Topic: cb550 top end diagnosis

[mikecb550]

Hello. I am currently preparing for a top end rebuild. I think the valve seals or rings are letting oil pass. The bike is blowing a lot of grey smoke.
I have been working over the bike looking for other possible causes for the oily smoke.
Prior to commencing the top end rebuild I would like to list some of the work done and readings, hoping that some experienced folks might recommend other possible causes or confirm the need for valve seals or ring replacement.

-work done and items replaced-

-checked valve clearances.
-new contact breaker plate (TEC).
-new points and condensers (TEC).
-set timing.
-adjust cam chain tensioner.

PD46A carbs-
-chem dip and soda blast.
-replaced o rings.
-replaced felt dust seals.
-new float needles.
-polished float needle valve seat.
-float hight set with clear tube method.

-new air box boots and carb boots.
-new stock air filter.
-replaced exhaust gaskets.
-synchronise carbs.

-4 new NGK D8EA plugs.

The PD46A carburettors have
-stock #90 main jets.
-#42 idle jets.(appears to be larger than stock #38)
air/fuel screw is out 2 turns as per manual.
bike has 4 into 1 exhaust.

I have run some compression and leak down tests and the results vary.
Here are the average results of repeated tests.
cyl.1. 150 dry, 170 wet.
cyl.2. 130 dry, 150 wet.
cyl.3. 140 dry, 160 wet.
cal.4. 155 dry, 178 wet.
the only leakage I could detect was in cylinder #2 and was a low %.
An example of temperature across the cylinders 1,2,3,4. At the bend in the header pipe. in degree celsius.
100,95,90,105.temperature results varied .

Any recommendations or advise appreciated.Mike.



 

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[calj737]

Aside from using the incorrect plugs (should be d7s, not d8) everything looks good. If you’re getting those compression readings, I don’t see the purpose of a ring job.

You swapped needles in the carbs, why? Stock needles have a very specific taper and aftermarket brands are not equal. Have you performed a plug chop to determine at what throttle position you’re getting gray smoke?

[mikecb550]

Thanks calj737.  The PO must have changed the idle jet size. Same with the D8 plugs. I matched the same plugs when replacing them (not knowing any better at the time).
First thing I'll do now is replace plugs with correct D7's. Also I'll try #38 Idle jets.
I should mention that my bike is an Australian import. I don't know if climate can influence the jetting?
If so, I wonder whether the #42 idle jet could have been stock with my particular bike?
More likely ,because it has an aftermarket exhaust, PO has tried to re jet.

While I'm on the subject, I had read on the sohc forum that I should be re jetting main jet for 4 into 1 exhaust , my 1977 bike being know as running lean. I will try that also.
I have not done a plug chop. I will look into it. Thanks again. Mike.

[mikecb550]

Ah sorry, you were asking about the float needles. I replaced the needles because carbs had a problem with overflowing. My PD46a carb needles have the rubber tips. Prior to my owning it, the bike had sat for a very long time unused and i thought maybe the rubber had distorted. Or at least that may be a possibility? It seemed to fix the overflowing.From memory I think the needles are OEM honda branded (i'll check).
I also replaced the drain plugs at the time because the taper had heavy corrosion.

[Little_Phil]

Had a smokey 78 550 that I was expecting to find worn or damaged bores. All I found was a couple of VERY worn exhaust valve guides. A surprise as I have some much older higher mileage 500/550s and never seen anything this bad. Expensive to replace guides and recut seats so found another head.

[calj737]

With stock airbox and mostly stock motor, moving your MAIN jet for the 4:1 is not likely to be needed unless its a Yoshi style pipe. Even then, its marginal. Temp doesn't have as much effect on jetting as altitude.