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Spike Morelli said:
That looks like an old Dyers "V" series blower set up. I'm looking also at those vacuum lines, are the secondaries balanced together as they should? Make for damned sure that the carbs have been "boost referenced" for proper power valve operation, don't block the power valves off. Yes, the idler pulley swing arm needs to be on the passenger side. The driver's side is the "pulling' side, and the swing arm isn't strong enough to deal with that tension.
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I have 2 x 4 barrel Holley 600cfm vacuum secondary carbs sitting on top of this GMC 671 blower, how would I go about balancing the vacuum on both carbs?

On the top of the carbs there is a vacuum diagraph with a small tail piece coming out of the top that a 2mm bore pipe could be attached to. Is this where a vacuum pipe ran to the distributor that had the vacuum advance and retard mechanism fitted? My distributor is a Mallory dual point type so it does not have a vacuum driven advance /retard mechanism, so I have plugged up these tail pieces. Is this the correct thing to do?

I have heard about the Holley carbs being modified for being used with a Supercharger, and I'm sure mine have not been modified. Do you know of a link that would explain how this is done please.
 
Heff, What is required, when using carbs, in this case Holleys, on top of a blower, is that they are "Boost Referenced". Here's why........with a normally aspirated engine, the carb will sense vacuum under it at idle and cruise, however, when you stand on the pedal, you open the butterflies and vacuum drops. The power valves in your Holleys are held closed by vacuum, and open during acceleration and low vacuum periods to enrichen the mixture at those times. BUT.....on a blown engine, the carbs are sitting atop a constant suction created by the blower's rotors directly below the carb(s), all the time. So the power valves are held closed full time with a blower, unless the carbs are modified for "Boost Referencing". Without the modification, your carbs will deliver a too lean condition at the worst possible time, acceleration. To accomplish this , the internal vacuum passages to the power valve are plugged, and a port drilled into the main body with a nipple on the outside of the carb, which is plumbed with either a hard line, or rubber vacuum hose, to the manifold BELOW the blower. At idle and part throttle, there is no boost ( most people don't realize this ), and is still in a vacuum mode at idle, holding the power valves closed. The blower has to overcome atmospheric pressure and the engine's own piston suction before delivering measureable boost. But once you nail the throttle, a blown engine goes into boost mode and the power valves will open. Carb shops can perform this modification to existing carbs, or better yet, you can buy brand new "Boost Referenced " carbs from Jegs, Summitt, Holley, etc. To "balance" the secondaries on vacuum secondary Holleys, you can buy the vacuum diaphram pod tops which have nipples, to run a vacuum line from one to the other to equalize the signal to open. Ford used these on their dual quad carbs from the factory, and any Ford guy should be familiar with them. I think the Holley part no. is 20-21, buy look it up, I may be mistaken.
 
Spike Morelli said:
Heff, What is required, when using carbs, in this case Holleys, on top of a blower, is that they are "Boost Referenced". Here's why........with a normally aspirated engine, the carb will sense vacuum under it at idle and cruise, however, when you stand on the pedal, you open the butterflies and vacuum drops. The power valves in your Holleys are held closed by vacuum, and open during acceleration and low vacuum periods to enrichen the mixture at those times. BUT.....on a blown engine, the carbs are sitting atop a constant suction created by the blower's rotors directly below the carb(s), all the time. So the power valves are held closed full time with a blower, unless the carbs are modified for "Boost Referencing". Without the modification, your carbs will deliver a too lean condition at the worst possible time, acceleration. To accomplish this , the internal vacuum passages to the power valve are plugged, and a port drilled into the main body with a nipple on the outside of the carb, which is plumbed with either a hard line, or rubber vacuum hose, to the manifold BELOW the blower. At idle and part throttle, there is no boost ( most people don't realize this ), and is still in a vacuum mode at idle, holding the power valves closed. The blower has to overcome atmospheric pressure and the engine's own piston suction before delivering measureable boost. But once you nail the throttle, a blown engine goes into boost mode and the power valves will open. Carb shops can perform this modification to existing carbs, or better yet, you can buy brand new "Boost Referenced " carbs from Jegs, Summitt, Holley, etc. To "balance" the secondaries on vacuum secondary Holleys, you can buy the vacuum diaphram pod tops which have nipples, to run a vacuum line from one to the other to equalize the signal to open. Ford used these on their dual quad carbs from the factory, and any Ford guy should be familiar with them. I think the Holley part no. is 20-21, buy look it up, I may be mistaken.
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Hi Spike. Thank you so much for all of this info. I know fully understand where you are coming from. Its been 17 years since I last had this engine up and running on the road and its all beginning to come back to me.

Secondary carb balancing.- yes I have the 2 little tail pipe nipples that come out of the vacuum pod and also have the vacuum line pipe with 90 degree rubber connectors on each end. I will make sure I fit these on.

I will look for a Carb shop that can carry out this modification. I will then run a vacuum pipe down from each carb to the blower manifold.

