Total Motorcycle Community Forums

OK sry this long thx in advance!

Before it got parked for winter, my '83 cb650sc was having fits where it would lose power and die. Otherwise it ran good and was pretty strong last time i checked. Did compression test this morning and its way low across the board. the engine was cold as it doesnt run, but ambient was a mild 55 deg. would that make a big difference?

anyhow i got
85\115\85\70....sucky.

i put oil in the fourth cylinder and it pumped up past 200.
so my rings are completely weefed i assume?

I guess i should have oiled the other 3 to see what i got but it seemed conclusive for rings.

i like to keep her but since this machine is over 30000 miles and maybe worth $1600 pristine, it aint likely worth the effortr. Mine needs minor work all around i'd say.

ahhh.. whadduyu think?
rebuild/fix? part it/salvage? or perhaps score a used motor?

Any cool engineswaps for this bike? maybe the 700s nighthawk ?

What sux is that i dont have a real good place to work on it now, and i lack bike specific experience

Personally, I'd rebuild. Redoing the rings isn't that complex of a job as long as you have a manual and the proper tools.

When I had my '83 GS650GL, I wound up checking all my rings (still good, thank god), but I gave the cylinders a light hone, replaced all the engine seals and cleaned up the head and valves. My cost to do all that was ~$100CAD for the gaskets/seals that I needed, $65 to get one cam cover bolt extracted by the machine shop, and a couple hours of my time, spread out over a couple days.

If you decide to take on the task of rebuilding, keep us updated on your progress. When you get it all done and back together, it will run stronger and have more power - mine did.

"weefed" I like that term. I only have automotive experience, but your rings do sound weefed based on your test.

Though I don't know if this would cause your bike to run fine then die out, I would think it would run consistantly a bit weak, but again I only have exp with cars.

Edit: 30,000mi shouldn't be nearly enough miles to weef out a nighthawk, perhaps you have other tuning or maintenance issues which wore out your rings or cylinders.

Sitting all winter without running will give a very inaccurate compression reading. The engine should be run up to temp before taking any readings you want to trust enough to base a rebuild on.

Was the bike hard to start, especially cold, before? Less snappy on the throttle? Using more oil than normal? Did the oil sell burnt when you changed it? Always ran with an air filter in place?

It sucks, but you should get it running before making any decisions like that. It sounds more like an ignition or carb problem. Bad ring symptoms don't come and go.

I have to agree. Id try and get it running. My test were low on my bike after it sat all winter. Once I got it running the compression came right up. Sounds like a fuel problem to me.

thanks for the replies.

OK, the carbs have sat with gas in em. I shot sovent into the drain plugs and down the throats. I cleaned the tank and 'fuel stick' as best i could. new gas is present in the bowls. I added ether while she was turning over.

same as last time it tried; a couple burps, but it won't run/

Can i check ignition timing just with the starter turning it over? i have an old timing light that has worked in the past, but it didnt seem to work on the bike whilst cranking.

When i had my truck battery hooked up to the bike, i noticed that the tach will 'twitch' randomly...Does that mean something? It reeks of a ground problem to me.

what do you think i should do next?

Id pull your plugs and check for spark . Id also pull the piolt jets and clean them, thier what lets the gas in at slow speed. Turn the jets in till they bottom out easy does it. Count the turns to bottom should be about 1-1/2. Use that as guide for when you put back . You might have the piolts that have a cap on them. They can be a B----- Sometimes I open the drains on the bottom of the tank and blow air in my fuel line coming from the gas tank that will help blow the crap out of those also.Do one at atime to get the most pressure.Good luck

I forgot to mention that there is spark, but it is orange. one of the coils was replaced in the fall for orange spark but now neither coil makes a hot blue spark. so as far as i know i may not even have ignition. I will post back in a few hours with test results for cdi.

I will post back in a few hours with test results for cdi.

fuelwise,I can't access the pilot jets due to bogus limiter caps, I must fanagle a way to remove the bowls with carbs still onboard. then i could hit the jets well enough i think to run at least./..if that was the problem.

thx again for reading and posting!!

Pulse generator checks out.

Coil test of primary lugs is good.
On secondary leads one was twice the ohms is shoulda been, and the other failed to show continuity.

Yellow spark at all four plugs. i couldnt get a continuity or impedance reading on two of the spark plug wires, however they still give a spark to the plug..??

So i think new wires are in order and the old ones may have spoiled the secondary coil resistance test.

would it seem logical to try a new spark unit if wires dont fix it?

The oil has oossed off of the rings while the bike was setting. Pour a small amount of oil in each cylinder to bring up the compression. This will help the engine to start, the oil helps the rings to seal the air fuel mixture in the compression chamber. If you still feel that you need to check the compression do it while the engine is warm. But I think the engine is fine. Good luck.