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Author Topic: Dohc Cb750f: possibly a valve issue  (Read 4277 times)

Offline crazypj

  • Posts: 13223
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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: 93
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

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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

Offline SquidHunter

  • Posts: 93
Re: Dohc Cb750f: possibly a valve issue
« Reply #23 on: Aug 20, 2018, 15:13:15 »
I did get a chance to ride today. Swapped to a colder plug to no avail. Also, I checked the spark when I broke down. No spark. After letting it cool off for a minute, the bike fires up and runs fine


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Offline advCo

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Re: Dohc Cb750f: possibly a valve issue
« Reply #24 on: Aug 20, 2018, 16:12:21 »
I did get a chance to ride today. Swapped to a colder plug to no avail. Also, I checked the spark when I broke down. No spark. After letting it cool off for a minute, the bike fires up and runs fine


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Sounds like you need to get some new coils. Have you checked the primary and secondary resistances on your coils?
« Last Edit: Aug 20, 2018, 16:14:08 by advCo »
"He broke the mirrors off his Cadillac, 'cause he doesn't like it looking like he looks back."

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Offline J-Rod10

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Re: Dohc Cb750f: possibly a valve issue
« Reply #25 on: Aug 20, 2018, 17:45:07 »
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)

There is oil in the ignition side of the DOHC. Timing cover has an o-ring to keep it in there.

If you pop that cover off while on the sidestand, you're going to have a mess in your floor.

Offline crazypj

  • Posts: 13223
  • Split personality, I fake being smart
Re: Dohc Cb750f: possibly a valve issue
« Reply #26 on: Aug 20, 2018, 18:41:15 »
Sounds like you need to get some new coils. Have you checked the primary and secondary resistances on your coils?
As it's heat related you would need to 'cook' them up to temp before checking as cold/room temp readings may be OK
'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: 93
Re: Dohc Cb750f: possibly a valve issue
« Reply #27 on: Aug 20, 2018, 19:15:35 »
There is oil in the ignition side of the DOHC. Timing cover has an o-ring to keep it in there.

If you pop that cover off while on the sidestand, you're going to have a mess in your floor.

It does for sure. Not a lot, but enough to make a mess.

I just got off the phone with Gary from power arc, and then Ken from Cycle X. 🤦‍♂️

Ken says I should go back to a lead acid battery. Gary says Kens charging system is the blame. Apparently it uses a ďshuntingĒ ground regulator, so while Iím running high rpms the excessive charge is over shunting and building up heat. The ignition has a built in safety feature that shuts it down with excessive voltage/heat. 🤷🏻‍♂️

Gary is supposed to send me a link from a study that was done. Apparently there was a similar issue with an aprillia race bike......

So, in the meantime Iíve got about 1500 bucks tied up in some shit that donít work....... or does, too good


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Offline SquidHunter

  • Posts: 93
Re: Dohc Cb750f: possibly a valve issue
« Reply #28 on: Aug 21, 2018, 00:08:13 »
As it's heat related you would need to 'cook' them up to temp before checking as cold/room temp readings may be OK

So I got the link. Apparently the problem is related to the charging system Iím using from Cycle X. The regulator is a ďshuntĒ r/r, and itís shunting excessive voltage in the high rpms. This is causing the motor to run about 20 degrees hotter than normal. Supposedly a series regulator/rectifier would be the fix. Itís 175 bucks!

The ignition and charging system are both working as they should, but apparently they arenít a good match on my bike. The excessive heat is triggering a safety mechanism, built in, to prevent a meltdown. Once it cools off itís back to business.

This presents a problem.....

Looks like itís time for a new bike


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Offline crazypj

  • Posts: 13223
  • Split personality, I fake being smart
Re: Dohc Cb750f: possibly a valve issue
« Reply #29 on: Aug 21, 2018, 16:19:37 »
The charging system on CB750/900 DOHC shouldn't be using a shunt, it's supposed to 'switch off' voltage to field coils. Shunt is used on permanent magnet systems as magnetic field can't be 'switched off' or varied. Either way, it shouldn't cause motor to run 20 deg hotter
'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