Running in / did it seize ?

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
Spanielsam
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Joined: Fri Jul 03, 2015 4:16 pm
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Running in / did it seize ?

Post by Spanielsam »

Hello all

I am after a bit of guidance please, sorry for the long post

After rebuilding my 1956 G80S and having the barrel re sleaved with new piston etc the bike has been running well. i have been limiting the speed up to about 45mph and trying to keep the load off the engine using the gears but a strange thing has happened twice now.

After about 50 miles from rebore the bike suddenly lost power and made a very "mechanical " whiring noise, i pulled the clutch in and coasted to an almost stop but let the clutch out before i stopped all together and the engine burst into life as if nothing had happened, no noises or smoke etc
I have continued to use it taking it gently and yesterday exactly the same thing happened, it has now done about 150 miles

I dont really know where to start, i have never experianced an engine seizing up and thought that it would be a catastrophic thing which makes me wonder, has it actually seized or has it just cut out? Would the force needed to turn the engine over at 40 mph Create the unusual whirring noise. I cant kill the engine on purpose to see what it feels like as there is no kill switch and i dont feel like pulling the ht lead off

I have good oil flow back to the tank and so i assume the pump is ok

I am loathed to start stripping it down again after it has been running so well but any advice would be appreciated, i just started it this morning and it appears normal

Apologies for the length of the post

Thanks in advance
Trevor
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SPRIDDLER
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Re: Running in / did it seize ?

Post by SPRIDDLER »

Stop the engine with the valve lifter. See page 9 of the Owners Manual. http://archives.jampot.dk/book/Owners_m ... ingles.pdf

A seize if that's what it was, isn't a good thing but it's not necessarily catastrophic when running in a rebored engine.
Can you advise which make of piston was used and what the clearance was between the piston and the sleeve?
'There is a tide in the affairs of men
Which taken at the flood............'
Spanielsam
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Joined: Fri Jul 03, 2015 4:16 pm
Location: LEICS UK

Re: Running in / did it seize ?

Post by Spanielsam »

Thanks for your reply

I usually stop,the engine with the valve lifter but i thought it would be a good test to see what it would feel like to have the engine turning over with full compression but no spark ie, how quickly do you slow down with tne engine acting as a brake , even with a closed throttle i imagine the the cylinder firing must help a bit ?

Unfortunately i do not have piston / clearance information as i handed the barrel over to T L Eng who i think are specialist in this work and did a very good job as far as i could see
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Mick D
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Re: Running in / did it seize ?

Post by Mick D »

Hi

It sounds like the engine has cut out, not seized - you're hearing the engine turning over but without any exhaust noise.

As it fired up again after you had coasted for a while my money would be on fuel starvation - my first port of call would be the strainer on the carb or any other fuel filter you have installed.

You can diagnose the fuel tap strainers, if fitted, by turning on the reserve next time it happens - if it fires up the main strainer could be blocked.

Sticking float valve could be another culprit.

Regards Mick
MalcW
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Re: Running in / did it seize ?

Post by MalcW »

Hi Trevor

How far had you ridden each time it happened? Had you made other journeys of similar length in between those events where it ran OK?

Malc
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Re: Running in / did it seize ?

Post by SPRIDDLER »

Spanielsam wrote:, how quickly do you slow down with tne engine acting as a brake , even with a closed throttle i imagine the the cylinder firing must help a bit ?
If the throttle's closed there'll only be sufficient volume of fuel entering to sustain a tickover so the bike should slow down pretty quickly. On level ground in 4th from 30mph perhaps 20 - 25yds., when it will slow to say 10/15 mph and start snatching (in 4th) but that's a very rough guess and not in any way accurate. I think you'd be better to use the valve lifter and see if it slows more quickly than you'd expect. Not easy to describe in quantifiable terms.
Unfortunately i do not have piston / clearance information as i handed the barrel over to T L Eng who i think are specialist in this work and did a very good job as far as i could see
T&L have a first class reputation.
I'll stick my neck out and suggest that it may not have been so much a seizure (and it possibly may not have been that at all) but merely what is lightly referred to as 'nipping up' and this is not unusual when running in a new piston and a bored engine. Be sure not to run the engine on a weak fuel/air mixture as this will cause overheating and possible nipping up.

