A couple of years back I helped a friend build a 1913 Speedster with a 4 valve Rajo overhead. Initially my friend bought a huge carb (1 3/8”) from Stan Howe but my friend used a huge “up and down” manifold (1 3/4”) which was clearly too big for the engine. It resulted in sporadic performance and eventually the carb and manifold were replace with a modified Model A manifold and Stromberg M1 carb.
But it smoked. With this set up the car ran well but smoked. The fumes were visible and didn’t smell like oil but rather burnt gas. On removing the cab, there was raw fuel in the horizontal portions of the manifold. Another clue was the slow om running adjustment had absolutely no effect. We could screw all the way in or out without any change in engine speed. We even tried to replace the M1 with a Tilletson (sp) with essentially the same results.
Again the slow speed adjustment had no effect. At this point we were ready to give up when we realized that if the idle adjustment is too agggressive then the throttle butterfly is no longer between the two venturi holes and the slow adjustment will no longer operate.
We reset the M1to a minimum throttle opening and started the engine and ran with minimum throttle while adjusting the slow speed mixture. Hey presto, the smoking had largely disappeared.
Maybe the smoke will return after a few miles, maybe it won’t.
Has anyone experienced anything similar?
You might just have the GVA open too far. The long brass tube holds the gas valve adjusting needle (GVA). That is what controls the fuel flow too the jet. Model A instructions would be open about a turn and a half from closed. Start and turn down to about 1/4 to 1/2 open depending on what engine requires. I would have to do some checking on the other adjustment, I can't remember if its a fuel or air flow adjustment. I don't do a lot with the Tillies just Zenith's on the Model A's I work on.