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Author Topic: Dohc Cb750f: possibly a valve issue  (Read 464 times)

Offline crazypj

  • Posts: 13121
  • Split personality, I fake being smart
Re: Dohc Cb750f: possibly a valve issue
« Reply #10 on: Jul 30, 2018, 20:44:38 »
It's not valves, hard starting when cold but runs OK when hot is a sign of tight tappets on shim motors. (valve seats and faces wear faster than cam and shim so they get tight over time/mileage)The ignitors are the weak point on 750 and 900. Can't remember if they are visible from under seat or side panel but if you pull one off you may find melted potting compound? If you upgraded, what did you use? (GM ignitors?)
'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: 50
Re: Dohc Cb750f: possibly a valve issue
« Reply #11 on: Jul 30, 2018, 22:07:17 »
Iíd say petcock, you should only check valves cold. Did you set the float height? That could affect it also.

Good. In that case they are in spec. Floats arenít adjustable on the 1980.


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Offline SquidHunter

  • Posts: 50
Re: Dohc Cb750f: possibly a valve issue
« Reply #12 on: Jul 30, 2018, 22:14:53 »
It's not valves, hard starting when cold but runs OK when hot is a sign of tight tappets on shim motors. (valve seats and faces wear faster than cam and shim so they get tight over time/mileage)The ignitors are the weak point on 750 and 900. Can't remember if they are visible from under seat or side panel but if you pull one off you may find melted potting compound? If you upgraded, what did you use? (GM ignitors?)

It starts every time, first bump, when cold. Itís craps out after riding for a while. Then it wonít start again until the motor cools down for at least 30 minutes or so.

No more ignitors. Iím using the power arc ignition, on the least aggressive timing curve.

I feel like my valves should be good because my compression is good across all four cylinders. Iím wondering if Iíve got one starting to stick when the motor heats up. The more I think about tho, the less that makes sense.

How about timing. Could heat relate to any timing issues?


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Offline crazypj

  • Posts: 13121
  • Split personality, I fake being smart
Re: Dohc Cb750f: possibly a valve issue
« Reply #13 on: Jul 30, 2018, 23:01:49 »
Timing wouldn't make it cut out until it cools down.Do you have a timing light? If so, take it with you and connect up when it cuts out. no spark, found problem. It really sounds like ignition overheating and shutting down, just because it's new, doesn't mean it isn't faulty
'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: 50
Re: Dohc Cb750f: possibly a valve issue
« Reply #14 on: Jul 31, 2018, 14:43:53 »
Timing wouldn't make it cut out until it cools down.Do you have a timing light? If so, take it with you and connect up when it cuts out. no spark, found problem. It really sounds like ignition overheating and shutting down, just because it's new, doesn't mean it isn't faulty

Thanks. Iíll try that. It does seem like an issue with heat. If the ignition is cutting out when it heats up, does that mean its faulty? Or is this a common issue with electronic ignition?


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Offline crazypj

  • Posts: 13121
  • Split personality, I fake being smart
Re: Dohc Cb750f: possibly a valve issue
« Reply #15 on: Jul 31, 2018, 15:17:14 »
If ignition was designed for bike it would indicate it's faulty. If it's something adapted, it may just be in a position where it doesn't get enough airflow. Anything half decent will have a warranty.I'll have to look up Power Arc, never heard of it.
« Last Edit: Jul 31, 2018, 15:20:13 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: 50
Re: Dohc Cb750f: possibly a valve issue
« Reply #16 on: Jul 31, 2018, 15:47:06 »
If ignition was designed for bike it would indicate it's faulty. If it's something adapted, it may just be in a position where it doesn't get enough airflow. Anything half decent will have a warranty.I'll have to look up Power Arc, never heard of it.

Itís an adaptation. Cycle X sells it


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Offline SquidHunter

  • Posts: 50
Re: Dohc Cb750f: possibly a valve issue
« Reply #17 on: Jul 31, 2018, 16:00:55 »
If ignition was designed for bike it would indicate it's faulty. If it's something adapted, it may just be in a position where it doesn't get enough airflow. Anything half decent will have a warranty.I'll have to look up Power Arc, never heard of it.

Or perhaps the plugs are getting too hot 🤷🏻‍♂️. Iím running iridium plugs. Never had an issue before, but maybe switching to a colder plug would help


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Offline crazypj

  • Posts: 13121
  • Split personality, I fake being smart
Re: Dohc Cb750f: possibly a valve issue
« Reply #18 on: Jul 31, 2018, 17:43:56 »
Nope. Plugs overheating would cause pre-ignition, knocking and banging. possible piston damage.  Iridium plugs are best as a fit and forget, they don't need changing for 100,000 miles. Unless your doing high mileage touring they are most likely a waste of money in a bike. The 'best' I ever use are NGK platinum 'race' plugs. Are you using carbon plug leads or stranded copper wire? Carbon leads only last a couple of years before they need changing. When possible I always use stranded copper with silicon insulation but you must use correct resistance plug caps with them or you get wrong reverse EMF back to ignition control unit (in the 300vdc range on a 12v system) The iridium plugs will also have built in resistors, probably 4.5~5 K-Ohm so you need around another 5~7 K-Ohm in leads or caps 
Severely unimpressed with the Cycle-X ignition page. Any time you have to use a smaller plug gap it tells me ignition is a 'low' voltage unit (around 20,000v, probably less?.) Modern ignition system (even a multi spark) should be in the 35~50,000v range and use a wider gap. The other thing is, you must use carbon resistor leads? ? ?
« Last Edit: Jul 31, 2018, 17:58:02 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: 50
Re: Dohc Cb750f: possibly a valve issue
« Reply #19 on: Jul 31, 2018, 20:23:25 »
Nope. Plugs overheating would cause pre-ignition, knocking and banging. possible piston damage.  Iridium plugs are best as a fit and forget, they don't need changing for 100,000 miles. Unless your doing high mileage touring they are most likely a waste of money in a bike. The 'best' I ever use are NGK platinum 'race' plugs. Are you using carbon plug leads or stranded copper wire? Carbon leads only last a couple of years before they need changing. When possible I always use stranded copper with silicon insulation but you must use correct resistance plug caps with them or you get wrong reverse EMF back to ignition control unit (in the 300vdc range on a 12v system) The iridium plugs will also have built in resistors, probably 4.5~5 K-Ohm so you need around another 5~7 K-Ohm in leads or caps 
Severely unimpressed with the Cycle-X ignition page. Any time you have to use a smaller plug gap it tells me ignition is a 'low' voltage unit (around 20,000v, probably less?.) Modern ignition system (even a multi spark) should be in the 35~50,000v range and use a wider gap. The other thing is, you must use carbon resistor leads? ? ?

Iím using what they sent me. Itís resistor plug caps for sure, and moroso wires. Seems to be high quality wires. I did gap the plugs at .22.

Thatís the only heat related problem I could find on power arcs website.

Still havenít had a chance to ride again, but next time Iíll pull the plugs and check for spark. It still has me scratching my head.

This is my first street bike, and definitely the first time Iíve dove into mechanics of a motorcycle. Iíve learned a lot, but I still feel like a total newb when something goes wrong. I appreciate the input.


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