Will I still get that " Hunting " tick over once this carb mod is carried out ( the revs on tick over range between 500 to 1000rpm ) . I run the mixture screws of both carbs ( 4 of to adjust, 1 on each of the primary barrels and 1 on each of the secondary barrels ) fully screwed in and then unscrew about 3 1/2 turns out. I have done this on all 4 mixture screws. This gives me a good steady idle at 700 rpm, no " hunting" but the exhaust fumes smell rich. If I screw in the mixture screws 1 full turn ( which would give me 2 1/2 turns out from being fully screwed in ) the " Hunting " starts, and the rpm ranges from 500 -1000 but the exhaust fumes do not smell rich.

Would this be the normal setting for a blown engine?
 
Any Roots blown engine likes the mixture a tad rich, especially on gas, because gasoline ( regardless of octane number) doesn't have the cooling properties that, say, alcohol, or E-85, and the blower case will run hotter, and the gas flowing over the rotors is the only thing that cools it, so towards the rich side is o.k. Better to change plugs, than pistons Now, "hunting", or "surging", is a sign of too rich a mixture, sometimes even too lean a mixture, especially when mechanically injected. Real blower pros know when they hear a "surging" street blown car, that it's not necessary, and the owner just wants people to think they're engine is "just so radical". My own blown gas dragboat has a vernier adjustable fuel shut-off handle that turns in and out to partially close off fuel to the injection outlet on the pump, to give me a clean idle and when leaned, helps build heat in the heads when idling. Just before making a pass, I turn it to full on. However you tune your blown engine, always let the spark plugs tell you where the mixture is, not the smell of the exhausts
 
If my explanation on "Boost Referencing" was rambling a bit, here is a sheet I sent to some customers who ask about it. ( Yes, I spelled "aspirated" wrong on the form, but left it)
 

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Spike Morelli said:
Any Roots blown engine likes the mixture a tad rich, especially on gas, because gasoline ( regardless of octane number) doesn't have the cooling properties that, say, alcohol, or E-85, and the blower case will run hotter, and the gas flowing over the rotors is the only thing that cools it, so towards the rich side is o.k. Better to change plugs, than pistons Now, "hunting", or "surging", is a sign of too rich a mixture, sometimes even too lean a mixture, especially when mechanically injected. Real blower pros know when they hear a "surging" street blown car, that it's not necessary, and the owner just wants people to think they're engine is "just so radical". My own blown gas dragboat has a vernier adjustable fuel shut-off handle that turns in and out to partially close off fuel to the injection outlet on the pump, to give me a clean idle and when leaned, helps build heat in the heads when idling. Just before making a pass, I turn it to full on. However you tune your blown engine, always let the spark plugs tell you where the mixture is, not the smell of the exhausts
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Spike, thanks so much for this info, and it makes sense to let the spark plug condition tell me what's happening inside the engine. Really appreciate your knowledge on blown engines.
 
Spike Morelli said:
Sure, I have posted these before...
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I presume you have the helmet because a hat would get sucked right in there?....can't imagine that would be good for the blower.
 
Just part of the mandatory safety gear, but you may as well look good, so it's painted to match the boat. Knock on wood, I've never been thrown out. They say if you jump out of an airplane, the average person will attain a velocity of approximately 120 mph....this thing goes faster than that. I'm not so much aware of the engine winding right behind me, as I am of the wind blast. Wide open throttle is intense, like an orgasm and a car crash happening at the same time .
 
Spike Morelli said:
Just part of the mandatory safety gear, but you may as well look good, so it's painted to match the boat. Knock on wood, I've never been thrown out. They say if you jump out of an airplane, the average person will attain a velocity of approximately 120 mph....this thing goes faster than that. I'm not so much aware of the engine winding right behind me, as I am of the wind blast. Wide open throttle is intense, like an orgasm and a car crash happening at the same time .
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Spike, that is one serious drag speed boat, a Blown Hemi of all things. impressive or what. Is this engine driving a water jet turbine or impeller?
 
Spike Morelli said:
Just part of the mandatory safety gear, but you may as well look good, so it's painted to match the boat. Knock on wood, I've never been thrown out. They say if you jump out of an airplane, the average person will attain a velocity of approximately 120 mph....this thing goes faster than that. I'm not so much aware of the engine winding right behind me, as I am of the wind blast. Wide open throttle is intense, like an orgasm and a car crash happening at the same time .
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I guess that works if a person has trouble choosing between the two.
 
The Heff said:
Spike, that is one serious drag speed boat, a Blown Hemi of all things. impressive or what. Is this engine driving a water jet turbine or impeller?
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Jet Turbine?????? Impeller?????? How dare you!!! V-driven propeller is the ONLY way to get the job done. Stay after class and write that 100 times.
 

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He He. V-driven propeller is the ONLY way to get the job done. V-driven propeller is the ONLY way to get the job done. V-driven propeller is the ONLY way to get the job done.V-driven propeller is the ONLY way to get the job done
 

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