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Author Topic: Dohc Cb750f: possibly a valve issue  (Read 361 times)

Offline crazypj

  • Posts: 13118
  • Split personality, I fake being smart
Re: Dohc Cb750f: possibly a valve issue
« Reply #20 on: Jul 31, 2018, 20:48:06 »
I'm still kooking through Power Arc and Cycle-X (in between other stuff) I would say a wire has cracked or maybe circuit board and when things heat up the expansion is enough to create an open circuit. Honda had similar problem in 1978 where bikes would run when cold or over 4,000 rpm but if revs dropped they would not re-start. I have no idea how many bikes were recalled, it was a lot.
Hopefully your still within the 6 month warannty? PA website is pretty worthless, 'technical information' isn't. Basically it's an optical trigeer capacitor discharge system. Sounds real good and has a ton of adjustability / programmability, but, self contained units always have problems from heat and vibration. Optical sensors can also suffer from dirt/dust if not properly sealed (shouldn't be a problem on DOHC 750/900)
« Last Edit: Jul 31, 2018, 21:19:07 by crazypj »
'you can take my word for it or argue until you find out I'm right'
Best thing I ever overheard
"yep, PJ's my boss, he taught me everything I know, just didn't teach me everything he knows"
Brian Morgan, 1982

CB360's,  http://www.dotheton.com/forum/index.php?topic=11736.0
XS650,  http://www.dotheton.com/forum/index.php?topic=11922.0

Offline SquidHunter

  • Posts: 46
Re: Dohc Cb750f: possibly a valve issue
« Reply #21 on: Jul 31, 2018, 21:35:37 »
I'm still kooking through Power Arc and Cycle-X (in between other stuff) I would say a wire has cracked or maybe circuit board and when things heat up the expansion is enough to create an open circuit. Honda had similar problem in 1978 where bikes would run when cold or over 4,000 rpm but if revs dropped they would not re-start. I have no idea how many bikes were recalled, it was a lot.
Hopefully your still within the 6 month warannty? PA website is pretty worthless, 'technical information' isn't. Basically it's an optical trigeer capacitor discharge system. Sounds real good and has a ton of adjustability / programmability, but, self contained units always have problems from heat and vibration. Optical sensors can also suffer from dirt/dust if not properly sealed (shouldn't be a problem on DOHC 750/900)

I was just reading through it as well. I would place money on a wire losing voltage somewhere. Iíve got to rewire everything again anyway, so Iím sure Iíll find something there. I had to do a temporary job, to get it home. However, it should have been good enough to finish out the riding season. At least that was the hope.

Another theory is oil may be getting on the optical sensor. I did notice a drip from where the wire comes out. Thereís only one way that oil can get in, and I can fix that. Iíll have to ride it some more and recreate the problem. Hopefully itís not a faulty unit, but it should still be under warranty. If not..... lesson learned and on to the next.


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Offline crazypj

  • Posts: 13118
  • Split personality, I fake being smart
Re: Dohc Cb750f: possibly a valve issue
« Reply #22 on: Aug 01, 2018, 13:01:47 »
If I remember right there shouldn't be any oil in the ignition side of CB750DOHC (I haven't worked on them since 1980's) Even so, something disrupting optical sensor would be a 'permanent' problem and not something that only happens when hot.You can try cleaning trigger unit. I didn't see any mention of trigger type (IR, UV, red LED, visible, etc). You don't have any stress fractures from overtightened mounting screws? (PCB may have 'white' area's around screws)
'you can take my word for it or argue until you find out I'm right'
Best thing I ever overheard
"yep, PJ's my boss, he taught me everything I know, just didn't teach me everything he knows"
Brian Morgan, 1982

CB360's,  http://www.dotheton.com/forum/index.php?topic=11736.0
XS650,  http://www.dotheton.com/forum/index.php?topic=11922.0