carb + stumble....
05-24-2009, 06:10 PM
#1
carb + stumble.... For all you carb guys out there that leaned out your jetting for a better a/f ratio during WOT and part throttle what worked for you on your double pumpers to correct tip in stumbles? Right now read a few posts on 50cc pump kits and a few others used green pump cams. I'll probably toss in the green pump cam tomorrow to see what it ends up doing, but was looking for suggestions as well before I toss in this 50cc kit on my quick fuel. Already have hollow screwed squirters. It will stumble on a quick stab on the primaries and if I snap it open to all 4 barrels or even roll into the 4 barrels it'll stumble and catch 2-3 seconds later. Just thinking of what the next step would be.
05-25-2009, 12:11 PM
#3
The right power valve (about 4 inHg lower than part-throttle vacuum reading) and bigger accelerator pump nozzles will usually correct a tip-in stumble. Going with a pump cam w/ faster dump also gets more fuel down the venturis quicker.
05-25-2009, 07:04 PM
#4
the power valve is fine, this is just a tip in stumble. I'll try out the pump cams and see about squirters but this carb should have large squirters in it already. Like I posted I have the 50cc kit, but not many responses from guys running a carb swap.
05-25-2009, 07:30 PM
#5
The power valve does a bit of helping during the transition to WOT, after the accel pump has done its job and the throttle shaft has stopped moving.
But your point that the PV isn't the total cause of the hesitation is true, and you've got some experimenting with pump cams and nozzles to do.
When you have it as dialed in as you can get it and it's answering the throttle well, grab a vac measurement at cruising speed and install a PV that's 4inHg lower than that number. You'll feel the difference, trust me.
But your point that the PV isn't the total cause of the hesitation is true, and you've got some experimenting with pump cams and nozzles to do.
When you have it as dialed in as you can get it and it's answering the throttle well, grab a vac measurement at cruising speed and install a PV that's 4inHg lower than that number. You'll feel the difference, trust me.
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05-26-2009, 05:31 AM
#10
If you look at the sticky at the top of this forum about who has tried a carb and dist on an LS, there is lots of info on carb set ups. I had another one about 50 cc squirters.
These LS motors with the open plenum intakes seem to need a pretty good shot off of idle to get them going. I think the high idle vacuum with stock cams is not helping matters.
I tried to tune it out without the larger squirter but was unsuccessful. I have yet to hear of anyone who has. In addition to the 50 CC pump I went up 4 sizes on the squirters, both front and back. I get no more hesitation when I nail it. The wideband confirms this.
What I still have issues with is coming lightly on the throttle. Unless I stab it a bit it goes way lean for a moment. The idle circuits on my DP are way too rich for this cam. I need to experiment some more with wire in the idle jets. Or maybe I just need a bigger cam to better match the DP
These LS motors with the open plenum intakes seem to need a pretty good shot off of idle to get them going. I think the high idle vacuum with stock cams is not helping matters.
I tried to tune it out without the larger squirter but was unsuccessful. I have yet to hear of anyone who has. In addition to the 50 CC pump I went up 4 sizes on the squirters, both front and back. I get no more hesitation when I nail it. The wideband confirms this.
What I still have issues with is coming lightly on the throttle. Unless I stab it a bit it goes way lean for a moment. The idle circuits on my DP are way too rich for this cam. I need to experiment some more with wire in the idle jets. Or maybe I just need a bigger cam to better match the DP
05-26-2009, 06:44 PM
#11
When setting your pump cam arm dont set it the .015" or whatever off of idle, try setting it a little later (like .030") to come in and see if that helps. I found that my stumble was being caused from the power valve being partially pressed at the given rpm and it wasnt getting the full shot. Also be sure you install the pump arm and diaphram kit otherwise it wont work correctly. I used the clear red cam.
Is this in a vacuum secondary or a double pumper carb? You have hollow screws but what sized squirters? I ha a #35 with a hollow screw and 50cc pump with clear red cam and it worked pretty perfect on my vacuum secondary 750. When i put my demon 750 on i had #31 squirters, pink cams, and 30cc pumps and almost had no stumble right out of the box. I didnt spend much timing tuning it, i am just installing EFI right now.
