Topic: Ignition timing on a DOHC750, 1980. NEED HELP.

[BenelliSEI]

So...... my son and a friend dragged home a very sad, 1980 DOHC CB750. Not my favourite bike, but I did the carbs. It’s running at idle, carbs sync’d nicely, but not quite right. So I put my timing light on it and cyl. 1&4 are spot on. 2&3 are wildly retarded. HOW DO I ADJUST THEM WITHOUT DISTURBING 1&4? Thanks, appreciate the advice.

[Tracksnblades1]

Oops, 1980 weren't for perfectionist were they....?
Manual just instructs you to time cylinder #1/4 static.
Then again at 3100 rpm for full advance....😎
No provisions for individually timing 2-3 was there.....?
😩

[Tracksnblades1]

Have you experimented with moving 2/3 pickup in/out or up/down within its hole clearance....?

[scottly]

If the 2-3 timing is "wildly" off, it may be that the pickup coil polarity is reversed. Each pickup coil has one wire with a white sleeve near the plug end as a marker.

[Tracksnblades1]

If the 2-3 timing is "wildly" off, it may be that the pickup coil polarity is reversed. Each pickup coil has one wire with a white sleeve near the plug end as a marker.

Wouldn't a reversed polarity here increase the gate strength...?

[scottly]

If the 2-3 timing is "wildly" off, it may be that the pickup coil polarity is reversed. Each pickup coil has one wire with a white sleeve near the plug end as a marker.

Wouldn't a reversed polarity here increase the gate strength...?

I don't know WTF you mean by "gate strength", but I know if the pickup polarity is reversed it Will affect timing.

[BenelliSEI]

Thanks for the suggest, today I’ll start by checking wiring. This bike has been chopped around a lot. The timing on 2&3 is far enough out that I can barely see the marks at idle. I’m guessing it 15-20 degrees BTDC when 1&2 are on the mark. I’ll see if there is movement in either pick up.

[BenelliSEI]

Swapped out coils, spark boxes and checked all the wiring. Conclusion is the #2/3 pulsar (black trigger box on ignition plate at crank rotor) has a mind of it’s own and fires whenever it feels like it! The boys ordered a Dyna System and we’ll just replace everything.....