craze1cars
Lieutenant Commander
- Joined
- Dec 26, 2004
- Messages
- 1,822
Re: Solas props
I think you have a factor here that you're blaming on the prop, that's not really a prop issue...because a prop would never cause what you describe here. BUT a 4bbl carb that does not have secondary flaps adjusted for the intended load will cause EXACTLY what you describe here. And going from being properly propped to being over-propped will exaggerate/reveal the problem because it adds substantially to the load on the engine by lugging it hard at low RPMS. It's basically a lean bog. The bigger & heavier your load, and the harder your prop bites, the more critical it is to slow down the openings of the secondary flaps, especially at low RPMS. On a Holley with vacuum secondary throttle plate you buy a spring kit and go to stiffer springs for the bellows, trial and error until you find the spring that gives you the best performance. On a Quadrajet with mechanical secondary throttle plates you will have some adjustment on adding a bit of spring tension for the secondary air flap, or if that's not enough maybe the vacuum choke pull-off needs to be adjusted by adding a restriction to the vaccum hose to slow down the release of the pull-off (choke pull-off has 2 jobs...it continues to control secondary air flap speed on a Qjet after it is done releasing your choke from cold starts)...to get rid of a bog you may need to add a second or so to the choke pull-off release time.
Other carbs adjust this differently. All that matters is that if you don't have these things set properly for your load, the air flap slaps open too quickly when you hammer the throttle at low rpms, which lets in a huge rush of air without enough time for the fuel to wick up and get mixed properly...resulting in a classic lean secondary bog where the boat falls flat on its face and stumbles...and then the fuel eventually catches up with the airflow and it takes off correctly after the "bog" is past. This is what many people describe as "feeling the secondaries kick in" is simply a classic description of an improperly tuned carb. You should NEVER "feel" the secondaries kick in on any 4 bbl carb. It should be smooth/fast/even power.
I'm not saying you need to adjust your carb for the Solas prop you had, because the Solas prop was obviously of incorrect pitch. But if you DID adjust the secondaries to open slower and not let in a big rush of air, you would not have experienced the "almost killed the engine" as you described. So really the prop indeed caused this, but only because the carb wasn't adjuster to handle the heavy load of that particular prop. So if you eventually find a prop that is of correct pitch, and it bites hard enough that you are still getting a lean secondary bog, it is entirely possible that you might just need to slow down the air introduction into the secondary side of the carb to get peak performance and good throttle response out of that particular prop. This is VERY VERY common when people put 4bbl carbs on boats. Most come out of the box with secondary flaps that open too quickly. I have no idea if your 4bbl is factory or not, or if it has been adjusted in any way.
Light cars with gears can get by with very loose secondary settings (fast opening). Heavy boats with hard biting props and no gears generally need much slower settings on the secondary side to avoid a bog. Many tuners don't get this concept and live with a boat that has a bog every time you slam the throttle open, car guys especially have difficulty understanding why their boat bogs, with similar setups on their hot-rod runs great...
And MAYBE...instead of messing with air flaps your accelerator pump just needs to be set to deliver a larger squirt than it does. That is another bog reduction adjustment. Some carbs have adjustments & options for this via linkage. Some do not. Depends entirely on what kind of carb you have how to get rid of a lean secondary bog, which is what you describe above.
Summary of what you need to do: Choose a prop first based on WOT RPM and overall handling characteristics. And don't worry about it if it causes a bog at part-throttle hammer-down, because this can be dialed out with a little carb tuning if you decide you like everything else about the prop.
I think you have a factor here that you're blaming on the prop, that's not really a prop issue...because a prop would never cause what you describe here. BUT a 4bbl carb that does not have secondary flaps adjusted for the intended load will cause EXACTLY what you describe here. And going from being properly propped to being over-propped will exaggerate/reveal the problem because it adds substantially to the load on the engine by lugging it hard at low RPMS. It's basically a lean bog. The bigger & heavier your load, and the harder your prop bites, the more critical it is to slow down the openings of the secondary flaps, especially at low RPMS. On a Holley with vacuum secondary throttle plate you buy a spring kit and go to stiffer springs for the bellows, trial and error until you find the spring that gives you the best performance. On a Quadrajet with mechanical secondary throttle plates you will have some adjustment on adding a bit of spring tension for the secondary air flap, or if that's not enough maybe the vacuum choke pull-off needs to be adjusted by adding a restriction to the vaccum hose to slow down the release of the pull-off (choke pull-off has 2 jobs...it continues to control secondary air flap speed on a Qjet after it is done releasing your choke from cold starts)...to get rid of a bog you may need to add a second or so to the choke pull-off release time.
Other carbs adjust this differently. All that matters is that if you don't have these things set properly for your load, the air flap slaps open too quickly when you hammer the throttle at low rpms, which lets in a huge rush of air without enough time for the fuel to wick up and get mixed properly...resulting in a classic lean secondary bog where the boat falls flat on its face and stumbles...and then the fuel eventually catches up with the airflow and it takes off correctly after the "bog" is past. This is what many people describe as "feeling the secondaries kick in" is simply a classic description of an improperly tuned carb. You should NEVER "feel" the secondaries kick in on any 4 bbl carb. It should be smooth/fast/even power.
I'm not saying you need to adjust your carb for the Solas prop you had, because the Solas prop was obviously of incorrect pitch. But if you DID adjust the secondaries to open slower and not let in a big rush of air, you would not have experienced the "almost killed the engine" as you described. So really the prop indeed caused this, but only because the carb wasn't adjuster to handle the heavy load of that particular prop. So if you eventually find a prop that is of correct pitch, and it bites hard enough that you are still getting a lean secondary bog, it is entirely possible that you might just need to slow down the air introduction into the secondary side of the carb to get peak performance and good throttle response out of that particular prop. This is VERY VERY common when people put 4bbl carbs on boats. Most come out of the box with secondary flaps that open too quickly. I have no idea if your 4bbl is factory or not, or if it has been adjusted in any way.
Light cars with gears can get by with very loose secondary settings (fast opening). Heavy boats with hard biting props and no gears generally need much slower settings on the secondary side to avoid a bog. Many tuners don't get this concept and live with a boat that has a bog every time you slam the throttle open, car guys especially have difficulty understanding why their boat bogs, with similar setups on their hot-rod runs great...
And MAYBE...instead of messing with air flaps your accelerator pump just needs to be set to deliver a larger squirt than it does. That is another bog reduction adjustment. Some carbs have adjustments & options for this via linkage. Some do not. Depends entirely on what kind of carb you have how to get rid of a lean secondary bog, which is what you describe above.
Summary of what you need to do: Choose a prop first based on WOT RPM and overall handling characteristics. And don't worry about it if it causes a bog at part-throttle hammer-down, because this can be dialed out with a little carb tuning if you decide you like everything else about the prop.
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