Box Stock

Jetting and timing

J

James A

New member
What jetting do yall recommend for green plate clone? The track we are racing at uses NKA rules. Do yall advance the timing on the flywheel? We have the ARC flywheel. I just noticed our motor doesn't sound like everyone else's.
 
If you don't have a pro built carb I would get one. Someone like Eric Vause or Jerry Dover would be good to go through. They will also offer advice on timing and such. There are a lot of little things that add up to big gains on these motors. Eric Vause has started doing informational videos on his racingcarb.com Facebook page that have good info. So far he has covered cylinder heads and piston rings. I use Kyle Heitz @ L&J Karting for my carbs and use 38-39 degrees of timing on green plate motors I have built.
 
James A said:
What jetting do yall recommend for green plate clone? The track we are racing at uses NKA rules. Do yall advance the timing on the flywheel? We have the ARC flywheel. I just noticed our motor doesn't sound like everyone else's.
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The jetting will depend on how the carb is worked because differnt carb builders will use different jetting set ups. Our Pro Carbs will use a .036 low speed with a .039 main jet for a Green plate. Timing will be 36-37 degrees and a .065-.070 coil gap with our 6619 flyhweel.
 
JPR57 said:
The jetting will depend on how the carb is worked because differnt carb builders will use different jetting set ups. Our Pro Carbs will use a .036 low speed with a .039 main jet for a Green plate. Timing will be 36-37 degrees and a .065-.070 coil gap with our 6619 flyhweel.
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Is that a typing error; .036" low-speed jet? The reason I ask is most I've seen recommend a .024" low-speed jet. A .036" is 124% bigger.
 
The hole in the restricter plate channels the air past the low speed circuit. They get most of their fuel from the low side that is why the larger low speed works good in a plate engine. We don't go that big but we also work our circuitry differently.
 
Ok....so I tore into the motor tonight. Somehow I put the stock flywheel key in and torqued it down when I put the motor together a few months ago. I took it out tonight and set it to 37°. I hit the carb from dyno cams as a blueprinted carb. I checked the old carb and it had a 40 main in it. I don't have the 61 and 64#drill bits to check exactly what it has in it now but to be on the safe side, I might order another carb for it.

Barry, what lower oring? And what advantage would I gain? Smaller pilot, the main jet?
 
The only way to accurately check the size of your jets is with a pin gauge set. They make a set with pin gauges .011" through .060" in .001" increments. Last time I looked you could buy them on eBay. The sets are not all that much money. The bigger tool houses will sell you individual pens in any size you want. Checking jet sizes with drills is a hit and miss proposition.
 
I just the motor back on the kart and finally hit it to crank. Sounds better. But won't really know till we get to the track and see what we got
 
Yakattack said:
Welding tip cleaners are a easy way also to check jet size...
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A company I worked for in San Diego, Truax engineering, had 250 injector Jets made for a rocket engine, very similar to carburetor Jets, by a company nearby. They made the Jets on a Swiss CNC lathe. Very very accurate. We flow tested all the Jets with water and found there were about 10 or 12 different flowrates in the batch. Now if you get that kind of difference in flow rates, with a precision CNC lathe making a jet, can you imagine what a difference in flow rate you would get with a drill!!! If you're serious about your racing, you will use reamers to size the Jets. Not perfect, but a lot closer than drilling. One other thing; the difference in area between a .038" jet and a .039" jet is just a little shy of 5.4%. Like I said, it depends on how serious you are about your tuning.
 
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