CB750 engine speeding up

[neat street]

Hi all
my F1 on standard setup has a small niggling start-up running problem which your thoughts would be appreciated on. when starting (with choke) starts and runs on first touch, take choke off and ride, after 15 mins or so, the engine speeds up from 1500, to 2500, reduce tick over adjustment screw, and all is great as normal, but when stored and restart, you have to readjust the tick over screw again to gain revs to 1500 and start the same process again?
any ideas please?
thanks
John

[K2-K6]

Initial thoughts on chasing tickover inconsistency is air leak between carb and cylinder head.

Reason behind this is that uncontrolled air effectively bypasses your carburettors to which you respond by making adjustments.

Are you confident that the rubber and clamps are in good order? Worth a close inspection to spot anything obvious.

Also,  if you spray a small amount of carb cleaner onto the rubber carb mounts (don't do this inside a garage) if it changes by sucking that in and burning it then it would confirm a leak.

[neat street]

Thanks for your reply, the carb rubbers and clamps are new and no signs of leaks.
thanks
John

[K2-K6]

Ignition advance retard mechanism working freely / ok?

Minimum amount of slack available in "pull" throttle cable? The manual specifies this measured at the twistgrip.

[neat street]

Thanks again, will check the throttle cable tomorrow, advance/retard freely working

[neat street]

Rubbers are correct, only changed due to old ones being a little hard. seems strange as to the running as she runs well and picks up as she should, just the annoying speed `pick up`? checked cables and all seems to be in tolerance.
thanks again for your reply`s

[K2-K6]

Just thinking this one through,  has it always been like this or has it changed to this status?

Reality check for me,  you can only set the main tickover screw to idle at correct rpm when this engine is warm. It has no throttle lift when cold starting and taking it off choke will need you to keep it running by operation of the throttle until it reaches normal temperature. 

If you adjust the main tickover screw with choke off and motor not warmed you will create the described symptoms.  It's just very manual on this era of bikes.

[neat street]

Hi, thanks again for your reply
 it has been like this since the rebuild, if you set the speed when hot, (after a 10-15min ride) at 1500rpm, when you then try to start up from cold, it will not run (with choke on/or off), increase idle screw so it will tick over, but then when nice and warm (15mins) it then increases revs? not sure if it could be a jet issue??? plugs good colour though. don't know how it ran before rebuild, as was not running too good then

[K2-K6]

See what others on here think if an F1 owner would look in and give opinion.

It sounds normal to me,  the error is being included when you adjust the idle speed without it being warm to operating temperature.

It really has no compensation for cold running,  you just have to modulate it yourself with the throttle / twistgrip until it'll tickover without interference. It will just stop otherwise.

Just set it as you want for rpm when it's warmed up and leave the adjustment alone,  that's how they work.

Some of the earlier twistgrip (K series) have a friction screw underside of handlebar,  you could tighten this so throttle would stay where you put it, could be used for warm up.  Don't know if it's fitted to your model.

[Spitfire]

My F1 is fine, standard set up, jets, filter etc. To start, full choke, start up, set off, take choke off immediately and that's it, but it will not tick over cold though, you have to rev it.

Cheers

Dennis

[Moorey]

Standard setup full choke when cold, when started 1/2 choke then within 1/4 to 1/2mile choke off. Ticks over fine

[neat street]

thanks for our reply`s
I will continue with what I do at present, just thought here would be an easy fix. I do the same as Dennis.
thank you all again
john

[K2-K6]

Compared to modern setup engines they are just very manual.

The very next model F2 K7 they put a choke cable on for ease of use,  along with different carbs that have a throttle lifting facility on operation of the choke.
These do the opposite to yours as the manual spec for "on choke" rpm is 2750 +/- 750 which alarms people as it sits there with elevated revs when cold.

[hairygit]

thanks for our reply`s
I will continue with what I do at present, just thought here would be an easy fix. I do the same as Dennis.
thank you all again
john

That's exactly how bikes were even brand new back in the day. No fast idle on choke, it is the rider that keeps control of the idle speed with the throttle until the motor is properly warm. Older bikes like these can seem very odd if you didn't ride them back in the day and have only ridden modern stuff. The brakes on these old Honda's were some of the best in their time, but younger riders used to modern brakes find them terrifying!

Sent from my POT-LX1 using Tapatalk

[neat street]

thank you all for your help