It is so difficult to diagnose your 'mechanical whirring' noise. If you had said it happened when you pulled the clutch in it could have been just the normal sound of the free clutch plates rattling around.
I'd check out Mick's suggestion of fuel starvation, adding to check that the vent hole in the petrol tank cap is clear.
'There is a tide in the affairs of men
Which taken at the flood............'
Spanielsam
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Location: LEICS UK

Re: Running in / did it seize ?

Post by Spanielsam »

Thanks again for your replies

I have been on various journeys, probably over 10 miles when its happened and there is no obvious thing like a long up hill or long down hill that has preceeded it

The fuel starvation has made me think and i will check that out, i suppose i could shut the petrol tap as i am riding and see if that has the same effect. It has a new carb from the club shop and i havent touched that internally

Apologies about my description of the sound but what with the wind noise and it only lasting a few seconds until i slow right down and the surprise factor, my head is a bit scrambled as both times there has been a car behind me

If it has just nipped up should i carry on and take it easy and see what happens or is it essential to strip it down, it sounds perfect again now

Thanks
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clive
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Re: Running in / did it seize ?

Post by clive »

Spanielsam wrote:...
If it has just nipped up should i carry on and take it easy and see what happens or is it essential to strip it down, it sounds perfect again now

Thanks
The issue with an overheating seizure or nip up is that in the course of it the piston expands to fill the bore. During this the bore may wipe aluminium from the piston and transfer it up the side of the rings. If a severe seizure this may lock the rings into their slots at that point, and this will cause the compression to be much reduced, although the engine will still run (yes been there) when it has all cooled down. Unlikely this has happened for a mild nip up you describe (mine was the result of hammerng a 500 single with a fully laden Busmar double adult up a long hill), however if it has you will experience less compression when kicking the bike over and once the engine is warmed up there would be a tendency for blow by to pressurise the crankcase. You would know this was happening because stationary on tickover you would see a pall of oily smoke drifting up from around the middle of the primary chaincase as oil is misted out of the engine breather.
If this is not happening I would leave well alone
clive
if it ain't broke don't fix
SPRIDDLER
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Re: Running in / did it seize ?

Post by SPRIDDLER »

Spanielsam wrote:Apologies about my description of the sound but what with the wind noise and it only lasting a few seconds
No need to apologise for the description Trevor, as you say, wind noise, surprise, distraction......even tracing the source of the noise whilst riding is hit 'n miss and can change according to how much fuel is in the tank, road surface...........
As regards whether to continue riding.........hmm, I really don't want to say. The thing to check first is for fuel starvation. If it's not that then the prudent thing to do would be to take the head off and check the bore although this won't necessarily tell you if the piston has picked up because you'd need to see the piston. It only takes another 15 mins to lift the barrel off............stuffing the c/case mouth with rag in case a ring has broken :(
If it was mine and bearing in mind your whirring noise rather than a partial lock-up and that it restarted and ran when you let the clutch out before you'd actually stopped I'd check for fuel starvation then ride it sympathetically to see if it happens again, being alert and ready to try and identify the noise more precisely. But I am a bit of a 'suck it and see' gambler rather than a prudent purist :oops:
'There is a tide in the affairs of men
Which taken at the flood............'
Mick D
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Re: Running in / did it seize ?

Post by Mick D »

Hi

To help with the diagnostics:

If the engine had momentarily seized, the whirring noise would be the clutch slipping, (as the rear wheel didn't lock up), - the deceleration would have been very rapid.

If the engine had become starved of fuel the deceleration would be as if you had slammed the throttle closed, (something you can replicate with care on the road).

Spriddler's suggestion to check the fuel cap vent hole is one I forgot to mention and should definitely be done.

Regards Mick
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