Topic: 1978 CB750F Dyna S timing issue

[majorzozo]

Hi all,

I have a 78 CB750F with about 100 miles since restoration. It has a Dyna S ignition.  It was timed statically and dynamically at full advance.  Per the instructions the "F" mark was not used only the advance marks.  In an effort to improve the idiling I hooked up a timing light to the #4 lead and while shining the light in the window at 1200 RPM it bounced between 1-4 and 2-3.  How could the 2-3 be showing in the window with the light attached to the #4 lead only?   At 2500 RPM with the light it was spot on for the 1-4 advance marks.

Other than being cold blooded the bike runs good at all RPM.s but how can I be getting the 2-3 and the 1-4 in the window (bouncing between the two) when the light is only hooked up to the 4 lead?  Bizarre. 

And again almost every direction I have viewed has you time the Dyna to the advance marks with not even mentioning to check the "F" mark at idle.

 Not sure on what to check other than maybe the common power wire connection.  Would going back to points provide a better idle?  Any advice is appreciated and thanks in advance..       
   


 
http://forums.sohc4.net/index.php/topic,191852.0.html

[Bodi]

If it was actually firing the "other" pair as well as the one wanted it would probably be spitting fire out of the carbs. Possibly your timing light is just too sensitive and picking up the other coil?
I don't know if the idle would be better with points. You should set the static timing at F and then reset to the advance mark with the engine at a high enough RPM to be at full advance. This is easily seen with a timing light, the marks shift as rpm rises then sit steady at full advance somewhere around 3000rpm. The low rpm setting isn't critical, doesn't have to be at F. You don't want to over advance though, engine damage can happen.

[newday777]

It sounds like the advancer shaft has been bent from someone turning the motor with the 17mm nut on the shaft while the plugs were in the motor and the compression in the motor took enough force to turn the motor that the shaft was bent. You can see it if you put a dial indicator on the shaft(without the plate and advancer on the shaft) HondaMan says if it has over .006 movement you have to straighten the shaft. It can be done by hitting the nut with little taps of a hammer  and spinning the motor slowly to find the high spot with the dial indicator. I've straightened 2 shafts this way that were out. One I got to .000, the other to .002 and it made a good improvement in idle and no bounce in timing light.

[HondaMan]

Yeah, what Newday said ^^^^ .

[PeWe]

Idle will work fine with both  advancer springs cut.
Just cut the loop off holding the advancer tower. Bend up a new from the outer wound. It can look good with help of 2 pliers.

I usually set the Dyna-S in middle position, start the engine and let it rev 1100-1200rpm which is the Idle I prefer.
You will now see that idle is stable with cut springs.
Timing light sensor on cyl 4 lead, adjust so F line up with case mark.
Check 2:3 and see it will match with F.
Full advance around 2700rpm.

If the advancer has a play when the small nut is not tightened.
Set it in most counterclockwise direction and tighten it.
This is where TDC will match T on advancer. F will then be correct.
Check ignition again, you will see that it will run even better when it is not too advanced any more.

Both my CB750's are like this.
Both have real TDC checked twice with piston stop and before that measure on piston with head off.

Both have Dyna-S and cut adv springs.
- K6  has Hitachi AD125-01 advancer (970cc , 107.5whp at last dyno Oct 2022, a smooth power curve)
-K2 has TEC 300 advancer
(almost stock, Cruzinimage K7/8 +0.50 pistons + No numbers 4-4. 58-59 whp. Might have a little better power curve now with clean carbs and K&N filter in stock box)

(Both have used points before.)
This is the way to get it run well from idle to top with smooth take offs

Uneven idle now is the carb sync.
Smooth running engine from idle to 3000 rpm (and up) make the ride much more comfortable.

Not fun with jerky engine on low revs . City riding and legal speeds.

[majorzozo]

Thank you all so much.  Hopefully I can get some time this weekend to check and adjust.  All of this makes sense and I will report back with the "cure".  Thanks again.   

[majorzozo]

Well sure enough the advancer shaft run out was .018.  Lightly brass hammered to .002.  When started didnt bounce around as much but still getting the 2-3 in the window sometimes with the lead on #4.  When I slid the lead closer to the spark plug connection no more 2-3.  Cheap timing light .  Re-syncd the carbs and runs alot better but will try tightening the springs as  suggested in an effort to get even better.  Like an idiot I stripped one of the carb top attach holes so will need to heli-coil.  Out of town for the next 3 weeks so looking forward to getting back on it.

Thank you all so much and Happy Holidays...