Dohc Cb750f: possibly a valve issue

SquidHunter

Member
I’ve got an issue that I can’t seem to put my finger on. After riding the bike for a while, and it warms up, it will sputter and die. It won’t start back up until the bike cools down. I’m thinking I might have a valve issue. Anyone experience this?


Sent from my iPhone using DO THE TON
 

1fasgsxr

New Member
My guess would be a fuel or fire issue. Im not familiar with the 750 ignition system but I would start there and carbs or petcock.
 

SquidHunter

Member
1fasgsxr said:
My guess would be a fuel or fire issue. Im not familiar with the 750 ignition system but I would start there and carbs or petcock.
I upgraded the ignition, and charging system. I’ve completely cleaned and rebuilt the carbs, the right way. Plugs and wires and electrical wiring all new. Freshly sealed gas tank and new petcock...... but, the petcock is eBay. I’m suspicious of the petcock. I do get flow from it, but it’s not a steady flow. It’s more like a chugging flow. Good then weak then good and weak again.

You really think that could be it? Seems like as long as the bowls stayed full it would be fine. Also, it will run fine until I get in traffic and the motor heats up. The valves seem to be in spec, but I’ve only checked when the motor was cold. Should I be checking with the motor warm?


Sent from my iPhone using DO THE TON
 

SquidHunter

Member
Also, it won’t start at all until after it cools down. Then it fires right up and seems to be fine


Sent from my iPhone using DO THE TON
 

SquidHunter

Member
Same with a cold start. Fires straight away with choke up. Doesn’t give me any problems until the motor gets hot.


Sent from my iPhone using DO THE TON
 

adventurco

Nick Ol' Eye
DTT BOTM WINNER
Check your fuel cap and make sure the vent is clean and allowing air to flow. Sounds like the carbs are starving of fuel once they start trying to draw it down after riding for a few minutes. Once you the bike shuts off and cools, the bowls will eventually fill back up and itll start up again.

Had the same exact issue on the XL350 with an aftermarket fuel cap.

I'd bet this is your problem.
 

SquidHunter

Member
advCo said:
Check your fuel cap and make sure the vent is clean and allowing air to flow. Sounds like the carbs are starving of fuel once they start trying to draw it down after riding for a few minutes. Once you the bike shuts off and cools, the bowls will eventually fill back up and itll start up again.

Had the same exact issue on the XL350 with an aftermarket fuel cap.

I'd bet this is your problem.
For sure not the fuel cap. I thought of this also. Pulled the cap and drilled a hole all the way through the top (it needs to be replaced anyway), just to check. Same issue. If it’s starving for fuel, it’s gotta be the petcock. The coils and ignition are all brand new. I just upgraded to an electronic ignition. Really strong spark.

The petcock they sent me is totally different than the stock, that’s why I’m betting on that. Especially seeing the fuel flow so erratically.


Sent from my iPhone using DO THE TON
 

adventurco

Nick Ol' Eye
DTT BOTM WINNER
SquidHunter said:
For sure not the fuel cap. I thought of this also. Pulled the cap and drilled a hole all the way through the top (it needs to be replaced anyway), just to check. Same issue. If it’s starving for fuel, it’s gotta be the petcock. The coils and ignition are all brand new. I just upgraded to an electronic ignition. Really strong spark.

The petcock they sent me is totally different than the stock, that’s why I’m betting on that. Especially seeing the fuel flow so erratically.


Sent from my iPhone using DO THE TON

Sounds like it, you should have steady flow out of the petcock or something is plugged up or not flowing correctly.
 

SquidHunter

Member
Thanks for the reply’s! This has me feeling much better. I’ll replace the petcock with a proper Honda unit


Sent from my iPhone using DO THE TON
 

esmoojee

New Member
I’d say petcock, you should only check valves cold. Did you set the float height? That could affect it also.
 

crazypj

Split personality, I fake being smart
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?)
 

SquidHunter

Member
crazypj said:
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. It’s craps out after riding for a while. Then it won’t start again until the motor cools down for at least 30 minutes or so.

No more ignitors. I’m 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. I’m wondering if I’ve 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?


Sent from my iPhone using DO THE TON
 

crazypj

Split personality, I fake being smart
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
 

SquidHunter

Member
crazypj said:
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. I’ll 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?


Sent from my iPhone using DO THE TON
 

crazypj

Split personality, I fake being smart
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.
 

SquidHunter

Member
crazypj said:
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.
It’s an adaptation. Cycle X sells it


Sent from my iPhone using DO THE TON
 

SquidHunter

Member
crazypj said:
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 ‍♂. I’m running iridium plugs. Never had an issue before, but maybe switching to a colder plug would help


Sent from my iPhone using DO THE TON
 

crazypj

Split personality, I fake being smart
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? ? ?
 

SquidHunter

Member
crazypj said:
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? ? ?
I’m using what they sent me. It’s resistor plug caps for sure, and moroso wires. Seems to be high quality wires. I did gap the plugs at .22.

That’s the only heat related problem I could find on power arcs website.

Still haven’t had a chance to ride again, but next time I’ll 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 I’ve dove into mechanics of a motorcycle. I’ve learned a lot, but I still feel like a total newb when something goes wrong. I appreciate the input.


Sent from my iPhone using DO THE TON
 
Top Bottom