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Frequently Asked Questions on MMSC |
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(603) 875-6714 |
Listed below are the questions (and their
answers) that we are most frequently asked about sled modifications.
"If you have something we should add to the site, just e-mail it to us and we can get it posted."
HOW DO I SELECT THE CORRECT SLED JET SIZE?
First, install jets that are large enough to insure that you are on the RICH side of the jetting vs. power curve (see graph). If the engine's jetting range is unfamiliar to you, it is best to start with jets large enough to cause the engine to "four-stroke" (indicated by very low power output, a flat "blubbering" high speed sound, and wet plug readings). If you do not know how to recognize these signs, remove the high-speed jets from the carbs and make a run, then you will have a feel for what four-stroking is.
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Jet Graph |
Next, proceed to test each jet combination by bringing the boat up to top speed, for at least a full minute, and taking an accurate speed and RPM reading. Cut the engine off clean, and without any further running of the engine, inspect each plug insulator (and piston dome if necessary) for color. Assuming the plug color shows you are still rich, drop down ONE jet size and re-test, comparing the speed and rpm with the previous run. Continue dropping down ONE size at a time until the performance stops improving OR until plug (or piston) color shows a lean condition.
HINTS:
GOOD engine power will be found about one jet size smaller than the last jet size which four-stroked. MAXIMUM power will then be found a couple of sizes smaller still. Warning: See the next hint for more information on this. Crankcase charged two-stroke engines enjoy a slight increase in volumetric efficiency with a mildly over rich mixture (due to the fuel's latent heat of vaporization cooling the intake charge). They tend to have a flatter air/fuel ratio range over which good power may be expected (as compared to a four-stroke engine). So, if maximum fuel economy is not imperative, running the engine at the "rich" end of the safe jetting zone is a good way to get some extra insurance against thermal engine failure.
A charcoal or dark brown plug insulator color usually indicates that a smaller jet size can safely be tried. Once the plug insulator begins to show a light chocolate or tan color, further leaning can be risky. A light tan or white color indicates too lean a mixture (assuming that the plug is of the correct heat range, timing is correct, etc.) and must be immediately corrected by jumping to a jet several sizes larger!
DO NOT CUT CORNERS! Jets may seem expensive, and this procedure is time consuming, but the alternative is poor performance and/or MELTED pistons. Now that's expensive! For jetting, as well as evaluating other tuning changes, consistent test procedures with good INSTRUMENTATION are the key to success. Learn to do this testing accurately and you will find this "poor man's dynamometer" your best friend in getting peak performance.
You can save a great deal of time jetting by using a dynamometer. This will allow you to see the power curve change with each set of jets. It is much faster doing all of the spark plug checks and jet changes in the garage.
I HAVE AN ENGINE WITH LOW HOURS. LAST SEASON I LOST A CYLINDER. - DO I HAVE TO BORE THE WHOLE ENGINE?
No, you do not need to oversize all the cylinders simply because you have to do one. Many people falsely believe that the engine will become so far out of balance (running with one oversized piston) that it will damage the engine and will intolerably rattle the sled. The fact is that the manufacturer's balancing specification are so loose that you will never tell the difference, it might even get better.
The fact is that the engines are usually compromise balanced to reduce perceived shake in the vetial (at the expense of the fore/aft plane). The added weight of a typical .015" or .030" oversize piston probably falls within weight tolerances for the standard piston!
And as for the increase in displacement, the power of the one "big" cylinder versus the others is negligible. The change is just too small to worry about even in a performance engine.
For example, an 800cc with a .015" oversized piston increases the displacement by only .22 cubic inches! At the most that piston might make 1/4 Hp more than the others! In fact, boring the single cylinder is all that the OEM's provide warranty coverage for. Keep the extra dough in your wallet!
