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Post by m40man on Dec 29, 2008 16:36:32 GMT
I'd be grateful for the opinions of the mechanical types out in DivvyLand please .... FIRSTLY:I've done a compression test on each cylinder on my CB650, as she seems underpowered compared to what I was expecting. So I have my readings, ready to compare. (They all are very close to one another.) The engine is quoted as having a 9:1 compression ratio. But can anyone tell me how I know from my readings how my engine compares to that? The compression readings are all something over 6BAR. But what does that mean, compared to the stated 9:1 please  .NEXT: If these readings are low, the possibilities could be worn rings / pistons (but there is no mechanical noise which I'd expect to hear - ie, no piston slap, knocking, etc. Or it could be the valves or the timing. I've done a static timing check & it looks spot on to my tired eyes. (I don't have a timing lamp). So I would suspect the valves. Is this a sensible suspicion? (Combustion seems good, perhaps a bit on the rich side - both coils are working, fuel is not a problem.) PENULTIMATELY: This is a SOHC 1980 air cooled Honda in-line 4. I reckon it should be on top side of the red (10300rpm) at about 110MPH but mine is hitting the lower red area of the tacho (around 9500rpm) at only about 80mph. So I expect I'd be the wrong side of the red at about 90mph. I haven't pushed it further because I've only just put her on the road after a while off the road & I'm being nice to her, though she seems happy to carry on revving. If the problem is something stupid like a bit of clutch slip, how are you supposed to detect that at those speeds . FINALLY: Am I just expecting too much? I used to have one of these nearly thirty years ago but honestly don't recall what speeds that one gave me at what revs. Ta for any help! |
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Post by cam7777 on Dec 29, 2008 17:07:11 GMT
If your readings are about the same across the cylinders, I suspect the problem is the nut behind the handlebars. Best thing you can do is find someone [1] who has an interest in these bikes and off load it quick. Find yourself a new project and move on mate, the problem you are experiencing is due to you thrashing riding your Fazer 1000, anything by comparison will seem slow. [1] -  |
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Post by cam7777 on Dec 29, 2008 17:18:26 GMT
6 BAR (88 psi) sounds too low to me.
Most motorcycles need a minimum of 100 psi in compression to run start and properly. Compression should be checked when the engine is warm, but that's kind of hard to do if the motorcycle is not running. So just hook up a compression gauge and open up the throttle all the way. Crank the engine until the needle on the compression gauge stops moving. This should take no more than 5 seconds. Very roughly, 100-120 psi is good, 120-140 psi is very good and 140 + psi is great. If you don't have this level of compression then you have bad valves and/or bad piston rings. If you want, you can add a teaspoon or so of oil to the engine through the spark plug hole. Then try the compression test again. If compression is higher, then it's likely you have bad rings. The oil will seal the rings and give you better compression. If compression is the same, then you have bad valves/seats and/or valve clearances out of service limits. If you get any bad readings, perform a leak-down test to further determine the source of the leak.
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Post by m40man on Dec 29, 2008 17:23:18 GMT
6 BAR (88 psi) sounds too low to me. Compression should be checked when the engine is warm, but that's kind of hard to do if the motorcycle is not running. So just hook up a compression gauge and open up the throttle all the way. Throttle? I cranked it on the starter with the kill switch off so the engine wouldn't fire (this works on most older bikes - unlike modern kill switches which also 'kill' the starter). I didn't think I should actually be running the engine So I didn't open the throttle at all. This was following the Haynes manual I have. Additional: Engine was hot (warmed up for ten minutes prior to testing). |
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Post by teejayexc on Dec 29, 2008 17:32:16 GMT
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Post by bmbob on Dec 29, 2008 17:37:02 GMT
Hi Martin
We do compression checks on our Aircraft engines, (flat 4 ) similar to a VW engine.
The readings we are looking for are around 80 psi, but mainly we want them to be all the same.
