Topic: High idle when warm.

[GroovieGhoulie]

Hey all, a recent issue has come up.

I have a '76 CB400F with no modifications from stock.  I recently went through the bottom end of the carbs and cleaned them out.  A recent issue came up.

When it's cold, it idles properly at 1200-ish RPM, however, after running around town for a bit, it idles around 2400 RPM and won't go lower.  I hear that people usually have the reverse problem, where it idles high when cold.  Any Ideas?

Also, my airfilter doesn't have the little spring clip that keeps it pressed against the filter hole.  Are these available from Service Honda or other US dealers?  I don't wanna have to try DSS as the shipping would ridiculous for such a small item.

[GroovieGhoulie]

Thanks for the pics.  I'll try that.

As far as the high idle, I will try it as I know where the adjustment screw is, but I wonder if it will make it not idle wel when cold.  Right now, I can flip the choke off and it runs fine after about 30 seconds or so.  I've heard it should be at leat 2 or 3 minutes until it run well when cold.

[jdpas29]

when you crank up with the choke out, the RPMs should be a little higher than what you'd want your bike to idle at.  my bike steadily climbs up to around 2000 RPMs over the course of 5 minutes or so and once it gets settled in on that steady hum, i push in the choke and the RPMs drop down to the appropriate level.

the only other thing i would say is (assuming that your idle screw and air mixture is right) you need to make sure that your throttle cables are snug and that they are not starting to stretch.  one of them may be close to braking.  i have seen instances where if the push cable was sagging, the RPMs would race until you really bury the throttle... then they creep back up again when you release that pushing pressure.

i hope that all made sense.   

[GroovieGhoulie]

when you crank up with the choke out, the RPMs should be a little higher than what you'd want your bike to idle at.  my bike steadily climbs up to around 2000 RPMs over the course of 5 minutes or so and once it gets settled in on that steady hum, i push in the choke and the RPMs drop down to the appropriate level.

the only other thing i would say is (assuming that your idle screw and air mixture is right) you need to make sure that your throttle cables are snug and that they are not starting to stretch.  one of them may be close to braking.  i have seen instances where if the push cable was sagging, the RPMs would race until you really bury the throttle... then they creep back up again when you release that pushing pressure.

i hope that all made sense.   

It sort of does that.  When I push on the throttle, the RPMs drop a little. Plus, the cables seemed a little "stretchy" when in place.  It's time for new cables methinks.

[eldar]

You will get that throttle action no matter what. It is good to have a LITTLE action like that. A millimeter or 2.
Now I do not know the 400 but many carbs have a high idle adjustment. This may be off on your carbs if you have it. It is off on my 750 but I dont worry about it since it just takes a little throttle blipping.

[bistromath]

I don't think your bike has a choke setup that's coupled to the throttle. My '75 550 doesn't. So you're going to either idle correctly when cold, or idle correctly when warm. I usually just hang on the throttle a bit until the bike is warm enough to idle well. Your other option is to adjust the idle speed screw with your thumb when you're sitting on the bike at a stoplight and it starts to warm up!

[eldar]

Yeah I was not sure about the 400. My 78K has a high idle adjustment. so some do, others dont.
My bike idles higher warm then cold so I keep my idle screw at proper idle for running when warm and just use a little gas when cold.

[TwoTired]

The 75 Cb400 I had and rebuilt did have a fast idle linkage coupled to choke operation.  The manuals show all the CB400 fours did.  This adjustment is separate from the main idle stop.

The main idle stop is adjusted for idle speed (1200 RPM) when the engine is at full operating temperature and choke fully disengaged.

You'll have to identify the linkage actuator for fast idle.   With the choke off fully the fast idle adjuster should have clearance of 0.012 inch.  When the choke is fully deployed, this will block open the throttle valve and force the engine to operate at higher speed. This adjustment is to be done after the carburettors are synchronized.

Cheers,

[GroovieGhoulie]

ADjusted the idle down when warm.  Now it stutters when really cold, but isn't bad, just keep the choke on.  Gonna get a set of feeler gauges and check the high-idle stop, good call TwoTired.

I'm shopping carb sync gauges.  Which ones are the best?  The Morgan Carbtune or one of the 4-dial types?

I'm also looking into timing lights to set the timing.  Are there special ones for motorcycles or would one from the local auto parts store work?

[TwoTired]

I'm shopping carb sync gauges.  Which ones are the best?  The Morgan Carbtune or one of the 4-dial types?

People usually swear by what ever they are using and are familiar with.  I've had my 4 dial vacuum guages since 1975.  And, they've worked well on all my Hondas.

I'm also looking into timing lights to set the timing.  Are there special ones for motorcycles or would one from the local auto parts store work?

The standard automotive strobe light will work fine.  Get the inductive type.  But, if you still have the stock points system, you can static time the ignition with an ordinary 12v 3w instrument bulb.  I soldered a couple of short 2-3 inch leads on a bulb and put aligator clips on the wire ends.  Clip across the points and with the ignition on the light will come come on when the points open as you turn the crankshaft.  This is where the marks should align on the timing window... which the little lamp will illuminate nicely if you made the wires long enough.

Cheers,

[oldbiker]

The CB400f does have an adjustment for cold (with choke) idle, as shown in the diagram.

Set the idle for 1200 rpm when fully warm using the adjuster at the top front between carbs 3 and 4.

Set the cold idle with the adjuster shown in the diagram. Now from cold, start with the choke and gradually reduce the choke setting as the motor warms up.