Slant Six Forum :: Air/Fuel readings

It's telling you that your accelerator pump shot is puddling on the intake manifold floor when you punch it a low RPMs.

A 4bbl carb allows very low manifold vacuum at WOT and low RPM so the fuel barely vaporizes and the intake air velocity is too low for atomized fuel to be swept along to each cylinder.

Because you are running headers, your intake manifold floor is cold and there is no way for the puddled fuel to easily vaporize back into the air stream. At higher RPMs, there is a bit more vacuum to vaporize the fuel and the higher air velocity is better at conveying the atomized fuel that doesn't hit the floor to the cylinders.

You need to add more fuel or a hot spot under the carburetor. See Factors Affecting Distribution in Holley Carburetors & Manifolds by Urich & Fisher, p. 13-14, )

Author:	Bren67Cuda904 [ Wed Feb 27, 2008 5:19 pm ]
Post subject:	Air/Fuel readings
Finally got a system that works hooked up. It wasn't as bad as I thought it would be. At WOT I was getting 13.8-14:1. Changed the rear jets from 57 to 58H and at WOT its now 13.1-13.2. To change them larger would require buying more jets as my jet kit starts at 64 Off idle hesition: At idle, punch the gas and it LEAN (18:1) bogs for a half a second and then goes. If I hold the brakes and push the RPMs up to 2500rpm (1/4 throttle) then punch it, the hesition is much less. What is this telling me? Shooter not large enough? Cam not large or long enough with not large enough shooter? Not enough info?

Author:	Shaker223 [ Wed Feb 27, 2008 5:29 pm ]
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I would agree. Check your pump shot and make sure it is working properly. Also check your squirter size too. It does sound like the one in there is to small.

Author:	dank10fenny [ Wed Feb 27, 2008 5:30 pm ]
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you sure its not more carb based? so when you get a weak/lean shot at take off your givin it too much breath? hence the stumle. and when you load the brakes before take your getting the carb more into a cruise state? im still learnin properties of diff cam durations and lifts though, so bare wit me

Author:	FrankRaso [ Wed Feb 27, 2008 7:01 pm ]
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It's telling you that your accelerator pump shot is puddling on the intake manifold floor when you punch it a low RPMs. A 4bbl carb allows very low manifold vacuum at WOT and low RPM so the fuel barely vaporizes and the intake air velocity is too low for atomized fuel to be swept along to each cylinder. Because you are running headers, your intake manifold floor is cold and there is no way for the puddled fuel to easily vaporize back into the air stream. At higher RPMs, there is a bit more vacuum to vaporize the fuel and the higher air velocity is better at conveying the atomized fuel that doesn't hit the floor to the cylinders. You need to add more fuel or a hot spot under the carburetor. See Factors Affecting Distribution in Holley Carburetors & Manifolds by Urich & Fisher, p. 13-14, )

Author:	MitchB [ Wed Feb 27, 2008 7:35 pm ]
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Quote: It's telling you that your accelerator pump shot is puddling on the intake manifold floor when you punch it a low RPMs. A 4bbl carb allows very low manifold vacuum at WOT and low RPM so the fuel barely vaporizes and the intake air velocity is too low for atomized fuel to be swept along to each cylinder. Because you are running headers, your intake manifold floor is cold and there is no way for the puddled fuel to easily vaporize back into the air stream. At higher RPMs, there is a bit more vacuum to vaporize the fuel and the higher air velocity is better at conveying the atomized fuel that doesn't hit the floor to the cylinders. You need to add more fuel or a hot spot under the carburetor. See Factors Affecting Distribution in Holley Carburetors & Manifolds by Urich & Fisher, p. 13-14, ) Even with port fuel injection, you have puddling. And not all the fuel that makes it into the engine enters into combustion. Some fuel is lost out the exhaust or down the cylinder bores. The latest car calibrations account for puddling and lost fuel. In the end, even if manifold vacuum and velocity drop very low, you have to add more fuel - until you reach the mechanical design limits. Mitch

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Author:	Bren67Cuda904 [ Wed Feb 27, 2008 8:40 pm ]
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I now have a LM-1 data logger with the RPM input. It also allows for 4 addtional input (5V) I would really like to add a throttle postion sensor to tie to the data logger. Any cheap easy ways to do this with a Holley carb?

