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Discussion Starter #1
Hey all,
Something I have always wanted to know how to do. Airbleeds, jets, and all the othe terminology...tell me what it all means and what each does.
We run CFM split Holley style dominators on our big motor. Was on the dyno yesterday, and it seemed REDICULOUSLY rich...stumbled like crazy through the mid range. We jetted it down 6 numbers, and definitely improved but was still stumbling.
I just want to know what to change, when to change, how it affects everything, etc. Any and all input is appreciated.
Thanks in advance!
 

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Got a couple years to spare? Dale has a reputation for building quality stuff, give him a call and get his advice first. Splits or dual carbs are a little more sensitive to tuning, and knowing how he calibrated them will give him the best position to help you. Learning to tune a single carb is a better place to start, not custom dials or splits.
 

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Without seeing the carbs, dyno sheets and AFR curves I would only be guessing, but if the midrange is rich be careful going down too much on the jet as it can make the top end too lean and hurt something. If you had to go down that much call Dale before changing anything more. And with splits depending on the castings he used and the mods made there may be some extra tuning circuits, so to tell you anything to change may hurt more than help. Pick up some Holley books, good place to start.
 

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Discussion Starter #5
Thanks for the input. I wasnt really being specific to our carbs...I just want to know about carbs I guess. In general, when do you change a jet, when and which air bleed do you change and when? Etc. I guess I'm looking for a basic overview of how race carbs work.
 

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When you dyno large motors with big splits on them, it is not uncommon to have to go down 20-30 pin sizes on main jet to get the fuel where it needs to be. Just make sure you put the fuel back in it before heading back to the race track.
 

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When you dyno large motors with big splits on them, it is not uncommon to have to go down 20-30 pin sizes on main jet to get the fuel where it needs to be. Just make sure you put the fuel back in it before heading back to the race track.
This is very true, i have seen a local racer with a 632 from sunset racecraft, dyno'd @ 1230 hp, when they sent it to him it had 8 jet sizes bigger then when they dyno'd it, and it ran really good, i think he went up another1-2 jet size till mph quit climbing......

I would really call Dale, and talk to him, couldn't meet a much nicer guy and very willing to help get you right on............
 

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RCM I would really call Dale said:
Dale will take the time to give you some insight on the care and feeding of your carburetors. He built them, knows how they are calibrated and can definately help you tune them. CFM's phone number is 330-723-5688.
 

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Discussion Starter #11
Thanks a bunch guys. I'll give Dale a call. I didnt want to bug him for something that seemed to be simple...but I understand where you are coming from. Will he remember the carbs, or does he have a number on them somewhere?
 

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What about the recovery of the fuel dumped in at low rpm in high gear (DYNO) 1 to 1 loading, from the accel pumps...????? Anything there?
 

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Discussion Starter #13
Basically, the motor ran perfect, idled great and all the way down to about 900 RPM. Ran fine up to around 3400. Once we would try to make a pull, any time it would get around that range it would sputter, pop, and just sound like crap. After we changed the jets, it helped alot but still did it. Luckily, on the last pull it cleaned itself up once it got to around 4600, and we took it to 6400. Im no carb guy...I'll call Dale this week.
 

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So how high are you going to run it on track? And it seems to me it would never see 3500 or 4000 in high gear....be careful not to try and tune it where it will never be run....what tranny and ratios do you run? Not the same....but for instance, a PS engine runs to 10600 on the track and falls to 8800 at the lowest...so why dyno and tune it at 7500?...just sayin...
 

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If it's a bracket style engine it will not likely see those RPM's, and can spend a fair amount of time in the burnout or around the pits in the lower RPM ranges. An automatic equipped car will be even more sensitive. If it's way fat in the lower and middle ranges it will carbon the plugs up and load the engine at the beginning of the run. Unless the carbs were just jetted way rich or the emulsion is set up wrong they likely need a little intermediate circuit adjustment. With 8 barrels it doesn't take much to be off a bit, if you have dyno sheets with BSFC's and even better O2 #'s send them to Dale, it will help him figure where to have you make changes.
 

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So how high are you going to run it on track? And it seems to me it would never see 3500 or 4000 in high gear....be careful not to try and tune it where it will never be run....what tranny and ratios do you run? Not the same....but for instance, a PS engine runs to 10600 on the track and falls to 8800 at the lowest...so why dyno and tune it at 7500?...just sayin...
That was my point above, idle pits not much mid range running in the 3500 to 5000 rpm range and lots of chances for reversion to mess you up.
 

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Kyle
Two things, what igniton system are you using on the dyno? What plug gap are you running? The bigger inch stuff tends to like a really small gap as the fuel load is greater on the bigger motors. We run our 632's at .024-.026. We run the 700 inch stuff at .020-.024 and the 932 inch stuff at .016-.018 that is with a digital 7 box.
 

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When you dyno large motors with big splits on them, it is not uncommon to have to go down 20-30 pin sizes on main jet to get the fuel where it needs to be. Just make sure you put the fuel back in it before heading back to the race track.
Exactly if its a 800 area engine they are almost impossible to dyno unless you have chimney type dyno room.Running even 5 inch pipe off the headers into a muffler changes the tune extremely.It takes a good dyno man to know what it needs there and the changes needed at the track...Be carefull.As others stated if Dale did your carbs he would be only guy i would take advice from.
 

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Kyle
Two things, what igniton system are you using on the dyno? What plug gap are you running? The bigger inch stuff tends to like a really small gap as the fuel load is greater on the bigger motors. We run our 632's at .024-.026. We run the 700 inch stuff at .020-.024 and the 932 inch stuff at .016-.018 that is with a digital 7 box.
This is good advice .dig 7 box by no means is a stout ignition for big inch or blown stuff....
 
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