Lots of discussion here and i believe that is a good thing, it should be noted some well-articaulted commentary is being made towards the combustion process and of course overheating. It must be said however most of this commentary is directed towards fuel injection, and while the end results are the same the methods in which FI are employed and how carbs work are like apples and oranges..Read pressurized fuel induction vs vacuum induced fuel induction..Two entirely different worlds...below is commentary from a very well respected and known expert in the field of carburation...It can make ones eyes glass over yet it explains the basic concepts and also explains why results can vary... Have a read..it might be illuminating...
Forget for a moment about all the info on A/F ratios and what is being posted about it on the websites.
It doesn't really apply as we tune for results not any particular A/F ratio. There are many things that effect the outcome, and carburetors are not a fixed metering device.
A/F ratios can and will vary considerably over the load/speed range of the engine depending on throttle angle, pressure differential above and below the throttle plates, airflow across the venturi areas, etc.
If you were way too lean the engine would be WAY down on power, "flat" on acceleration, hesitating, and bucking and kicking in protest. A/F ratios at light engine load can run pretty lean and the engine show no ill effects. Running w/o vacuum advance throws a monkey wrench into the equation because lacking another 15-20 degrees timing at light load means that it will not effectively burn a leaner mixture and require MORE fuel to make things happy.
So timing is also a big player in this deal as it allows for a much leaner mixture to be effectively burned, especially in the "normal" driving range and why engine vacuum is used to add more timing as it is a load sensing device.
There are also many other contributors to not being able to run effective ignition advance and observing some ping at light engine load.
I've had dozens of Pontiac 455's in here for custom tuning, many were very sensitive to timing and fuel at light load and I've had a few that were NOT happy adding any advance to them at all with the vacuum unit. We found that these engines had small camshafts in them and or camshafts with early intake valve closing points. Some had a LOT of quench as well. Others ran hot due to poor combustion efficiency, some had combinations of problems all making for a poor end result as far as getting them tuned was concerned.
In any and all cases we give the engine the timing and fuel that it wants. If you had to pull the timing back to 8 initial and 30 total and it woln't tolerate any more at light load a flag goes up telling me it's making too much cylinder pressure to the octane even though the static compression ratio is 9.3 or so to 1.
I'd also add that 74 main jets and 41 rods are already pretty "fat" in that carb number, assuming it has the correct rebuild parts in it (fuel inlet seat diameter, float height, PP hanger arms not bent down too much, etc).
I typically set those up with 73 main jets and 42 primary rods for set-ups like yours and they are flawless everywhere.
Where to go from here. As mentioned in any and all cases we tune for results, not any particular A/F ratio, especially at light engine load. I've seen engines have perfect results with the A/F ranging from 13 or so clear to 17 to 1 at very light load, so 15.5 to 1 is not outrange the effective range, and that number is also effected by how much timing advance you are using. LESS timing doesn't provide effective combustion so the results are altered vs adding more timing and seeing improved or better numbers on your gauge.
Audible detonation tells me it's not happy with something, and it may be more than just the A/F ratio the carb is trying to provide.......continued....
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And a postive note to our old carbed RV's..things are not as they seem to be.
All of the idle fuel must flow past the main jets. Most idle tubes are only .030-036", some even smaller. You'd have to be using a jet/rod combination that has less metering area with the rods fully seated to provide a restriction to the idle fuel
The idle system however does feed the main system. As the throttle angle increases and the engine sees a pressure differential change above and below the throttle plates fuel will be introduced from the transfer slots in addition to the holes under the mixture screws.
So yes, idle fuel has an impact on main fuel delivery to the primary side of the carburetors for "normal" driving.
The big difference for a Q-jet vs many other square flange designs is that it gets fuel much sooner (lower throttle angle) from the boosters due to the small primary bores and triple venture areas created by the rings around the boosters.
The design of the boosters and venture areas also greatly increase efficiency and atomization and one of the reasons Q-jets (and Thermoquads) are excellent street carburetors and rival the very best fuel injection set-ups for efficiency and fuel economy.
I've actually built scores of Q-jets to replace both factory and aftermarket Throttle Body style fuel injection systems and not once to date has any of those folks complained about increase fuel consumption or not as user friendly anyplace. Matter of fact most absolutely LOVE the switch back to a carb.
Case in point. A very good friend of mine owns a Chevy 1 ton dually crew cab with a 454, TH400 and 4.10 gears. It came from the factory with a throttle body FI system.
Despite his best efforts and everything with the factory ECM working correctly he got HORRIBLE mileage and not overly impressive for power output. We selected a 1985 dedicated 454 Motorhome Q-jet and he installed a factory cast iron spread bore truck intake. We also built him an HEI distributor with a custom advance curve and vacuum advance.
He had to purchase a fuel pressure regulator and set up a return system for the high pressure factory fuel pump, but overall the swap was relatively easy. He couldn't believe the improvement that the carburetor and HEI had over the TB and ECM set-up. Fuel economy went up, as did throttle response and power output clear across the load/speed range.
The only possible negative was that he had to "pat" the throttle once in the morning to set the choke, otherwise he said the new set-up was better to the FI in every other area.......Cliff
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. My experience led me to choose a cooler-opening thermostat with the argument that I wanted full coolant flow sooner than later and running too cool was never a concern.
I wasn't complaining. Just, you know....poking with a stick.
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