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Sit N' Spin
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Discussion Starter · #1 · (Edited)
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".

Timing -

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.

Vacuum -

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.

Tachometer -

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.
 

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Good read

Although you've covered most of the basics, you left out a very important timing factor. The curve. To say you want all the timing in at a certain point is OK, but the rate that the advance comes in is also a critical. A lot of the jet boats out there are also ski boats. A good ski boat pulls skiers up/out with a steady, gradual pull. The timing curve, rate of advance, plays a huge role in a smooth transition into the power range. If the timing is too sudden, inexperienced skiers will have the rope jerked out of their hands. If it is too slow, the boat will bog down and the skier will be playing a balancing act try to stay up. Each boat/engine combination will have a different "curve" requirement. Maximum RPM, or Max speed, does not necessarily relate to the best drive/ski ability. Basics are defiantly a requirement, but overall use needs to play into the equation as well and a happy medium can be reached for both speed and drive/ski ability.........Ray
 

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Sit N' Spin
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Discussion Starter · #3 · (Edited)
Thanks Ray. The main point that I was trying to make is that when tuning a jet boat engine, MPH is not the information to go off of when doing so. There are way too many other variables that affect MPH and engine power is just one of those things. While MPH is most certainly affected by engine performance, due to many other variables that affect MPH it is definitely not a very good indicator of engine performance. It may tell you that you're down on power, but it has no way of telling you how much power you're down on to give an accurate indication as to what could be the cause of it. I can't tell you how many times I've seen guys wonder why they were down 3-4mph from the last time they took the boat out and were quick to write it off as some sort of an engine or pump issue, yet the engine and pump were spinning the exact same RPM at WOT that they've always spun every trip out, indicating that engine and pump were definitely not the cause of the loss of MPH.

However, since the load the jet drive applies to the engine is purely based on how fast the engine spins the pump and how much water is flowing through the pump, the only thing affecting RPM and vacuum is the load vs. engine power so the information obtained from these two things is the most accurate to go off of when tuning a jet boat engine.

In regards to the timing curve and jet boats that are used primarily as a ski boat...I honestly don't see how that would affect a skiier's ability to get up. The only thing that the timing curve should affect in a jet is throttle response from idle to full advance. One would think that it's all in how the driver brings the RPM up. Since everything on a jet boat is all about generated thrust which is directly related to RPM, if you're pulling the rope out of the hands of the skiier just back off the throtte, which will decrease RPM, which in turn decreases available thrust.
 

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In regards to the timing curve and jet boats that are used primarily as a ski boat...I honestly don't see how that would affect a skiier's ability to get up. The only thing that the timing curve should affect in a jet is throttle response from idle to full advance. One would think that it's all in how the driver brings the RPM up. Since everything on a jet boat is all about generated thrust which is directly related to RPM, if you're pulling the rope out of the hands of the skiier just back off the throtte, which will decrease RPM, which in turn decreases available thrust.
And drops the skier due to lack of consistent acceleration. For an experienced skier it is no big deal, but with kids trying to get up, or inexperienced adults, a steady pull is required. I've seen it both ways, and I'll curve the distributor a little more carefully and make the boat more drive/ski ability friendly.........Ray
 

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I agree on some of what your saying, but some is pure and utter bull$hit to say the least.
So tell me, did you learn all of this before or after you lunched your engines? (yes I said engine with an S on the end)
So when is pump learning 101? I think you lunched 2 pumps before you got it to actually work without it eating its self up huh?

BTW, hows the copper plumbing on the boat holding up for ya?

I usually skip over posts by idiots, but these last few caught my eye and some of what you say just makes me wish I had your knowlage.......NOT!!

Ok, for all you newbies, flame away......I can take it, Im wearing my big boy shorts today. :)sphss
 

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Sit N' Spin
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Discussion Starter · #8 · (Edited)
EngineS? I don't know where you're getting your information but...the second engine has 15 hours on it and is still running strong. The first one failed due to faulty machine work (block needed to be align honed and ended up eating up the main bearings...machinist swore up and down after asking numerous times if the block needed to be align honed that it didn't need it...trusted the machinist...never make that mistake again).

2 pumps? Actually no...1 pump (very first pump I'd ever built...long before I knew anything about jet drives, how they work and how they load an engine) and 1 impeller. That impeller casting was way too thin for the shit I put it through. Nothing to do with how the pump was built or set up.

Exactly what in this thread and my other thread do you think is bullshit?

Widowmaker...I agree to a point. Running an advance curve or locking out timing is just a matter of preference. I prefer to run a curve since I do pull skiers and it makes the throttle response a bit less sensitive and more controllable between 2000 and 3000 RPM, which is where I pull skiiers up at.
 

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Resident Ford Nut
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I agree on some of what your saying, but some is pure and utter bull$hit to say the least.
So tell me, did you learn all of this before or after you lunched your engines? (yes I said engine with an S on the end)
So when is pump learning 101? I think you lunched 2 pumps before you got it to actually work without it eating its self up huh?

BTW, hows the copper plumbing on the boat holding up for ya?

I usually skip over posts by idiots, but these last few caught my eye and some of what you say just makes me wish I had your knowlage.......NOT!!

Ok, for all you newbies, flame away......I can take it, Im wearing my big boy shorts today. :)sphss
Ouch ;)



" B]I agree on some of what your saying, but some is pure and utter bull$hit to say the least.[/B]

I have to agree with that Tim. I was going to ask Jetaholic how many boats he has tuned beyond their expectations ? I'll tell ya this ...I can give you several ways to make my boat rev higher but go slower as can at least 4 dozen guys reading this post.

You can tune your boat for economy,mph,quickness or a comprise of all three. The 109 mph 9 passenger station wagon is compromise of all three.

