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Leak Down Mystery

 
Post new topic   Reply to topic    Honda305.com Forum -> Honda Dream - CA72 | CA77 | CA78
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Dr. Frankenstein
honda305.com Member


Joined: 02 Feb 2010
Posts: 541
Location: Charlottesville, VA

PostPosted: Sat May 11, 2019 12:30 pm    Post subject: Leak Down Mystery Reply with quote

I did a leak-down test on my '62 CA77 today because I was only getting 75 - 85 lbs compression out of each cylinder.

I fed 40 lbs air In, and I heard air escaping out of the oil hole, which the manual says is due to stuck/burned/bad rings, or a warped/scored cylinder.

This bike has recently had the entire engine rebuilt, with new gaskets and seals. I installed a set of used but nice pistons, with the correct rings. The pistons measured to spec all the way down, and weren't cracked or otherwise damaged, nor were the cylinders. I honed the cylinders before installation to a nice cross-hatch pattern, careful not to take too much off.

When I was having the engine rebuilt, I had the valves and seats redone professionally at my friends commercial engine shop; we pulled 150 lbs off each valve seat area/compression chamber in the head.

What am I missing? The only other thing I can think of would be the O-rings that go around the knock pins, which I replaced with aftermarket but same size rings...

Valve guides? My professional mechanic friend says he doubts that's it, and he used to build race car engines...

I am open to suggestions.
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Tim Allman
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Joined: 02 Dec 2014
Posts: 296
Location: Guelph, Ontario, Canada

PostPosted: Sun May 12, 2019 12:04 pm    Post subject: Reply with quote

A compression or leak-down test can tell you only about the parts that form the combustion chamber. Other things like valve guides and piston pin retainers are irrelevant to the test unless of course they are either very damaged or missing. Problems with the green O-rings would give only an oil leak.

So, it must be a problem with the pistons, rings, valves or valve seats. I don't see any alternative to disassembling your engine and checking the rings for breakage, the ring gaps and the sealing of the valves. Given that you hear the escape of air from the crankcase my money is on a ring or cylinder.

However, the fact that both cylinders are low but (reasonably) even puzzles me. It's possible to break a ring during assembly but it would be unusual from my experience to break rings in both cylinders.

One other thing worth looking at is how much of the iron sleeves project above the block. Ideally they should be slightly proud or even. Too high and you get oil leaks. Too low and the head gasket won't seal. However, in this case you would hear the leaking from the head gasket area rather than from the crankcase.

Maybe others with more experience than I can weigh in.
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LOUD MOUSE
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Joined: 15 Aug 2005
Posts: 7760
Location: KERRVILLE, TEXAS

PostPosted: Sun May 12, 2019 12:51 pm    Post subject: Re: Leak Down Mystery Reply with quote

Do you have time to call?. ...............lm


Dr. Frankenstein wrote:
I did a leak-down test on my '62 CA77 today because I was only getting 75 - 85 lbs compression out of each cylinder.

I fed 40 lbs air In, and I heard air escaping out of the oil hole, which the manual says is due to stuck/burned/bad rings, or a warped/scored cylinder.

This bike has recently had the entire engine rebuilt, with new gaskets and seals. I installed a set of used but nice pistons, with the correct rings. The pistons measured to spec all the way down, and weren't cracked or otherwise damaged, nor were the cylinders. I honed the cylinders before installation to a nice cross-hatch pattern, careful not to take too much off.

When I was having the engine rebuilt, I had the valves and seats redone professionally at my friends commercial engine shop; we pulled 150 lbs off each valve seat area/compression chamber in the head.

What am I missing? The only other thing I can think of would be the O-rings that go around the knock pins, which I replaced with aftermarket but same size rings...

Valve guides? My professional mechanic friend says he doubts that's it, and he used to build race car engines...

I am open to suggestions.
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Dr. Frankenstein
honda305.com Member


Joined: 02 Feb 2010
Posts: 541
Location: Charlottesville, VA

PostPosted: Sun May 12, 2019 1:49 pm    Post subject: Reply with quote

Unfortunately no; my grandson is down here and I only have time to check this quick.

I had a thought though - could I have rebuilt the engine 'out', like 90/180/360 degrees out? I had the timing mark at the top of the cam sprocket when I rebuilt it - and I am pulling an even 80/85 lbs on each cylinder, with oil...recheck the timing, maybe...?
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LOUD MOUSE
honda305.com Member


Joined: 15 Aug 2005
Posts: 7760
Location: KERRVILLE, TEXAS

PostPosted: Sun May 12, 2019 5:30 pm    Post subject: Reply with quote

Too bad. ..............lm


Dr. Frankenstein wrote:
Unfortunately no; my grandson is down here and I only have time to check this quick.

I had a thought though - could I have rebuilt the engine 'out', like 90/180/360 degrees out? I had the timing mark at the top of the cam sprocket when I rebuilt it - and I am pulling an even 80/85 lbs on each cylinder, with oil...recheck the timing, maybe...?
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Geoff Hastings
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Joined: 22 Feb 2017
Posts: 229
Location: Epsom, Surrey

PostPosted: Mon May 13, 2019 6:53 am    Post subject: Reply with quote

I haven't seen you mention checking your valve or tappet clearance. If your freshly ground valves and seats are being held off by too tight cam followers/valve clearance then you'd get those symptoms. A rough check of rings is to check compression with dry bores then squirt some oil into the bores, if it improves the compression then you have a ring problem.
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