what cfm carb should i get?
10-13-2010, 09:00 PM
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what cfm carb should i get? I have a old zz3 crate motor, it's been rebuilt with speed pro .125 dome std bore. comp roller 236. 240. 570 590. 1.6 roller rockers, dart pro1 heads 230cc runners 2.08 valves. edelbrock performer rpm air-gap. I have a holley 750 double pumper but it run's way to rich what would be a good carb for this motor? or am i doing something wrong with the double pumper? Tired of burning plugs HELP!
10-14-2010, 03:53 AM
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that 750cfm holley should be perfect... get a holley tuning book and read it front to back...
you'll probably need to change to smaller jets, and/or smaller discharge nozzle, different power valve for your application... i usually do away with the power valve and just use a plug then jet accordingly..
a good carburetor isnt a bolt-on and go piece, you have to tune it..
you'll probably need to change to smaller jets, and/or smaller discharge nozzle, different power valve for your application... i usually do away with the power valve and just use a plug then jet accordingly..
a good carburetor isnt a bolt-on and go piece, you have to tune it..
10-14-2010, 12:47 PM
#4
750 double pumpers, (both Holley and BG) are notorious for running rich at idle. Assuming you have the correct power valve installed and your ignition system is good, the idle feed restrictors are likely too big for your motor. The IFR's are really sized for bigger motors like big blocks. A simple fix is to insert a piece of wire into each idle feed restrictor port in the main metering blocks. Many articles and pictures are around on the web to show you which ports I'm talking about. This is a classic fix. You should have a big enough strand of wire to allow each idle mixture screw to be about 1 1/2 turns out from fully seated. It will take some experimentation to get the right balance of idle quality and off idle driveability.
10-14-2010, 03:26 PM
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Start all carb tuning by gettin your timing correct. That is a pretty big cam with HUGE cylinder heads on a 350. You are going to need a ton of timing advance at idle. I take it you are not running pump gas? 20-24* mechanical advance at 900RPM idle, with an additional 15* vacuum advance should be a good starting point. Once you get enough timing advance, you will be able to turn your idle screws back in so the tranfer slots aren't too exposed...this is most likely why you are running so rich.
10-14-2010, 10:13 PM
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Start all carb tuning by gettin your timing correct. That is a pretty big cam with HUGE cylinder heads on a 350. You are going to need a ton of timing advance at idle. I take it you are not running pump gas? 20-24* mechanical advance at 900RPM idle, with an additional 15* vacuum advance should be a good starting point. Once you get enough timing advance, you will be able to turn your idle screws back in so the tranfer slots aren't too exposed...this is most likely why you are running so rich.
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10-15-2010, 12:23 AM
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i would lock out the vacuum advance...
i usually turn the air mixture screws all the way in, then back them out 1 1/2 turns, then bring my idle up to where i want it..then start fine tuning the jetting....
71 jets sound kind of fat for a 350... seems like i remember a solid starting point at 68 jets for a decent built 350, and seems like a #32 to 36 discharge nozzle..
i usually turn the air mixture screws all the way in, then back them out 1 1/2 turns, then bring my idle up to where i want it..then start fine tuning the jetting....
71 jets sound kind of fat for a 350... seems like i remember a solid starting point at 68 jets for a decent built 350, and seems like a #32 to 36 discharge nozzle..
10-15-2010, 06:48 AM
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Your combo isn't much different than mine. I have a stock 350, 10.8:1 compression, ported AFR 190's, Air Gap and a Mighty Demon 750. Looks like we have the same/similar cam. I am running 22* initial, 52* at idle with vacuum and two light springs in my HEI. My HEI only has 15* mechanical advance, so my timing at WOT is 37*. It took me a ton of messing around until I settled on this.
My carb has 76/80 jets, 37 squirters, 8.5PV (fron only), blue pump cams and I did mess with the IFR's to lean out the idle some. Again, it took an entire summer of messing around to get to this point.
My carb is a little bigger than yours, but with dome pistons, you probably have more compression. Go up slowly with the timing so you don't ruin anything. Bump up your base 5* and put in clean plugs (use the cheapest you can find. I run Autolite 3923's). Warm it up and go for a ride and see how it acts. Take notes and only change one thing at a time. Don't mess with the carb until the timing is squared away. You'll get there!
My carb has 76/80 jets, 37 squirters, 8.5PV (fron only), blue pump cams and I did mess with the IFR's to lean out the idle some. Again, it took an entire summer of messing around to get to this point.
My carb is a little bigger than yours, but with dome pistons, you probably have more compression. Go up slowly with the timing so you don't ruin anything. Bump up your base 5* and put in clean plugs (use the cheapest you can find. I run Autolite 3923's). Warm it up and go for a ride and see how it acts. Take notes and only change one thing at a time. Don't mess with the carb until the timing is squared away. You'll get there!
