Carb'd LS2 accel pump size
08-18-2008, 04:37 PM
#1
Carb'd LS2 accel pump size Just fired the LS2 crate motor this week, need to get an exhaust before I can do any serious tuning. But I am trying to get it running well enough to get it home from the muffler shop. I have a question about the bog right off of idle.
I read where a lot of guys recommend the 50 cc pump. Have also been reading about larger nozzles and pump cams. Just wondering if the 50 cc pump is absolutely necessary or if anyone has gotten rid of the bog with just other tuning? Don't mind buying what I need, just want to make sure it is really needed.
Motor is an LS2 346/440 crate motor with a new Holley 750 DP, manual choke and mechanical secondaries. I switched to 72 primary/76 secondary jets cause that is what a lot of guys seem to be using. I am running on the #2 MSD ignition pill for now. Idle mixture screws are 3/4 turn. Started with 1 1/2 turns but the smell o meter says 3/4 works better for now.
What happens is the car use to bog with even the slightest hit of throttle, then backfires through the open headers on decel. I moved the pink cam one notch so it come on quicker, then adjusted the primary accel pump to give it maximum travel. It doesn't bog off idle like it did, but will if I put too much into it.
So what is the consensus, 50 cc absolutely needed or will tuning the 30 cc stocker get me there? Any one tried larger nozzles? The carb came with a #28 primary and #31 secondary.
BTW, once I get the exhaust on I can hook up the wideband.
I read where a lot of guys recommend the 50 cc pump. Have also been reading about larger nozzles and pump cams. Just wondering if the 50 cc pump is absolutely necessary or if anyone has gotten rid of the bog with just other tuning? Don't mind buying what I need, just want to make sure it is really needed.
Motor is an LS2 346/440 crate motor with a new Holley 750 DP, manual choke and mechanical secondaries. I switched to 72 primary/76 secondary jets cause that is what a lot of guys seem to be using. I am running on the #2 MSD ignition pill for now. Idle mixture screws are 3/4 turn. Started with 1 1/2 turns but the smell o meter says 3/4 works better for now.
What happens is the car use to bog with even the slightest hit of throttle, then backfires through the open headers on decel. I moved the pink cam one notch so it come on quicker, then adjusted the primary accel pump to give it maximum travel. It doesn't bog off idle like it did, but will if I put too much into it.
So what is the consensus, 50 cc absolutely needed or will tuning the 30 cc stocker get me there? Any one tried larger nozzles? The carb came with a #28 primary and #31 secondary.
BTW, once I get the exhaust on I can hook up the wideband.
08-18-2008, 04:48 PM
#2
you dont need the 50cc pump-try and get a green cam, at least in the pri side, also helps in the sec side-you have to buy a whole kit to get them, unless you can find some loose-you might want to move up to a 31 on the squirter, maybe even 35, to big and you will need a 50cc pump
sometimes with the idle mix screws in less than 1 turn, can cause off idle problem-when your in the transions circuit, your still pulling from the idle circuit, so if the screws are in pretty far, you get a reduced shot from there
you might try and open the sec blades a tiny bit, bring some control back to the idle mix screws
def try a cam change though-w/o exhaust, kind of hard to make adjustments, once you get that fixed, you will get some more control i think
sometimes with the idle mix screws in less than 1 turn, can cause off idle problem-when your in the transions circuit, your still pulling from the idle circuit, so if the screws are in pretty far, you get a reduced shot from there
you might try and open the sec blades a tiny bit, bring some control back to the idle mix screws
def try a cam change though-w/o exhaust, kind of hard to make adjustments, once you get that fixed, you will get some more control i think
08-18-2008, 09:35 PM
#3
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i have the same crate motor in my 63 , good on you if you can get it going well on the 30cc pump, i changed mine to 50cc after trying a few changes and not fixing it and now i have instant throttle response
08-19-2008, 04:56 AM
#4
I tried and tried to get my off idle stumble to go away. the only way i found was with alot of timing at idle (like 28 degrees) and a really rich idle mixture which led to a pretty high idle. I ended up just buying a 50cc accelerator pump and all works great now.
