Hanging idle

[Windmill John]

Due to me making some assumptions….. and being a little blasé, having rebuilt many CV carbs on BMs, I’d set up my idle incorrectly on my CB250.
I won’t go into details, but due to a blocked idle circuit on one side, I ignored something else.

Whilst looking on a BM forum I am a member of, Rob on the site had posted an excellent block of helpful text.
Even though I’d done most things, when I took a breather and went back to basics, I resolved my hanging idle issue.

Below is a link to the thread. I asked Rob if he was happy for me to post here. He was and here it is. Ignore the choke part as it refers to Bing carbs, but the principle is the same.

See the post by Rob Frankhamr:

https://www.bmbikes.org.uk/viewtopic.php?f=15&t=30706

[ancientdad]

It requires you to log in to read.

[Windmill John]

Ah, they’ve changed that, you used to be able to just view.

Here is Rob’s post from that thread:

There are a number of reasons why this might happen:


1) Idle mixure settings are probably the most obvious but often occur in parallel with other issues...


2) The advance mechanism bering slow to return. This is difficult to diagnose, even with a strobe light because the revs of the engine can result in the advance being slow to return just as much as the advance being slow to return can be the cause of the revs hanging. Make sure the advance returns effectively when the engine is off and, if the condition persist, consider replacing the return springs.


3) Mis set engine timing. This should be set as accurately as possible at idle. A couple of degrees out can cause the revs to hang, especially when the engine is hot.


4) An air leak at the carburettor or the carb/head connection. Check the condition of the rubbers between the carb and the head and all gaskets in the carb. On particular source of issues is the choke on the piston carbs. Each choke is a vertical piston valve operating in a gallery in the carb body. At the lower end of the piston there is a rubber disc which closes off an auxiliary jet. After time, these rubbers can fail causing all sorts of issues with tuning.


5) carb float settings.


6) Carburettor balance. Unbalanced carbs can have this effect. The carbs need to be balanced at idle. Unfortunately the piston carbs don't have balance take offs in the same way that the CV carbs do so you will have to balance by ear or using an external vacuum gauge setup. Some people advocate plug chopping but I don't.


7) Other plug settings. Needle height and jet sizes.


8) exhaust leaks. A badly leaking exhaust can have the effect.


9) Mis set valves.


10) Low compression.


Part of the problem is that there is a strong likelihood that more than one of these conditions will apply. As one issue starts to take effect, you tune the engine around it to achieve the best results... or rather, you detune the other factors to compensate for the original issue. This means that, as time goes by, the engine gets more and more out of tune and harder and harder to correct.


So... what to do...


a) check the condition of the advance and the ignition timing. Check the valve settings. Check the compression of the cylinders. Check for exhaust leaks. Make sure all of these 'non carb' related causes are eliminated before touching the carb.


b) Check the condition of the rubber tubes between the carbs and the head for air leaks, replace if suspect.


c) Check the needle height and the carb float height.


d) reset the carbs to intial settings and ensure that there is some play in the cables. Make sure that the chokes are closing properly.


e) Retune the carbs starting from scratch... i) Set the mixture screw settings by ear
ii) Balance the carbs at idle (you should also balance the carbs off idle but that's not part of this issue)
iii) Test ride the bike to see if the issue is improved.


f) If it is, go one to complete the final tune of the bike... if it's hasn't then you need to dismantle, clean and check the carbs taking into account jet sizes, needle sizes and all gaskets/'O' rings. then repeat from (c) above...


Hope that helps


Rob