Timing on 1/4 not showing

[KJO750]

This feels like another “Weak Spark” help thread but I’ll go ahead anyway because Ive ran out of knowledge.

In short I’m having trouble with my 1980 cb750f not running on all four cylinders. 2/3 seem to get to temp pretty quickly while 1/4 are cold.

So far I’ve tested :
-Primary and secondary resistance on coils - within spec for 3ohms coils.
-Swapped coils to cylinders
-12v to Black / white wire when ignition on. Yellow on RHS blue on LHS (while sitting on bike)
-Swapped cdi spark packs with each other and with another set of known working ones. Also tested 12v to 0 test in workshop manual. Mine test drop to 1v not 0 ?
- Tested resistance across yellow and blue wires on pickup harness 530 ohms on each
-Sprayed Aerostart product into 1 and 4 with now bang.
- all 4 Plugs have blue spark when grounded on rocker cover bolts
-D8ea plugs with new leads and Ngk 5ohm resistor caps
Bike is running brand new rs34 balanced mukinis

I can find timing Marks for 2/3 when timing light is connected to 2/3 leads. I cannot find 1/4 mark when connected to 1/4 - it’s not even close. I only screw/spring on auto advance unit .

With a little throttle assistance ( cracked open a little) I can get bike to run 2/3 but then dies as soon as let go.

Mukinis were purchased from Mukini OZ and are jetted for dohc cb750f

I’ve searched and searched for previous help that everyone here has previously posted so I apologise if this is another one of the same threads.

Any help or instruction? I’m happy to learn

 

[KJO750]

Forgot to mention I have compression tested motor and got 150 140 145 150 psi across cylinders

 

[amc49]

If you are saying the advance unit is only sprung on one weight then you need that fixed, the spark on the missing spring will scatter all over the place.

The pickups need to have the same close gap too or one won't read accurately. Closer is better but you absolutely cannot make contact there.

 

[KJO750]

Thankyou. I actually haven’t noticed if the advance unit is sprung on both sides. I’ll look at that this morning and report back.

 

[KJO750]

If you are saying the advance unit is only sprung on one weight then you need that fixed, the spark on the missing spring will scatter all over the place.

The pickups need to have the same close gap too or one won't read accurately. Closer is better but you absolutely cannot make contact there.

I’ve checked the advance unit and both springs are accounted for and working.

How else is it possible that I see 2/3 timing marks with light and not 1/4? Timing light is firing correctly on those leads.

I will re check TDC on 1 at compression. Other that that I don’t understand how it can be so far out?
Thanks again

 

[KJO750]

Problem Solved!

When I relocated CDI unit I have inadvertently wired my extension of the blue pulse wires around the wrong way. Through investigation of why I was roughly 90 degrees out on 1/4 while 2/3 were spot on is what led me to re check my work. I’m posting this in case it can help others.
Thanks amc49 for your input

 

[amc49]

I almost said that but very rare to encounter it so said nothing. The two wires off the pickup have to be right, one way (wrong) throws timing off a whopping amount. Never checked it on these bikes but on Ford car electronic ignition it retarded by 25 degrees, dunno why that is stuck in my head all these years though.

Congratulations on following rule #1 for trouble, or , ALWAYS suspect your own work. It is not a sign of weakness, rather the mark of a detailed worker. Most cannot go there and why so many things stay broke.