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Old 10-14-2012, 07:03 PM
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Incorrect timing curve for a roots style blower motor.
Blower motors love lots of timing at idle. but conservative timing when under boost WOT.
the timing curve is a lot different than a N/A motor.
lack of idle timing makes it back fires thru the carbs. and generally load up and run like pooo.

lack of fuel system fuel flow (you are staving the carbs for fuel) under boost heavy throttle and or lean carb jetting.

The bigger the camshaft the more the blower motor will want generous timing at idle.
Many blower motors want full locked out timing 34-36deg at idle and part throttle
but need the timing to retard progressively as throttle and boost is added WOT.
that what a boost retard box is for.

typcial is 34-36deg at idle and part throttle light cruise and progressivily retards to 26-28-30deg timing under boost depending on the boost- compression ratio and fuel octane used.

a stock timing curve will not work. You need to double the fuel flow volume
fuel pump(s) volume fuel lines size
filters and fittings size tank pick up tube(s) are all critical. stock stuff will not work.

stock carb jetting will not work. metering rod power step spring choice is critical.
start with the sfiffest spring 8" vacuum rated.
750cfm carbs are best. for a ton of reasons. Blowers need big carbs.

stock jetting for a N/A motor will not work.

start carb jetting tuning plenty rich and work from there. Lots of fuel flow volume. not high pressure.
fuel pressure 6psi. two carter p4070 fuel pumps 72GPH rated each work really good thru two separate 3/8" fuel lines.
once for each carb.

what cam? what blower what pulley ratio?

Last edited by F-BIRD'88; 10-14-2012 at 07:10 PM.
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