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Carb Syncronization, 55 BN1

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Gray_Cat

Gray_Cat

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I installed rebuilt carbs on my 55BN1 and have encountered syncronization issues I can't seem to overcome. The car starts fine (1 push of the button) but I can't get it to idle properly below 1800 RPM. The slide action in the rear carb seems to be functioning properly. The slide in the front carb is going up much higher in relation to the rear carb and causes the engine to race. The engine runs great above 1800 RPM but I did notice some very slight backfire - the timing etc. was untouched. Everything was renewed in both carbs except the butterflies, is it possible the one in the front carb is the culprit? I've adjusted the jet nut in all manner of lean to rich and it doesn't seem to make much difference on the front carb. The springs seem fine and the correct amount of dash pot oil (the real thing) has been added. I've tripled checked the linkage etc. as well and seems to be in order. The vacuum is hooked up - is there any possibility the crossover pipe between the front and rear (manifold) carbs has anything to do with this? I've worked a lot with SU's in the past (distant) but this has me mystified. Any help is appreciated!
Oh, any suggestions as to a good shop manual, the original one leaves a great deal to be desired. Many thanks! /ubbthreads/images/graemlins/thirsty.gif
 
Gray,
Obviously, air leaks at the carb or manifold flanges or at the crossover tube will screw everything up, as could improper float levels.

You don't mention actually synchronizing the butterflies. With air filters removed & the slides held up, loosen the shaft coupling so that the shafts can move independently. Make sure that the choke /fast idle adjustment is well clear of it's cam & that the choke mixture levers are completely down with some slack. Back off the choke cable adjustment if in doubt.

Back off the idle stop adjustments completely (air space between the adjuster screws & stops) & look at the butterfly seating in the carb bores. If everything is seating completely, drop the slides & advance the throttle stop screws until they just begin to clamp a piece of paper. Advance the screws an EQUAL number of turns from this point, maybe two, or as needed to get a preliminary idle adjustment.

With the engine idling, use a piece of small rubber hose to listen for an equal air hiss in the carb throats. First experiment a bit to see how the idle stop adjustment affects the sound. Then, adjust the idle stop screws equally or to obtain the same sound at the desired idle rpm. Tighten the shaft coupling & re check the sound. If by chance you can't get equal adjustment, there may be something wrong with the butterflies, or the shaft bushings in the carbs "could" be worn.

Also check that the piston/air valve return springs have approximately the same tension & free length.

Then proceed with mixture adjustments as in the book. Lastly reset the choke pull wires & fast idle adjustment so that the first part of the pull only operates the fast idle cam/throttles & further movement pulls the choke levers. Set fast idle at about 1200 rpm with the cable completely pulled.
D
 
Many thanks Dave. I think I may have not left the shaft connecting linkage disconnected when making the initial adjustment. I will do that and check out the butterflies as well. Thanks again!
Roy
 
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