compression

[Mike]

After tinkering with my monza for a number of years, I had a realisation today that in all that time I'd never measured the compression. I dug the gauge out of the tool drawer, and first test gave a maximum of 75psi

* A small amount of oil into the bore raised that to around about 95psi;
* petrol in the intake port did not significantly leak past the valve (can't easily check the ex valve in this manner with the engine in the frame)
* Exh valve clearance was tight, so backed it off and reset it to 0.07mm
* Intake valve clearance was spot on at 0.05mm

Current measurement is a maximum of 105psi. When testing more modern bikes, I would expect to see 140psi+ - what should I be aiming for on this standard 250?

I'm currently suspecting that I've won myself a job of removing and inspecting the head, barrel and rings next weekend. The exhaust port is pretty gummy with, so the valve seat could probably do with a bit of a clean.

[Ventodue]

Well, quite a lot depends on what piston you've got in there, Mike, and therefore what the compression ratio is. For example, I see some Monzas had a compression ratio of only 8:1 (ref. Mick Walker). Which would give you a theoretical maximum compression reading of 117psi (see below).

So, given the fact that you've already diagnosed a bit of ring wear, maybe 100psi ain't so bad after all ...

Anyways, the important thing is, how does it run? Especially, how does it start? I see you've already sorted a tight exhaust valve, so that should improve things!

Comp.jpg

[Mike]

Now I check Mick Walkers book, I can indeed see that it's being quoted at 8:1, which given normal atmospheric pressure of 14.7psi, that would give a maximum possible pressure of 117.6 psi, so I suspect you're right - maybe it's not so bad. It'd obviously benefit from a stripdown at some point in the future, but probably not urgently.

> Anyways, the impotant thing is, how does it run? Especially, how does it start?

It has never ran particularly well, but then, I've just done a bunch of work (new carb & ignition), and it's definitely running better, but still to do a road test to confirm whether all of the prior badness is gone. I think I'll pop the tank back on and get it on the road for a test run, rather than worrying about the rings right now.


Edit - I only just spotted the table you helpfully posted after I did the maths there. Good to be able to check my homework

[bikester250]

Ventodue,
Could you tell me the source for that chart?
I've heard all kinds of stories about the accuracy of using compression gauge readings to estimate compression ratio. Some positive -others negative.
Regards,
Phil

[Jordan]

I think cylinder compression gauges can be confidently used on multi-cylinder engines, where one cylinder may be revealed to have a fault due to its reading being notably less than the others. The readings in themselves have little meaning, until compared to each other.
It may be that readings taken from a single cylinder engine could be useful, especially if logged over time. Then they can be compared to previous readings. A sudden drop in pressure could indicate that a valve has become leaky.
Otherwise, I don't think I'd like to read too much into individual measurements, on engines that may have been rebuilt many times to possibly non-standard specs. In these cases, comparing to other engines' readings could lead to wrong conclusions.

[Ventodue]

Could you tell me the source for that chart?

You can find it here, for example.

http://www.healey6.com/Technical/Nomina ... essure.pdf

But please be careful. The values quoted there are, as I said, theoretical. The actual compression readings you get from an individual engine can be influenced by a number of other factors - valve timing, for example. Even relative humidity or altitude can come into play.

As Jordan has said, taking compression readings is a simple way of getting a basic idea of the relative well-being of an individual cylinder. But not much more.