I have dual strombergs supplying a common intake plenum. If i shut of the idle fuel needles on one carb the engine dies but if i blank off one carb the engine runs fine. my question: why cant you shut off the idle needles of one carb and use just the idle needles of the other carb to control idle fuel. If you block off one carb with plate and shut off fuel to that carb it works perfectly yet just shutting off the fuel needles on one carb kills the motor. It doesnt make sense to me and hoping someone can shed some light. just trying to understand how these circuits work together on operation.
I'll hazard a guess. When you add a plate under 1 carb the engine see 100% of air thru the othr carb and fuel gets metered thru that carb for 100% of the air. Then if you run the idle richness screws in on one carb and the engine dies it is because the other carb isn't supplying 100% of air and the amount of fuel metered thru the running carb is less than 100% of what the engine needs. The carb that has the idle richness screws adjusted fully in must be supplying a substantial amount of the air. With duals it is normal to run them at 1:1 linkage, so both carbs are giving idle air and both must give metered fuel for that air. Thats just a guess, a theory to work from. Is it overly rich at idle and that is why you want to close one of them off? What engine, a flattie?
Its on a banger...its not overly rich, but while tuning the idle needles in, at about 1.25 turns out on all 4 needles the motor tries to stumble and wants to die. Turn them out to about 1.5 turns and it smoothes back out and idles fine but idle RPM is a little high and no adjustment on idle screw left, in fact you can take it completely out. So I am trying to get the idle down by turning the idle needle screws in. When I run 1 carb only the idle is where it needs to be with room to spare on the idle adjustment screw and the needles are about 2 turns out. I think the problem is in to much air and not enough fuel when shutting off both needles on one carb. Its the only thing that makes sense because when I block off the carb completely everything works perfectly. So in review, the car runs and idles perfectly on one carb and plenty of adjustment on idle screw, but idles a little to fast with both carbs with no adjustment on idle screws. Carbs are linked 1:1. The car runs very well and is very driveable, Im just trying to fine tune as best I can and get a feel for how these 97s operate. Thanks for your thoughts on this.
Have you tried to disconnect the throttle linkage to set the idle seperate on each carb and then reconnect to linkage? That is what I do on similar when I can't get the idle down to where it should be. It sounds like the linkage is keeping the throttle part way open if you can remove the throttle screw - something is anyway, that choke arm isn't keeping it on 'high idle' is it?
have you adjusted the two carbs so that each supplies the same amount of air at idle (sync) ? Beside the worn shafts it could be only one carb is in service at idle.
I think this is correct, at least on the right path. Carburetors don't inject or pump fuel, but rather they allow fuel to be ****ed into the engine by vacuum force (putting aside accelerator pumps for the moment). If you kill the fuel flow of one carb, theoretically you'd have to at least double the fuel flow of the remaining carb and that would have to be accomplished with half the air flow in that carb. Seems like it would have to have some seriously big idle circuits to enrichen the mixture enough to compensate.
Another thought is that the engine is a 4cyl making maybe 40ish horse power, the strombergs came from an engine making maybe 100ish horse power. the 100hp engine fuel requirements will be over twice as great because the air flow is over twice as great and the air signal to the carb circuits much greater. The idle circuits in the stromberg expect an air flow with 4 pistons per cycle and the banger has 2, couple that with dual carbs and the banger may not be getting hardly any fuel. What makes carburetors so interesting is that they are designed to supply for horse power not cubic inches. Horse power is calculated in pounds of fuel and pounds of air. The carb p***ages are designed to accomodate a certain amount of air flow and that is what moves the fuel. When I studied carburetors the professor would have us design a 1 barrel 'tube type' carb that would supply a 125hp irregation pump and design it with a 3/4" throttle blade and show all circuit sizes etc; then redesign with an 11/16ths throttle blade.
Thanks for all the replies....I checked with my supplier and told him what Uncle Max says and he gave me a few things to try first and If it doesn't work he will take them back and replace throttle shafts and bushings at no cost. I purchased these about 6 mos ago and only have about 75 miles on them. He said they check all units pretty close and change shafts and bushings on about 70% of all reconditioned 97s. The fact that he will repair them if needed shows me he is a (very well known) standup guy. So I will be dinking with them this weekend and decide what to do then. On another post Uncle Max recommended 71 PVs and 42 mains on an engine very similar to mine in construction. I am currently using 67 PVs and 45 mains....there is a huge difference in PV sizes here so I am going to try Maxs recommendations just for ducks and see what happens I will post whatever comes of this thanks again guys. You all gave me stuff to think about. Ive done tons of carb work and mods on S and S carbs for Harleys in the past, but these Strombergs circuits are a little different so I am trying to absorb this info as it comes.
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