6-71 blower trouble

OK guys, this is where I stand. I'm getting closer!!!!!! I have it idling at 1500 once it gets to operating temp with no roll. The idle screws are only out 1/2 of a turn, which was one of my problems. I was starting at 1 1/2 turns and had a Detroit roll of 800 to 4000. I have done the changes on page 17 except for the plunger springs. They are on back order so the stock ones are still installed. Idle timing is 18 Deg. and 28 Deg at 3500. I can kill it by covering only one carb (no vacuum leaks). Fuel pressure is 6psi. I may be starving it some with 1 -6an feeding both carbs, but I'm only trying to idle it. I will convert it to two -6an's before F-bird gets a chance to chew my butt out. Here is my concern now: If I just touch the throttle the motor goes to 4500, and I mean just breath on it. If I just start to close one of the choke plates the idle climes uncontrollably. Does this mean I am starving this motor??? Keep in mind there is no drag on this motor. It is on a run up stand.


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idle screws 1/2 turn out and it runs better is making me think your running rich
on the idle circuit.
maybe blown power valve or leaking by on the bowl seats.
something is definately amiss there.

Ignition timing:

if you have the mechanical advance set up with a very fast curve (all in by 2500) it tends to be unstable and bounce around making idle surge all the worse.

You need lots and lots of idle base timing and a stable advance curve with less advance travel.

Something like 24 -26 inital base and smooth advance to 32+/- total at say 3200rpm.
Again, light weight quicky advance springs are unstable, and contribute/compound the idle surgeing.

For now for tuning carb set up purposes I'd like you to lock out the mechanical advance and run fixed spark timing of 28-36deg at idle.
Give it all the base idle timeing it wants (up to 36deg) as you advance the timing the rpm will pick up. then readjust the carb throttles further closed ( evenly between both carbs to get thenidle back down. Then when the blower case is warmed up start adjusting the idle mixture screws (evenly between both carbs) for best idle quality and rpm. Again readjust the throttles ( idle speed) as required to get the rpm back down as you gain rpm from adjusting the idle speed screws. it should idle nicely at 1000rpm. As it warms up it will like further leaning of the idle mixture screws.

You need two vacuum gauges to tune it. One connects to the intake manifold below the supercharger. the other connects to one of the carbs vacuum port.
Each will act a bit differently.

Use the first vacuum gauge that is on the intake manifold as a guide to adjusting the idle mixture screws. Best idle vacuum.
It is imperative to get the 2 idle speed screws and the 4 idle mixture screws adjusted evenly.

A AFR gauge/O2 exhaust sensor is a great tool for setting up the carbs.

The exhaust must be leak free. The O2 sensor should be mounted on the header collector. There must be about 20" (or more) of exhaust extension pipe after the 02 sensor on the exhaust to avoid the sensor reading free air /oxygen contamination drawn into the exhaust from exhaust reversion pulse at idle.
A heated type 02 sensor is best.
A wide-band type is best. (Innovate motorsports)
A simple narrow band system is great too. bit accuratly is limited to the 14.7:1 "stoichiometric ratio" AFR zone.
You want a idle mixture AFR somewhere in the 13:1 area. ( idle mixture adjustment screws)
You want the part throttle cruise AFR ( no boost part throttle cruise driving)
some where around 14.7:1 zone 13:1 to15:1 is good.
(Primary metering rod cruise step size. and primary jet combo.)

As soon as you start to get into the throttle beyond this, boost starts being created and you want to fatten up the AFR (fuel inrichment)
blower motors need a bit richer power AFR than N/A motors like.
You want to be 12.5:1 or a bit richer at part throttle (now with some boost comming in)
(primary metering rod power tip/primary jet pair size.)
metering rods should be raising out of the jet (rich power mode) at this throttle point

Further throttle (secondaries are opening) needs progresively further richer AFR as boost comes in.
10.5 to 11.5:1 power AFR zone up to full throttle high boost.
The AFR must remain power/ rich 11:1 ish at WOT as the rpm increases.
(secondary jets) A 10:1 full power/WOT AFR is not too rich. 9:1 is a bit to fat.

The 2 carbs accelerator pump shot need to be nice and fat to feed the big internal volume of the blower and intake manifold when you give it throttle.
Big shooters and max accelerator pump volume shot on the adjustable accel linkage. More pump shot volume is better.

Do a google search and bone up on tuning with a AFR gauge.

The purpose of the second vacuum gauge connected on the carb above the blower is to tell you if the carb's metering rods and down/lean or up rich power mode based on the vaccuum reading.

You want the metering rods down/lean at idle and light part throttle driveing ( no boost) .. You want the metering rods up/rich for heavy throttle up to full throttle.
At WOT max rpm and max boost the blower is pulling hard on the carbs and starts to create a vaccuum under the carbs. If this vaccuum exceeds the rating of the power piston/metering rod spring and pulls the rods down/lean, the engine goes lean. Lean is bad at WOT. This must be avoided. this is the purpose of boost referencing the power valve/power piston action referenced off the intake manifold below the blower instead of at the carb base.

Do not drive the engine with the timing set locked at 36deg.

On premimum pump gas:
The supercharged motor will want less WOT timing than 36deg under heavy throttle under boost. It will want around 27 to 32deg. This can be achieved using a "boost retard" ignition timing box that progresively pulls back the spark timing as the boost comes up. Or simply setting the fixed spark timing for 27 to 32deg.

High octane racing gas and or water/methanol injection will allow more spark timing, more boost and more power without detonation.

Detonation must be avoided.

You'll find that locking out the distributor advance curve for dialing in the carbs setup makes things a lot easier. You'll find that the blower motor wants generous idle spark timing to idle best. More is better. Don't be shy. Give it all it needs at idle.

Do not wack the throttle open when the blower is cold or you will get a haircut (back fire) Let the carbs/blower case and manifold warm up completely before attempting to wing the throttle open.
The accelerator pump shooters will need to be dialed in as well. (bigger shot)
The AFR gauge will show you what the motor needs for accel shot volume when you wack the throttle (warmed up engine) and watch the gauge.
You can drill out the shooters with small numbered pin drills or buy
new assortment of larger sizes.
Again the blower needs a big shot of fuel enrichment when the throttles are opened. Rich is better.

There are a few easy different ways to temperairly or permanently "lock out" the mechanical advance in the distributor.
Depends on what distributor you have.

What distributor are you using?

Covering the carb(s) is not going to tell you whether you have a vacuum leak, because you're shutting off the air flow through the carb(s) and that stops the fuel flow.

Those Edels really don't like more than about 5.5 pounds, and that's if you have the floats set properly (you DID reset the floats per the manual, right?). If you have an adjustable regulator, wind it down to about 5 pounds.

I'm running one -6 line from the pump to the regulator, then split to two -6s, one to each carb. Fuel starvation has not been a problem.



What kind of throttle linkage do you have? Progressive is much better than 1:1. With 1:1, you're opening up a lot of carburetion with not much throttle movement.

MSD 85551

It blows my mind what you know about this blower stuff. It is going to take me a little time to digest all you said.

Right now I have a 1 to 1 solid rod. When I get this monster in the truck I will go with progressive. I may be at 5.5 psi the needle bounces between 5 and 6 so it is kind of hard to nail down to 1/2 lb readings.

this is how you lock out the advance on that distributor.

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