Author Topic: Post top end rebuild multi mystery  (Read 847 times)

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Offline JBMorse

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Post top end rebuild multi mystery
« on: February 28, 2016, 02:43:47 PM »
Hi,
I recently did a top end rebuild on a CB550 engine going in my 500/4. I've got it all together and it's not running or sounding good at all. Bad compression, almost no compression on cylinder 2, and cylinder 3 or 4 is smoking.

Here's what was done:
Top end gasket set including valve seals and head gasket
New STD size rings
Honed cylinders
lapped valves
put it all back together
set timing, valve clearances, cam chain tension, all the 3k mile tuneup stuff.

Here are the compression numbers (From an automotive style tester)
            dry       wet
1.         65       100
2.        0!!        barely above 0, maybe 10
3.          120     140
4.        90          115


Also, all the plugs are black (except #2 which was barely firing). So my mixture seems to be very rich. Here are the intake/exhaust details:
'78 CB550 engine
Carburetors from the '71 CB500, stock jetting, clip on 4th groove from the top
Uni Foam filter
4-2 exhaust (Mac I think) that is probably somewhat close to stock backpressure
Pilot screws almost 2 turns out

I rechecked the valve clearances, they are good. I also checked the cam chain tension, I think it's fine. Everything seems to be in order, but it's not running well.

Whoa that's a lot of info to sort. Basically I'm dealing with poor compression (especially on #2), smoking on cylinder 3 or 4, and a seemingly very rich mixture. Here are some questions:
1. I don't have a leakdown tester, but I'm inclined to take this engine apart again and get a valve job done. Do I need to buy a new head gasket if I take it apart again?
2. Should I wait to change any carb settings until I have the compression sorted? Will that affect the appearance of the plugs?
3. If I discover a broken ring or something, can I expect to do an overbore?
Maybe I'm getting ahead of myself on that one!

So it's unfortunate the thing isn't running after all this but I honestly don't mind tearing into it again. At this point I feel like a good day in the workshop will be enough to get it all apart and hopefully find the source of the problems!




1971 CB500K

Offline BPellerine

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Re: Post top end rebuild multi mystery
« Reply #1 on: February 28, 2016, 03:19:37 PM »
is your cam timed right?
1978 CB 750K ard and webers
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Offline JBMorse

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Re: Post top end rebuild multi mystery
« Reply #2 on: February 28, 2016, 07:31:50 PM »
Yup, the cam was timed correctly.
I took the whole top end apart this evening and didn't find anything obviously wrong. I wonder if I just need to have the valve seats recut. All I did was lap them, and I'm not sure they're all sealing properly. Good opportunity for a 3 angle valve job.

So now I am wondering whether to order another head gasket. Does taking the head off render it no good? Also, according to the part numbers, the CB500 and CB550 head gaskets are the same. Really? I would have thought the bore size would make a difference.

I still don't know what to do about the jetting. I will try to get good compression and deal with it after that.
« Last Edit: February 28, 2016, 07:38:50 PM by JBMorse »
1971 CB500K

Offline mystic_1

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Re: Post top end rebuild multi mystery
« Reply #3 on: February 28, 2016, 07:48:05 PM »
That zero reading is pretty alarming.  Even with valve surfaces that resembled the Moon, you should still get some compression I would think.

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Offline calj737

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Re: Post top end rebuild multi mystery
« Reply #4 on: February 29, 2016, 02:43:08 AM »
That zero reading is pretty alarming.  Even with valve surfaces that resembled the Moon, you should still get some compression I would think.
+1 I would perform a leak down test prior to disassembling anything further to identify the location of the compression leak. The only thing that makes much sense is that #2 valve is stuck wide open, or the rings are not sealing at all. Or, you had a major air leak at the spark plug hole when you threaded the adapter in. The carb jetting has nothing to do with compression tests.
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Offline strynboen

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Re: Post top end rebuild multi mystery
« Reply #5 on: February 29, 2016, 06:12:41 AM »
have you bend some valvets..vas the valve cover hard to press Down??
its easy to get a valve lifter, bend if you not vas care to hold them all up.before press the cover down and have full slock in the valve adjusters.carefull Mount the cover ..and then adjust the valvets dovn to spec..
i kan not speak english/but trying!!
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Offline Redline it

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Re: Post top end rebuild multi mystery
« Reply #6 on: February 29, 2016, 07:12:02 AM »
New Std rings? and honed cylinder? if you ringed it just because you were already in there, cool. Usually though, it's done because of specs, or being still within them enough and the cross hatch is thin or gone. You didn't mention ridge, reamed. That's the important thing to do, if there was any ridge at all, and honing will smooth it out some, but it'll still be there to meet the first hard driven sharp edges of the new rings and they don't give much. If you used a ridge reamer, a half turn too much, or less can make a cylinder no bueno, like right now.  On the leakdown test, (it kind of is a technical operation, time consuming that has a 1 or 2 extra arms and hands semi requirement to complete the mission, so) to get the results of what you want, to know where the leak is, rubber tipped air nozzle or rubber hose even, taped to the threaded end of the air hose, go up from 10psi to 30max and you'll hear where it's leaking, easy.