I just serviced the two WCFBs on a 354 hemi... now the engine starts and runs smooth, especially at a fast idle, but once it warms up it starts to idle rough and stalls. I cannot seem to correct the problem with the mixture screws. I confirmed fuel supply. Does this sound like a vacuum issue... or timing issue? Perhaps the metering rods are not pulling down with the engine vacuum? Any thoughts?
Fuel pressure to high maybe? My two did the same thing when I had a cheap gauge that read wrong, 5 pounds now and runs great. Posted using the Full Custom H.A.M.B. App!
When the engine is warmed up and idling rough (and assuming the problem is carburetor related) I would first determine whether it is running rich or lean. If the engine is running rich, creating an intentional vacuum leak will make it run better with the additional air. That means typically pulling off a vacuum hose, but not one that would affect the vacuum advance and ignition timing - because that introduces another variable... Rich mixtures could be internal leakage, plugged air bleeds causing siphoning of fuel, or too high of fuel level in the bowls. If the engine is running lean, a shop rag placed over the carbs air horn will richen up the mixture slightly and then make it run better (acting somewhat like a choke plate). Lean mixtures could be plugged internal passages or vacuum leaks. Maybe also fuel delivery problems after running for a period of time - for example dirt in the fuel tank that plugs the fuel filter, then drains back when the engine is off for a period of time. Once you know whether it is rich or lean, you can proceed with some direction. First make it run right when warmed up, then address any cold startup issues. Cold vs. warm is a factor. Colder engine temperatures like richer fuel mixtures whereas warmed up engines don't need as much fuel because the fuel atomization is better at operating temp. The automatic choke can add another variable though, as it richens the mixture as well as increases idle speed when it is applied. That increased throttle opening can also apply the vacuum advance which changes the ignition timing. If changing the air/fuel ratio in both directions (rich & lean) does not improve how it runs at operating temp, then look at the ignition.
The metering rods are not part of the carbs idle circuit, they are part of the main circuit. One more thing, a vacuum gauge is a great diagnostic tool. How it behaves when the engine is struggling to run can be extremely helpful. Two WCFB's on a 354... Hmmm... Is this a 300B or a Chrysler Marine application?
You called it: 300B So... thanks for the advice with diagnosing carburation. Ultimately, I believe I had multiple compounding issues. I did have an issue with the filter bowls not filling (even with good fuel pressure) that seems to be fixed now: removed a secondary filter upstream of the pump. I had both carbs off multiple times, reset the floats (found one to be misadjusted). Cleaned vacuum ports... again. Chokes are working beautifully, which was the first indication of good vacuum function... manifold vac. gauge reading ~15mmHG at idle. I was convinced it was carburation, so I proceeded with blinders. Once I checked every possible fuel/air related item, I had no choice but to rule it out. So, as advised, I moved onto ignition. Low and behold - coil output to the distributor is unstable after about 25 minutes of idling. Ballast resistor or coil itself? Resistor is reading about 2 ohms cold... which I am considering an issue. Hard to say if it cooked the coil... everything runs nice cold. Diagnostics on-going. The service manual I have states the resistor should read between 0.5 and 0.6 ohms, but the catalogs of various suppliers list a 1.4 ohm resistor for this application. Perhaps this is due to differences with modern coils? If the 1.4 ohm resister results is 7-9 volts at the coil, should I consider it good enough, despite the manual? Thanks again.
Looks like it was the ballast resistor after all. Once it heated up to a certain point, its voltage output to the coil became unstable and the coil was not firing properly. The ballast bypass was not incorporated until alternators were introduced in '60 (I think). Therefore, it would not start up until the ballast and the coil cooled down again.
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