Author:	LUCKY13 [ Wed Feb 27, 2008 8:54 pm ]
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Go with the Blue, or the Orange pump cam in the number 2 possition. It needs a more agressive pump, and maybe even a bigger quirter. But start with the cam first. The main problem is your still to lean and your trying to cover up for that. When the jetting is right it will not take as much pump shot to work. If your out of bigger jets(which you need to be richer) you can take a piece of wire and run it from one main air bleed to the other. What I do is make it long enough to wrap around the air vent tube so it cant get loose and go down the inside the carb throat. After wraping it around the vent tube stick both ends down in each main air bleed (one end in each bleed). This is doing the same thing as installing a smaller main air bleed and will make it richer. Not something you want to do for permanet fix, but it will show you if your going in the right direction. The primaries are what needs to be richer. You need to get down to around 12.5 A/F at WOT. You can drill out the Power Valve channel restricters also to get more fuel without adding bigger main jets. But this doesnt help the off idle leaness or hessitation. You dont need heat, you need fuel. With the colder intake a good carb with a annular booster setup would help give even more power and make the throttle response and drivability much better than it is. It will also make the torque convertor work better and give better fuel milage. WIthout the heat the fuel does not evaperate as good, the annular boosters will make the fuel drops smaller to help this and it will also create more power. I have even noticed that on the slant six with its cylinder distribution problems that a carb with annular booster gives better distribution because the fuel is in smaller drops and this lets it turn in the intake better without dropping out of the air and becoming flowing rivers on the bottom of the intake. From where you are at the carb setup and fuel mix will add another 30 HP. That may seem like a stretch, but it is not. It will take more than just getting it richer to get it there. After the fuel is right you will find the timing to be much more forgiving, but more responsive to the right changes also. Just getting the primaries richer is going to help a ton though. Jess

Author:	Doctor Dodge [ Wed Feb 27, 2008 11:13 pm ]
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What is the current power valve's rating? Do you have a vacuum gauge in the car? If so, what is the idle vacuum reading, what is the vacuum reading when the carb goes lean? What is the vacuum reading when the engine "goes" again? I think you need a higher rated power valve. DD

Author:	Bren67Cuda904 [ Thu Feb 28, 2008 4:34 am ]
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Quote: I now have a LM-1 data logger with the RPM input. It also allows for 4 addtional input (5V) I would really like to add a throttle postion sensor to tie to the data logger. Any cheap easy ways to do this with a Holley carb? I'l have to lookup the PV. Anyways to do the above?

Author:	Bren67Cuda904 [ Thu Feb 28, 2008 5:01 am ]
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Quote: 570 Street Advenger. Orange cam 59 primary 58H secondary 45 power valve 31 nozzle Purple spring. I still need to get the vac. readings

Author:	LUCKY13 [ Thu Feb 28, 2008 12:58 pm ]
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The Holley site list this carb as having 54pri & 65sec jets OTB. With no PV in the back there should a big spread in the mains jets like this. You can log each set of three cylinders A/F seperate cant you? I just wander if the three that are feeding off the secondaries mainly are running lean at WOT. Orange cam, which hole? #2 would give more squirt. Jess

Author:	Slant Cecil [ Thu Feb 28, 2008 2:29 pm ]
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Bren, I need jets smaller than 64, between 61-63. You have any? I have squirter discharge nozzles if you need some, up to #37 and a few drilled larger than that. I also have some pump cams and sec dia springs.

Author:	Bren67Cuda904 [ Thu Feb 28, 2008 2:50 pm ]
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I am missing that bunch of jets also. I'll have to look to see what shoots I have. I got all the springs and cams too. I've been looking at the 80cc pump kits but I'll exhaust testing with what I have at this point.

Author:	Doc [ Thu Feb 28, 2008 3:33 pm ]
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Quote: Quote: 570 Street Advenger. Orange cam 59 primary 58H secondary 45 power valve 31 nozzle Purple spring. I still need to get the vac. readings My guess is that it goes lean around 8 to 12 inches of vacuum and "comes-back" at 4.5 (a #45 power valve should open at 4.5 inches) DD

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Author:	argentina-slantsixer [ Thu Feb 28, 2008 4:53 pm ]
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I'm with doc. Also my experience on big holleys over slants is that they require at least a #28 squirter and an aggresive cam on the accel pump, all of that matched with a 10.5 power valve (in my case) you should get readings on cruising vac, and then go with the largest (highest) power valve rating. I've used 12.5 with GREAT success but moved back to 10.5 for cruising economy.