Topic: Low compression and oily plugs (82 650sc)

[Doogs]

Hello, I recently pulled my head and cylinder off my 82 650sc due to low compression and oily plugs.

The tops of the cylinders had oil residue and coming out the exhaust ports.

The plan is to replace the head/valve stem gaskets and adjust the valve clearance.

Before I do that though, I'd like to think that the oil was mainly coming from the head gasket but wanted to check if replacing the piston rings would be worth while.

[Doogs]

JPGs

[fizzlebottom]

It would have been valuable to do compression tests before and after adding a little oil to each cylinder to determine whether your compression issues are related to worn rings.

But if you didn't do that and you've got the rings out in the open already, I don't see any reason not to do a ring job at this point. I'm typically of the mind that replacing 40 year old items before they fail is a good idea vs. simply not replacing parts that aren't broken. Unfortunately you can't really diagnose the rings now unless you throw it back together and re-test compression.

Beforehand, just take your bore measurements and make sure you don't need to bump up sizes at all.

[Vintage Hondaman]

Hi , I also have a 1982 650 SC Nighthawk , originally an import from USA , I too have low compression and oily spark plugs - so it looks like I will have some work to , - just on another related question , did you clean your carbs out (I have ) and did you find that you have the rubber passage plugs fitted inside your float chamber and mounted over the idle jet tower - some carbs had these rubber plugs some did not - I did not find rubber plugs fitted to my carb - but I think they  should have been there and may have been removed by a previous owner - did you find these plugs on your carbs ?

I am wondering if my black spark plugs are actually due to an over rich mixture which may be a reason for the "oily" appearance

[fizzlebottom]

Hi , I also have a 1982 650 SC Nighthawk , originally an import from USA , I too have low compression and oily spark plugs - so it looks like I will have some work to , - just on another related question , did you clean your carbs out (I have ) and did you find that you have the rubber passage plugs fitted inside your float chamber and mounted over the idle jet tower - some carbs had these rubber plugs some did not - I did not find rubber plugs fitted to my carb - but I think they  should have been there and may have been removed by a previous owner - did you find these plugs on your carbs ?

I am wondering if my black spark plugs are actually due to an over rich mixture which may be a reason for the "oily" appearance

No such rubber plugs that came with these carbs, so you don't have to worry about trying to find any missing parts there.

Wet dark plugs could definitely be a rich mixture. Do you have the original Keihin 118 main jets and 35 slow jets in, or have there been changes? And what are your mixture screws set at?

[Doogs]

Hi , I also have a 1982 650 SC Nighthawk , originally an import from USA , I too have low compression and oily spark plugs - so it looks like I will have some work to , - just on another related question , did you clean your carbs out (I have ) and did you find that you have the rubber passage plugs fitted inside your float chamber and mounted over the idle jet tower - some carbs had these rubber plugs some did not - I did not find rubber plugs fitted to my carb - but I think they  should have been there and may have been removed by a previous owner - did you find these plugs on your carbs ?

I have been through my carbs. I did not have any rubber plugs like you are referring to. All I did was replace the jets, orings/bowl gaskets, drain screws (mine were fubar), and do a general cleaning.

Pictures or diagrams would help.

[HondaMan]

All I did was replace the jets...

Now, you're scarin' me.
That could be your issue. If you have the OEM jets, put them back in, first thing.

[fizzlebottom]

Now, you're scarin' me.
That could be your issue. If you have the OEM jets, put them back in, first thing.

Agreed. No reason to replace the jets in these carbs. They might have run lean from the factory, but replacing jets is gonna do nothing but cause problems. I'm really hoping you won't say you're also running pods and got rid of your stock airbox.

[Doogs]

Stock air box is still there, heard about all the complications from pods. The jets were stock Keihin.