Carb tuning
09-05-2013, 04:23 AM
#42
Registered
iTrader: (3)
They where 1350 Sterling to start with. I use most of Mike's recipe. I run cams from Hollman Moddy custom ground. They are very small but will run to 7500 and idle at 700 rpm. My ignition system is fully programable it has boost retard, retard for curb idle (neutral) and i run a timing curve; 36 degree total. I run race stripped 10.71:1 Mooneyham blower at 1.1 over drive. Special surface gap sparkplugs. I tune with Innovative O2 AFR meters in the collectors. I run blower shop intercooler forward motion feed only. I have single stage waterpumps raw-water only at 150 degree water temps. Twin in line modified 1150 Holley 3 circuit carbs(power-valves front and rear) 80 Mains Squared .080 PVRC 4.5 and 5.0 above the blower with the intermediate and idle circuits this equates to a 112 main jets at full throttle acceleration 12.9:1 AFR
09-05-2013, 10:25 AM
#46
Registered
iTrader: (3)
Dan, think of the carb as having 3 power circuits. Primary main, primary power valve, and secondary main. Primary mains are doing the work until 3500RPM. After that, the Power valve opens, adding fuel, and the secondarys come in, again adding fuel. In order to fix your rich spot, fuel needs to be pulled from somewhere. You cant pull primary jet, because you'd be lean at 3500. If you pull any more secondary jet, you'll be lean at WOT. So there lies a spot in the curve where there just is simply too much fuel being introduced. Powervalve circuit. If you remove fuel from there, and don't make any other changes, this will lean the wot AFR also. So, you would need to add more secondary jet.
So lets say you remove 4 jet sizes worth of fuel from the power valve circuit. You would then go up 4 sizes on secondary main jet, so your WOT AFR stays the same. So, you will go from a current setup of 88P/92S, to like a 88/96 Setup. Or 88/98 possibly. That is much more of a traditional jetting configuration, and fuel distribution will be better. Carb's can give very linear fuel curves. But they need to be set up properly.
Most people don't talk about power valve channel restrictions. When modified, they can have a huge effect as far as fuel changes. For example. Lets look at a holley #88 Main Jet. Drill size is .104. Now lets look at a #83 jet. Drill size is .094. So, theres a difference of .010 between the two. If I told you to pull 5 main jet sizes, youd expect a substantial change in AFR correct? Theres a lot more math to it, as you have to figure total flow area, etc. But as a quick guideline, reducing the PCVR by .010, would be like pulling 6 jet sizes worth of fuel from the overall flow. So you'll have to make that up if you want to keep the WOT afr the same.
To sum it up, lets go to the river, install a .010 wire in the pcvr's, and record the data. My guess is you'll prob end up with 88P/96S, no more 10's at 4000RPM, Better fuel distribution, and a safe wot AFR.
This is stuff we can talk about on the phone or at the dock. I am writing it here, that maybe it can help someone else out.
So lets say you remove 4 jet sizes worth of fuel from the power valve circuit. You would then go up 4 sizes on secondary main jet, so your WOT AFR stays the same. So, you will go from a current setup of 88P/92S, to like a 88/96 Setup. Or 88/98 possibly. That is much more of a traditional jetting configuration, and fuel distribution will be better. Carb's can give very linear fuel curves. But they need to be set up properly.
Most people don't talk about power valve channel restrictions. When modified, they can have a huge effect as far as fuel changes. For example. Lets look at a holley #88 Main Jet. Drill size is .104. Now lets look at a #83 jet. Drill size is .094. So, theres a difference of .010 between the two. If I told you to pull 5 main jet sizes, youd expect a substantial change in AFR correct? Theres a lot more math to it, as you have to figure total flow area, etc. But as a quick guideline, reducing the PCVR by .010, would be like pulling 6 jet sizes worth of fuel from the overall flow. So you'll have to make that up if you want to keep the WOT afr the same.
To sum it up, lets go to the river, install a .010 wire in the pcvr's, and record the data. My guess is you'll prob end up with 88P/96S, no more 10's at 4000RPM, Better fuel distribution, and a safe wot AFR.
This is stuff we can talk about on the phone or at the dock. I am writing it here, that maybe it can help someone else out.
09-05-2013, 10:58 AM
#48
Registered
Join Date: Aug 2005
Location: Concord, North Carolina
Posts: 71
Likes: 0
Received 0 Likes
on
0 Posts
Dan, think of the carb as having 3 power circuits. Primary main, primary power valve, and secondary main. Primary mains are doing the work until 3500RPM. After that, the Power valve opens, adding fuel, and the secondarys come in, again adding fuel. In order to fix your rich spot, fuel needs to be pulled from somewhere. You cant pull primary jet, because you'd be lean at 3500. If you pull any more secondary jet, you'll be lean at WOT. So there lies a spot in the curve where there just is simply too much fuel being introduced. Powervalve circuit. If you remove fuel from there, and don't make any other changes, this will lean the wot AFR also. So, you would need to add more secondary jet.
So lets say you remove 4 jet sizes worth of fuel from the power valve circuit. You would then go up 4 sizes on secondary main jet, so your WOT AFR stays the same. So, you will go from a current setup of 88P/92S, to like a 88/96 Setup. Or 88/98 possibly. That is much more of a traditional jetting configuration, and fuel distribution will be better. Carb's can give very linear fuel curves. But they need to be set up properly.
Most people don't talk about power valve channel restrictions. When modified, they can have a huge effect as far as fuel changes. For example. Lets look at a holley #88 Main Jet. Drill size is .104. Now lets look at a #83 jet. Drill size is .094. So, theres a difference of .010 between the two. If I told you to pull 5 main jet sizes, youd expect a substantial change in AFR correct? Theres a lot more math to it, as you have to figure total flow area, etc. But as a quick guideline, reducing the PCVR by .010, would be like pulling 6 jet sizes worth of fuel from the overall flow. So you'll have to make that up if you want to keep the WOT afr the same.
To sum it up, lets go to the river, install a .010 wire in the pcvr's, and record the data. My guess is you'll prob end up with 88P/96S, no more 10's at 4000RPM, Better fuel distribution, and a safe wot AFR.
This is stuff we can talk about on the phone or at the dock. I am writing it here, that maybe it can help someone else out.
So lets say you remove 4 jet sizes worth of fuel from the power valve circuit. You would then go up 4 sizes on secondary main jet, so your WOT AFR stays the same. So, you will go from a current setup of 88P/92S, to like a 88/96 Setup. Or 88/98 possibly. That is much more of a traditional jetting configuration, and fuel distribution will be better. Carb's can give very linear fuel curves. But they need to be set up properly.
Most people don't talk about power valve channel restrictions. When modified, they can have a huge effect as far as fuel changes. For example. Lets look at a holley #88 Main Jet. Drill size is .104. Now lets look at a #83 jet. Drill size is .094. So, theres a difference of .010 between the two. If I told you to pull 5 main jet sizes, youd expect a substantial change in AFR correct? Theres a lot more math to it, as you have to figure total flow area, etc. But as a quick guideline, reducing the PCVR by .010, would be like pulling 6 jet sizes worth of fuel from the overall flow. So you'll have to make that up if you want to keep the WOT afr the same.
To sum it up, lets go to the river, install a .010 wire in the pcvr's, and record the data. My guess is you'll prob end up with 88P/96S, no more 10's at 4000RPM, Better fuel distribution, and a safe wot AFR.
This is stuff we can talk about on the phone or at the dock. I am writing it here, that maybe it can help someone else out.
