Archive for June, 2012

Accelerator Pump Jets

June 14th, 2012 No comments

So, last October I went to Thunderhill and had some carb tuning done at MCE. They did a nice job – leaning it way out and boosting the HP and torque about 15 points each. However, once they were done, there was a big hesitation right at the beginning of accel that was really disturbing on track. I asked Scott about it, and he suggested that with it leaned out, it wasn’t getting enough gas right at the first push of the throttle, so the accelerator pump probably needed bigger jets. It’s taken me 9 or 10 months to finally get around to replacing these things! I had some in the garage already. The existing jets were 28 (primary) and 31 (secondary). I watched this video here, which gave me a great instruction on how to pick and install new ones.


I tried out 31 in front and 37 in back and took a test drive. Better! but still some hesitation. So I put a 37 in front. Great! the car is off the line like a mountain goat. I may try out 37/40 at some point, but the 40s I have are a different shape, and I don’t have the necessary drilled screw, per the video.


I’m looking forward to getting this on track again to try it out. There’s some chance of me going next weekend to Laguna. However, it’s a 90dB sound limit! I’ve been metered at 94.5 at Laguna before but then passed the 92 limit by just lifting a bit when I passed the booth. Not sure about 90. We’ll see how it shakes out!



There was still some initial lag. I took a look at the throttle action and pump actuation. I saw that not much was happening at the beginning of the throttle action, so I have moved the cam to position 2. I reset the clearance at WOT to .015. The jet seems to start much earlier now and that may be what I need. I also learned from the video that I should have the 50cc pump diaphragms in there for these .037 nozzles. Not sure if that’s what I have! I’m on the borderline though if not, so I think I’ll run this and see what happens.

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