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RPM & Speed increased from 3200rpm 18knots 3500rpm 22knots

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Bob & Sue

Contributing Member
I was coming in from fishing Thursday running at 18knots 3200rpms just barely on plane with tabs down when the rpms went to 3500 and jumped to 22knots engine was runiing fine it almost felt like it jumped up on plane drive was up a couple of clicks water was smooth as glass. It was dark and I was standing with my hand on the throttle I just thought maybe I bumped the throttle up but not sure could anything else cause this?
 
Bob & Sue said:
........ running at 18knots 3200rpms just barely on plane with tabs down when the rpms went to 3500 and jumped to 22knots engine was runiing fine it almost felt like it jumped up on plane drive was up a couple of clicks water was smooth as glass. It was dark and I was standing with my hand on the throttle I just thought maybe I bumped the throttle up but not sure could anything else cause this?
Click to expand...

Water conditions may have changed slightly, allowing the hull to get up further out of the water.
What ignition system? A slight change here may have also caused this.
Hard to say without knowing more.

Question: why are you running at an RPM that does not get the hull up further out of the water?
That would be a very inefficient RPM and speed to operate at, IMO.
That is just barely within the RPM range where Detonation potential may be somewhat greater, and of the two most efficient hull speeds, it's certainly below true planing speed, as per your comment.

Just a friendly FYI on that!

.
 
RicardoMarine said:
Water conditions may have changed slightly, allowing the hull to get up further out of the water.
What ignition system? A slight change here may have also caused this.
Hard to say without knowing more.

Question: why are you running at an RPM that does not get the hull up further out of the water?
That would be a very inefficient RPM and speed to operate at, IMO.
That is just barely within the RPM range where Detonation potential may be somewhat greater, and of the two most efficient hull speeds, it's certainly below true planing speed, as per your comment.

Just a friendly FYI on that!

.
Click to expand...

I was coming into a no wake zone and had slowed down about 1/8 mile from the entrance I was running on plane at 3500rpms just prior to this about to throttle down when this happened.
It's a 1999 mercruiser 5.7l Thunderbolt V.
Do you know what the rpm advance curve is for this system?
Thanks for the FYI always appreciated!!!!
 
Bob, there will be a number associated with the ignition module. The number indicates the advance that the module provides.
It will not include BASE advance.

When strobing the timing marks, the actual curve/limit seen by the timing light, can't help but include BASE.
It's while looking at these curves, and doing the math where it can be confusing.

The important advance, IMO, will be at/neat 3.2k rpm or so.
At 3.2k rpm, this curve below shows approx 22*.
Add a BASE of 6* (for example) and the actual advance is 28* @ 3.2k rpm.
The TA (total advance) occurs where the diagonal line becomes horizontal.

The TB V tends to continue the advance further than some others.
Example:
Take a look at the 26* @ 3.8k rpm.... this equates to 32* @ 3.8k rpm, if a BASE of 6* were to be used.
This equates to 34* @ 3.8k rpm, if a BASE of 8* were to be used.
These are still somewhat conservative numbers due to the non-quench style combustion chambers of Merc's SBC engines.


This came up under a search for the TB V.... so verify this for yourself.
It should be found in the Merc Service manuals.
http://www.boatfix.com/merc/Techbk/95/95HGB4.PDF
http://www.boatfix.com/merc/Bullet/95/95_11.pdf




2009-08-17_155950_7.4firingorder+timingcurve.jpg

Also be careful that specs for smaller ski boats are NOT used for Cruiser Boat engines.
These systems bring the advance on much earlier than for our cruiser engines.
Apples/Oranges.


FYI, while searching for an advance curve image, I came upon many forum threads where information is either lacking, or is incorrect, and/or both.
Please use caution when viewing some of these forums and the information provided.
Example:
We'll often see an advance number being thrown out there on a forum, such as 30* or 32* or 34*.
Since Ignition advance is progressive, advance numbers are meaningless without an associated RPM.
IOW, we can not suggest a TA of 28* or 30* without accompanying this number with an RPM.

This is just too dang important to fool around with.
So use caution.

.
 
