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different carb tune for 2 step?
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DRR Sportsman
Picture of Ron Gusack
posted
First time using a delay box for me and the car didn't leave clean when the line locks released. It sounded way rich even though the afr looks lean. The 2 step popping seems louder on some runs compared to others too. One out of five runs was clean and that was leaving at 2800. I started at 26 and went to 28. Does the carb need a slightly different tune than it had when footbraking? My afr is always on the lean side.
 
Posts: 459 | Location: Maryland | Registered: January 23, 2007Reply With QuoteReport This Post
DRR Sportsman
Picture of FootbrakeJim
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Ron, I don't know what fuel you are using, or how tight your converter is, but if I can assume you were staged at WOT, 2800 is a really low launch RPM. There is not much vacuum in that situation to get fuel moving, and you have already used up your accelerator pump shot going to the mat. If your converter is somewhat tight, that sounds like the makings for a stumbling leave.
I was just getting ready to post a carb tuning tip for a similar situation that I experienced, running a Pro Tree index class over the weekend. I will just put it here instead. What solved my problem might fix yours too. For me, the starter was not using Auto-Start, and he was flipping the tree almost immediately after both cars got staged. Every time I staged last, my car struggled to get fully onto the converter before the tree would flash, and it was scattering my R/T's all over the place. I too thought it sounded really rich, not crisp at all. While this was different from your situation in that I still had the pump shot working for me, the end result was the same, not a clean leave. I am running a 1050 on methanol. Like you, my jetting is slightly lean, but my shooter nozzles are huge, so I really suspected it was sort of a fat stumble. In my case, the air was very good, and kept getting better, down around 1,000' DA. But my issue kept getting worse. Even though it sounded like an overly rich mess trying to flash the converter, the fact it got worse as the air got better told me it was actually a lean condition, made worse by the dense air quality. So I decided to take a swing at something, and here is what cured things for me: A simple 1/8 turn OUT (rich) on all 4 idle mixture screws. In your scenario, you have a big hole of "air" to try and cover up, until your boosters see enough vacuum signal to fill that hole with more fuel. (Especially since you said your AFR is on the lean side). I would guess if you had a high-speed data recorder, it would show a lean spike right at launch. So it stands to reason that a slightly richer idle mixture will ease the transition and give the engine a "head start" toward the proper air-fuel ratio until your RPM (vacuum signal) can catch up with the throttle opening. So that is my advice - You may need more than 1/8 turn, but it was amazing what that small adjustment did for mine. Immediately cut the time to reach flash RPM about in half, and made it a much more crisp launch.

This message has been edited. Last edited by: FootbrakeJim,


Dan "Jim" Moore
Much too young to feel this damn old!!
 
Posts: 1041 | Location: Farmersville, TX  | Registered: December 05, 2002Reply With QuoteReport This Post
DRR Sportsman
Picture of Ron Gusack
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I've worried about how the carb would react to all the pump shot being used up since I started on this 4 wheel line lock journey. I know 2 good racers that use it here locally with good results but I'm pretty sure neither of them are using a 1050 like I am but both are on gas like I am. My logger shows the afr dangerously lean when I'm on the 2 step but any misfire will cause it to read lean and this is misfire extreme. The afr drops like a rock as soon as the 2 step releases and then goes up. Converter stall is in the 5200 range but holding this car over 3200 is a challenge. I know the boosters are flowing well before 2600 so I'm up on the main circuit. I could fatten the idle up though.
I have a screen shot showing a pair of logged runs but it's on Photobucket, so no good.

This message has been edited. Last edited by: Ron Gusack,
 
Posts: 459 | Location: Maryland | Registered: January 23, 2007Reply With QuoteReport This Post
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