08-19-2008, 09:14 AM
#5
Thanks for all of the replies
You guys running the 50 cc pumps, what cam and nozzle size are you running? also are ether of you running a mechanical secondaries? If you are, I assume you only needed the larger pump on the primary side?
From the book I have the green cam has a smaller shot than the pink one I have now. It also said the double pumper idle circuits are typcially jetted rich for big cams. It recommended a small piece or wire in the idle ports to restrict the idle mixture a bit. Sounds like a low buck plan to get a bit more out of the idle mixture screws. Although the carb does seem sensitive to the idle screw adjustments.
You guys running the 50 cc pumps, what cam and nozzle size are you running? also are ether of you running a mechanical secondaries? If you are, I assume you only needed the larger pump on the primary side?
From the book I have the green cam has a smaller shot than the pink one I have now. It also said the double pumper idle circuits are typcially jetted rich for big cams. It recommended a small piece or wire in the idle ports to restrict the idle mixture a bit. Sounds like a low buck plan to get a bit more out of the idle mixture screws. Although the carb does seem sensitive to the idle screw adjustments.
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08-20-2008, 04:57 AM
#8
I put mine in the primaries since mince a vacuum secondary carb. I finally got a dual wideband setup and put it on my car. Idle was 14.6-14.8 when from idle to WOT revving it it went to 13.6 and at WOT under load it was 12.8 so the jetting in the carb in such is actually really close.
08-20-2008, 02:11 PM
#9
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Same here. 50cc in the primaries on a iron 6.0l and 750 mighty demon. I still go just a lil lean but only in fourth or fifth. Thinking bout putting a green cam on the secondaries and see what the wideband says.
08-21-2008, 06:56 PM
#10
id change the squirter. some say its just a band-aid, but id recommend a going up a couple of sizes in the front. maybe move the 31 to the front and get a 33-35 for the rear.
if your carb doesnt have hollow squirter screws, pop in a set while you are messing around with it. really helps the fuel flow. with a 50cc, you might introduce too much fuel into the motor and end up with the same problem you have now.
if your carb doesnt have hollow squirter screws, pop in a set while you are messing around with it. really helps the fuel flow. with a 50cc, you might introduce too much fuel into the motor and end up with the same problem you have now.
08-26-2008, 10:30 AM
#12
Muffler shop is taking it's time with my exhuast. Haven't had a chance to play with any of this but sounds like I will need the larger pump.
This site is good for the lazy man who doesn't want to do his own trial and error experiments!
09-06-2008, 05:02 PM
#17
A quick update. Ordered a 50 cc squirter, a cam set and a pair of #35 nozzles. I haven't put the larger pump on, but switched the primaries to a #35 nozzle with a hollow screw and the larger brown cam (#336). Put another #35 nozzle on the secondaries and moved the stock pink cam to the #2 hole. I also went to a #4 pill on the MSD ignition box.
I still have some work to do, but this seems to have gotten rid of the worst of the bog. Still have less than a half hour on the car so far from done. The exhaust shop forgot to put on the bung for the wideband so that should be the ultimate test of the tune.
Did learn one thing today. I need some stickier tires. I have no idea how fast this car is because hitting the gas makes the back end try to out race the front!
I still have some work to do, but this seems to have gotten rid of the worst of the bog. Still have less than a half hour on the car so far from done. The exhaust shop forgot to put on the bung for the wideband so that should be the ultimate test of the tune.
Did learn one thing today. I need some stickier tires. I have no idea how fast this car is because hitting the gas makes the back end try to out race the front!
09-07-2008, 08:37 AM
#20
A 1970 Datsun 240Z. 2400 pounds, 3.7 rear gears and only 225 tires.
Found the O2 bung last night, so should get the wideband on it soon. Also modified the timing curve to be more like Shawn's. The extra timing at idle sure makes it easier to start. I'm not sure what motors the stock pills are suppose to be good for, but they sure don't have much advance at low RPM's.
Found the O2 bung last night, so should get the wideband on it soon. Also modified the timing curve to be more like Shawn's. The extra timing at idle sure makes it easier to start. I'm not sure what motors the stock pills are suppose to be good for, but they sure don't have much advance at low RPM's.