Topic: Weak/no spark cb750f

[udadni]

I just got a free CB750f from a friend. Two years ago, it started easily, but now i can't get it to spark. I've checked 3 and 4, and I got 4 to spark once. I haven't been able to check 1 or 2 yet because i'm doing this by myself and haven't been able to get a good visual angle on either of those when kickstarting.

I've been reading the forums here, and on the older SOHC/4 forum, and what i've read leads me to believe that either my coil wires are ruined, or the coils themselves are bad. When I unscrew the plug covers from the wires, the hole that the contact in the cover screws into has bits of wire jutting out of it, and some of the wires look fairly corroded. The plug covers themselves look clean on both contact points. (I didn't have a volt/ohm meter when i was doing this, so i can't say for sure. Is there any way to check to see if the coil wires are good using an ohm meter? Also, do you guys know if the 750f is a 6 volt or 12 volt. Can I try to jump it using a car?

[frostypuck]

It's a 12 Volt, but not such a good idea to jump it off using a car. I would look at buying a new battery first. Before anything else. They sell them at Sears, Walmart etc. You'll need to slow charge the battery after adding the acid. Probably set a charger to it's lowest setting for around 10 hours. THEN try to start the bike.
If you top off the existing battery with d.i. water and slow charge it, it may or may not have survived the last 2 years without corroding the plates inside. And may be a waste of time. If you have a bike shop nearby, purchase the battery from them and have them fill and charge it for you, and pick it up the next day. That'll probably be in the $40 neighborhood, versus doing it yourself w/ a Wally-world battery for about $25. The choices are unlimited
Good luck,
Chris in Boston

[Gordon]

I vote that we no longer offer assistance to people who have SOHC4's given to them for free!

At least until someone decides to give me one, that is... 

[frostypuck]

I was at a British Bike show over the weekend, one of the vendors was trying to give away a '74 750. (Along with a GT380, and a couple basket case Kawi's) None of the half-dozen sohc enthusiats that were there ended up taking it home with them. But that was only because all of us rode to the event, no one had a truck to haul the thing home. We have an illness.
Chris in Boston

[eldar]

Well check the plug caps to see what the resistance is. Should be 5000 ohm but there has been some suggestion that they started out as 10,000 but go with the 5000 mark. If all 4 are within about 500 ohm, then they should be good. Now snip about HALF an INCH off the ends of the wires then put the caps back on.

Now pull the points cover and check your timing and the condition of the points and condensers and maybe clean the ...no DO clean the points!  Clean the plugs or better yet, get new ones if they are still the ones from the bike.

Do a test on the battery and make sure it is good and charged or get a new one.

Then when testing the spark, use the electric start. Works much better.

[udadni]

Thanks for the info. Here's an update: I was dinking around again, and decided to try and test for spark again. I got 4 and 3 to spark consistently, though it wasn't the most awe-inspiring spark i've seen. I wasn't able to set up a clear shot for 1 and 2. The battery thing definately sounds like a good call, I'll head up to skagit powersports and get them to prep me a battery.

I'll test resistances once i get batteries for the multimeter i'm borrowing.

[mick750F]

   From the description of your spark plug wires I think I'd try cutting off about a half inch. Re-install caps and try starting again. It's worth a try before throwing money at it. Just my 2¢ worth.

Mike