10-15-2010, 08:48 PM
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alright, tomorrow im am gonna put the gold springs in the h.e.i which has a digital m.s.d module with a pertronix coil 60k volts. advance the hell out the timing and maybe try putting the jets i have in there, 71 in the back and 68 p front? set the idle screws at 1 and 3/4. this guy was telling me he can clean foul plugs? i'm using autolite racing 3924 with 45 gap
10-19-2010, 07:08 PM
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anybody know how to get rid of the choke blade? i don't use it and it would prob help get more air in the engine! so i've been reading all kinds of articles and stuff on the web and i'm sure i can tune it. so what i think my biggest problem was I had my fuel pressure up to 10 psi! they say anything over 7 will open the needle and seat and flood the motor at idle? and to figure out which power valve i need i should check it with a vac. gauge with it in drive and go 1-2 steps down from that reading. they say blocking it off makes you jet it up and lose power in the mid range. it has #41 squirter's in the back, before i go buy the whole kit $100 bucks i figured i'd ask you guys with more experience!!
11-12-2010, 04:42 PM
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it's a silverado '92. I changed the front jets to 68 and the back ones to 78! put the idle air screws at 1 1/2 out all four. so then i take a look at my floats with the pump on and wow looked like a water hose! set the floats so they don't leak out. put ar-3923 plugs and fired it up, drove down the road cruising and back pulled the plug and OMG it's black with a tan tip!!! throttle response is great! pancherj, the vac. can was out like 40 turns! i got it out at 5 turns right now. and the motor seems to not want much timing! but this is on pump gas.. 94
11-15-2010, 07:19 AM
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You said that it seems like it doesn't want much timing...what do you mean? How was it acting. If you went for a light cruise and it turned the plugs black, I would suspect it has too little timing advance (also, the tan tip and black base is another possible indicator of timing issues). How does it idle (RPM and vacuum reading)? What is your fuel pressure set at?
You have the makings of a solid performer...you just need to fine tune it. Go back up to my post where I shared my settings. That should be a good ball park start for you. Tell us what your timing advance is without vacuum canister, with vacuum canister, total advance without vacuum and what RPM total is reached. After this, I would return the carb to near stock. You have a 10 jet split from front to back...that is pretty big. Stock jets, power valve and pump cam should be a conservative starting point.
You have the makings of a solid performer...you just need to fine tune it. Go back up to my post where I shared my settings. That should be a good ball park start for you. Tell us what your timing advance is without vacuum canister, with vacuum canister, total advance without vacuum and what RPM total is reached. After this, I would return the carb to near stock. You have a 10 jet split from front to back...that is pretty big. Stock jets, power valve and pump cam should be a conservative starting point.
11-17-2010, 08:58 PM
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You said that it seems like it doesn't want much timing...what do you mean? How was it acting. If you went for a light cruise and it turned the plugs black, I would suspect it has too little timing advance (also, the tan tip and black base is another possible indicator of timing issues). How does it idle (RPM and vacuum reading)? What is your fuel pressure set at?
You have the makings of a solid performer...you just need to fine tune it. Go back up to my post where I shared my settings. That should be a good ball park start for you. Tell us what your timing advance is without vacuum canister, with vacuum canister, total advance without vacuum and what RPM total is reached. After this, I would return the carb to near stock. You have a 10 jet split from front to back...that is pretty big. Stock jets, power valve and pump cam should be a conservative starting point.
You have the makings of a solid performer...you just need to fine tune it. Go back up to my post where I shared my settings. That should be a good ball park start for you. Tell us what your timing advance is without vacuum canister, with vacuum canister, total advance without vacuum and what RPM total is reached. After this, I would return the carb to near stock. You have a 10 jet split from front to back...that is pretty big. Stock jets, power valve and pump cam should be a conservative starting point.
my biggest concern is that i can't run on pump gas??
11-18-2010, 06:49 AM
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Getting "chopyy" and popping through the intake sounds like the timing is retarded to me. Detonation sounds more like a ping (similar to a header leak). To advance the timing, you should be turning the distributor counter-clockwise (when you are standing at the front of the truck). You really need to get the timing sorted out. Buy a cheap timing tape or just make your own temporary version with masking tape and a black marker. I prefer to use the dial-back timing lights. However, the newer ones seem to not work as well with MSD and other ignitions. Mine is ancient and it always works fine!
11-30-2010, 09:11 PM
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I got the mid weight springs in my hei. I was told i should run one loose one and one mid? and also just have one arm with a middle spring to get the timing to advance earlier? the carb is new with 71 jets up front and 81's in the back. I know a little about tuning carbs, I tune my brothers 600cfm easy! just seem like this combo is very hard to tune! maybe to big of heads for stock c.i.d? when rebuilt all we did was have the machine shop put new bearings and clean it up. crank and rods are stock forged and didn't need turned. maybe lock out the vacum advance? right now i'm running pump gas 94 with 32 total..
i have a 400 with a 750 dp... at the strip we run 74 X4 with 28X2 with i not sure what cams... but the old thing runs good.... it does not have any hicups or any thing... and its runs about 12.6-1 AFR after all the discharge is clear...
now on the street we run 71's and 78s and we keep the discharge the same for simplicty.. its runs 14-1 on a cruse and 12.3-1 wot
all i can say is change one thing at a time and get a wide band....
btw we went from 12.4 - 12.15 going square with the jets...
