Dohc Cb750f: possibly a valve issue

crazypj

Split personality, I fake being smart
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)
 

SquidHunter

Member
crazypj said:
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. I’ve got to rewire everything again anyway, so I’m sure I’ll 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. There’s only one way that oil can get in, and I can fix that. I’ll have to ride it some more and recreate the problem. Hopefully it’s not a faulty unit, but it should still be under warranty. If not..... lesson learned and on to the next.


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crazypj

Split personality, I fake being smart
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)
 

SquidHunter

Member
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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adventurco

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SquidHunter said:
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?
 

J-Rod10

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crazypj said:
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.
 

crazypj

Split personality, I fake being smart
advCo said:
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
 

SquidHunter

Member
J-Rod10 said:
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 “shunting” ground regulator, so while I’m 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 I’ve got about 1500 bucks tied up in some shit that don’t work....... or does, too good


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SquidHunter

Member
crazypj said:
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 I’m using from Cycle X. The regulator is a “shunt” r/r, and it’s 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. It’s 175 bucks!

The ignition and charging system are both working as they should, but apparently they aren’t a good match on my bike. The excessive heat is triggering a safety mechanism, built in, to prevent a meltdown. Once it cools off it’s back to business.

This presents a problem.....

Looks like it’s time for a new bike


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crazypj

Split personality, I fake being smart
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
 

SquidHunter

Member
crazypj said:
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
I’m running Cycle X’s charging system. It’s the same as a Norton. It’s a shunt R/R. Apparently it’s fine until the battery is fully charged and the rpms are high. It’s running hotter for sure.

Worst case scenario, I’ll just go back to stock.


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themotoworks

New Member
I've had this problem on a few bikes, take a look at the plugs when it craps out. I had one running so rich at one range that if I was in traffic running in that range long enough, it would start losing idle, only run when the throttle was over 1/4 open, and eventually die alltogether, let it cool down completely, it would fire back up. once I realized what was going on, I retuned it and got it a lot leaner on idle and just a bit above, ran fine afterwards
 

SquidHunter

Member
themotoworks said:
I've had this problem on a few bikes, take a look at the plugs when it craps out. I had one running so rich at one range that if I was in traffic running in that range long enough, it would start losing idle, only run when the throttle was over 1/4 open, and eventually die alltogether, let it cool down completely, it would fire back up. once I realized what was going on, I retuned it and got it a lot leaner on idle and just a bit above, ran fine afterwards
When I originally wired up the ignition system, I was running way too rich. I ended up going back to the stock jetting, because it fouled my plugs.

I’ve pulled the plugs when it happens, and if anything they are showing lean. The problem is they aren’t getting spark. After it cools down I’ve got plenty of spark. It’s a safety mechanism that’s being tripped in the ignition.

Here’s a copy of the link that power arc sent me. It’s pretty interesting
http://www.bayarearidersforum.com/forums/showthread.php?t=394141&page=4



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SquidHunter

Member
Spoke with Ken at Cycle X today. Super solid guy. He’s going to ship me a series R/R. If that doesn’t fix it, then we will know without a doubt it’s the ignition.

The theory, according to Power arc, is this will cool down the charging system. Hence cooling down the motor. We’ll know within about a half hour of riding when it arrives. Fingers crossed


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VintageMBike

New Member
SquidHunter said:
Spoke with Ken at Cycle X today. Super solid guy. He’s going to ship me a series R/R. If that doesn’t fix it, then we will know without a doubt it’s the ignition.

The theory, according to Power arc, is this will cool down the charging system. Hence cooling down the motor. We’ll know within about a half hour of riding when it arrives. Fingers crossed


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I bought that same Cycle X setup for my 82 Honda cb 900F (after reading one of your posts in this forum). No offense to the nice guys at cycle x, but the advertising was misleading: NEW POWERFUL HONDA CHARGING SYSTEM WE DEVELOPED" when it was really Norton Brit Bike technology.

I havent had any issues with the Cycle X charging system, and I know 900f's run a bit hot anyway, but time will tell. Fingers crossed I dont have any issues.
 

irk miller

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VintageMBike said:
I bought that same Cycle X setup for my 82 Honda cb 900F (after reading one of your posts in this forum). No offense to the nice guys at cycle x, but the advertising was misleading: NEW POWERFUL HONDA CHARGING SYSTEM WE DEVELOPED" when it was really Norton Brit Bike technology.
I don't see where you got your quote from on their DOHC charging system page, but they can still say they developed the system for the DOHC Hondas even if they are using "Norton Brit Bike technology." The Wassell part of the equation is the way they encase the windings, not in the technology. If we want to get technical, they are using Hippolyte Pixii technology. Regardless, according to your quote, they say they "developed" the system, not "invented" or "patented" the system. Developing technology is when you take existing tech and adapt it , ie. make them for Hondas.
 

teazer

Active Member
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I didn't know he was the one who invented the permanent magnet alternating current generating system. Learn something new every day.

Isn't it just a Lucas RM23 (or whatever the model number is) that we were fitting to our Trumpets and Beezas all those years ago? Admittedly I would never have though of fitting one to a Honda, but maybe I lack imagination.

Lucas used to dump surplus power as heat through a single Zener diode with a big heat sink. I'm guessing that's what they still do in maybe a more sophisticated package of some sort.
 

crazypj

Split personality, I fake being smart
kopcicle said:
fsck'n Honda iggy bits
You've probably seen hundreds with the guts melted out, I know I've seen a few dozen
teazer said:
I didn't know he was the one who invented the permanent magnet alternating current generating system. Learn something new every day.

Isn't it just a Lucas RM23 (or whatever the model number is) that we were fitting to our Trumpets and Beezas all those years ago? Admittedly I would never have though of fitting one to a Honda, but maybe I lack imagination.

Lucas used to dump surplus power as heat through a single Zener diode with a big heat sink. I'm guessing that's what they still do in maybe a more sophisticated package of some sort.
Didn't Tesla invent AC generator? I wouldn't think of fitting an old LUCAS system on CB900 either. I doubt it's something 'clever', probably still a zener diode but without the heatsink?. I never had any real issues with zeners, vibration always broke the terminal pin before they burned out or did anything nasty which totally sucked as the zener was always fine but impossible to get a wire back on 99% of time
 

irk miller

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crazypj said:
You've probably seen hundreds with the guts melted out, I know I've seen a few dozen

Didn't Tesla invent AC generator?
No, Hippolyte Pixii did in 1832, more than 30 years before Tesla was even born.

 
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