Low Compression

Yorkie · Sep 17, 2023

I recently picked up a non-running ‘71 Commando that had sat since 1988. It has 22K miles on it and has not been horribly abused but it has really low compression. I can turn the motor over by hand. The valves move and have correct clearances. The pistons are also moving. I was curious as to the groups thoughts as to why the compression would be so low - stuck rings?

concours · Sep 17, 2023

Corrosion/wear/damage on the valve seats will do it.

Failed head gasket.

concours · Sep 17, 2023

Put a tablespoon of oil in each spark plug hole, re-test, report back.

Onder · Sep 17, 2023

Will it run?....oh sorry almost as bad as another oil thread!
I remain firmly in the 'tear it down, inspect and repair as necessary' camp.
Why risk damaging a good core?

Fast Eddie · Sep 18, 2023

22,000 miles is more than enough to wear out important things on a Commando if ridden hard, not maintained perfectly, etc.

And sitting for 35 years can bring its own woes. Was it sitting in a dry, heated shop… or a cold, leaky barn? Makes a big difference.

I‘m in the ‘start taking things off to have a look’ camp, def the head and barrels…

baz · Sep 18, 2023

Do you know the history of why it was laid up?
Was it running ok when laid up?
It's more than likely stuck rings
If it were corrosion on the valve seats it'd likely be only low compression on one side
You may be able to fire it up and the rings would jump out and make a seal
But why risk it
Personally I'd strip the top end and that way virtually the whole engine can be checked

Shelby-Right · Sep 18, 2023

Yorkie said:
I recently picked up a non-running ‘71 Commando that had sat since 1988. It has 22K miles on it and has not been horribly abused but it has really low compression. I can turn the motor over by hand. The valves move and have correct clearances. The pistons are also moving. I was curious as to the groups thoughts as to why the compression would be so low - stuck rings?

My old Matchless had the wrong piston ½ down the bore at TDC it had 4.1 to 1 and 3mm ring gaps , it ran ok , you might need to give yours a run , see if the comp comes up .

L.A.B. · Sep 18, 2023

Yorkie said:
I recently picked up a non-running ‘71 Commando that had sat since 1988.

Which could have the old type slotted pistons that should be replaced, regardless.

marshg246 · Sep 18, 2023

There is a YouTube channel where the guy gets all kinds of engines and tries to make them useful again. Often, they have low compression. At first, he doesn't waste time on carbs and such, he sees if he can get spark and if so, puts some oil in the cylinders, squirts gas in the intakes and sees if he can get it to fire at all. If so, he'll cleanup the carb(s) and see if he can get it to run. He did that with a VW with only one cylinder that had decent compression. When he was done, all four had decent compression.

I DO NOT recommend this and I always take them apart, but it is an option. It's unlikely to hurt anything that you wouldn't replace anyway if you rebuild it properly.

Onder · Sep 18, 2023

In my youth, I put in time working in an auto scrap yard. Lots of old tin there. Often the oil on the dipstick was clean as a whistle. That was how I learned that dirty oil will settle out over time. That
which settled was now a layer of hard dirt not just in the bottom of the pan but everywhere oil had
been.
...like the inside of your crank. Here you are worried about compression and now what happens to
the journal surfaces when you start coursing oil through all the galleries?
All these all brit bikes are just big lawn mower engines. Pulling it out of the frame and putting it back
is more of a bother than tearing it down. Just do it.

Deckard · Sep 18, 2023

L.A.B. said:
Which could have the old type slotted pistons that should be replaced, regardless.

I agree with LAB. Not only about the pistons, but '71 750's also had issues with valve guide seals that also may not have been addressed.
Best to pull head and cylinder to inspect. Pretty easy to remove in situ.

Yorkie · Sep 18, 2023

Thanks for all the comments. Off with its head. I’ve just retired so time is plentiful.

Fast Eddie · Sep 18, 2023

Yorkie said:
Thanks for all the comments. Off with its head. I’ve just retired so time is plentiful.

Cool.
Keep us posted.
With pics !

Low Compression

Yorkie

concours

concours

Onder

Fast Eddie

baz

Shelby-Right

L.A.B.

Moderator

marshg246

Onder

Deckard

Yorkie

Fast Eddie

Similar threads