04-20-2006, 02:32 PM
Airtight Fuel system
The Fuel system must also be airtight. If the fuel tank, tank lid, fuel lines or filter has a leak then the same kind of tuning issues as described above will occur. While you can check the fuel system by using the same method of brushing soapy water onto the suspected offending area while blowing into the tank, I generally find it better to simply remove the tank from the car and submerse it in a sink of water while blowing into the vent line and plugging the fuel line. If bubbles are present you've got a leak.
An engine at full throttle produces about 6 to 8 psi in the vent line and conversely in the tank. By blowing really hard you're also able to produce about 6 to 8 psi for a couple of seconds. If you're able to blow hard enough to cause the tank lid to open slightly and leak, it's time to replace the tank lid spring or bend more preload in the spring. The entire system (fuel tank, fuel filters, fuel lines, tank fittings, etc.) must be airtight to achieve consistent repeatable performance. I always that my fuel system is airtight when it's new but also recheck it every time I rebuild a car after a race. It easy insurance plus soapy water is a good way to thoroughly clean the tank. Just be sure it's totally dry before using it by flushing out any water left in the tank with fuel before installing it in you car.
Tuning in a Pinch
The pinch test.
Many racers find it difficult to properly tune the low speed needle. We've developed a method that makes it easy, even for beginners, to adjust the low speed needle to within 5% of peak performance and it's done right on the bench. We call it the pinch test and here's how it works.
Step 1. Start your engine and warm it up for about 30 seconds by revving the engine from idle to mid-throttle while holding the wheels off the ground.
Step 2. Allow the engine to idle for 10 seconds. If the low speed needle tuning is so far off that it won't idle, simply bump the idle position higher until the engine will reliably run at a higher idle.
Step 3. With the engine at idle, pinch and hold the fuel line near the carburetor, cutting off the fuel flow and carefully listen to the engine rpm.
If the low speed needle is set correctly, the engine should increase rpm only slightly and then die.
If the engine increases several hundred rpm before dying, then the low speed needle is too rich and needs to be turned in.
If the rpm doesn't increase at all and the engine simply dies, the low speed needle is set too lean and should be richened or turned out.
After doing this a few times you'll get the hang of it, and tuning the low speed needle will become easy. The only way to get that last 5% performance on the low speed tune is through careful track testing. By evaluating the engine as it accelerates from a dead stop or out of slow corners during actual race conditions is the only way to get those last few percent of power from your engine. Variables like the clutch setting, gearing, traction, atmospheric conditions and even the car's weight will have an effect on this last 5% of tuning performance, and the optimum setting can only be reached by careful on-track tuning.
The Fuel system must also be airtight. If the fuel tank, tank lid, fuel lines or filter has a leak then the same kind of tuning issues as described above will occur. While you can check the fuel system by using the same method of brushing soapy water onto the suspected offending area while blowing into the tank, I generally find it better to simply remove the tank from the car and submerse it in a sink of water while blowing into the vent line and plugging the fuel line. If bubbles are present you've got a leak.
An engine at full throttle produces about 6 to 8 psi in the vent line and conversely in the tank. By blowing really hard you're also able to produce about 6 to 8 psi for a couple of seconds. If you're able to blow hard enough to cause the tank lid to open slightly and leak, it's time to replace the tank lid spring or bend more preload in the spring. The entire system (fuel tank, fuel filters, fuel lines, tank fittings, etc.) must be airtight to achieve consistent repeatable performance. I always that my fuel system is airtight when it's new but also recheck it every time I rebuild a car after a race. It easy insurance plus soapy water is a good way to thoroughly clean the tank. Just be sure it's totally dry before using it by flushing out any water left in the tank with fuel before installing it in you car.
Tuning in a Pinch
The pinch test.
Many racers find it difficult to properly tune the low speed needle. We've developed a method that makes it easy, even for beginners, to adjust the low speed needle to within 5% of peak performance and it's done right on the bench. We call it the pinch test and here's how it works.
Step 1. Start your engine and warm it up for about 30 seconds by revving the engine from idle to mid-throttle while holding the wheels off the ground.
Step 2. Allow the engine to idle for 10 seconds. If the low speed needle tuning is so far off that it won't idle, simply bump the idle position higher until the engine will reliably run at a higher idle.
Step 3. With the engine at idle, pinch and hold the fuel line near the carburetor, cutting off the fuel flow and carefully listen to the engine rpm.
If the low speed needle is set correctly, the engine should increase rpm only slightly and then die.
If the engine increases several hundred rpm before dying, then the low speed needle is too rich and needs to be turned in.
If the rpm doesn't increase at all and the engine simply dies, the low speed needle is set too lean and should be richened or turned out.
After doing this a few times you'll get the hang of it, and tuning the low speed needle will become easy. The only way to get that last 5% performance on the low speed tune is through careful track testing. By evaluating the engine as it accelerates from a dead stop or out of slow corners during actual race conditions is the only way to get those last few percent of power from your engine. Variables like the clutch setting, gearing, traction, atmospheric conditions and even the car's weight will have an effect on this last 5% of tuning performance, and the optimum setting can only be reached by careful on-track tuning.
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