Thanks to both of you for the great replies. A little of my history kinds of dates me but in 1966 I owned my first race car a 1947 Fiat with a 288 cubic inch chev in it running in B/altered it ran 10.32 141mph we ran Vortex magnetos then I moved up to fuel funny cars and dragsters for 10 years. I really appreciate the detailed responses I receive on this site. I am thinking of putting in a new ignition system am looking at the complete MSD Marine setup any opinions or should I just wait till the thunderbolt v dies, I am going to pull the distributor and check the advance curve.
Please see post I just left about this site.

http://www.marineengine.com/boat-fo...nk-a-great-Group-of-Contributors-on-this-Site
 
The timing chart I posted is straight out of the mercruiser sevice manual for that year motor and includes the variablea with the spark controls such as the knock detector.

The graphs Rick posted are also correct but do not include the knock control and idle control that may also be present.

Most all of mercs standard motor timing specs have a max advance of 32 dwgrees of Total advance that includes the base of 8. So 8 + 24 =32.
The key is when you achieve this. This is what varies between most engine models and there respective use, inboard vs outdrive, heavy vs light boat.......
 
kghost said:
.......... This is what varies between most engine models and there respective use, inboard vs outdrive, heavy vs light boat.......
Click to expand...
Amen to that!
Bob, if you should go with another ignition system, do not use the specs suggested for the TB V unless it also includes management for knock sensing.
Those numbers come in way too early for any standard type system.
I'd play this on the more conservative side due to the OEM engine build.

You'll have choices between conventional Electronic, HEI mechanical advance, and HEI EST, etc.
 
Bob & Sue said:
I am going to pull the distributor and check the advance curve.
Click to expand...
Bob, there is no mechanical advance apperatus to your distributor.
The distributor itself is nothing more than a "locked" 1:2 triggering mechanism.
These are EST (electronic spark timing), so all advancing is accomplished within the ignition module.

About all you can do, is to check the module function, and/or replace it.


kghost said:
.......... This is what varies between most engine models and there respective use, inboard vs outdrive, heavy vs light boat.......
Click to expand...
Amen to that!
Bob, if you should go with another ignition system, do not use the specs suggested for the TB V unless it also includes management for knock sensing.
Those numbers come in way too early for any standard type system.
I'd play this on the more conservative side due to the OEM engine build.

Since you have HP Racing experience, you no doubt know what the Full Dished piston offers in this version of the Merc SBC engine with a wedge area within the combustion chamber.
No Quench Effect or Squish what-so-ever. :mad: :mad:
This is one reason why we must be rather conservative with ignition advance for this version SBC.
This is a pet peeve of mine, but that's another story for another day! :)

Bob, you'll have choices between conventional Electronic in either mechanical or EST, HEI mechanical advance, and HEI EST, etc.

.
 
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Do you have a good coffee supplier on the left coast? We here on the other side could use some of what you got..........lol

I am dragging my a$$ today...........sucks gettin old...........
 
If this was a one time occurrence i would ignore it. I would be looking for water in the fuel. No water then i would think you were on a wave or wake of another boat coming down.
Just for giggles...dump your fuel-water sep. into a clear container and let sit. If you see water on the bottom that could account for the sudden jerk of the rpm's.
 
Master mechanic said:
If this was a one time occurrence i would ignore it. I would be looking for water in the fuel. No water then i would think you were on a wave or wake of another boat coming down.
Just for giggles...dump your fuel-water sep. into a clear container and let sit. If you see water on the bottom that could account for the sudden jerk of the rpm's.
Click to expand...

I think I'm going to put a new fuel filter on along with my spare prop just had it rebuilt time to have the present one balanced. See if it happens again.
 
RicardoMarine said:
Bob, I could possibly help you with the coffee!
But I'm afraid that the free mechanical work would involve a plane ticket and some very good meals! :D

.
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I agree, But Rick you are a hop skip and a jump away.........Me on the other hand, 3000 miles +
Besides I only really work on fresh water boats........hate barnicals they hurt and cause a lot of bleeding so I gave that up......

The fresh water stuff is very clean to work on......

As far as your ages, I have a few more years to catch up but not too many......

I typically only help others with issues I do not really work for $$ on boats any longer and thus spend more time on my own boat and motorcycle when the weather is appropriate.
 
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