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Carburetor Leaning out on Long left hand sweeper
I am experiencing a lean condition on a long left hand sweeping turn at my local road race track (Spring Mountain in Nevada). Carb is 775 Race Demon running on a 408W. I am running dual power valves with spring loaded needle & seats. Road race floats. Fuel pump is an in tank 255lph. I don’t think it is a pump issue as the fuel pressure (at regulator) shows good at 6.5psi. It does it when the tank is full too. When I go around that turn I basically just keep the engine at 3700 - 4000 rpm or so in 3rd gear at part throttle as I am at the limit of mechanical grip. I can feel the engine slowly lean and have confirmed it with an AFR monitor – 15:1. Once I get on the straight the AFR comes back down to normal under WOT – 12.5:1. What is going on here? The fuel feeds from the passenger side of the carb. Is the G-force resulting in lack of fuel? Or is it that with two power valves I don’t have enough “jet” to run the engine at that sustained RPM with the primary bowl only?
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Do you have circle track floats?
Holley Performance 116 13 Wedged Nitrophyl Float Primary Circle Track | eBay |
Thanks for the response. Yes - road race floats.
http://www.summitracing.com/parts/qft-16-14/overview/ |
Does your carb sit level?
Float levels? |
2 Attachment(s)
Well that is interesting. No it does not sit level. It is tilted backwards. Levels are at the 3rd setting which is recommended for race on the Demons.
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for racing take out the power valves--your probably backing out of the throttle enough that manifold vacume builds up and closes the power valves---add about 4 or 5 jet sizes for starters when you remove the PV
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If there is no load on the engine it would be the same as cruising, lean is not going to hurt anything. If you put a load on the engine that is a different story. If you are making a sweeper and just cruising through, or coasting, and not accelerating, I wouldn't worry about it, it would be similar to getting off the accelerator going into a corner, but not as much. Just another way to look at it. If it is lean to the point of missing and upsetting the car, you need to find out where you are in the carb circuit and richen it a bit.
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I can feel it trying to "lay down" and there is some slight missing in the transition back to full throttle coming off the apex. I was under the impression that most road race carbs come with dual power valves due to the large amount of part throttle the engine sees. I know the Race Demon RR comes with dual power valves. Maybe I will try to remove the rear PV and jet up accordingly. Or I could maybe try a 1:1 linkage? That would result in all blades open under part throttle and thus allow the engine to access both front and rear jets while the PV's are closed? I hear 1:1 is not so great for the street though. Vehicle does see some street and I drive it to the track.
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I'm using power valves front and rear in both JBL's with no problem and 1-1 linkage. Just have to find what works for you. I would try the 1-1 linkage first since it is the easiest then go from there. Jet extensions on the rear might be a good idea. I'll also add, I modified the PV inlet with a cover so when it opened under acceleration the inlet was always covered.
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Thank you for walking me through some good ideas. I do not have jet extensions but will buy some along with a new road race float to accomodate them. Vector1 - how streetable is the 1:1 linkage. I know that is subjective but would like to hear your thoughts. I actually already have a little baffle that covers the power valves and keeps the inlet sucking fuel from the bottom of the bowl.
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I'd get rid of the "spring loaded" needle and seats, and fit normal .110-.130 viton tipped needle and seats. Check and adjust float levels.
Fit road race floats and fit jet extensions to the rear with a notched float in the rear bowl. The jet extensions only help under hard acceleration when the REAR bowl volume can uncover the mains/pvcr. No benefit fitting extensions to the FRONT. |
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Something else you might check is your amount of ignition advance. If you have a vacuum advance hooked up, unhook it and the effect will be to richen the a/f ratio I believe. |
Timing is mechanical and is 30 at total. I will revisit the springs to see when it's all in by. However too much timing typically will manifest itself in a white hot strap and not so much in the air fuel ratio. Regardless a good point to revisit. I have to believe that the vehicle is under part throttle (primaries only)at higher RPM for so long that is draining the primaries and not touching the secondaries. Further exacerbating the issue is that the secondary driver side jet is uncovered (turning left) when I do get back into full throttle. Going to 1:1 linkage should help the first issue as engine will pull from both primary and secondary and jet extensions will help the latter. FYI. This is a 540 HP 408w in a 3000 lb Griggs prepared 1986 Saleen Mustang. Cobra is gone now. I miss that car
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I'd say you need larger needle and seats.
It is quite possible that you're starving for fuel on the primary side since essentially only one needle and seat is supplying the engine. With the carb open where the secondary main system is supplying as well, the 2 needle and seats must be just adequate. The needle and seats could be the same size from 390-850 CFM, a 390 DP is good for a potent 4 banger etc, or restricted V8 classes. Run the largest you can while still having fuel level control at low speed. |
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