Engine Compression

[luckyd20]

Can I get the numbers for the engine compression. Both my cylinders are at 115psi. On my clymers manual <123psi is poor compression. 123-145 normal and >145 Good. I road the bike for about 10mins to warm it up.

[georgesgiralt]

Hello !
Did you remove the carbs to measure the compression ? If not, this could explain the low figure.
If yes, you have to find where you are loosing the pressure.
For this put a dab of motor oil in the spark plug hole just before taking the compression. If the figure goes up, the leak is at the valves. Otherwise, the rings also are at fault.
Then, if the figures are still so low, maybe opening the engine ? ....

[luckyd20]

I did not remove the carbs. I'll try that next time. Thanks!

[bruce_launceston]

Hi George, I think you meant that after putting engine oil in the spark plug hole and reading higher compression that the rings are at fault. The oil will help seal the rings, not the valves. Cheers

[georgesgiralt]

Yes, you're right, I was mixed up in my mind !

[flybot]

Also, you normaly check this with a fully warmed up engine, and the throttle wide open when cranking with a "normal" carb. I guess thats why the carbs come off on these- slides.

[montmil]

...I guess thats why the carbs come off on these- slides.

And those butterflies.

Ask George about his "stinky fingers". NOT what you're initially thinking, lads. It's motorcycle related.

[luckyd20]

So the carbs must be off correct?

[montmil]

So the carbs must be off correct?

No need to yank them entirely off. Too much work.

Remove the air filter plastic tubes, then...

Loosen the hose clamp at the cylinder head and pull the carbs back enough to allow unrestricted air to enter the heads as you run the compression test.

Easy button up afterwards.

[Bob_Roller]

I've always questioned this, why remove things that are always in place when the engine is operating ??

Remove both spark plugs, on a fully charged battery, operate the starter with the throttle fully opened, until a maximum pressure is reached on your pressure indicator .

My experience, is about 10-15 compression cycles, you will have your maximum pressure .

A big factor that may not be realized, is the accuracy of the compression gauge, I had one from Snap-On, that had a 30 psi error, on the low side !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[Barry]

The carbs don't have to be removed but the engine cannot create enough vacuum at cranking revs to lift the pistons on CV carbs so you should ideally prop them open.

Are you saying Bob that if the pistons are not raised the compression figure realised is still the same it just takes longer to get there.

[Bob_Roller]

That's what I've seen in the few compresson checks that I've done on my '81 R65 .

[georgesgiralt]

Couldn't get the same figure with carbs on versus carbs out... Later was higher whatever drain I put on the battery...
Maybe it depend on the general condition of the engine ?

[Olliepup]

Chris Harris on Youtube demonstrates a leakdown test procedure for an airhead engine that looks like a good way to judge engine problems. He has several other repair procedures for airheads that were informative. Brace yourself for his salty language.
Wavy

[Barry]

For what it's worth the factory BMW workshop manual for the R45/65 specifies slightly different pressures to Clymers

Good      140 and above
Normal    121  - 142
Poor        below 121

It also says engine must be hot, battery fully charged, both spark plugs out and carbs removed.