Holley 950 hp on BBF. I have 11-12 Hg of vacuum, intial timing set at 15 btc, 6.5 pv. It's only the front passenger side doesn't do a thing, the other 3 are working as they should. Iv'e had the float bowl off and metering block off and blew them out with air, reassembled and it's still the same. Any ideas ?
I have them exactly 1 turn out. My wideband hovers around 14-15 :1 at 900 rpm. I can turn them out more but the afr goes down to 11-12:1. forgot to mention the idle transfer slot is perfectly square.
Turning either primary mixture screw in to full seat should stall the engine, symptom of defective power valve?
The left side works normally as do the two rears, the power valve in the rear is blocked. The right side does nothing, whether it's bottomed out or removed altogether. It's pulling the fuel mixture from somewhere. The engine runs great, very crisp throttle response. I'm kinda puzzled. I may change out the power valve tomorrow see if it makes any difference.
You either are getting enough fuel from the 3 operational idle jets or its leaking in , it's up to you to find it . You may have to pull a well plug to find the problem , stick a pencil eraser over the offending idle air bleed , if no change then you know there's a blockage or vacuum leak....
Power valve feeds both sides of the carb at the same time...so changing power valve is a waste of time, that isn't going to change anything. You've either got a blockage in the main body or metering block, or maybe an incomplete drilling in either the main body or metering block....this drilling issue is something to definitely check if this is a new carb acting this way.
Not on a 4 corner idle carb it won't. Engine can idle fine with any 2 of the 4 mixture screws completely closed if the other two are open enough to provide enough of the idle emulsion mix to keep the engine happy. Doesn't matter if they are fronts, rears, or one of each.
This was my first thought, an incomplete drilling or possibly a burr from casting/machining. It should definitely do something. How's the needle look?
Run the idle screw all the way in GENTLY back it out 2 turns. While idling slam your hand or something to seal the top, this will suck crap out of the circuits. Sent from my LG-TP450 using The H.A.M.B. mobile app
Odd that. If you remove the idle screw it becomes a direct vacuum leak and the engine RPM should have jumped up. Try taking it out again to confirm, if it doesn't suck air then the idle vacuum source and possibly the transfer slot vacuum source are plugged, maybe the throttlebody gasket or something. The circuit origin is those two holes seen beneath the throttle blade and it extends up thru the throttlebody to those 2 holes, side by side, at about 7 o'clock looking at the mating face of the throttle body. The metering block mates to it and that is right where the idle mixture screw is located supplying fuel for both idle & transfer slot. If you pull the screw out then that should be a direct vacuum leak as the passage will run from the missing screw down to the hole below the throttle blade and open to manifold vacuum.
Just for the fun of it I swapped the metering blocks around. Before the swap it was the right front mixture screw that didn't do anything, after the swap it's the left rear that's unresponsive. At least now I'm getting somewhere. I suspect somethings not right with the metering block? I also plugged off the front P/V just to rule that out. It's also a used carb. When I remove the mixture screw it does pull air through but doesnt change the rpm.
I have found there are at least two causes of this problem. One is excess material on the metering block gasket that restricts or blocks the idle passages. If you look on the metering block you will see a long square passage that has two restrictors at the bottom. CHeck these with a small wire and make sure they are clear. The idle well could also be blocked from debris or congealed gas. Carb cleaner (spray) will usually open this well. I have had to remove the well (from the top)and clean it in the past.
I have a friend that has a 440 6 pac. He was having a similar problem and I took the metering block and soaked it in immersion carb cleaner (a brand new pail) for 24 hours. Cleaned it up with hot water and blew out all the passages after that. After reassembling the carb it ran like new. I suspect that over time a build up of crud was restricting passages than wouldn't come clean with whatever method he was using to clean things up.
I'd replace that primary metering block with a new one... Contact Holley and see if they are willing to replace it free of charge....