After I do a run to check all the temps and pressures we whip the aircfaft in the hangar, drop the spark plugs out and do a compression check while the eng is hot.
I agree with everything Clive has said, especially the bit about compareing it to your Fazer.
Ride safe
Regards Bob
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Post by teejayexc on Dec 29, 2008 17:40:26 GMT
Hi Martin We do compression checks on our Aircraft engines, (flat 4 ) similar to a VW engine. The readings we are looking for are around 80 psi, but mainly we want them to be all the same. After I do a run to check all the temps and pressures we whip the aircfaft in the hangar, drop the spark plugs out and do a compression check while the eng is hot. I agree with everything Clive has said, especially the bit about compareing it to your Fazer. Ride safe Regards Bob Don't tell him that Bob  , he'll be wanting the 'bugger to fly now  |
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Post by m40man on Dec 29, 2008 17:53:09 GMT
Goodo, that what I was doing, but I didn't open the throttle. I don't understand how opening the throttle impacts the results on a non-running engine It sounded like Clive was saying run it, which is what surprised me. I guess I mis-interpreted that. Your link says "100 PSI is the minimum. It should be 125 to 150, and some engines have as high as 170 PSI or more. Yes, an engine can run with lower compression, but it will run very poorly." Nothing could be further from he truth with this engine. It starts & runs well. I had to make my own rubber washer for a down-sized converter for my 12mm plug threads. So maybe I'm not getting a good seal? The engine isn't grossly underpowered anyway - I was just expecting a bit more. So maybe nothings amiss except my seal isn't good enough? I think I'll take her for another run tomorrow, & note some speedo readings against engine revs. (Of course, all this relies on the speedo being accurate !) I'll oil the seal & check the compression again too, in case a leak is the problem with the low readings. Has anyone fitted wings to a bike & managed take-off ? (Not counting Superman, obviously  .) |
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Post by cam7777 on Dec 29, 2008 19:25:20 GMT
I usually remove all the plugs when doing the compression tests, definitely NOT running!
It could be that you haven't got a good seal on your testing gauge, possibly giving you false readings.
if your engine starts and runs fine I wouldn't worry.
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Post by scarecrow on Dec 29, 2008 22:37:55 GMT
My friend had a similar kind of fault on his Kawasaki 650 it had been stood for a while, turned out after a lot of searching some carb diaphragms had perished & they cost a fortune. so he had some made out of an inner tube I think? then waved it goodbye.
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Post by bmbob on Dec 30, 2008 9:43:32 GMT
Martin....you need the bike on the centre stand ,all plugs out........so engine NOT running, select a gear (2nd will do), then do each cylinder in turn.Before you do the comp. check, rotate the wheel to find T.D.C. then take it out of gear, then do the comp. check. (if you leave it in gear it will turn the back wheel!)
Finding T.D.C.is important 'caus you want the valves closed.
Lol we'd have to be a bit nimble doing comp. checks on our Aircraft ...with the eng running and a girt big propellor thrashing round at 2400 Rpm !!
What we actually do is one guy holds the propellor while the other does the comp. check, when we get close to 80 psi the guy holding the prop. can actually rock it slightly, that way we will get TDC exactly and take our reading.
HTH
Ride safe
Regards Bob
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Post by m40man on Dec 30, 2008 9:51:35 GMT
Martin....you need the bike on the centre stand ,all plugs out........so engine NOT running, select a gear (2nd will do), then do each cylinder in turn.Before you do the comp. check, rotate the wheel to find T.D.C. then take it out of gear, then do the comp. check. (if you leave it in gear it will turn the back wheel!) Cheers Bob. Almost what I did: I had kill switch 'killing' the ignition, plugs out, but I didn't TDC the piston first. Doesn't turning it on the starter do enough? (It'll hit TDC a few times whilst turning on the starter). IE, I don't understand why you have to get it to TDC before turning it over? And is opening the throttle important? Again, I don't understand that bit & didn't do that  . |
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Post by General Gman on Dec 30, 2008 10:53:09 GMT
standard compression from the Honda manual is 170 psi, so I'd say you have a problem there, mate (if indeed the gauge is sealing properly)
I'd be wanting to see at least 130-140 psi on a motor that old - could well be tired rings or your valves are buggered and not sealing correctly (it'll still run ok, but won't be terribly efficient) or you could well have a leaky head gasket.
