holley 4160 vac secs blow back probs ???
hi all. i am in the process od trying to dial in my engine and timing with fueling to start my engine. i have a blown 351w with weiand 671 and 2x 600 holleys.
have sorted the ign at 16d at idel and curving to 28 d all in at 3000 rpm. my engine is really tight to turn over so i am having probs with getting it up to good rpm to start it (has been running breiefly before but timing ws wrong). while i am cranking every now and then i get a blow back through the top of the carb?? is the normal if the engine is turning for a few revoultions without starting with a blower ?
16° might not be enough initial. You can go higher, up to 24° or so if the starter will allow you to, w/o the starter becoming sluggish w/the engine hot.
Total of 32° or so, all in by 2500-3000 RPM or thereabouts.
A lot depends on the CR, the cam specs, how much boost you're making, the blower ratio, etc.
16 deg inital is not enough for a blower motor. They want as much initial timing as you can throw at it. More is better.
Lock out the distributor advance curve and set the fixed timing at 28° btdc.
If you back fired it replace the the power valves.
The power valves must be boost referenced.
Do not open the throttles when cranking to start.
Pump the throttle 1 or 2 times , get your foot off the throttle and start it up.
When it fires, Catch the throttle when it starts and hold at 2500rpm till it warms up.
Do NOT wing the throttle open when the blower is cold or it will back fire.
Let it warm up.
The timing must be 28 to 32 BTDC at idle or it will backfire etc etc.
Fixed locked out timing is best to get it up and running and dialed in.
richen up the carb jetting to find the tune. 68-70 primary---76-78 sec jets
The blower must warm up or it will back fire.
Stock 600 Vac sec holleys (1850) will need pri and sec jetting, boost referenced power valves may need idle circuit (idle feed restriction) and air bleed tuning.
Will need larger accelerator pump shooters,cams and may want 50cc pumps.
Remember , keep you big fat foot off the throttle when cranking/starting.
Last edited by F-BIRD'88; 10-25-2010 at 03:06 AM.
hi guys thanks for the info.
ok i have
jets as . primary = 70s and sec 72s
i have the purple spring in the vac secs
shooters in the primarys have .25 stamped on
i have what i think is the orange cam on the acc pump ( dont think its brown as its too light in colour)
power valves are 65 ( need to check vacum when running to double check they are half of manifold vacum !) they are the anti blow back protected ones, they are not leaking.
i have removed the ball bearing from the vac secs as advised in my supercharger book on holleys (good idea ?)
how much fuel pressure should i set my regulator at ? its at 6psi
also i do not have a dizzy. i have a omex ecu. this is what i have to use below !!
50 i can put any btdc figures in ( left numbers are throttle and below rpm )
so what do you think on a fresh engine just to bed it in and get it running ?
the carbs dont go bang ( like an explosion ) they have pressure coming out of the top which looks like air and fuel mix which is obviously pressurized< is it beacuse its not started and there is a fuel build up and its escaping out the top ??
the engine is very hard to turn. have drained my battery. brand new reallly
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