Topic: 1973 CB750K won't start

[drewgooo]

Hey everyone,

This would be my first post here and I would REALLY appreciate any help, guidance, or suggestions that anyone has to offer!

I bought a 73 CB750K about a month ago and have been having a hell of a time diagnosing what my issue is and why this engine will not fire. I was able to ride it for about two weeks after purchasing the bike and then, out of the blue, it shut off and haven't been able to get it fired up since.

I have rebuild the carbs, tested for compression, replaced the plugs and wires, tested the coils..all the basic stuff.

The kickstart is does not return when kicked and it will lock up after one or two kicks but I am able to turn the crankshaft manually just fine...I'm thinking that the spindle might be a little bent but not sure why it locks up.

The electric start works great..other than the fact that the bike won't actually start. I know I've got a spark, compression and gas... I have hit a wall a figured it was time to reach out to the pros lol.

Thanks in advance to any suggestions or tips!!!

-Drew

[70CB750]

Does the spark fire on compression - in another words, is your timing correct or 180 degrees off?

Had me puzzled on 2 stroke engine once 

[70CB750]

Ha  the Kill switch was my first thought, but he says there is a spark.

[drewgooo]

Thank you for the reply calj737.

I have 12.5v at the BLK/WHT wire at the coils with key in the ON position. Coils are sparking and I apologize for not being a bit more detailed. The motor cranks but will not catch. It has backfired once but aside from that, it just cranks without any indication of spark.

What has me puzzled is that I've pulled all the plugs and verified that they are all generating a nice blue spark.

I have confirmed flow to the carbs but have not confirmed to the plugs. How would I check that fuel is getting to the plugs?

70CB750: I was HOPING that was my problem but no such luck. timing is set correctly.

[madmtnmotors]

Blue points wire to coil serving 1/4? Yellow points wire to coil serving 2/3? Have you static timed the ignition? If so, using what method?

[70CB750]

After cranking it, the plug has to be wet.

[drewgooo]

Ok, I will definitely check that in a few hours. My neighbors don't seem to appreciate the sound of a struggling engine at 6am lol.

I have not static timed the ignition. I may have spoken a little early regarding the timing..I have been assuming its alright just due to the fact that it was running well prior to all this...and yes, blue to 1/4 and yellow to 2/3.
 
Would there happen to be a link or something like that describing how to check the timing is set correctly?

Thank you everyone

[madmtnmotors]

It is described in the manuals, available here:

http://manuals.sohc4.net/cb750k/


There are also many threads about it here on the forum. Static timing is explained in great detail from many sources:

http://bfy.tw/Vte

[drewgooo]

I appreciate the links! I'll check back in a little later once I can verify my timing is correct.

Thanks!!

[drewgooo]

ha wow..i feel a bit stupid now.

That second link is pretty funny though lol.

[madmtnmotors]

ha wow..i feel a bit stupid now.

That second link is pretty funny though lol.

Good to know you brought your sense of humor, you'll need it from time to time around here! 

These motors are pretty basic, most problems boil down to properly metered fuel or correctly timed spark (I usually focus on spark first, unless the carbs have been sitting for a year or more). They are pretty tolerant of abuse and neglect, but they can try your patience attempting to revive them! 

[madmtnmotors]

I usually use an ohmmeter to static time the ignition. More often than not, dynamic timing proves my static timing to be spot on.

Before you start, did you remove the points cam from the advancer shaft? If so, was it oriented properly when reinstalled? I'm not sure if these can be inserted on the advancer shaft 180 degrees out, but I know the Dyna trigger can be, so I'm guessing the points cam can be installed incorrectly as well. If you did not remove it, and the bike was running before, then this is a non issue.

Static timing:

>Ignition off
>Spark plugs removed. This prevents having to fight engine compression while making adjustments
>Connect the ohmmeter (or continuity tester) to opposite sides of the 1/4 point assembly. Closed will read continuity (or some low ohms value if using an ohmmeter), open will show no continuity (or "1" or infinity depending on type of ohmmeter used)
>Align the 1/4 "F" mark with the timing mark on the case by rotating the crank clockwise using the BIG nut. Loosen the three screws attaching the points plate to the engine.
>Rotate the entire points plate until the points begin to open (thus signaling the coil to "fire"). Tighten down the points plate.
>Rotate the crank a couple of times and verify that the points are opening at the moment the "F" mark aligns with the timing mark.
>Now align the 2/3 "F" mark with the timing mark on the case by rotating the crank clockwise using the BIG nut.
>Connect the ohmmeter (or continuity tester) to opposite sides of the 2/3 point assembly. Closed will read continuity (or some low ohms value if using an ohmmeter), open will show no continuity (or "1" or infinity depending on type of ohmmeter used)
>Now loosen the 2/3 SUB PLATE. Not the screws for adjusting the gap, but the screws holding the sub plate that the 2/3 points are attached to.
>Rotate the 2/3 SUB PLATE until the points begin to open (thus signaling the coil to "fire"). Tighten down the 2/3 points sub plate.
>Rotate the crank a couple of times and verify that the points are opening at the moment the "F" mark aligns with the timing mark.

Now put the plugs back in, connect the spark plug wires, turn on the fuel, make sure the kill switch is in the "run" position, and fire it up!


EZPZ

[drewgooo]

Just got it fired up.

It seems that the screw holding point was loose on the 2/3 coil so it was just staying closed.

THANK YOU all for the replies..It most likely would have taken me another few weeks of banging my head against the wall before I even would have thought to check those things.

[madmtnmotors]

Very cool. I should have included "check point gap first", but if you run down the 3000 mile tune up procedures, it would have been covered. Sure feels good when they respond to your efforts!

I, too, have had the points come loose leading to a poor or non-running condition. The last time it happened I had actually forgot to tighten down the main points plate. The main points plate needs to be securely fastened to maintain a proper ground connection from the engine. As I was riding the engine began to sputter and stall. By the time I had coasted to a stop I realized what I had forgotten (just completed a tune-up), so I reached for the number 2 JIS screwdriver and was back on the road in less than a minute! 

[70CB750]

Excellent!

Just got it fired up.

It seems that the screw holding point was loose on the 2/3 coil so it was just staying closed.

THANK YOU all for the replies..It most likely would have taken me another few weeks of banging my head against the wall before I even would have thought to check those things.