Box Stock

Jetting question

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Lazyb

Member
I’ve just started racing go karts but I’ve been in asphalt racing for years. I recently changed the carb on my predator and think the jetting is way to fat. It’s awful sluggish on acceleration on restarts. But on the stand it’s awesome. My quest is on the low side jet, up to what rpms does it have effect? I’ve tried to research and have had no luck. My temp gauge was loose during the race and don’t think I got an accurate head temp to know if the high side needs dropping but have a new thermocoupler on the way.
 
really good question, "up to what rpm does it have effect?" i hope we get an answer. i think its about vacuum. I've been told to pull the lower rubber off the low speed jet to get a richer low speed (idle) and better acceleration. was told to watch my plug and keep my rpms higher at idle to prevent loading the plug, and lastly, change my clutch engagement higher if needed to allow for the higher idle to prevent premature ware. i've done this and seen an improvement coming off the corners where i had to get out of the gas and i restarts
 
It’s a blueprinted carb I got for an intended mini bike build and decided to try it. Like I mentioned I think it’s too fat on the low side but would like to know a rough idea of the rpm range of the low side jet so I know if that’s where my issue really is. If it’s a 50/50 split then I know I’ll need to change both jets
 
The low-speed needle feeds continuously throughout the RPM range. The high-speed feeds continuously throughout the RPM range but the high-speed dump to is in a venture. As air flows over a venture he there is a pressure drop, and that pressure drop increases as the airflow goes faster. This means the low-speed fuel flow is linear and the high-speed flow is a curve, meaning the faster the air flows through the carburetor, the more fuel flow.


Regardless of everything else you may have heard, or experienced, you want the clutch to slip, on the starts, at peak torque RPM.
 
.022 is a starting point on low speed. Every builder will do their carbs how hey think is best . Some can make a .032 work some cant. Doing a plug check,cutting the motor off at rpm , is better than head temp
 
Here's an interesting spreadsheet.

On the left you enter your current jet sizes, on the right you enter your new jet sizes and below you see the overall percentage change plus the change in the high speed jet and low-speed jet flow. I

It needs some field testing.

Notice the change in the highest fee and low-speed jet sizes, compared to the jet sizes on the left.

1605238399063.png
 
Ok first you do not have an accelerator pump in these carbs... you are running 100% off the low side at idle, these carburetors are basically 2 carbs in one... the low side is your idle and transition carburetor ie accelerator pump... the fuel that feeds the low side comes from the well area around your emulsion tube... and is drawn into the low side through the low jet and is dumped/distributed into the carb venturi via the 4 small pick holes and the main low speed idle hole at the rear... despite what you might be told.... the low side on these carbs delivers zero fuel once you have transitioned over to the main venturi and the main jet... as the fuel well that feeds it is empty... ie no fuel only air.... the low side is completely out of the picture by about 3000 rpms the emulsion tube on these engines is critical... it is to your carb what the cam is to your engine... NEVER remove the lower o-ring on the low jet... that all but eliminates the low jet... in a properly built and jetted carb as soon you blip the throttle off of WOT the fuel well around the emulsion tube should refill... and then you have fuel to feed it during the transition back to wot... i get my emulsion tubes blank and drill them for the application... never believe what the internet tells you about etubes if the person can't tell you exactly what they do and how they do it... jmho.... this is just the basics... the low side circuitry can be manipulated and the converging and diverging angles of the main venturi should also be addressed... a well built carburetor will make or break you....
 
Thanks ponytl that’s what I needed to know. I did know the low jet wasn’t in use all throughout the rpm curve. Just didn’t know where the breaking point was. I’ll have to run it down the road and do a plug test
 
Lazyb said:
Thanks ponytl that’s what I needed to know. I did know the low jet wasn’t in use all throughout the rpm curve. Just didn’t know where the breaking point was. I’ll have to run it down the road and do a plug test
Click to expand...
I wonder, what is the mechanism that stops the flow to the low-speed jet at the top end? There is a hole in the throat of the carburetor near the bottom of the throat. There is a clear path to the source of the fuel. Explain to me the theory that says it doesn't flow fuel through the full RPM range.
 
When the butterfly is closed it allows air to pass through the idle jet and pulls up fuel. When you are wide open throttle there is no air going past the idle jet so no fuel is picked up.
 
Wrong, the fuel tower will continue to feed fuel to the low speed jet, where it will mix with air, then this mixture will flow to the pick off holes behind the throttle plate and the pilot screw ahead of the throttle plate, whether the throttle is slightly open by the idle speed screw, or wide open. The low speed jet will always be supplying fuel, it doesn’t magically stop
 
The reason I ask is because if I’m running fat on idle and fat on wot , from what your explanation is, I could change the low jet to a smaller one and possibly fix my issue from idle to wot
 
My 2cents . Vacum and pressure must be equal in all directions . There for the pressure differential in the carb body should provide fuel flow at the low speed fuel holes . Now there is a reason the low speed holes are near the edge of the throttle blade when it's closed or slightly open to provide a higher differetial of pressure . Same with the venturi full throttle pressure differential too draw fuel .
The low side should lose some of that ability , likely not all at wot .
I can see both sides and must say the general consensus is it is still in play too some degree .
I would change the low speed jet . All you can do is try it .
 
Lazyb said:
The reason I ask is because if I’m running fat on idle and fat on wot , from what your explanation is, I could change the low jet to a smaller one and possibly fix my issue from idle to wot
Click to expand...
My theory, a carburetor that's tuned, correctly, for racing will not idle well, a carburetor that's tuned to idle, correctly, will not race well.

These carburetors were not built for racing, they were built for engines that run at a near steady state RPM. They do that very well. We asked them to do something different, they need to be tuned different to meet those purposes.
 
In a carburetor, when you're talking about fuel and air flow, you must remember this; there is no "draw", there is no "suck", there is only a "pressure differential", which means the fuel and air get pushed, from a high-pressure area to a low-pressure area.

You might think that this is inconsequential, but it's not, it makes a difference.
 
Ok some other thoughts . The tilly or walboro carb have the idle hole on the bottom ; the clone /predator have them up on the side . This should not affect the pressure differential factor , correct ?
Though it should affect the amount of the pressure differential due to it being higher above the fuel level .
The last thing is the air bleed . There must be something there because there are rules govonering it .
 
EXCELLENT CONVERSATION !! Any other experts out there can elaborate on the rest of the functions of the fuel and air flow operations.. how about e tube height into the venturi. ?
 
Here's a little insight .
Those holes in the E tube that were blocked off were done to pull more fuel through the main jet or you can say causing a stronger signal in the venturi . Air bleed restriction diameter have the same effect larger = leaner smaller = richer mixture.
 
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