Blower roll
05-29-2017, 11:43 AM
#3
Registered
have to keep adjusting the idle mixture screws and also give the motors as as timing while maintaining your desired idle speed.then you get to do it some more on the water,,I usually strap the boat to the dock.and then you get to play w/ it going in and out of gear.usually for gear change,you turn 1 screw out a little more than the others.
05-29-2017, 11:57 AM
#4
Platinum Member
Platinum Member
You talking about "lope" or "surge" at idle/low speeds?
Several things come into play.
1) Centrifugal timing advance - if your advance springs are letting the weights move anywhere in the "idle zone", then your timing will rollercoaster with the surging, making it impossible to tame. Use springs stiff enough to keep the timing firmly on "base timing" until 1,500 rpm OR totally lock your timing "all the way in". I prefer the first, since hot weather, low vapor pressure fuel, and 32+ degrees of initial timing make for some "hard to start" situations. If you are already a "race fuel motor" then its not as big a concern.
2) Power valves on carbs - if you are not boost referenced, and run higher number powervalves, you can run into idle instability in the carb metering. Boost referencing is recommended on powervalve equipped carbs - and boats don't need PV's in the secondaries, just upjet them.
3) Blower intertia - the additional parasitic drag and rotational momentum of a blower - combined with varying belt tension can be "just enough" to START the surging cycle. If you don't have #1 or #2 contributing to it, it will usually self-dampen the surge out, but if you DO have #1 or #2 contrinbuting, you have a problem.
BUT - I LIKE a little "dependable" Blower Surge. I think it's cool, and it also allows you to time your shifts to ease drivetrain shock.
XOXO
MC
Several things come into play.
1) Centrifugal timing advance - if your advance springs are letting the weights move anywhere in the "idle zone", then your timing will rollercoaster with the surging, making it impossible to tame. Use springs stiff enough to keep the timing firmly on "base timing" until 1,500 rpm OR totally lock your timing "all the way in". I prefer the first, since hot weather, low vapor pressure fuel, and 32+ degrees of initial timing make for some "hard to start" situations. If you are already a "race fuel motor" then its not as big a concern.
2) Power valves on carbs - if you are not boost referenced, and run higher number powervalves, you can run into idle instability in the carb metering. Boost referencing is recommended on powervalve equipped carbs - and boats don't need PV's in the secondaries, just upjet them.
3) Blower intertia - the additional parasitic drag and rotational momentum of a blower - combined with varying belt tension can be "just enough" to START the surging cycle. If you don't have #1 or #2 contributing to it, it will usually self-dampen the surge out, but if you DO have #1 or #2 contrinbuting, you have a problem.
BUT - I LIKE a little "dependable" Blower Surge. I think it's cool, and it also allows you to time your shifts to ease drivetrain shock.
XOXO
MC
05-29-2017, 02:37 PM
#6
Registered
05-29-2017, 05:36 PM
#7
Registered
iTrader: (3)
try turning the mixture screws in a tad. What is it idling at RPM wise out of gear?
As for powervalves, even if the power valve was opening at idle, it will have no effect on the idle itself. Powervalves add fuel to the main well, just like main jets do. You boat wont idle any different if you went from 82 jets, to 92 jets. The only way a powervalve will make it idle bad, is if the diaphram is ruptured and raw fuel is being sucked past it.
As for powervalves, even if the power valve was opening at idle, it will have no effect on the idle itself. Powervalves add fuel to the main well, just like main jets do. You boat wont idle any different if you went from 82 jets, to 92 jets. The only way a powervalve will make it idle bad, is if the diaphram is ruptured and raw fuel is being sucked past it.
05-29-2017, 05:43 PM
#9
Registered
iTrader: (7)
Will it help with 4-6 deg or more base timing. With good tuning you can tame it down but 90 % of the SC engines I've heard surge. Some patients adjusting air mixture screws certainly help.
