Compression test.

[sirch345]

thats an interesting situation to be in. You needed to put some oil in it so not to crank it dry but that oil will mask bad rings. If its not worth rebuilding you dont want to screw around with carbs and whatever else is wrong it just to find out the rings are flogged. I think id be inclined to try and wash some of that oil out with petrol and redo the comp reading. If the compression starts to drop... Anyone know the right reading for these?

According to the Honda Shop Manual:

Chris.

[Aly]

Thanks Chris, but the spec'ed reading wasn't exactly my point. More that this gauge reading cant be trusted as an indicator of engine health. That its exactly on spec only shows the top might be good. Although like MK_WF mentioned excess oil will drop the chamber size - thus making extra compression and maybe compensating for leaking valves. So all in all a kind of worthless reading? I'm only harping on about this because I often struggle with diagnostics and find myself changing more parts than warranted (more cars than bikes in my case).

[sirch345]

Thanks Chris, but the spec'ed reading wasn't exactly my point. More that this gauge reading cant be trusted as an indicator of engine health. That its exactly on spec only shows the top might be good. Although like MK_WF mentioned excess oil will drop the chamber size - thus making extra compression and maybe compensating for leaking valves. So all in all a kind of worthless reading? I'm only harping on about this because I often struggle with diagnostics and find myself changing more parts than warranted (more cars than bikes in my case).

Some very valid points there

As for a compression test, I feel the difference between the two cylinders is more important than trying to hit the spec's in a workshop manual. So as long as both cylinders are reading a similar reading, if they are both not bang on the quoted figure in the shop manual I wouldn't be concerned. That is obviously not possible to do with a single cylinder engine, but is possible on multi cylinder engines, cars and motorcycles etc for that matter as you already know

Chris.

[MK_WF]

Yeah, the absolute reading is not to be trusted unless you have an expensive calibrated kit.

I had ordered a new kit from China for my Pitbike as the old gauge didn't have an adapter for the small 10mm plugs.
Funnily the reading was unexpectedly low.
Then I hooked the new gauge to my RD where I could counterprobe with the old gauge (known to be in the ballpark).
What should I say: New gauge 6.5 bar, old gauge 11.5 bar - measured on the same engine within 5 min.

My conclusion: Keep the low reading gauge when I'm going to buy any new bike or car. Then do the compression test under surveillance of the owner and haggle down the price due to the "obvious engine damage" he has just witnessed :-)

[Donut 77]

Mmm.. very interesting and slightly worrying. I get your point Martin. I guess it would only be on the first compression stroke and with the precise amount of oil - as you say, if it’s less then it won’t hurt and much more will definitely not crank. I think most of it did end up coming out of the front exhaust port.

I have no choice now really other than to continue until it runs and see.., then tackle it then. Sort of ‘head in the sand’ approach

If the rod is bent and if it even runs, I assume compression would drop significantly as the piston only reaches part way up the cylinder?

[tony.mon]

I wouldn't worry about bent rods.

Depending which part of the stroke the piston was in when you tried to turn it over, the valve overlap- which is where both inlet and exhaust valves are open for a short part of crank rotation after the exhaust stroke ends and the inlet stroke starts.
If that is as the engine starts to turn, any oil will be blown out of one or the other of the ports.
In any case, these engines don't rotate very fast as they try to start, so crank speed won't be up to much by the time the piston stops, even if you have put in too much oil and it's just before compression stroke. I'd be surprised if the rod bent under those stresses.

As you're proposing, get it running and then see what you have to do.

[Donut 77]

I’d tend to agree Tony. Someone on here described these engines as ‘tractors’