Author Topic: Compression Tests in Honda 4 Bikes - What is an acceptable level?  (Read 553 times)

Offline paulbaker1954

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Recently I  have got rather interested in Compression level testing on my 500 4 mainly down to trying to diagnose some running issues.

The Technical official Honda 500 Maintenance Manual indicates a "good level" of 170 PSI and anything under 140 indicating worn rings, poor valve seating etc

So off I went with my pressure tester and got 135 PSI on all 4 pots and started to panic, why are these levels so low compared to the Honda spec given that I rebuilt this engine only 2k miles ago with new pistons and rings and honed bores and all measured up to within official Honda sizes for a standard bore (56 mm) engine. Also the engine runs well with no smoke etc indicating worn rings or anything else wrong.

So why the large discrepancy.

I think there are a number of factors at play here

1) The compression tester I used was a fairly cheap unit   but like many commonly available that has a long length of rubber hose. This significantly increases the volume of air being compressed and can drive down the readings on these small capacity cylinders as the additional volume of air is significant compared to the swept cylinder volume.

If you look at the Honda maintenance book picture the  guy there is using a rigid metal small bore pipe connected to the gauge NOT a rubber hose. Also the Honda manual says nothing as to whether this is a dry or wet test but logic would assume dry as nothing says otherwise.

There is an excellent post on the USA forum about this that end up getting very technical but a really interesting read for the highly technical amongst us.

http://forums.sohc4.net/index.php?topic=39971.0

2) This was a dry test so I put some oil down the bores and the readings whacked up to 175 but I think I put way too much oil in there which was why such a jump. I thin I should have added just a few small drops !!

So now to my million $ question on these 4s what would be a sensible "good" level to get when using the type of gauge easily available and thet we all use.

My experience seems to lead me to around 135 psi on a dry test and that it does not rise dramatically on a wet test with a few drops of oil would indicate a pretty good condition engine. But that said I wonder what an acceptable lower level would be to indicate a top end problem.

Be very interested in others opinions here and how this thread develops.

A) Does anyone else have and readings from their bikes that may help?

B) Does anyone know what sort of increase is OK between a dry and a wet test?
« Last Edit: May 08, 2018, 08:53:21 AM by paulbaker1954 »
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Offline Seabeowner

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Re: Compression Tests in Honda 4 Bikes - What is an acceptable level?
« Reply #1 on: May 08, 2018, 09:31:23 AM »
Paul,
Only have some limited information: I have a Schrader compression tester that was bought a VERY long time ago. It has a rubber bung with a hole, a short (3") length of steel pipe, maybe 5mm dia and the gauge.
I have 4 bikes and can only ever remember using it once in the last ten years. On the 500/4 that I use the most.
Engine warm, WOT, all plugs out, electric start, no added oil to bore, 148-155 psi across the 4. No 3 cylinder was the worst. This was a UK bike with an engine that looked like it had a fairly hard life and was smoking a bit on No 3.
Since then, I stripped it to replace the primary chain, but also checked the bores (which were in spec). Home honed the bore, swapped around with a bunch of old piston rings I had and threw it back together. Much less smoke, but not perfect.
My main criteria:
Does it start/go:  YES
Does it make a lot of worrying mechanical noise:   NO
Does it smoke like a trooper:   NO
Does it go 80MPH:     YES
Does it produce 50bhp and do 100mph:    Probably not.
Does it give a lot of pleasure with not too much maintenance:  YES

Phil
1971  CB500K0
1973  CB500K1
1975  CB550F1
1978  CB550K

Offline kevski

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Re: Compression Tests in Honda 4 Bikes - What is an acceptable level?
« Reply #2 on: May 08, 2018, 02:31:34 PM »
engine should be up to temp, all plugs removed, turn over on starter for around ten seconds if all four are within the same figure say around 15 different, keep using it, you can run at much lower readings than Honda say.

Offline matthewmosse

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Re: Compression Tests in Honda 4 Bikes - What is an acceptable level?
« Reply #3 on: May 09, 2018, 05:24:31 PM »
I have had a cb550 get to indicated 100mph with no top rings, other than the damage done by the rings hammering the head and hard starting when hot, the only clue to me things were not well was oil leaks, I had no compression tester at the time. There was no damage done by running in this poor condition other than loosing oil over my boot. My own use of the tester has had limited success since I bought one, it can help point to a weak cylinder if one has low compression and a test with and without oil could help spot valve vs bore issues but on my bikes - mostly cb500 and 550 I found valve sticking was easier to spot through checking tappet clearance a few times when things were not  right, not a full Feeler gauge test, just wiggling the rocker when checking points - the right valve showed up then when the compression tester wasn't telling a very consistent story. I think a lot of that is that a affordable compression tester or timing light used occasionally and spending it's life in a garage is not going to be the best calibrated bit of kit. In all honesty whilst I have a lot of Draper tools gizmos for helping set up and diagnose faults I find I distrust it so much I tend to check other ways whether it's compression or timing - last few engines I compression tested - dry test, got readings around 90psi on all 4 pots but ran perfectly and started well. Stripped down and ring gap measured at 3 points in the bore they were apparently within tolerance going by the book and no carbon build up to indicate blow by of combustion products past the rings etc. As you say losses to long hose etc plus calibration of the gauge - I wouldn't panic based on a reading. Of more use to me has been a inspection camera small enough to shove down the plug hole, spotted a partial seizure of a piston with one of those, saw the scoring on the bore before it got bad and could hone and new piston before it got costly on a strimmer. I wouldn't spend my money on cheap gauges again - carb balancers that the needles bounced around too much to read, compression tester that I am convinced does not read accurately, tyre pressure gauges that 3 different ones vary their readings of the same tyre by 15 psi. There's a lot of dud readings off cheap or old gauges.
Got a 500/4 with rust and a sidecar and loadsa bits. nice and original and been round the clock

Offline Tomb

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Re: Compression Tests in Honda 4 Bikes - What is an acceptable level?
« Reply #4 on: May 09, 2018, 07:56:41 PM »
At my work I overhaul PRV's (pressure relief valves). I test them on 12" Budenburg calibrated gauges (click)

If anyone wants gauges tested I'll do it if post is paid. I'll give a 10 psi increment correction up from 100 to 200
Tom
'73 CB550 with CB500 engine café racer
'62 CB77 Sprinter
'70 CD175
'78 CB550 with sidecar
'80 Z50R
And a load of old Yamaha 1100's