Compression testing advice

[freeridenick]

From what I've read compression testing should be done when the engine is warm, but can effective results be had from a cold engine though?

I'm being lazy really as I don't want to build my bike back up to find I've got to stop it back down again to resolve any issue.

[bigtwinthing]

it is better at normal operating temp, but as you've said, not always practical, the results hot or cold will show up a difference though.

do you think you have a problem with the valves etc?

[freeridenick]

The CCT went on the front cylinder and the head needed replacing. That work was done by the guy I bought my bike off, so the valves are all OK. I just want to make sure there's no other damage.

The guy I bought it off is a close friend and he couldn't see any damage to the piston that would make him suspect a bent rod, but if I build the bike back up and there is a problem I've wasted my time.

Is there a 'tolerance' with compression or should I expect both cylinders to be exactly the same?

[AMCQ46]

hot or cold you should be able to see if you have any problems just the same, you will just get a lower reading on both cyl when cold as the engine wont be spinning as fast and the components wont have expanded to their running sizes.

the comparison front to rear is the best indication of problems more than the absolute level, but it is normal for there to be a 10 to 20% variation

[kitsun]

From what I've read compression testing should be done when the engine is warm, but can effective results be had from a cold engine though?

I'm being lazy really as I don't want to build my bike back up to find I've got to stop it back down again to resolve any issue.

Warming up the engine is to improve the seal. you can do it cold by squirting oil through the spark plug hole and turn the engine over a few times. this will coat the cylinders and improve the seal and the compression would be the same as when the engine is warm

kit

[freeridenick]

Thanks for the info. I think I'll try it cold and use the oil tip.

Just need to find somewhere to rent a compression test tool for a day.

[sirch345]

I agree it doesn't really matter if the engine is hot or cold for a compression test. The important bit is (as already mentioned ) the difference between the two cylinders.

Chris.

[turbo_billy]

......and have the throttle wide open as you spin it over

[AMCQ46]

And both plugs out

[tony.mon]

Testers aren't expensive...
Google or Amazon, from about £13

[freeridenick]

Cool. £13 makes it not worth renting.

The oil's been drained from the engine. Will there be enough residual oil left to not damage the engine or is it best to fill it up? There's not much engine speed on the starter.

[tony.mon]

Cool. £13 makes it not worth renting.

The oil's been drained from the engine. Will there be enough residual oil left to not damage the engine or is it best to fill it up? There's not much engine speed on the starter.

As long as you aren't cranking it for long or testing it dozens of times it won't need oil. You'll probably do a wet test anyway, and with the other plug out there's next to no pressure on the big ends and mains, compared to normal running.
If there's not much speed on the starter and the battery's ok, check the main starter lead- they often start to fail at the lower end, or at the starter bolt itself.
Run a bit of thick-gauge auxiliary wire alongside and see if it speed up, but be careful- it may get hot quickly. If it gets hot quickly the main starter lead or motor bolt connection is failing.

[freeridenick]

Tested the compression and I get 10 bar on the front and 11 on the rear. 10% difference cold (with a bit of oil in the cylinders) seems OK from what you've all said.

Are those figures what everyone would expect? Works out at 145 and 159.5 psi, manual says 164 psi with a warm engine.

[bigtwinthing]

Ask someone with a boroscope to have a look, all Snap on Dealers have them, slip him £20 for a look.It will show up piston, bore and possible valve damage, well worth it.

[AMCQ46]

Tested the compression and I get 10 bar on the front and 11 on the rear. 10% difference cold (with a bit of oil in the cylinders) seems OK from what you've all said.

Are those figures what everyone would expect? Works out at 145 and 159.5 psi, manual says 164 psi with a warm engine.

That is about what my engine reads, but I didn't put any oil in the bores... Do you have a reading before you added oil?

If it is valve damage you are checking for then those reading already tell me the valves are fine, adding oil is just a way to see if the piston rings are slightly worn