Ok, I’m playing catch-up reading through the recommendations and I’m starting to get a feel for how the carb works. Correct me if I’m wrong, but it seems like your part recommendations are to get more fuel into the primaries, and the vacuum secondaries to open sooner with the softer spring. Both make sense to me, but I wonder if I’ll get less engine efficiency and more fuel burn with that setup. Just thinking out loud.
Sorta Kinda...
By lowering the cut in point from a vacuum standpoint on the power valve, it will hold it out longer (actually improving the part throttle running) and stepping up the jet size will enrich the top end more, so when you change the secondary cut in it will be fatter... You just sort of need to trust me on this LOL... These are the reasons a mechanical secondary carburetor works so much better... But what you have will work quite well with the changes I have recommended.
I’m not understanding why disconnecting the vacuum line helps the situation. If you don’t mind explaining it or explained it somewhere else I’d appreciate learning more about it.
So the idea behind a vacuum advance is as such. NOTE: On a car...
While the engine is under a heavy load, there is no vacuum signal for the most part so the engine can "pull a gear" with slightly lower ignition timing, as the engine speed increases and the load decreases, the vacuum signal comes up and pulls in more timing pneumatically.. This promotes a little better fuel mileage and increases the rate of burn allowing more fuel to be used to generate horsepower and keeping the throttle response nice and crisp. If you put your foot back in the gas, the vacuum signal drops, taking out some timing to prevent preignition (knock).
Inside the distributor (speaking for your GM HEI), there are centrifugal weights and springs on an apparatus that increases the timing based on RPM. This is typically between 12 and 20 degrees from base. Really depends on application, the distributor from a motorhome is a little different than one from a Camaro. That said, let's use 20° as an example. So you set the initial advance at 10°BTDC, at over 2,500RPM the timing is going to be at 30°BTDC, not more, a good running 454 likes about 36°BTDC at anything over 3,000RPM. The vacuum advance won't pull in, rendering it virtually useless. The problems start when you have the engine set up to use the vacuum advance, it almost becomes a vacuum retard, because just about the time she really wants more timing, it starts falling off, making tuning a nightmare.
Now move on to a jet boat engine.
There is literally no engine load at lower RPM, whether the boat is moving or not. There is no need to reduce the ignition timing under acceleration because the torque required has a linear curve (meaning once the engine is out of torque increase, it stops accelerating...or... The torque continually increase with the speed it turns.). With the engine at wide open throttle,
there is no vacuum signal, so it won't pull in anyway.... It will only work when it doesn't need to...
The best way to get it set up correctly would be figure out how much advance your distributor is set up for, use the lightest possible springs in an advance curve kit (
https://www.summitracing.com/parts/SUM-G5212/ )and set the base timing at idle accordingly to get you to that 36° sweet spot (+/-). Eliminating the signal to the vacuum advance. You literally only need to reduce the ignition timing for starting. A jet can't pull a gear. It will easily rev right up to the point where there is no more torque available to continue accelerating....
I certainly hope this helps. Let me know if you still have questions. And yes, this is why I asked you what method you used to time the engine... The original Chevy truck specifications will do you no good whatsoever.
GT
