Running rough

[lcochran]

My problem. 1975 Cb750F, excellent condition only 20,000 miles.  Drove it to work(100miles round trip) and then parked it for about 2 weeks :(. Last weekend kicked it over and it cranked right up but was running really rough, so i pulled the plugs, replaced them and it still ran rough. Did a spark test and found on #3 from left it had no spark.  Replaced the spark plug wire, still no spark.

Whats going on? can someone tell me how to fix this please. dying to ride.

Leston

[LukeM]

Unless I'm mistaken, there are 2 coils, one for 1 and 4 and one for 2 and 3.  If you have good spark on everything but plug #3, then your coils are good. I would suspect the spark plug wire is loose at the spark plug end.  The plug caps push onto the wire and tighten down.  One trick you can try if you have enough wire is to trim off about 1/4 or 1/2" off the spark plug end of the wire, and re-attach the cap.

If you have no spark for 1 AND 3, your motor would be running VERY badly, if at all. I'd look at the little 2 wire cable connecting to the low voltage side of the coil.  There should be one connector for each coil.  Sometimes some corrosion builds up there, leading to weak spark. Also, it's possible that a grounded spark plug or wire will show no spark as well. If none of that pans out, then it's probably coil replacement time.

Good luck, and let us know how it goes.
Luke M

[lcochran]

Well once i found that 3 wasnt firing, i didnt check the others. I'll do that and go from there. How hard is it to change coils?

[vern401]

it's not hard they are or should be under the gas tank. unplug the spark plugs from the coil you are replacing. Unplug it's wiring harness wire/s. Unbolt it and put ine new one in using the revers order from above.

[LukeM]

What he said.  My thinking on this is/was: if you've got a dead coil, your motor is firing on 2 out of 4 cylinders. You'd have a VERY hard time getting it started, and it would run VERY rough, if at all.  It's more likely the spark plug cap that connects the plug wire to the plug is loose.  You can do a google search on how to cut off the plug end of the plug wire, and refit the cap (the little prong end just pushes into the metal section of the wire).

To test for spark, get a known good spark plug, place the plug cap on it, and lay it against the engine or a good ground. Turn the key and run switch on, and crank away.  It's a fairly narrow gap, but if you're in a garage, you'll be able to see it.  You may have to hold the plug tight against the engine.  Use a piece of wood or rubber fuel line hose, so you don't shock yourself.  Don't ask me why I recommend that. :-)  1 and 4 fire at the same time, as do 2 and 3.  I think the firing order is 1 3 4 2, if I recall.  Refer to the manuals on this site to be sure.

However, if you do wish to remove/replace the coils, the above procedure works just fine. Good luck.
Luke M

[vern401]

Very true if the coil is not firing at all it normaly effects both cylendars. Still check all 4 cylendars for spark. The origional plug caps do go bad and can be replaced. I would find the root of the problem before just replacing parts.