indicated) that the bike is not correctly geared for road use.
Kind regards,
John

Don and John,
Thanks for comments.

My '55 (long stroke) is smooth as you'd expect for a 500 thumper, but does not exhibit any annoying vibrations.
The '56 (short stroke) which this thread is about is smoother than the '55 in all areas, even at motorway speeds save for the very aggressive(read: painful) vibration at high speed (60mph) through the seat only. Foot pegs, handle bars etc are still smooth. In fact, my long stroke vibrates throught the foot pegs much more than the short stroke.

As for gearing on the short stroke? It has the AMC box in it, and seems to be very well geared for the road. My '55 L.S. is terrible. 1st is short, 2nd is too high, 3rd is ok, 4th is too high after 3rd. This means I have to rev out 1st and 3rd more than I'd like. At any rate, both are happy doing 55-60mph, but I wouldn't want to do it for long stretches of open road.

As for the heavy piston? I couldn't comment as I don't know what is in it. I can say that it is the most epervescent revver that I have ridden in old bikes. Quite akin to my DB32.

I've been working too much, and haven't had any bike time this week, so I'll check for engine bolts and more importantly, Primary drive chain tension etc.

Just had a thought. Might be something simple like rear wheel balance. Will check that over the weekend too.
Cheers all.

Matchymarty wrote:Just had a thought. Might be something simple like rear wheel balance. Will check that over the weekend too.
Cheers all.

don't ignore front wheel balance too

A badly fitted rear tyre is about all I can think of, which really is the simple cause, and it will give you the ride you describe. I had the same thing happen to the front wheel of my 350. I took it off the trailer after a 400 mile journey. Rode it and frightened the life out of me. So bumpy I dare not go faster than 30 mph. Prior to trailering the bike 400 miles to the Scottish Jampot at Kelso, I had fitted a new tube. I used washing up liquid to ease the tyre on. I am sure the tyre was still slippery, pulled up tight to the front stop of the trailer and braking the car, must had moved the tyre. It had never happened before.

Ok, so just got magneto back from being o'hauled and got the CS running again.
Have to say, after spending a fair bit of time on the '55 G80CS LS and the '49G80S recently, (some 1200 miles since the bum number has been out of commision) a quick test ride just now to make sure the mag is running right, has shown up just how bad the vibration is.

1. At idle, the engine occilates heavily in the frame.
2. Engine responds well, but as revs increase, vibration increases whether stationary or going, and vibration is terrible through the seat.
3. All engine/frame bolts seem to be secure
4. Primary chain tension is ok as per my other bikes.

I'm beginning to suspect flywheels out of balance???