Weird Results From Leak-Down Test

[Dobs]

Has anyone done a compression or leak-down test on their VFR1200? I just did a leak-down test and noticed something weird. It looks like the rear left (#2) and front right (#4) cylinders leak about 5% into the adjacent cylinder (I can hear air coming out the open intake valves on the adjacent cylinder). Interestingly, when I test the adjacent cylinders (#1 & #3) the leakage is practically zero (maybe 1%), so the leakage appears to be in one direction only. I was wondering whether this is a feature of the VFR1200 or that I have 2 blown cylinder head gaskets. I know I would be very unlucky to experience blown gaskets on both heads, so I'm thinking it must be a feature of the 1200. Something to do with the sealed crankcase?

Should I be concerned? Is 5% leakage worth worrying about? I also did a compression test and got 210, 185, 210 & 195 PSI on cylinders 1,2,3 & 4 respectively. I work this out to be a difference of 12% between the best and worst cylinders. Is this acceptable?

Graham

[spud786]

Some bikes come with decompression assemblies to aide in turning the motor over on start up, (meaning they slightly hold a valve open on some cylinders). I dont dont if the 1200 uses such technology, but its a thought.

[FJ12Ryder]

I can't think of any reason that there would be leakage with the piston at TDC. I'd double check to make sure I was at TDC, and then also do a compression check on all 4 cylinders. Something doesn't sound right.

[Tamworth]

If the bike is running well then there's no problem. Why are you doing a leak down test?

[Dobs]

If the bike is running well then there's no problem. Why are you doing a leak down test?

It wasn't running well (difficult to start, down on power). I suspected I had a burnt exhaust valve due to a leaky exhaust gasket (had had the silencer off and on several times without replacing the gasket and it was definitely leaking) so I did compression and leak-down tests (on a cold engine) when I could get to the front cylinders while doing the valves.

After my initial post, I had accepted that I had about 5% leakage on 2 cylinders (which wouldn't have been a problem) and reassembled the bike and started it up to make sure everything was ok. I let the engine warm up then decided to try another leak-down test on one of the rear cylinders that was previously leaking. To my surprise, that cylinder was now perfect - no detectable leakage, same as the cylinder adjacent to it. I suspect the exhaust valves on that cylinder weren't seating properly by just rotating the engine by hand, they needed the force of combustion to fully seat them. Either that or the valves needed to expand slightly with heat to seal fully.

So, the moral of the story is.....don't trust compression or leak-down tests done on a cold engine.

Thanks for your replies guys.

G.

[fotodadi]

so...if it ain't broke, don't fix.

[gswanson]

My take on this is that you possibly had ring blow by on the two cylinders and that they were leaking into the crankcase. When you tested the engine warm, the rings were better seated thus less leakage. I have done a lot of leak down tests and have found that the results will vary depending on the technique used. If you went slightly past TDC and reversed your rotation to get back to TDC there is a good chance the rings were not seated properly. If your engine needs combustion chamber pressure to help seat/seal the valves you have bigger problems such as weak valve springs or bent valve stems.