Topic: 1975 CB400F won't idle

[Mainer4]

975CB400f
Just rebuilt the top end due to low compression in one cylinder resulting from rust and abuse of PO.  While I had the carbies off I did a mechanical breakdown and cleaned all the bowls, idle and main jets and float valves.  I checked the C clip position on one needle and confirmed it to be in the factory (middle) position.  I then balanced the four carbs using the 1/8" drill method.  Putting everything together and firing it up the engine refused to idle down and continuously runs at a minimum of 3500 RPM no load.  The throttle cables are fine, the mechanical stop is completely back out.  The chokes are all wide open and adjusted for proper opening.  The throttle linkage to the choke is correctly adjusted so there is no activation at all when they are open.  The only way I can get the engine to slow down is to close down the air metering valves.  Factory says they should be open two turns with 1/2 turn range.  If I shut them all down to 1.5 turns the engine slows down but the exhaust smells very rich (as indeed it would).   

Looking for opinions on what could be causing this.  Please note, as mentioned above, there is no mechanical constriction of the throttle mechanism.  I have not checked the timing yet, that is on my list of possible causes, but I ran out of time.  Theoretically the ignition timing should not have changed between when I tore it down and put it back together.  The ignition advancer does seem to be working properly, but again I will have to check for certain with a timing light.

One thing I did notice which may have some bearing on this - although this problem existed before I tore the engine down.  The piston skirts on 3/4 of the carb pistons have a small chunk missing from the very bottom, almost as if they were worn this way by bottoming out within the carb body.  I have a spare carb set which I am going to check.  It may be possible that when the throttle is completely closed there is still enough of an opening in 3 of the 4 carbs that there is an unbalanced condition.  This unbalanced condition may show up throughout the throttle range and I may have to use vacuum guage balancing rather than mechanical if this is true- but I will see what the list has to say first before I go that route.

[TwoTired]

Okay opinions...
Have you checked the fast idle cam isn't still actuating with the choke off? I note there is also a fast idle speed adjuster as well as a main idle stop.

The main idle stop - stops position of the main linking pivot rod for all the carbs.  However, it is possible to run the individual carb slide adjustments toward one end so the slides cannot fully seat even with the main stop disengaged.  This, perhaps, is another artifact of the drill bit method rather than the light/no light method while looking through the carb bores to verify total closure.  And then fine tuning balance with the Honda recommended guages after installation.

To review, the slides are supposed to control all the air for combustion.  Alternate air leaks between the head and carb slides will disrupt that control.  Since these bikes idle on the rich side anyway, leaks of this type will increase the idle speed.  And can often be found by putting some viscous fluid on the suspect area and observing RPM changes.  However, 3500 RPM would be a LOT of leaking, I imagine.

You might want to verify the slide orientation.  The cutaway should be on the air filter side, an easy check.

If none of the above applies to your situation then continue with the slide deformation investigation as described in your last paragraph.  It is important that each of the 4 carbs meter air and fuel in the same way for the engine run and pull smoothly.  I know of no reason why 1 slide of the 4 carbs should be different in any way that could present a different mixture than the others.

Cheers,

[oldbiker]

If you are using the static (1/8 drill) balance, I hope you realise that does NOT mean all four carbs are 1/8 open when the twist grip is rolled completely closed. This check is done by making sure all four carbs are opening the same amount but when the twist grip is rolled shut the carbs should show no gap at all.

[Mainer4]

An update to my earlier post.  I have confirmed that the slides are not closing completely.  I have backed off the choke linkage completely, and even loosened the throttle cable stop eccentric.  Still remaining open.  I think this leaves two possibilities.  One, I have somehow misadjusted the throttle cable or two, I completely misadjusted the slides during the balance adjustment.  Either way, it explains why it races and won't idle.   

[Mainer4]

AFter I took the carbs off I realized that from the airfilter end they never close, but looking at the engine side they are closed.  So I knew it wasn't an issue with the linkages or even my set up.  I decided the problem had to be the float valves allowing leak by so I took the valves out of the seats and dressed them.  I readjusted the floats to a more conservative angle and put the whole thing back together again.  Now runs pretty well.  Put a set of vacuum guages on it and that is as far as I got, but it appears I do need to do a little fine tuning yet, and then figure out why #2 cylinder isn't running right.