I Think I Figured It Out!!!

[Muscle racer]

ok, here's my new toy. Im super excited.  Its my first bike but I grew up around bikes. The guy that sold it to me said it wont need anything for another 40k miles, but since I brought it home, its been in the garage bc I keep finding small things that need to be done. Its a 78 k with 19k miles.

 It still doesnt run right. feels like it doesnt have power. I pull the throttle all the way back and it wont go past 20 mph. I changed the gasket on carb#2 bc it was leaking...fixed, I changed the accel pump bc it was still leaking(just alittle)fixed, the ignition switch was working, replaced it...fixed. I put seamfoam, as instructed, and it didnt do anything. I cranked it up today and just ran it for a few min, then touched the exuast and only #2, #3 were hot. Im hoping I have figured it out!

I went to buy new spark plugs but didnt have the part #, so didnt do it yet.

All you cb750 gods out there, PLEASE help me get this bad boy on the rd. Im itching for it.
I know its probaly something easy and simple but Im learning as I go.

help me out!!!!

thanks

[LukeM]

So much for the "aww, it won't need anything for a long time" adage.  EVERY motorcycle needs regular attention.  Welcome to the wonderful world of motorcycles.

If cylinders 2 and 3 are hot, and 1 and 4 are cool to the touch, I'm guessing you have issues with one of your ignition coils.  They fire every time.  When they are not on the power stroke, they fire again during the exhaust stroke. It saves having a distributor on the bike.

Check your coil connections for cylinders 1 and 4.  You can test it by pulling the spark plug for either 1 or 4, pressing it to a ground (like the engine block or head) and crank the starter.  You should see a faint blue spark jumping across the plug. If not, you may have a shorted spark plug, an open connection from the coil to the plug, or a bad or broken connection for the low side of the coil.

This has been discussed many times on this forum.  Do a search for "ignition coil" at the home page, and see what you can find.  I'm sure we'll be able to help you with your problem.

Cool looking ride.  It'll be fun to get out there and buzz around.
Luke M

[lcochran]

I just had this same issue recently.  My 1-4 werent firing, but 2-3 were.  I pulled off my ignition cover, to see that i had the dynatek electronic ignition which operates with 2 magnets sending electricity- one for each pair of cylinders.  There are two lines leaving the ignition, both feed a coil under the gas tank.

After you've done the spark plug to ground test and you see you have no spark.  Take your tank off and take  off your coils. Check each for resistance across the coil.  In my case i had 3 ohm coils, so the resistance should be 3 ohms.  Your's may be different but the manual will tell you.  In my case i had 3 ohms of resistance, which told me to go further down the line.  Where i eventually found the ignition was broken.

I'm willing to bet either one of your coil's under the tank is no longer working, or that your ignition isn't sending spark to the coils.
Good luck.