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Matt, Mechanical Engineer
Category: Honda
Satisfied Customers: 22729
Experience:  BEng hons Mech engineering, in auto industry 22 years
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My Honda CBF 250-4. After starting and settling at tick over

Customer Question

My Honda CBF 250-4.
After starting and settling at tick over rpm of 1500 the tick over increases to 3500.
On a journey when dipping the clutch to change down at a junction from a speed of say 5000 rpm the tick over stays at 5000 and causes problems.
The carb is a Keihin VEA2 and has been removed and cleaned in an attempt to cure the problem but to no avail.
Fro Brian Simpson email:***@******.***
Submitted: 1 year ago.
Category: Honda
Expert:  Shantal-Mod replied 1 year ago.
I'm Shantal and I'm a moderator for this topic.
I've been working hard to find a Professional to assist you with your question, but sometimes finding the right Professional can take a little longer than expected.
I was checking to see if you had already found your answer or if you still need assistance from one of the Professionals.
Please let me know if you wish to continue waiting or if you would like for us to close your question?
Thank you,
Customer: replied 1 year ago.
I have not found a solution to my original question regarding the excessive rpm. yet and need a professional solution SAP.Another query about this carburetor is the purpose of the electrical carb heater. The circuit diagram shows an external thermostat. Since there is no voltage on the heater I wish to trace the wiring back to the control thermostat but cannot find the location of the device. Can you supply info about the purpose and operation of the heater and thermostat?
Expert:  Matt replied 1 year ago.

Hello Brian

whats most likely is that an air / vacuum leak is opening up as the engine warms and everything expands and so allowing in extra air

The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane around each joint in turn. If the engine rev's up you've found your leak.

Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.

Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

Expert:  Matt replied 1 year ago.

regarding the carb heater element then this typically warms as soon as the ignition is turned on and then time out via a relay unit

Customer: replied 1 year ago.
Hi Matt.
Thank you for your reply to me engine query . Video's showing this procedure are available on youtube. also a paragraph in the service manual outlines this procedure. which I conducted 2 weeks ago with no improvement. However I ran through the procedure again, alas with no improvement. Theoretically I would think that an air leak would weaken the mixture and slow the engine rather than speed it up. The problem still exist and I am despirate to cure it and put my machine back on the road, any more sugestions?
Customer: replied 1 year ago.
Regarding the carburetor heater. My wiring diagram shows a simple thermostat controlling the carb heater. Since there is no voltage on the heater with the engine either hot or cold I looked for the thermostat but have been unable to find it. This carb is insulated against engine heat by the two rubber connecting hoses and runs extremely cold, My present thoughts are of the vacuum operated piston jamming due to the cold and causing the revs to increase. There is very little info on web sites on this Honda model. Once again I am frustrated by not being able to troubleshoot these two problems. HELP!! Brian
Customer: replied 1 year ago.
Hi Shantal, Cannot give a rating yet since my problems have not been resolved. I hope that your satisfaction guaranteed clause kicks in here. Brian
Expert:  Matt replied 1 year ago.


I'm going to opt out here and get another expert involved

don't reply to this post otherwise it'll lock back to me

good luck