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ED WALCOT 10-14-2012 04:30 PM

Blower back fire.
I am having a problem with my 426 cube inch motor . the motor has a bds blower systems with two 650 carters on top . W hat is going on is at an idle the motor run fine . Once I drive the car on the street i get passed one third throttle the motor back fires in the blower . I have retarted the timing to 32 d. total still have same problem . Also I have installed a HUASE FUEL PUMP . I STILL HAVE THE SAME .Help there must be some one out there that has run into this problem .

T-bucket23 10-14-2012 04:34 PM

Start with checking the float levels. It sounds like you are going lean. Make sure both carbs are actually getting fuel. If the secondary one is not it will lean out as soon as the blades begin to open.

techinspector1 10-14-2012 04:57 PM


Originally Posted by ED WALCOT (Post 1599097)
Also I have installed a HUASE FUEL PUMP

I have no idea what this is. Can you be more specific? Have you monitored the fuel pressure at the carb inlets? Is this a late Hemi (426)? 10-14-2012 07:00 PM

Power enrichment (power valves if it was Holley) systems are not getting correct signal or jetting too lean

F-BIRD'88 10-14-2012 07:03 PM

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?

F-BIRD'88 10-14-2012 09:05 PM

Its very likely the spark plugs are now carbon-fuel fouled and will need to be replaced.
Blower motors tend to like a spark plug that is slightly cooler heat range than
what you would using in a stock motor. 2 to 3 heat ranges cooler.
Racing plugs are not used on the street. Too cool .. Hot plugs are not used. Too hot (detonation, priignition)
Ignition faults and vacuum leaks must be eliminated. blower intake gaskets.
When the timing curve and carbs are set up right the plugs will no longer foul and the
backfireing and poor drivability will be cured.

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