Carefully inspect your gasket. A "hanging chad" (most likely ove the idle air bleed) could be the culprit. I spent a lot of time finding one of these on an OT car one time. It helps if you have a known good gasket to overlay on the gasket you're using.
From this test you learned there is nothing wrong with either the main body passages nor the baseplate...it is in the metering block for sure. Blockage of some kind between idle feed restriction, main well, and the idle mixture needle. I wouldn't expect much idle speed change from so small a vacuum leak as that, especially when it is rich enough to run off the other three needles. Being a used carb, metering block might need a good 24 soak in GOOD carb cleaner to clear out some kind of hard deposit/varnish, or it may just be easier to replace it. https://allcarbs.com/product/holley-950-hp-metering-block/ https://allcarbs.com/product/hp-billet-metering-block/
Good thinking. Odd it didn't change the rpm. The metering seems to have the problem, I'd guess it is new enough that the idle restrictor is at the top of the circuit. Looking at the face of the metering block (it'll be turned around, don't mix up the left & rights) the passage(s) you need to deal with get the fuel supply down low, about 5 o'clock there's a rectangular recess with 2 holes, the left hole is the supply fuel that is drilled into the main well; the right hole supplies the circuit for idle & transition fuel, the hole connects to a passage that rises straight up internally to near the top of the metering block. Spray carb cleaner into it down low and should come out up high at the 'idle restrictor', this is a removable jet with about a .031 hole that supplies fuel for both circuits, that passage is wide open, a channel milled into the metering block. It drops down to 2 holes, the outter hole supplies fuel to the idle mixture screw, the inner hole is for the transition circuit. So there you have it, the idle circuit is mostly wide open for inspection, the only concealed section is where the fuel rises after it gets it supply and terminates at the restrictor. I would spray into that lower left hand hole, it should come out the main jets and bubble around the PV holes if you have it out. That means the circuit has supply fuel. Spraying into the lower right hand hole it should come out at the top where the restrictor is located means the internal passage is clear. Since this is a used 950, back when they first started using aluminum metering blocks there was a machining problem, the idle circuit supply (the rectangle at 5o'clock with 2 holes) could leak fuel into the vacuum source for the distributor - a stock holly the passage is nearby - when they released those metering blocks they had a special metering block gasket, the PV hole is round, not a tombstone shape and that gasket sealed off the vacuum from the idle fuel. When people rebuilt their carb they would use the normal tombstone shaped PV opening and THAT would let the idle circuit fuel bleed straight into the vacuum source. It'd both kill the idle circuit on that side and cause extra rich idle. They've since moved the supply on the newer metering blocks.
Good info. The IFR's are non -removable in this carb. What I did notice in the channel you mention is the one thats giving me trouble has tarnished like color running the length where as the other side still looks like new. The carb as far as I can tell by the casting numbers on the metering blocks was built in 2014 and is made of the regular pot metal Holley used. It also has no provision for ported vacuum, although it does have the channel and the hole for it, it's just not drilled or tapped. I have a gallon of Chem-dip that I'm going to let the parts soak in for a day, reassemble and try it again. If that dont work I might have to buy another metering block.
I believe as mentioned above your issue is in the metering block or metering block gasket. You can verify metering block passages with compressed air or a can of brake clean/carb cleaner. Just make sure the innies and outies are all opened up. Then inspect your gasket to see if maybe it's on backwards or just incorrect. You would see a hole in the main body that needs to connect to the metering block, and the gasket would not have the hole if this was the case. Been there done that (my fault on assembly).
I think that's exactly whats going on. I set the mixture screws to about 3/4 turns out and according to my wideband does show the mixture going leaner at idle, so they do work, sort of. I also decided to use the carb at last weekends old time drags in place of my QFT-750 AN. My previous best was an 11.43 @ 116 .22 mph. My new best is an 11.31 @ 117.89, so it did pick up a bit. With a bit of tinkering with shooters, jetting, etc, It should be in the mid to low 20's at close to 120 mph.
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