Topic: CB750 won't run when engine hot

[Scott]

Hello
I rode my bike CB750 (1974) for about 1 hour, switched it off to wait for the ferry, 3 or 4 minutes latter I tried to start it but no go.
I rolled down the ramp and onto the ferry.  I got it started by putting full choke,  the bike ran for 5 or 6 seconds then cut out.
Pushed the bike off the ferry (up hill, man I'm unfit)
After 10 mins tried to start the bike again, once again it would just crank over until I put the choke on full.  Then it would run but only briefly, long enough to ride it 30 to 40 meters.
When it cut out it didn't splutter and then cut out, but cut out instantly as if ignition had been turned off.
Using the speed of the bike to turn the engine over ( before I coasted to a stop) I tried putting the choke on full and twisting the throttle but it wouldn't fire back up.
This happened 4 times in all. 
So now I'm up to 40 maybe 50 minutes since I first shut the bike down and this time it ran for the 45 minute ride home.
Thanks for any thoughts on this
Scott

[phil71]

It's somewhere in the fuel supply. Start at the fuel tap and work your way to the floats..

[Xnavylfr]

Check to make sure your cap is still venting good!!!


Xnavylfr(CHUCK)

[flybox1]

+1

It's somewhere in the fuel supply. Start at the fuel tap cap and work your way to the floats..

[Scott]

Sorry I forgot to mention that the very first time it happened I opened the fuel cap to make sure that it didn't have some sort of vaccum lock or similar.
Also the fuel level in the tank was quite high as I had just filled it up an hour earlier.
But I have some new fuel line on order so maybe I will change that first before pulling the carbs off to check the floats
Thanks
Scott

[fishslayeryo]

Just a random thought. I had issues starting my K1 all of a sudden. Ended up my petcock is not working now in On, but will work on Res....one more to my list of things to fix:) Good luck finding your issue.

[Scott]

thanks mate

[Jules083]

Have you checked the valves lately? If they are tight they could be holding themselves open just enough when the motor is hot to not have good compression. Higher RPM's it would still run ok, but starting is where it would really show.

[Scott]

Mmm valves, not lately or ever.
Only had the bike for a short time but now I reckon is the time to check the valve clearances and I guess it will start to eliminate possable courses of the hard to start.
The main thing that has me stumped is that it runs briefly and then cuts out again
Scott

[CycleRanger]

It does perhaps sound fuel related, however is the battery fully charged?  Have you ever changed the condensers?
Check the wiring, make sure all the connections are tight and clean. 
Never hurts to check the electrics.

[Spanner 1]

Slow-ass fuel delivery due to bunged-up petcock.... IMO  ... need to drain the tank and take the p/c off and clean the passages..... g'day !

[apehanger 550]

  I'm new to 4 cyl Honda's ,but not 2 cyl. Occasionally, if the points are not installed just right,they can ground out on the points cover when the bike gets hot. First the spark gets weak as the points spring gets closer and closer to the case cover and then the bike dies. Probably not your problem,but it may pay to check for spark when your bike is hot.