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Cylinders 1&4 not firing

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broot

Cylinders 1&4 not firing

Hey folks. My brother's 1980 cb750k recently started dying when it was idling. Upon closer inspection it appears cylinders 1&4 aren't firing. After looking at the owner's manual we thought it may have been the ignition coil which connects to these two cylinders. Checked resistance across both primary and secondary and they seemed good (roughly 24 ohms on primary side and 13k on secondary). When we have the bike going we get no voltage on the secondary side of the suspect coil and a high voltage on the secondary of the good coil. The CDIs were recently replaced and when we swapped them around we still had hot 2&3 and ice cold 1&4. Is it possible the ignition coil is toast despite good resistance measurements? Could it be the pulse generator? Any help would be greatly appreciated!

TOOLS1

Re: Cylinders 1&4 not firing

Administrator

Yes, and yes to both questions. However I would suspect the pulse generator.
TOOLS

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1976 CB 750-A X 2
1977 CB 750-A X 4
1977 CB 750-K
1976 CB 750 F
1981 CB 750
1966 Kawasaki SG 250
1981 KZ 750 LTD
1973 CB 350
1979 CM 185 Twinstar
1982 Honda XL 80
South of Eden (Kansas City MO)

broot

Re: Cylinders 1&4 not firing

Thanks for the quick reply! Luckily we've got a parts bike to swap the coil and pulse generator out of. That pulse generator looks like a bugger to get at!

Lucky 1

Re: Cylinders 1&4 not firing

In reply to this post by TOOLS1

The CB750 does not have a pulse generator(magneto).
Some Hondas like the VF1000R V4 does have a pulse generator (magneto).
The CB750 has an alternator that makes 1/2 wave ac
and the rectifier makes it into 12volts DC.

On a Roadstar Adventure.

Lucky 1

Re: Cylinders 1&4 not firing

In reply to this post by broot

What are you talking about?

Which side of the bike?

IF you had a book you could find out the name of
the parts before you take apart the wrong parts.

On a Roadstar Adventure.

broot

Re: Cylinders 1&4 not firing

This post was updated on .

The book I have has a wiring diagram for the cb750k. On this wiring diagram the pulse generator is wired into the spark unit and the spark unit is wired into the ignition coil. The wiring diagram I have looks more or less like this one:

http://schematicwiring.com/wp-content/uploads/2010/10/Honda-CB750KDOHC-Electrical-wiring-diagram.jpg

Rodgil

Re: Cylinders 1&4 not firing

The pulse generator disconnects at the oil pressure switch (on top of the gearbox, below the carbys), at the neutral switch (next to the front sprocket) and at the connector near the starter solonoid, next to the battery. You can test the pulse generator at this connector, see the service manual available on this site.

1982 CB750F
1978 Triumph Tiger 750
197? Yamaha DT175
197? Hodaka ACE 100

broot

Re: Cylinders 1&4 not firing

Thanks!

broot

Re: Cylinders 1&4 not firing

In reply to this post by Rodgil

Checked the resistance across the yellow and blue wires coming from the pulse generator. Got a good reading on the yellow and an open on the blue. The blue connect to cylinders 1&4 so this would make sense. Looks like we may have found the problem.