I ran my stock cylinder flat track ported motor with a 39 mm kiehin carb, PSI hi rev pipe, pump gas with DGH needle in the center and a 55 slow jet then a 180 main. Now Its set up for 100 octane fuel and a ESR air box eliminator. How much should I adjust my jetting.
The safest approach is to put about a 200 main jet and work your way down to the jet that makes the best power. If you end up with a main jet that is smaller than your current 180, there is a good possibility that your new set up may not be flowing as well as the old one or making as much power as your old intake system.
You need a way to test your before and after performance level any time you make ANY changes to your engine or re-jet your engine. Contrary to popular folklore and myth that has been passed down for ages, especially over the internet the last 20 years, a spark plug is not a sensor of any type. A spark plug CANNOT tell you when you have the main jet that makes the most power. A spark plug CANNOT indicate optimum air fuel ratio. A spark plug CANNOT tell you when you have the metering needle in the correct position. Do not become another one of the guys that waste their time cutting spark plugs apart and looking for the "max power mixture ring". I can set up an engine that runs like crap and produce spark plugs that have the perfect "max power mixture rings" that forms at the base of the porcelain cone that surrounds the center wire. I can set up an engine that makes max power and make it produce spark plugs that will not have the "max power mixture ring".
A plug chop only reveals the operating temperature of the center wire, porcelain cone surrounding the center wire, threaded shell and ground strap. The ideal spark plug temperature is influenced by many factors and optimum power is not on that list. When you have a well designed and tested engine package you will be able to find the main jet that produced max power without the engine experiencing detonation. A spark plug will collect very small metal fragments from the piston, head and top of the cylinder when the engine is experiencing detonation. Hopefully you monitor the spark plug frequently and carefully when making small changes and catch the engine at the onset of detonation. A well developed engine package will tolerate some detonation without causing a piston failure but it must be noticed and corrected before it gets severe enough to cause a heat related piston failure
One of the best ways I have found to find the main jet that makes the best power and acceleration is to find a smooth steep uphill that has good traction where two bikes can run side by side in a roll on drag race. The other way is to use a dyno. If the dyno operator is going to use an O2 sensor to tune your two stroke, I would recommend that you load up your bike and go to another dyno where the dyno operator knows something about tuning two strokes. I use O2 sensors on 4 stroke tuning on the dyno and I use O2 sensors mounted in the pipe during two stroke engine development on the dyno but DO NOT USE A SNIFFER TUBE IN A TWO STROKE EXHAUST and do not try to use the same A/F ratios on a two stroke that works on four strokes.
The roll on method of testing involves lining up side by side with another bike that has a performance level and gear ratio (sprockets) that is ideally the same as yours. Get both bikes rolling in 1st gear, shift to 2nd and both bike riders MUST roll on the throttle simultaneously and shift through the gears at the RPM that produce the best acceleration. The roll on method is very effective as long as one rider is not trying to get a jump on the other rider and prove he is a better rider. We are trying to determine how the two bikes acceleration compares as they go through the gears and how many bike lengths difference there is when you are
about half way through the RPM range in top gear or lesser gear if you do not have a long enough drag strip.
Start with a huge main jet (
one that is large enough to make the engine misfire as you approach max RPM in the lower gears) and a new spark plug. Make a few roll on runs with each jet change. (
or as many runs as it takes until both riders are rolling on the throttle and shifting gears consistently enough to get an indication of the performance change resulting from the jet change) Look at the spark plug under a 2x to 5x eye loop after each jet change and series of associated roll on test. The spark plug will remain ghost white even when the jet is big enough to have a rich misfire. The spark plug will remain ghost white even when the jetting is a little rich or the engine has the jet that makes max power. If there are microscopic pepper like specks that collected on the porcelain cone that surrounds the center wire your engine has experienced detonation. If you see detonation specs on the spark plug, stop immediately and determine why you are experiencing detonation. Correct the problem immediately. The spark plug may start to get a little color after many (25 to 100) roll on passes with a main jet that is close to making max power if the head temp and spark plug heat range are correct for each other.
If your spark plug indicated detonation and the performance level change was a noticeable improvement from leaning the carb one jet, you have something wrong with your engine package. The octane of the fuel may be too low, the ignition timing may too advanced, the compression ratio may be too high, the pipe may have too much restriction, the heat range of the spark plug is too hot or you may be trying to tune an engine package that was designed/built by someone that did not know what they were doing.
The performance level will increase noticeably at first when the carb is leaned down from the main jet that produced a rich misfire at high RPM. Continue to lean it down as long as the performance increases and the spark plug does not show traces of detonation. If you lean it down a jet and do not see a performance change you are at or very near to the jet that produces max power. Look at the spark plug for traces of detonation specks. If there are none, lean it down another jet and perform the roll on test. If the performance still does not increase and the spark plug is free of detonation specks, you have found the jet that provides the mixture for max power. If you use your engine for only drag racing, leave the main jet that you last tested in the carb. If you want to jet the bike for reliability and are willing to accept a 1% to 2% loss in peak power, richen the main jet 2 jets from the last jet you tested.
