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Author Topic: Dohc Cb750f: possibly a valve issue  (Read 2767 times)

Offline SquidHunter

  • Posts: 89
Dohc Cb750f: possibly a valve issue
« on: Jul 30, 2018, 13:09:12 »
Iíve got an issue that I canít seem to put my finger on. After riding the bike for a while, and it warms up, it will sputter and die. It wonít start back up until the bike cools down. Iím thinking I might have a valve issue. Anyone experience this?


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Offline 1fasgsxr

  • Posts: 1499
Re: Dohc Cb750f: possibly a valve issue
« Reply #1 on: Jul 30, 2018, 13:25:38 »
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.
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Offline SquidHunter

  • Posts: 89
Re: Dohc Cb750f: possibly a valve issue
« Reply #2 on: Jul 30, 2018, 15:36:42 »
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. Iíve 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. Iím suspicious of the petcock. I do get flow from it, but itís not a steady flow. Itís 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 Iíve only checked when the motor was cold. Should I be checking with the motor warm?


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Offline SquidHunter

  • Posts: 89
Re: Dohc Cb750f: possibly a valve issue
« Reply #3 on: Jul 30, 2018, 15:38:44 »
Also, it wonít start at all until after it cools down. Then it fires right up and seems to be fine


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Offline SquidHunter

  • Posts: 89
Re: Dohc Cb750f: possibly a valve issue
« Reply #4 on: Jul 30, 2018, 15:41:09 »
Same with a cold start. Fires straight away with choke up. Doesnít give me any problems until the motor gets hot.


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Re: Dohc Cb750f: possibly a valve issue
« Reply #5 on: Jul 30, 2018, 16:05:17 »
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.
« Last Edit: Jul 30, 2018, 16:08:30 by advCo »
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Offline SquidHunter

  • Posts: 89
Re: Dohc Cb750f: possibly a valve issue
« Reply #6 on: Jul 30, 2018, 16:16:47 »
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 itís starving for fuel, itís 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, thatís why Iím betting on that. Especially seeing the fuel flow so erratically.


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Re: Dohc Cb750f: possibly a valve issue
« Reply #7 on: Jul 30, 2018, 16:34:39 »
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 itís starving for fuel, itís 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, thatís why Iím betting on that. Especially seeing the fuel flow so erratically.


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Sounds like it, you should have steady flow out of the petcock or something is plugged up or not flowing correctly.
"He broke the mirrors off his Cadillac, 'cause he doesn't like it looking like he looks back."

74 CB360 - Luna - http://www.dotheton.com/forum/index.php?topic=63294.0 - Sold
82 GS550L - Tracker-ish - http://www.dotheton.com/forum/index.php?topic=67229.0 - Sold
74 XL350 - The Turd - http://www.dotheton.com/forum/index.php?topic=70252.0 - Sold
Suzuki FA50 "No-Ped" - http://www.dotheton.com/forum/index.php?topic=71189.0
73 Suzuki RV125 -http://www.dotheton.com/forum/index.php?topic=73875.0
'97 BMW F650st - http://www.dotheton.com/forum/index.php?topic=75732.0

Offline SquidHunter

  • Posts: 89
Re: Dohc Cb750f: possibly a valve issue
« Reply #8 on: Jul 30, 2018, 18:50:35 »
Thanks for the replyís! This has me feeling much better. Iíll replace the petcock with a proper Honda unit


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Offline esmoojee

  • Posts: 169
Re: Dohc Cb750f: possibly a valve issue
« Reply #9 on: Jul 30, 2018, 19:40:49 »
Iíd say petcock, you should only check valves cold. Did you set the float height? That could affect it also.