"The most efficient way to tune a jet boat engine is to tune for max RPM, not max MPH" Question: If I tune for max rpm at WOT what does that do to my cruise economy? Or are you talking about a particular setting and then working around that setting?

I'll leave it there so I don't go off on the wrong track. Maybe I'm reading more into what you wrote.


p.s. As Widowmaker said.. "Lock the timing out". At least on any high performance engine. And yeah I can pull a 115 lbs women up with 935 hp available and not pull the ski rope out of her hands.

Sleeper CP :D
 

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Sit N' Spin
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Discussion Starter · #10 · (Edited)
Ouch ;)



" B]I agree on some of what your saying, but some is pure and utter bull$hit to say the least.[/B]

I have to agree with that Tim. I was going to ask Jetaholic how many boats he has tuned beyond their expectations ? I'll tell ya this ...I can give you several ways to make my boat rev higher but go slower as can at least 4 dozen guys reading this post.

You can tune your boat for economy,mph,quickness or a comprise of all three. The 109 mph 9 passenger station wagon is compromise of all three.

"The most efficient way to tune a jet boat engine is to tune for max RPM, not max MPH" Question: If I tune for max rpm at WOT what does that do to my cruise economy? Or are you talking about a particular setting and then working around that setting?

I'll leave it there so I don't go off on the wrong track. Maybe I'm reading more into what you wrote.

Sleeper CP :D
I probably should've been more clear on that, which kinda gets into carb jetting.

When I'm tuning an engine, I'm tuning for mainly a compromise of all three. The very first thing on a carburetor I dial in is the idle mixture for max vacuum, then fine tune that so it doesn't die when I chop the throttle to idle. Once that's dialed in, I tune the primaries first and foremost.

When I'm tuning the primaries on a carb, I'm jetting for max vacuum. Once I achieve that, I jet the secondaries for max possible RPM without altering the primaries (tuning around the economy jetted primaries). Once that's all said and done, I then increase throttle until vacuum starts dropping off by a noticeable amount for a small RPM increase. I read the gauge to find out how much vacuum I'm pulling when that starts to happen, then select a power valve that's rated to open at that point. This way the power valve is only coming on when the pump starts to load the engine by a considerable amount and not at cruise.

So to clarify, when I'm tuning my cruise setting I rely on vacuum, and WOT tuning I tune to max out the tach.
 

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That helps....but am I to assume you're discussing a typical recreational boat or a 80+ mph river racer ?

If the latter, then there are better ways to tune a boat.

S CP :D
 

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I probably should've been more clear on that, which kinda gets into carb jetting.

When I'm tuning an engine, I'm tuning for mainly a compromise of all three. The very first thing on a carburetor I dial in is the idle mixture for max vacuum, then fine tune that so it doesn't die when I chop the throttle to idle. Once that's dialed in, I tune the primaries first and foremost.

When I'm tuning the primaries on a carb, I'm jetting for max vacuum. Once I achieve that, I jet the secondaries for max possible RPM without altering the primaries (tuning around the economy jetted primaries). Once that's all said and done, I then increase throttle until vacuum starts dropping off by a noticeable amount for a small RPM increase. I read the gauge to find out how much vacuum I'm pulling when that starts to happen, then select a power valve that's rated to open at that point. This way the power valve is only coming on when the pump starts to load the engine by a considerable amount and not at cruise.

So to clarify, when I'm tuning my cruise setting I rely on vacuum, and WOT tuning I tune to max out the tach.
Why is it you keep on going back and editing your posts? Do you not place the corect info in it the first time? Dude, you are a toolbag. Quit giving out bunk info. You rubbed me the wrong way ever since the HB days when you were a newbie and didnt know dick....imo..you still don't.

carry on, im off my soapbox and getting popcorn........
 

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Why is it you keep on going back and editing your posts? Do you not place the corect info in it the first time? Dude, you are a toolbag. Quit giving out bunk info. You rubbed me the wrong way ever since the HB days when you were a newbie and didnt know dick....imo..you still don't.

carry on, im off my soapbox and getting popcorn........
yep
 

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Glendale Arizona
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Why is it you keep on going back and editing your posts? Do you not place the corect info in it the first time? Dude, you are a toolbag. Quit giving out bunk info. You rubbed me the wrong way ever since the HB days when you were a newbie and didnt know dick....imo..you still don't.

carry on, im off my soapbox and getting popcorn........
Take off those kid gloves Tim. I sense you're holding back and it's gonna eat you up inside man. You need to let it fly dude. Get it out there......You know......... say what you're really feeling.........


:(:(
 

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Sit N' Spin
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Discussion Starter · #16 · (Edited)
That helps....but am I to assume you're discussing a typical recreational boat or a 80+ mph river racer ?

S CP :D
There was a reason why I titled it Jet Boat Engine Tuning Tips For The Lake/River Rats...meaning a typical lake boat. If the title was interpreted wrong I apologize.

Futs...you're right. I don't know anything...I guess the fact that I have a good running boat that does close to 80 while only turning 5200-5300 without a shoe and a ride plate while having decent (as decent as it can be in a jet boat) fuel mileage just happened by pure luck.

If you know so much and think I don't know dick, you have anything to back that up? Prove it with some corrections to my post. Explain what info in my post is incorrect/bunk and explain why it's wrong.
 

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ATTA BOY
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Why is it you keep on going back and editing your posts? Do you not place the corect info in it the first time? Dude, you are a toolbag. Quit giving out bunk info. You rubbed me the wrong way ever since the HB days when you were a newbie and didnt know dick....imo..you still don't.

carry on, im off my soapbox and getting popcorn........
Got ya covered.

 
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