Low compression diagnosis/leakdown (Paddy's Bandit)

[Pete.]

I did want to make this a how-to for leakdown testing, but found that my leakdown tester is misssing (I vaguely remember lending it to someone but can't remember who).

Anyway, I knocked-up a fitting to put air into each of the cylinders on Paddy's Bandit, which is destined to be a first big bike for one of my in-laws once I've got the engine sorted.

The principle of leakdown testing is simple - you put air into the cylinder, making sure the valves are closed, and see if/how much escapes and where from. You have two pressure gauges tee'd together with a 1mm restrictor in between them, feed air into one gauge, and plumb the second gauge into the cylinder via the spark plug hole. The first gauge shows the supply pressure and the second shows a lower pressure, which varies depending how much the system is leaking. The difference is expressed as a 'leakdown percentage'.

I didn't bother with the gauges because I only have the one, so I made the fitting and fed air directly into the cylinder. There are three places you can lose compression - past the rings, past the exhaust valve and past the inlet valve. You simply listen to hear where air is coming from to tell where you have a problem.

Here's cylinder No1. Quite clearly leaking very badly past the exhaust valve.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Pete.]

Cylinder 2 and 3 leaking but not as badly as 1. Don't think you can tell from the video but cylinder 3 is also leaking very slightly past the rings - not enough to be a bother though.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Pete.]

No4 cylinder is perfect - no leaky valves and just the tinyest hint of it getting past the rings, which you'll never stop completely anyway.

So the head is coming off, and the cause of the leaky valves will be obvious I'm sure. Either they are badly burned or slightly bent, to be leaking so much.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Pete.]

Well I guess we have our answer - burned valves and seats. Didn't look too bad at first but when they were cleaned up the damage is easy to see.

Strange thing is, the valves on 1,2 and 3 cylinders look in very good nick (but the seats are badly pitted) whereas the valves on 4 are in a right state yet it sealed up nicely. Dunno if they will grind out - I'll have to get some grinding paste tomorrow and have at it with the sucker-stick.

Anyway, some gratuitous pics to stop you falling asleep...
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[P.]

If anyone can get it going, its you Pete.

Informative for sure

[jimspeed]

those seats look bad mate, what do you think the cause was? as surely the bandit would be new enough to accept unleaded fuel from new?
looks like you have plenty of time on the sucker stick there tho.
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Hyosung cruise 125(passed test on, sold) Kawasaki el 252 (better than expected but sold on) Kawasaki GPZ500S first "big"bike.(sold) ZZR600 E5..Z750 2007,ER5, currently on a 2008 Enfield bullet electra x and loving it..
,"Alpha-9: Is there any correlation between dyno rod and dyno kits?"

[Pete.]

I dunno the cause - Paddy inherited it with the running problem. Could be the gaps were tight sometime or it could be just the 60k miles. Won't take me long to fetch them down with a bit of power and some decent paste, but I'll have to see how far the valves sink into the seats.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Fizzer Thou]

This is the leakdown tester that I utilise.

https://i666.photobucket.com/albums/vv23/WiNot_Rhencullen/Workshop/Picture419.jpg

With the combustion chamber full of compressed air,the dial shows the amount of air pressure lost as a percentage.

I listen for escaping air at the carbs for inlet valves leaking,exhaust pipe for those valves and the oil filler cap for piston ring leakage.

When we performed the compression test on Paddys bike when he was here the other Saturday,a small syringe full of engine oil bumped up the compression test by quite a similar amount on each cylinder,but still not good enough for decent combustion.But what the tools do perform is an indication of where to look next.

With that amount of wear on the valve seats I would also be looking at the service figure for valve spring length.I bet that they are substantially under what the limit says in the manual.
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Just talk bikes.What else is there?

Always have a 'Plan B'

[Pete.]

I'll check the free-length as a matter of course, but I never bother with installed height on an ordinary roadbike especially an old hack like this. If I can get the valves reasonable without sinking them in too much that'll do for me.

I did notice evidence of some kind of foreign matter having been cycled through the head on 2 and 3, some small dents in the top of the combustion chamber. I think this is debris that has fallen down from the plug well when the plug was removed, especially on 2 because the drain hole was plugged solid.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Vincent]

[Pete.]

Cheers Chris, I'll let you know. Ground a few in and they look fine with no significant change of shape. I was quite surprised.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good