Here is a good one for you blower guys. I have a 8.5 to 1 468 BBC w/hyd roller cam. Motor is on a run up stand and did run very good. Other than the cam this motor is pretty much stock. I have installed a USED Weiand 6-71 on it. It came off a SBC so I had to buy the Intake, pully parts, and belt to make it fit the BBC. The intake is new. The carbs are matching 600 Holleys. I am running 39 tooth on the blower and 35 tooth on the crank, 9 percent. Here is the problem: As the motor turns over I can feel the vacuum pull through the top of the carbs but as soon as the engine starts, that vacuum pull goes away and it dies. The RPM is very high which I am sure is due to a lean condition. It is just burning the fuel I pumped in it to get it started. Carbs are fresh but not mine so I changed to a single 850 that I took off a running race car to eliminate the carbs as the problem. No change to the running condition. It acts like a vacuum leak so I siliconed the blower to the intake and the carb adapter to the blower. NO CHANGE AND GETTING FRUSTRATED. Got any ideas??????? Sarge

Running a 6-71 on a 468 at 9% under-driven you are making absolutely no boost. You need to run 1:1 just to make 2 pounds. In my opinion, a 6-71 is too small for a BBC. To make any appreciable boost you're going to have to over drive the blower by at least 10% (5 pounds). The faster you spin the blower, the more heat you add to the charge. Heat lowers your detonation margin.

Also, the interface between the blower case and the manifold is critical. You should use a gasket made for that purpose and carefully torque the mounting bolts. You do not want to distort the blower case as you can cause the rotors to be misaligned, put odd loads on the bearings, and have leaks.

I'm guessing that you are actually restricting the motor's breathing with that small, under-driven blower.

check the pop off valve
if its open itll do as you stated.
must be fully closed.
most used blowers valves are shot .
at least the ones ive seen.
heres my instructions for as 6-71
http://www.holley.com/data/Products/Technical/W538.pdf
if you dont have them
pop off valve instructions
http://www.holley.com/data/Products/Technical/W510.pdf

I'm not trying to build any boost really. This is going to be for show and sound. I have 3 go fast cars so don't really need another one. What is the blower supposed to be torqued at??? I will try changing pullies but don't really understand what that has to do with loosing the vacuum signal after the engine starts!!!

The pop off valve is located in the intake and is new.

Probably has nothing to do with it... just an old hot-rodder making an observation... I'll go back to my room now.

Oh yeah, here are the torque specs from BDS that I used with my 6-71.

My 454 is naturally aspirated.
Holley 750 (3310) on an RPM Air Gap intake.
My son and his buddies stopped by yesterday and wanted me to fire it up.
It's got an electric fuel pump, so I'm reasonably sure that the bowls were full.

Usually it's two pumps, and kit the key ... and it fires right up. It will die if you let off without giving it a minute to warm up a little.

Well, it's been quite cool here lately, and the temp in the garage was around 50° F. Two pumps, crank ... nothing.
4 pumps, crank ... nothing. WTH?

So I snapped the choke closed, cranked breifly, and away she went.

The moral of the story?
454's like fuel ... we all know that.

I know ... it's probably MUCH warmer than 50 in Texas, but you've got a lot more distance (and volume) between the carb(s) and the intake valves than I do.

Try the choke, and see if it helps.

Thanks for the torque specs. Any idea I get from anyone is worth looking into. This motor runs but only for a couple of seconds then looses its suck from the carb so the idle circuit is not receiving a signal which means no fuel. Sarge

bigger fuel line to the carbs from the pump?
I dont really understand your reasoning by going through all the trouble to put on a blower if your not gonna use it?

Thanks for the message. A lack of fuel is why it won't stay running, I'm sure, but the lack of fuel is because there is no vacuum through the top of the carbs to signal for fuel. Sarge

Not that I doubt you, but I just don't see how that is possible if the blower is turning. On mine, I can produce "suck" at the top of the carb just by turning the blower input pulley by hand (with the belt off, of course). Roots blowers are positive displacement, meaning if they are turning air is being moved. There has to be a huge leak somewhere between the carbs and the blower. Do you have all the vacuum ports in the carb bases plugged? Did you check the gaskets between the carbs and the adaptor plate? Is there a gasket between the adaptor plate and the blower case? (Ooops, just re-read your original post about the silicone... probably not the best solution.) Does the adaptor plate have any vacuum taps and are they plugged? Could the adaptor plate be warped, or even the blower case? Maybe a crack in the blower case or one of the end plates not sealed?

When I start my 6-71 SBC cold it takes a lot of gas to get it started and keep it running. It won't idle on its own for probably five minutes until it gets some heat built up. Until then I have to keep jazzing the accelerator pumps.

IAC (idle air control) valve?

Vince

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My project 34
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give weiand tech support a call.
if you dont have any vacum at the carb opening something is really wrong.
did you do a leakdown test on all cylinders?

How can the RPM be too high if it dies when it starts?

My guess is your starving it, swap the pullies top to bottom so you will be at least 9% over.

According to the BDS charts, they don't even reccomend a small bore blower at that ratio underdriven. Wonder why?

http://www.blowerdriveservice.com/techcharts.php

What camshaft is in this motor? What is the inital timing at idle?. idle rpm?

At what rpm are you attempting to warm up the motor at?