Compression - lack of!

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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Compression - lack of!

Postby RogGillingham » Mon May 13, 2019 9:36 am

1951 18S. Still trying to get engine going but Peter who is helping me suggests compression is poor, he can't stand on the starter as he would expect! Have measured it with meter and we get about 5 Bar. Can anyone tell me what I should be getting please? Looks like it's all coming apart!!
Many thanks for any help you can give.
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Re: Compression - lack of!

Postby SPRIDDLER » Mon May 13, 2019 11:33 am

That's 72 psi and you need around 100 psi (6.8 bar) for a cylinder to fire.
Is the decompressor holding the zorst valve open? Free the cable off completely - if you can't get at the cyl head end remove the cable from the h/bar lever. A valve just a tiny smidge open or with a damaged seat will reduce compression. LATER EDIT: Omitted to mention earlier that (pretty obviously) it could be a leaky head gasket... but you'd probably have noticed oil weeping or heard it squeaking.
Did you do the test (at least half a doz kicks) with the throttle wide open? Presumably the engine was cold?
Put a few cc's of oil in the plug 'ole. If the comp improves significantly it's a problem with rings. If not it's valve trouble.

If you want to work out what it ought to be here's a guide...

The theoretical compression pressure is atmospheric pressure (14.7 psi) multiplied by the compression ratio (say 8 : 1).
This gives a theoretical cylinder pressure of 117 psi. (8 bar)
In reality the compression you measure, particularly at kickstart speeds will be reduced by valve overlap and increased a little by heating of the air being compressed.


Have just looked up your '51 details, comp ratio is 7.24 : 1 without a comp lowering plate fitted so expect around 106 psi (7.3 bar)

http://archives.jampot.dk/book/Owners_m ... Manual.pdf
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Re: Compression - lack of!

Postby RogGillingham » Mon May 13, 2019 7:31 pm

Spriddler, many thanks. It hasn't started for 50 years so could be several things not right. I'll go through the things you suggest. I was hoping to get it going then see what leaked etc and only take it apart to sort what needed doing but looks like I'll have to take it apart and check it all out but we'll see!
Thanks for promptness of reply.
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Re: Compression - lack of!

Postby g80csp11 » Mon May 13, 2019 7:50 pm

even at 72PSI you should be able to get the bike running
id suggest that the problem is spark related and not lack of compression , ive known many bikes that run with low compression run some diesel in the bore to free up stuck ring over night , just a few cc's will be enough then bow out by kicking over with plug out

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Re: Compression - lack of!

Postby SPRIDDLER » Mon May 13, 2019 10:07 pm

g80csp11 wrote:even at 72PSI you should be able to get the bike running

I bow to your professional knowledge Thrifty Peter. (And you've never been known to go to 100 when you could get away with 72 ;) )

Cripes! 50 yrs since it last ran? Yes, stuck rings is a possibility. If you have to remove the head and the cylinder it really isn't a major job.
Presumably you've checked for a spark, cleaned the points and carb, new plug, fresh fuel, drained the crankcase of wet-sumped oil etc?
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Re: Compression - lack of!

Postby Mick D » Tue May 14, 2019 7:02 am

Hi

Just trying to start an engine after 50+ years risks causing serious damage - much more so should it actually start :o and your restoration costs will go through the roof.

If it were me I'd strip the engine to it's component parts, clean and inspect everything, (especially the oil ways), then decide what degree of restoration to embody. There may be little else required to get it running and you'll have peace of mind and will have saved a bucket of cash.

Regards Mick

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Re: Compression - lack of!

Postby SPRIDDLER » Tue May 14, 2019 8:28 am

Mick D wrote:Hi

Just trying to start an engine after 50+ years risks causing serious damage - much more so should it actually start :o and your restoration costs will go through the roof.

If it were me I'd strip the engine to it's component parts, clean and inspect everything, (especially the oil ways), then decide what degree of restoration to embody. There may be little else required to get it running and you'll have peace of mind and will have saved a bucket of cash.

Regards Mick

Very wise, Mick, and undoubtedly the 'proper' approach, but it does depend upon what experience, skills and facilities are available. I have to admit that I don't have a lot of patience so I'd take pot luck by doing the routine stuff to get it running and seeing what transpires.
However, if no history is known one has to be circumspect about the reason it was put aside and not run for 50 years....... :?
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