As I've read through posts on various message boards and have spoken with various lake/river jet boaters there seems to be a bit of misinformation on how to properly tune a jet boat engine. I've seen many mistakes be made due to some people going off of the wrong information when tuning, while others just simply "trial & error" tune without knowing exactly what's going on in regards to the type of load placed on the engine by a jet drive.
One of the most common mistakes I've seen made is tuning a jet boat engine by going off of max MPH. While MPH is certainly what we're after, there are WAY too many things about the dynamics of a jet boat (i.e. hull hydrodynamics, hardware setup, etc etc) that affect MPH that you'll end up chasing your tail by doing it this way. MPH is not just about RPM, and there is no direct relationship between RPM and MPH. So rule number 1 for tuning jet boat engines...
FORGET ABOUT MPH!!!
MPH, due to the many variables that can affect it, is not a direct indicator of engine power. It can be said that -
Less top MPH does not always point to a drop in engine power, but a drop in engine power (RPM) will ALWAYS lead to a drop in top MPH.
The most efficient way to tune a jet boat engine is to tune for max RPM, not max MPH. Think of your tachometer as your horsepower meter. Any adjustments made to the engine that affect the engines performance will show up on your tach reading. By tuning for max RPM, you eliminate A LOT of variables that tell you nothing about engine performance.
Your 3 best friends when tuning a jet boat engine are -
1) Timing light...just a regular one...not the Total Advance or "dialback" type as Duane from Hi-Tech Performance has just informed me that they have proved that they cannot be trusted for accuracy(thanks Duane!)
2) Timing tape...this goes on the balancer itself with the "0" right on the stock #1 cylinder TDC mark on the balancer
3) Vacuum gauge
4) Your tachometer
Some claim that "tachs are for wussies". I highly disagree with this. Having one is the most accurate way of knowing how the engine is performing. Remember...the tach is your "horsepower meter".
Setting ignition timing refers to setting the time at which the spark plug fires relative to piston positon and crankshaft degrees of rotation. Example...most Chevy V8s idle around 10*BTDC, or 10 degrees Before Top Dead Center. This means that the crank position is at 10 degrees before the piston is at top dead center of its travel up the cylinder when the spark plug fires. The reason the spark plug has to fire BEFORE the piston gets to the top of the compression stroke is due to a time lag between the time that the air/fuel mix is ignited and the time that the burning air/fuel mix reaches maximum cylinder pressure. In order to ensure that the burning mixture is reaching maximum cylinder PSI at a certain point in travel of the piston to apply the proper downforce on the crank to achieve max power (usually AFTER top dead center, when the piston is making its way down the bore), the mixture has to be ignited BEFORE the piston gets to the top and starts to go back down the bore. The higher the RPMs increase, the earlier the spark plug has to ignite the mixture to ensure that max cylinder PSI is being made at the same point after top dead center in piston/crank travel. This is where the mechanical advance comes in. As RPMs increase, the mech advance mechanism advances the timing to make this happen.
*Note: This is why higher octane fuels require more timing advance. The time lag between ignition and max cylinder PSI is greater due to the fuel having a slower burn rate, which requires the air/fuel mix to be ignited even earlier, requiring more advance.
A base timing setting can be obtained on the trailer since load does not affect the reading itself (exception to this would be if you run a vacuum advance which IMHO has no purpose in a jet boat. I advise to lock out the vacuum advance mechanism and take it out of the equation). The timing advance mechanism is centrifugal, so it works purely off of RPM and RPM only. First, remove the number 1 spark plug and place your thumb over the spark plug hole. Then rotate the engine by hand until you feel air pressure under your thumb that is over the #1 spark plug hole. You are now on the #1 cylinder compression stroke. Keep rotating the engine until the mark on the balancer lines up with the number "0" on the timing tab. Install the timng tape by placing the end of it marked "0" on the mark on the balancer, then wrap it around the balancer. Once this is done, reinstall the spark plug.