Is this in a vacuum secondary or a double pumper carb? You have hollow screws but what sized squirters? I ha a #35 with a hollow screw and 50cc pump with clear red cam and it worked pretty perfect on my vacuum secondary 750. When i put my demon 750 on i had #31 squirters, pink cams, and 30cc pumps and almost had no stumble right out of the box. I didnt spend much timing tuning it, i am just installing EFI right now.
Last edited by Shawn MacAnanny; 05-26-2009 at 06:50 PM.
05-26-2009, 08:10 PM
#12
I have 33 and 35 squirters right now. It's a double pumper, the vacuum secondaries have no place on a performance engine and will always cost 10-15hp over a double pumper since the secondaries will never open up completely. I just talked to quick fuel on the phone and will try 35 squirters up front first, and bought 37s and 40s for the rear to try out. They stated the pink cams give the most pump volume, we'll see what happens when it stops raining and I'm not working. I had zero stumble at all until I started jetting down to get the part throttle and WOT to better readings. It was extremely rich at the track, so I'm just reeling it back in to where it won't load up and part throttle is in the upper 13.XX-14.1 range and wot is 12.8-13.1 on the wideband. I'm sure the squirters will help, then I got the tech's name to shoot for another option.
05-27-2009, 06:40 AM
#13
just my .02, what i found on mine was first, it worked best with a green cam, just idling and giving it a little gas, sounded much better-it acted like a bad coil or something, was rough, couldnt believe it was a fuel issue, found it accidently by pushing down on the accel pump by hand while giving it gas
the green cam is more agress. than the pink, but doesnt have the longer duration-after switching to the green cam cured off idle problem, i had a lean tip in-i used the LM-1 to see what was going on-i figured going from light cruise to power, it needed help in the transion slot, which adds fuel untill the boosters started flowing-i read my idle vac, got an old P/V that was 6.5, played with the spring untill i got it to around 8.5 or 9-that cured the problem-i could actually see the tip in problem on the LM-1 in the RPM graph, besides feel it-i dont use a 50cc pump, the squirters are small enough they dont drain the pump-i run a Procharger, so i have a lot of other mods on the carb, more for upper end fuel flow, but the driveability is perfect-there may be other ways of getting there, but this worked for me-the intake design is part of the problem, fuel sticks to the runners on the way down, so by the time the fuel hits the comb. chamber, it is a lot less-FI has the injector right at the head, so the problem is not there-ok, im done, lol
Nice looking Camaro!!!!!!!
the green cam is more agress. than the pink, but doesnt have the longer duration-after switching to the green cam cured off idle problem, i had a lean tip in-i used the LM-1 to see what was going on-i figured going from light cruise to power, it needed help in the transion slot, which adds fuel untill the boosters started flowing-i read my idle vac, got an old P/V that was 6.5, played with the spring untill i got it to around 8.5 or 9-that cured the problem-i could actually see the tip in problem on the LM-1 in the RPM graph, besides feel it-i dont use a 50cc pump, the squirters are small enough they dont drain the pump-i run a Procharger, so i have a lot of other mods on the carb, more for upper end fuel flow, but the driveability is perfect-there may be other ways of getting there, but this worked for me-the intake design is part of the problem, fuel sticks to the runners on the way down, so by the time the fuel hits the comb. chamber, it is a lot less-FI has the injector right at the head, so the problem is not there-ok, im done, lol
Nice looking Camaro!!!!!!!
Last edited by forcd ind; 05-27-2009 at 06:43 AM. Reason: add
06-01-2009, 10:36 PM
#14
swapped from 33/35 squirters to 35/37 squirters and still have a lean stumble. The air quality sucked so I tossed back jets to get the WOT back where I needed for pinks all out. Upon stabbing the throttle it'll hit upper 14:1 on the wideband. Still on the pink/pink pump cams. Didn't try the green pump cam or the 50cc setup since I didn't have much street time to test before the track. Thoughts?
