Starting Issue - CB750 (help!)

[Ndgrimm]

Hey everyone, I'm hoping that someone here can help me out in getting my 1976 CB750f back up and running. I am stumped. I will preface this post with the fact that I am still new to working on bikes, so feel free to tell me I'm doing something totally stupid if I am!

About 3 or 4 days ago I had the bike running wonderfully and would start on the first kick every time. Then, my buddy and I were working on the frame, welding a hoop on the back and grinding some of the old seat tabs off, and I went to start it up after we were done and it wouldn't start up. As we were working on the frame we were jostling around the bike to work loose a piece of metal rather than just finishing cutting it. This might have caused part of the problem.

When I turn the key on, it appears that all the electrics work (speedo lights up, oil light comes on, headlight, electric starter works, etc...) Because I figured it couldn't be a messed up connection (e.g. from the welder's sparks burning through a wire or something), my first thought is that the jostling caused the engine to flood. I have noticed that if I have the bike running in the past and I work the back shocks up and down a little bit to jostle the bike, the engine starts running funny as though extra gas is going to the engine (is this normal?). As I said, the electric starter worked, however, it would just keep turning the engine over but not start it. Same with the manual kick start. So I went ahead and pulled out the spark plugs to see if they were wet from the engine being flooded and they indeed were. After pulling all the spark plugs out, I made sure the key was turned off and proceeded to hold the throttle open and kick the engine over a couple times in hopes of clearing out the engine. Then I let the bike sit for about a day and a half to ensure that the gas had evaporated...

Yesterday, when I got home from work I figured that all the gas for sure had to have evaporated, so I put some new spark plugs in it. I also checked the oil to see if it smelled like gas and it kind of did so I did an oil change on it. After all of that, I opened up a float bowl to ensure there was gas in the carbs and there was, then I tried to start it with the same result (none). I did notice when I would have the choke on, used the electric starter, and opened the throttle, it would sound closer to starting but still would just keep turning over.

What am I doing wrong/not doing?? I'm super bummed because the bike was running SO well before this week...

 

[HerrDeacon]

Have you checked the fuses? Did you weld on the bike with everything still connected to it, like the battery?

 

[Ndgrimm]

Yeah, all of the fuses are still in good shape. I did have everything still connected, but the bike was off. Could this have screwed something up?

 

[Reinhard]

Sounds like you welded your points shut. Rule number one when welding on a running bike. Disconnect the battery

 

[Ndgrimm]

Reinard, thanks for the reply. Good to know. So even though the battery is still working (speedo still lights up, and the electric starter still turns the engine over), the points could be welded shut? Maybe I just don't know what you you mean by the points being welded shut, however, it sounds like that would make the battery stop working... Is that correct?

 

[DesertKZ]

Nope. Battery could be fine and points welded closed. Open the cover, rotate the motor and see if they're opening. Also double triple quadruple check nothing else came loose near the battery.

 

[Ndgrimm]

Ok, I pulled off the cover, rotated the motor and the points are both opening and closing. So that's a relief.

I have been checking through all of the electricals and still can't find anything that is abnormal. I am stumped!

Everytime I try and kick over the motor it still smells a lot like gas even with the petcock turned off. Could it still be flooded? And if so, how long do you think I would have to leave the plugs out to let the gas evaporate? I thought a day and a half would be enough but maybe not...

Thanks again for your help, guys!

 

[CarbsAndCylinders]

Your bike would not be flooded that long, plugs in or out. Sounds like you have done something to the electrical system. Have you checked to see if the plugs are sparking?

 

[Reinhard]

I would double check fuses with an ohm meter, not by eye. See if you have power to your coils. Check for spark. Check all your connections at your coils and points. Check that you have power going into and out of your ignition switch, which it sounds like you do since you have lights. Check that you have spark on the points when you crank the motor over. It smells like gas because the bike's trying to start. I doubt if you have spark.

 

[Ndgrimm]

Ok, I checked to see if the plugs were sparking and it appears they aren't. I held one against a grounded piece of metal and gave it a few kicks and couldn't see anything. So I looked in the shop manual and the first thing the troubleshooting section says is that the ignition coils could be bad. So I used an ohm meter to check the resistance and got the following readout:

I think that means it's reading 7ohms. What should it be, 3? Couple questions: I tested with the ignition off, does it need to be on? It shouldn't matter since I'm just measure resistance, correct?

The only other thing to do that I forgot to last night is check the spark on the points when I crank the motor.

Again, thanks a ton for all your help, guys!

 

[Sonreir]

Measure between the black/white and the yellow (or blue). You're just measuring the core right now. Also, clean up any connectors before you do your measurement. Corrosion will affect the resistance reading.

 

[Ndgrimm]

OK here are the measurements I got. I'm still learning this whole electronics thing so I may have measure some things incorrectly...

I was getting .6 ohms just touching the black and red leads together so do the measurements have to be adjusted by that amount?

5.3 ohms between yellow/blue wires and black and white wire
.9 on 200m DCV setting (so would that be mili-volts?) when touching coil screw and ground.
.2 on 200m setting when touching yellow or blue wire and ground.
.9 ohms when touching yellow wire connection at points to yellow wire going into coils. My thought was that this would test the condensers.

I also checked to see if there was any spark at the points and there wasn't. I took a closer look at them though and it looked like there were small circles of different colored metal where they touch each other, almost like the metal was burnt. I'm not sure if this is normal from the spark that normally occurs or if it was from me being dumb and not disconnecting the battery when I was welding...

 

[Bert Jan]

points look good. 9 out of 10 you fried something. My guess is the coils.

 

[Sonreir]

Resistance readings from coils seem OK to me. Discoloration on the points is normal.

With the ignition on and the kill switch set to run, it's time to start checking voltages. You should have 12V at the black/white wire, the yellow (or blue) wire as it leaves the coils, and at the points.

 

[Reinhard]

Touching the frame of the coils only tells you if you have continuity to ground. The coils are tested by measuring resistance between the caps. The other things you are doing are either wrong or not going to tell you anything if your points aren't working. Put down the meter for a second and get back to basics.
1. file your points back down so the are square to one another. 2.Soak a business card with gas and pull it through the points. 3.Blow out the points with compressed air. Something as small as a grain of sand can make your points malfunction. 4.Reset the point gap. It does'nt matter right now what you set the gap at as long as both are the same. Use a piece of a beer can to set the gap if you don't have feelers handy.5. Put a SMALL dab of grease on the advancer shaft. 6.Make sure all of your connections are hooked up.
7.Turn on the ignition and crank the bike then tell me what happens.

 

[Ndgrimm]

Ok, I decided to go through and test the entire wiring of the bike because I wanted to actually understand everything that was going on. So I began by tracing the power from the battery all the way to the coils. Long story short, I got to testing the kill switch (which I removed when I first got the bike and just wired the black and the black/white wires together in order to 'bypass' the switch.), and noticed that everything else that was connected to the black wire was getting power perfectly, but the kill switch wasn't receiving any. So after seeing that, I decided to cut open the heat shrink wrap around the wiring from the kill 'switch' down to the closest connection that I could find which did have power. Sure enough, underneath the wrap, the black wire had shorted out but I couldn't see it until I opened up the wrap. So having found the problem, I redid the wiring and...the bike is up and running just as it was before I got in this situation.

Thanks for all your help and advice, guys! It definite helped!