Hi, I just purchased a KPV motor to try and run some f100 races. The motor runs great once on the track, but struggles to get going coming out of the pits for a hundred feet or more. A friend of mine turns in the low speed needle 1/2-1 turn in until he gets out of the pits, but he also has a blueprinted carb. Right now my carbs set at 5/8H and 1.5L (not sure about pop off pressure) and have a Horstman 3 pipe with 10/80 gearing. Does anyone have a rough approximation for what a stock carb should be set at, and do I to need to be turning in the low needle until I get out on the track? The other strange thing I’ve found is that the engine idles when I come back into the pits at 1800rpm. Not sure if that’s normal but all I’ve ever run is PRD.
Mike, it sound like you have a possible air leak, I would make sure all gaskets are good, and also doing a cleaning of the carb, do a rebuild on the carb, and start from there. Start with low speed at 2 full turns out and High speed at 1/2 turn, and move them in from there.
Not sure how different stock to blueprinted settings would be, I would imagine they would be the same, but we always ran our KPV carbs at about 1.5 on the low and .25 on the high, and would open the low to 2-2.25 on the track once we got rolling. It won’t get off the grid unless it’s turned in.
I did find an air leak at the cylinder, so I threw some new gaskets on and rebuilt the carb while it was off. I won’t get a chance to run it till the next race though. From what I could find online though, a non-blueprinted carb runs at 1 1/2 low and 3/4 to 1/2 on the high. Seem to be a little different from all the blueprinted one, but then again, this is just going off of shop manuals.
You have to check your pop off pressure and fulcrum arm height. There is a way to set up that carb so your engine will idle and still run good on the track. If your carb is set that way you’re going to need needle settings that are very different than conventional wisdom.
If you can’t get going coming off the grid is it too rich or too lean? If you choke the airbox does it pick up or get worse.
FAA certified jet engine and aircraft technician,
Nicholson Speedway class champion 2001,
Yamaha KT100 Service Center,
41 years karting experience