You could get a leak-down test done, but you won't have the kit to do it at home.
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Post by m40man on Dec 30, 2008 13:35:24 GMT
Well you live & learn - opening the throttle whilst testing was the best advice given:  All the pots are much of a muchness, so I'm very happy. So I'm going to concentrate on tweaking the carbs (currently a bit rich) & otherwise will ride her & enjoy  . Thanks for all the input, folks . |
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Post by Flymo on Dec 30, 2008 14:06:48 GMT
Well that seems like a very respectable figure 170+ Maybe the old girl you used to have just seemed more exciting in your youth  Flymo |
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Post by General Gman on Dec 30, 2008 14:54:07 GMT
I would have though you should be able to get about 110 on the clock of a CB650.
Have you checked the gearing Martin, or the clutch ?
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Post by General Gman on Dec 30, 2008 14:55:41 GMT
Oh yeah... and is it the standard tacho / front wheel ?
Does the bike have more to go, or is it genuinely in the red ?
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Post by m40man on Dec 30, 2008 16:09:58 GMT
Very good questions, GM. I've checked the sprockets - they are standard 16/40 front/rear. The engine certainly wants to carry on into the red & I don't doubt beyond it. The tacho looks like a good match for the speedo - but when you peel back a rubber grometty thing on it, it's base does have a different lip on it compared to the speedo, so who knows - it might be a replacement off another CB of the era? I'm trying to find a close-up of a CB650Z tacho' on the web, for comparison. Too many are the US versions, though, or the custom or Nighthawk, including on hondacb650.com & similar sites . Excellent suggestions though - cheers . |
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Post by teejayexc on Dec 30, 2008 16:34:42 GMT
Well you live & learn - opening the throttle whilst testing was the best advice given: All the pots are much of a muchness, so I'm very happy. So I'm going to concentrate on tweaking the carbs (currently a bit rich) & otherwise will ride her & enjoy . Thanks for all the input, folks . Martin; The reason the throttle must be wide open is to ensure that each cylinder fills completely. If the throttle is partially open, a vacuum will be generated in the inlet manifold and the pressures indicated will be lower than expected. |
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Post by cam7777 on Dec 30, 2008 21:00:47 GMT
Sounds like you have a good engine with 170 psi across the cylinders. Result! |
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Post by m40man on Dec 30, 2008 22:43:47 GMT
Sounds like you have a good engine with 170 psi across the cylinders. Result! Yep - no oil-in-the-bore type tricks (or messing with Photoshop ;D). I was truly surprised . |
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Post by CD on Jan 20, 2009 14:30:27 GMT
Some strobe lights allow you to check the revs.
If the speedo is accurate you could work out (from the gearbox specs) what the revs should be for a particular gear and compare with the tacho reading at that speed.
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Post by darren on Feb 3, 2009 19:12:26 GMT
Sounds like you have a good engine with 170 psi across the cylinders. Result! Yep - no oil-in-the-bore type tricks (or messing with Photoshop ;D). I was truly surprised . I usually do a wet test (oil in bore) this gives me an idea of lost compression, but yours is pretty darn good dry |
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Post by yorric on Feb 3, 2009 22:43:07 GMT
Martin
Try using your sat nav to cross check indicated (speedometer) speed against real (GPS) speed. Its interesting what difference you can see on most vehicles. Mine all seem ok at speeds up to 60mph & grossly over estimate road speed at top speed.
Another area to check out is the air filter - is it clean? If not you could be strangling the poor thing!
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, he'll be wanting the 'bugger to fly now 
.)
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