this info was copied and pasted right from dub jetts website..... read it and learn..... he has allllot of info on his site. hope this helps. and i hope dub doesn't mind me pasting this info from his site..... >>>>> the motor god>>>
http://www.jettengineering.com/
Tuning your full tuned pipe is normally very easy if a few simple rules are observed:
Your engine must be in good condition and be timed for full tuned pipes.
You must have a good carburetor and tank system--if your engine is not running properly on its stock muffler, it certainly will not run on a full pipe.
You must have some idea of the rpm range you can expect with your chosen prop - if you think you should be turning 20000 rpm, but the engine is actually only capable of 16000, then you will surely fail to get the tuning correct.
You should do your tuning on a test bench with good fuel and plug--use at least 20% oil, which means adding 4-6 oz. to standard sport mix.
Make sure your engine is well broken in before running these tests.
Run pipe pressure, placing the tap at the front part of the largest section of the pipe (at the rear of the divergent cone). Ignore bulges toward the rear of the pipe.
Follow these steps:
Choose the propeller you wish to tune, eg. 10x6, then get a propeller with 1" more pitch, eg. 10x7. Start with the bigger prop
Determine the expected rpm range by running your engine on its standard muffler. You will very seldom obtain more than 1500-2000 extra rpm with a full tuned pipe. In the case of JETTs, where the standard muffler is also tuned, then the expected rpm increase may be only a few hundred. If the prop is very large, out of the recommended range for the JETT, then you may obtain larger than expected boosts with the full pipe. For example, a JETT .46 will not pull a 11x6 very well in its stock condition, and you may find that it turns the prop only 12000. With the full pipe, tuned properly, it turns 13500. However, the same .46 will only turn 300 more with the 10x6. (these are examples only)
Using the larger prop, install the pipe at its longest length (nothing cut off) and run the engine up to peak. (peak is where the engine will not go any faster when you continue to screw in the high speed needle--it sags and drops off rapidly) open the needle rapidly after peaking to avoid hurting the piston and liner. Record the rpm. Let the engine cool. Cut the pipe or header by 1/2" (12 mm) and repeat the test. The engine rpm should increase. Cut the header again, and repeat the test. Continue at ever decreasing amounts cut off until the engine does not pick up any more rpm, or it actually drops off slightly. At this point, your system should be tuned for the size smaller prop, as you have cut off the pipe too much for the larger prop.
This method is not foolproof, but will work well if all other systems are in good shape. You are looking for a system that will run smoothly as you advance the throttle. Pipes that are too short have difficulty "jumping" up on the pipe and will struggle, overheat and blow plugs when operating at full throttle--use smaller props until the engine stays on the pipe, even at rich settings. If you are blowing plugs, something is wrong--look!!!!!!
Pipes that are too long show little or no jump when coming on the pipe, ie., they are always on the pipe. This jump is usually at about 2000-3000 rpm below peak. Eg., as you advance the throttle slowly to, say, 12000, the engine suddenly jumps to 13500 and will not run steadily between 12000 and 13500. The amount of jump determines the boost of the system. The more boost, the more problems you will have.
Try to keep the "jump" or boost to a minimum while producing the power you need.
JETT engineering 9-16-98
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