Using the timing light, aim the light at the balancer and increase RPM until the numbers on the timing tape stop advancing, then adjust the distributor until the number that corresponds to the amount of advance you wanna start with lines up with the "0" the timing tab. Example...if you wanna run 34* total advance, adjust the distributor so that the number 34 on the timing tape on the balancer lines up with the "0" on the timing tab. THIS IS YOUR BASE SETTING!!! That setting will get you on the water for additional fine tuning. If you have the correct stop bushing and springs, when you rev the engine the mark should stop advancing at or close to 3000 RPM, and once you've dialed in your base setting the timing will be somewhere around 10-12* at idle.
However, load DOES affect how much timing the engine wants. Some people say to advance to 36* total and leave it there, while others say to advance until you get the max possible RPM, then back it off a couple degrees for a detonation safety margin. I am a huge fan of this. What the RPMs do in relation to advancing the timing will tell you how much advance the engine wants. Some people fear that they'll detonate by doing it this way. If you're detonating, that will drop performance and you won't get an increase in RPM.
Engine vacuum is a direct indication of engine load. Remember...engines are self driven air pumps. The amount of air they have to ingest to carry the load will drop your vacuum reading. Engine vacuum is directly related to how far you have to open the throttle plates to achieve a certain RPM given a certain amount of load. High vacuum = throttle plates barely open = minimal load whereas low vacuum = throttle plates open more = moderate to max engine load. All vacuum tuning must be done on the water so that the engine is under load.
Vacuum gauges come in handy for tuning idle mixture and cruise jetting. For idle mixture, you adjust the mixture screws evenly to achieve highest possible vacuum. Once you've done this, reset the idle SPEED screw to idle. After doing this, you'll wanna rev the engine to roughly around 3000-3500, then chop the throttle to idle. If the engine dies or acts like it wants to die for a second, richen the mixture screws up by 1/4 turn on 2 corner idle carbs (carbs that have only 2 idle mixture screws) and 1/8 turn on 4 corner idle carbs (carbs that have 4 idle mixture screws). Repeat the rev test until you can chop the throttle and it falls quickly and smoothly to idle without bog or hesitation.
For primary jetting, with a vacuum gauge hooked up and a long enough hose to reach the driver seat so you can read the gauge, get up to your normal cruise RPM and take a baseline vacuum reading. From there, you try to jet the primaries down 2 sizes and repeat the test at the exact same RPM.
If vacuum increased, you're going in the right direction. Try to go down 2 more sizes and retest.
If vacuum decreased, the motor wants more fuel. Go up 2 sizes from your original jet size you started with and retest.
Keep playing with the primary jet sizes and checking vacuum with each size at the exact same RPM until you achieve max possible vacuum. This will give you the most efficient cruise mixture and best cruise fuel economy.
The tach is used for dialing in the secondary jets along with best ignition timing. Once the primary jets are dialed in, you want to jet the secondaries for max possible wide open throttle (WOT) RPM. At wide open throttle there's almost no vacuum to speak of so you can't really go off of vacuum at WOT for tuning purposes.
Get a baseline WOT RPM reading. Jets get up to max RPM the instant you stab it to the floor so you only have to hold it there long enough for you to get a good reading on the tach. Go 2 sizes up on the secondary jets, then retest.
RPM increase - You're going in the right direction. Keep jetting up in 2 size increments until you see a drop in RPM, then go back down 2 sizes
RPM decrease - Go 2 sizes down from original jet size and retest
For timing advance I start at about 32*, get a baseline tach reading, then advance in two degree increments and retest at WOT. Dial in the advance for max possible WOT RPM, then back it off a couple degrees for safety.
You may have to go back and forth between the timing and the carb jetting (more advance can make the motor want more fuel) to get it perfect. But these tips will definitely get you close to where only small adjustments are necessary for a perfect fine tune.
Remember...forget about MPH when tuning a jet boat engine. When tuning a jet boat engine for max power, it's all about achieving max possible RPM. MPH is a completely different subject.