Originally Posted by ackpht


Results: with ported vacuum driving the vacuum advance, throttle modulation off idle is MUCH improved, and the need to brake torque at stops is reduced (though not eliminated). Manifold vacuum is 15" at idle, ported vacuum is 12" (this is with the throttle blades completely closed). When the throttle is opened, both drop, though manifold vacuum drops much more than ported does, probably because the ported side has all of Earth's atmosphere to suck on. Both vacuum levels then increase as engine RPM increases (all this is with no load). The spark advance is thus not being retarded so much off idle when using ported vacuum, and so it makes sense that power comes on more gradually. That was a very insightful suggestion, thanks! beer

I have also noticed a "chug-chug" mode when starting warm without opening the throttle- there isn't enough vacuum to advance timing, so the engine chugs along like a steam engine at low rpm until it either dies or you open the throttle a little, which develops vacuum, which advances the timing, and then the engine idles smoothly with the throttle closed. Is this why they had throttle stop solenoids "back in the day"?

Carb numbers are 3437S front and 3858S rear. The dies used to make the stampings are very different, so I suspect they are not of the same vintage.

Now if I can make it smoke a little less...


Something is not right as you should have NO vacuum at the front (PORTED) opening at idle. If you look at the two pictures below, In the 1st pic you will note the side port is much lower than the front one. That is because the side port sees manifold vacuum for an external use such as a choke pull off. The front port is higher as its opening is ABOVE the butterflies and NOT exposed to manifold vacuum. The vacuum at that port is created by a "Venturi" effect. IE: as more air passes by the opening it begins to try and draw air in creating vacuum.

The second pic shows the path for the lower port. The Front port cannot be seen as it is ABOVE the butterflies. If yo have the carb off, and open the primary butterflies you should see a small round hole that feeds the front port. Blow air or carb clean through it wink
It cannot have any vacuum being drawn at idle unless, A. the butterflies are too far open or, B. the butterflly for that side is not closing in conjunction with the other side

Your chugging when starting is likely due to your not opening the throttle a bit when starting. These cars are not fuel injected and need to be dealt with differently than today's vehicles. The other possibility is that fuel is percolating in one or both of the carbs

I am not trying to be offensive but you have a 25K motor with mismatched carbs and are asking for more expensive problems if you don't either A. get a pair of carbs meant for a dual 4 street setup. Or B. Drop back to a single 4 intake. Most will not notice with the air cleaner your running. Either way you will need to spend some $$ to do it properly. If you happen to have a nice wife🙄🙄🙄 PSST, Santa's coming wink XmasPeng XmasPengs XmasPengs Actually you might need this one XmasTruck beer

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Last edited by TJP; 11/22/23 11:15 PM.
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