1979 cb650 Cylinder #1 not running

If it's broken or just needs tweaked

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Posts: 48
Joined: Tue Apr 15, 2014 3:16 pm

1979 cb650 Cylinder #1 not running

Postby ScottBowl » Tue Jun 05, 2018 7:29 pm

I'm looking for some opinions on where to go next with this one!


Found this old '79 in a shed about an hour and half away from home. The old guy was able to get it started but only with the choke halfway out and it sounded horrible, unable to idle and died as soon as the choke was pushed back in; I suspected carb troubles. Noticed the ever present head gasket leak. Other than those two majors it looked to be decent.

Decided to buy it (600$) since I've owned two of these before (both '81s with CV carbs) and thought it might make for a decent flip. Brought it home and dumped the old oil out which was extremely runny almost like black water.

First I cleaned the carbs (PD50A);
I took the bowls off, the floats and float needles out, took the emulsion tubes out. The pilot screws came out too, Pilot screw for cylinder #1 that tiny o-ring had flipped and was deformed. Replaced it with a new one. I didn't go any further, didn't touch the small spring loaded side cover which I believe to be an air cutoff valve and did not remove the throttle needles or open the top covers. Admittedly I probably should have been more thorough... once everything was removed I used carb cleaner and compressed air (110 psi compressor, not an air duster can) on every opening I could see, then cleaned all the parts I removed and put them back in.

Pilot screws were set at 1.5 turns out.

Put them back on the bike, put the gas tank back on. Overflowing from #1 and #4; probably just float needles sticking. #1 has a Persistent drip when the petcock is engaged; maybe I should check that brass overflow tube for cracks. #4 bowl seems to be behaving, I have both of them a few good taps with the back of a screwdriver to try and free up the needles.

I put fresh oil in, cranked the bike over and it ran just as bad as when I was at the old guy's house. So much for a quick flip :) It's possible to get the bike running on about half choke, if you pull the choke higher the revs shoot up to 4-5k and if you push it lower it just falls on it's face and dies. I have 2 or 3 pipes that are warm out of 4. Sometimes it's #2, #3 and #4, with #3 and #4 going out intermittently leaving me with the two middle cylinders running or #2 and #4 running. #1 seems to never run. I suspected bad coils; pulled #1 and #4 and checked them both at the same time, nice bright spark. Thumb test shows compression on #1. Plug #1 was wet but somewhat oily.

I'm trying to formulate a plan on where to go next. My instinct is to get a real compression tester and check cylinder #1, or just take the valve covers off and check for stuck open valves... Maybe the float level is so high on #1 that it's flooding the cylinder? Ignition timing or bad ignition box? Blown head gasket leading to bad compression?

Thanks for reading!

Posts: 48
Joined: Tue Apr 15, 2014 3:16 pm

Re: 1979 cb650 Cylinder #1 not running

Postby ScottBowl » Wed Jun 06, 2018 1:23 pm

Some news:

I found a slight cut on the yellow wire going to the right coil which I taped up. Some data:

Ignition coil readings (same readings for both coils tested individually):
between small blades going to wiring harness 2.5 ohm
between plug cables 12.28k ohm

Plug Caps (not sure which reading belongs to what plug): 5.1k, 5.1k, 4.7k, 4.5k

"Pulser Generator" readings:
Measured at connector located next to the battery box
Between yellow wires: 515 ohm
Between Blue wires: 492 ohm
Gap set to 0.406mm between pins and crankshaft on both positions.

Is there a test for the spark unit?

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