Topic: Trying to figure out my 1985 CB650SC

[Mountains187]

on: October 21, 2019, 04:51:27 PM »
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I have a 1985 Nighthawk 650 that I have been troubleshooting since I bought it about 3 months ago. Thing have progressively worsened since I've had it. Two weeks after I bought it, it got a point that it wouldn't even start anymore.

1-Replaced fouled plugs on that one, seemed to be both oil and fuel fouled. The plugs got her running good but the idle was very inconsistent.
2- Everyone was saying pilot jet, so I gave it a small seafoam treatment. That really made it run worse.
3- removed carbs to clean, internals of carbs were spotless as the previous owner rebuilt them.
4- After reassembly, synced carbs but now running worse but backfires and barely staying running so I synced again to make sure I didnt mess anything up.
5- Replaced spark plugs again. Ran compression test on all cylinders (all 90-95psi).

That last part was today. After I first replaced the plugs, I got the bike running and it kept itself running without issue but the amount of white smoke coming out the exhaust was lets say noticeable. I was really worried because it was more than it had ever put out before. Started it up an hour later and no smoke so I took it for a ride. Seemed to drive and run fine, minus the idle fluctuation at the end of the ride.

Not really sure what to think, this is where I was the last time. Runs fine and then takes a dump in a few days after almost minimal riding. The PSI on the compression test is way under what is listed in my Clymer Manual, but it also only seems to tell you to be worried in they are not all close to each other. I'm at a point where if it goes back to the same problem its either headed to the Honda Dealer for some professional help or to Facebook Marketplace because I do not have the knowledge in this area to keep guessing at solutions.

[Maraakate]

90-95 PSI sounds very low.  Did you do the test correctly?  I.e. with carbs off or throttle WIDE OPEN crank it over a few times and average out a few readings.  The numbers will read a bit lower if the bike is cold (which yours will be since you said you can't get it running correctly)... probably about 10-20PSI lower than what you're expecting.  Are you using some el-cheapo compression tester?  May not be accurate if this is the case...

PO rebuilt the carbs... do you mean a full rebuild kit or just cleaning + new float needle, gaskets, o-rings?  Could be mystery aftermarket jets in there.

How did it run when you first got it?  Was it rideable but just in need of a tune up?  Have you checked and adjusted the valve clearances, timing, point gap, and spark consistency?  You say an 86 so it may be CDI.  If you can find a copy of the factory service manual there are testing procedures for the CDI.  Clymer omits this useful information because people neglected to test them with fresh batteries.  If you find the information on the test procedure use fresh batteries in your meter!  Ideally use an analog meter, but a digital will let you know if it's in the ballpark but a fresh battery is extremely important.

Are you using a fully charged, fresh battery?  Can't answer for the CB650SC, but some of these bikes will run like absolute garbage with a weak battery but may start.