trouble with the Nighthawk - just did warm compression test

[Iahawk]

my 84 CB650SC Nighthawk that I picked up and 'refreshed' 2 years ago has a problem. I knew it used oil as I regularly topped it off but I wasn't aware of the smoking until a buddy of mine rode it and I rode behind him. At times it smoked enough to keep the mosquitos away. (Kind of cool to watch your own bike going down the road).

First thought was worn rings so I did a compression test this morning. Cold test (I know, should be done warm...but) cylinders 1-4 were 120, 60, 120 and 120. Whoops. Re tested cyl #2, wet, and it showed 100 psi. Bad #2 cylinder. I may do a leakdown test but need to buy the kit. More than likely worn ring or cylinder.

Parts are few and far between and oversize rings don't exist for this bike (but I can source standard sized rings).

A parts bike and an engine swap is an option..but would still need to spend $750 - $1000. Maybe a used engine? Rebuilding existing one is really not an option given the availability of parts.

So everything on the bike is in excellent condition...all except the engine!!
What would you do?

[Stig / Major Tom]

I had the impression that you had found a very good source for even the smallest vintage Honda parts? In Europe?
Nevertheless  - that's a tough call. Seems you really like this bike!
Would a used engine  on a shelf, be a good investment.  Or would you have another smoker?
Be interested to see how this works out for you Hawk.
Stig

[Iahawk]

many parts are available, just not oversize rings. I might get one of those cheap boroscopes to look into the cylinder to see how the walls look. You're right about the used motor...anything will be 40+ years old for this bike and a gamble.

Bummed because I absolutely love riding this..so comfortable and I love the power (apparently less power than it should have!).

I could part it out at some point. There are a lot of valuable parts that would sell easily but that would also be a pain.

I could also just keep riding it while I start looking for another old bike to refresh. Hmmmm....

[Stig / Major Tom]

Yeah. Keep riding as is. Stay out of your mirrors.  "What smoke?!"
Stig

[Ruffus]

hawk, this motor was used in several HONDA models so I think there will be parts available.
I would take the head and cylinder group off and measure #2 .
Could be a failing oilring too (standard will do, otherwise you should a remarkable noise from inside) together with maybe worn out valveshaft gaskets.
All in all would be a shame to abandone such a beautiful bike just of labour investment.

[Iahawk]

Ruffus, I just ordered a boroscope so I'll start with taking a look in the cylinder. Unfortunately this was a fairly unique motor...only used in 83-85 CB650SC. The 82 and earlier CB650 used a completely different engine and the similar 84-86 Nighthawk 700S used a similar motor design but it shared no parts.

No oversize rings available and even a head gasket was questionable. Honda doesn't offer it but I do see an aftermarket one available on CMSNL.com. Most people on the Facebook Nighthawk page don't tear into these for lack of parts. A motor swap seems to be the common fix to a bad motor.

But I'll start with the boroscope and maybe pick up that leakdown test kit I've been eyeing and give that a try, too. Bike still seems to run fine and I keep an eye on the oil before and after each ride.

[Ruffus]

..hawk yes, good move to have a look inside before further action.

What I meant is: HONDA did not and does not produce pistons rings on their own.
So a subcompany is producing a variety of sizes of rings for several customers and under NONAME too.
Once you have the measures it will be possible to get a set of fitting rings.

[Iahawk]

ordered a boroscope yesterday..it showed up this morning. My flight to Chicago today is delayed a few hours so I stuck the scope in cylinder #2. Surprisingly it looked great!

What I did see that was odd was both intake valves (I believe, kind of hard to tell when you're moving the scope around) had a large drip of oil hanging from it. Now I did put a few squirts from my oil can into the cylinder for the wet compression test but I don't think the oil would end up a drop on each valve.

Does this point to worn valve guides leaking oil into the cylinder? Or more evidence of worn rings allowing oil in the cylinder? I think I'll still pick up a leakdown test kit as that should be able to help pinpoint the leakage.

[Ruffus]

My guess is on valve guide gaskets (the little oilseals 7x12-14mm in your cyl head). If open, change ALL 16 of them.
Warm up the motor by driving and have a look again with the endoscope. Compare with neighbor cylinder.

Inexpensive treatment first.

[Iahawk]

I like the way you think Ruffus! I'll do just as you suggest when I'm back in town later this week (taking another look at the cylinders when engine is warmed up)

[Iahawk]

I'm not sure what to make of my warm compression test numbers on the old Nighthawk (?)

Cold test,  cyl 1-4,   120, 60, 120, 120.

redid it with a cold and wet (oil in cylinder) test, cyl # 2 was 100.

Went for 31 mile ride today, came back and removed tank and plugs for warm compression test.

Warm test = (cyl 1 - 4), 100, 110, 105, 115

Always thought warm test would give higher readings? Used same compression tester, did same process with 6 compression strokes per cylinder. Same as the cold test. I'm a little confused..

Will put the scope back into the cylinders after the bike cools down a lot more...don't want to melt my cheap little scope!

[Ruffus]

Just a wild guess.
Is there a chance when you connect your comp tester with #2, the O-ring/ gasket between tester and sparkplugthread isn't snugly tight?

I'm still inclined to be more on the valve theory.

[Iahawk]

I did put the boroscope in the cylinders yesterday..everything looked normal in all 4. I'm starting to think the large oil drips in #2 were from me squirting the oil in for the wet test. It was dry and I could clearly see the cross hatching on all cyinders.

at this point, since the bike runs so well, I'm going to leave it alone and just ride it. (other than the cam chain tensioner spring that I've been delaying for a year...that does need to be replaced)

Maybe it's better to be ignorant and happy! 

[Ruffus]

Wise move👍🛠😅

[Stig / Major Tom]

Maybe it's better to be ignorant and happy! 

peace out man