Strange Change on RPM's - any ideas?


toofast

toofast

Active member
I can't seem to find the thread, but somebody else posted a "sudden" change in efficiency or RPM's, (Can't Remember)....Anyways, my story sees similar.

I maintain pretty detailed records of fuel burn, GPH, MPH, RPM, etc. I have the factory installed "mechanical RPM" gauges and then two smartcraft monitors directly connected to the engine. I went on a 80 mile trip this weekend. On the way out everything was perfect...that means when I throttle up my engines, I would bring the lever's up together...and always my engines run about the same RPM on cruise, burn about the same GPH and stay in "sync".

On the trip home, we departed the marina...all was well...ran about 5 miles, then anchored out for an afternoon swim. As we left, I started out and all of a sudden my STBD engine is about 200 RPM's behind my Port at the same "throttle input". Makes no sense, so I think I have seaweed or something on my props...so I stop, everthing is perfect...much be my imagination.

Start up again, same thing...STBD engine nees about "1 1/2 inches" more throttle input to maintain the same rpm's as the Port engine...both gauges read this way. So I think maybe something slipped in throttle linkage...NOPE, at least not that I can tell...at the engine both throttles seem to advance exactly the same distance.

Now it gets worse...so I then end up cruising this way...meaning I have way more throttle into the STBD engine to keep the RPM's the same...and guess what, I am not burning a TON more fuel in my STBD engine...however they are now the same exact RPM. (A ton more fuel means 2 GPH, but for my boat that is way off.)

So now I think somehow my gauges are messed up? How could the fuel burn be so different. And of course, when I got back....my STBD engine did burn more fuel on the way home, vs on the way there when they both burned the same.

Any ideas...where to look first. I find it hard to believe my factory tach and smartcraft tach went at the same time. Or do they feed of the some comon point, such as the ECM, even though they are connected in different ways.

OH, I also checked the trim, they both seeem to be trimmed the same and both props show no damage.

Thanks in advance for some more ideas...
 
TooFast:
I had a similar problem this year. Like you, it is my Stbd engine. And, like you, it started all of a sudden.
My dealer did a tune up and new plugs, and it helped significantly, but did not resolve the problem.
In order to get equal tach and throttle, I need to trim Stbd higher than Port.

So, doing that, I wanted to make sure I was not going to hurt the u-joint in Sbtd. So, whil on the lift, I measured the drives to the transom rub rail with both drives all the way down. (There was almost an inch difference between the two. Then I raised them to where I now needed to run in order to maintain equal tach & throttle. The difference btwn down and trimmed was only about an inch, and the difference between the two while trimmed was less than an inch.

(I do not have the exact measurements with me, they are at the lake.)

Still, I am not convinced the problem is solved. I would appreciate other input, as well.
 
is it possible that the trim gauge is wrong? one may not be trimmed as much as the other? you could trim to the point you run at and then measure the rams on the drive to see if they are the same. also, bad fuel filter could be causing this. i remember loosing rpms suddenly on my 01 31PC. the fuel filter was clogging up.
 
It is possible that one trim gauge is wrong. However,
***The amount of time I use each trim button is the same (using the thumb button);
***The distance the gauges move is the same for both;
***The amount of rise is about equal for both in that period of time;
***And, I still need to raise Stbd further in order to achieve throttle / tach equalibrium. And, when fully trimmed in this manner, the amount of rise is only about an inch for Port and 1-1/2 to 2 inches for Stbd.

The fuel filter possibility you riased is worth looking into. And yet I believe they changed those when the did the tune-up a couple weeks ago. Still, I need to confirm that assuption.
 
Castines said:
is it possible that the trim gauge is wrong? one may not be trimmed as much as the other? you could trim to the point you run at and then measure the rams on the drive to see if they are the same. also, bad fuel filter could be causing this. i remember loosing rpms suddenly on my 01 31PC. the fuel filter was clogging up.
Click to expand...

I did try to measure the distances of the trim...but I was in the water and it was not exact...but it did not look like they were off. In fact manually moving the engine with the buttons on the dash did not make a massive difference. BUT you bet I will try to figure out how to measure the RAMs next weekend.

Plus I have trim on the smartcraft and they showed the same as the gauges, ".7" in my case for each side...so my guess is not the trim....but will look deeper.

As for the fuel filter...guess I need to follow my typical advice...KISS. So for sure I will remove, inspect and/or change the filter next weekend.
 
I have the same issue, my starboard runs lower rpm than my port at same throttle position and it also trims up slower than my port. Additionaly, I noticed a higher burn rate on my port engine.

I am planning to check them when the season is done, nothing I can do mid season. :)
 
Is the power steering running off of Stbd?
If so, that can cause some mof the power drain. And yet, I never had disparity in RPM in prior years.
 
LBI4fun said:
I have the same issue, my starboard runs lower rpm than my port at same throttle position and it also trims up slower than my port. Additionaly, I noticed a higher burn rate on my port engine.

I am planning to check them when the season is done, nothing I can do mid season. :)
Click to expand...

Did your STBD always run lower RPM's? But you have a higher burn on your PORT, which really would drive me crazy.

Also, how much lower is lower...if you have the same Livorsi throttles...my STBD is a full 2 inches advanced to run the same RPM.
 
the difference is only about 150 to 200 max and it only started this season, I usually adjust the throttle accordingly. The difference in burn rate is about 5 gallons at time of fill up. I usually fill up once i hit a quarted so it really did not affect me but it does bother me a bit to not be exact.
 
Seas-The-Day---

Do you happen to have smartcraft and if so, do you notice a fuel burn difference?

Like you before, this is driving me crazy...how can something change in an instant and there not be a problem...that will lead to more serious problems. Something has to be wrong.
 
I do not have smartcraft. Still, if you notice my logs on the thread called "Now I remember why I love my Formula...", the burn is equal in my left and right tanks except for the generator which runs off Port (I believe) and consistently burns about .34 - .40 GPH in addition to the Port engine.

I find it interesting that for those of us having this issue, it is consistently with the Sbtd engine! ???
 
I also noticed that my Port drive trims a bit faster than my Starboard in either direction. I went through recalibration on SmartCraft and same results, I also checked them in the back and there is definitly a difference.

Fluids and all look fine.
 
Well, I managed to get ahold of a tech at Mercury Marine...I called three times before someone was willing to chat.

He said most likely it is engine related...

He would recommend a compression test then leakdown test...as well as it could be a fuel pressure issue.

So...a clogged filter could affect the fuel pressure, so Castines was right on...

But for sure he admitted something is not right...

As a side note, he also did not like that my WOT RPM was only 4500 under normal load...he said to be careful with that as if I start lugging the engine with a heavy load...bad news is around the corner. Bummer that I did not "fight" this issue earlier when the boat was delivered...every day I live and learn.

Anyways...if we all have "engine" failures on relatively new engines...man what a bummer! Good news is that Formula-Guard would cover 100% the engine failure...but of course we have to pay to get a good diagnosis first...then they will kick in.

We should make sure and stay in touch with this...
 
LBI4fun said:
I also noticed that my Port drive trims a bit faster than my Starboard in either direction. I went through recalibration on SmartCraft and same results, I also checked them in the back and there is definitly a difference.

Fluids and all look fine.
Click to expand...

Interesting. My Port drive trims faster, as well. Just a little. But, I also found that my Port drive -- all the way down -- is about an inch higher at the cavitation plate than my stbd drive when it is all the way down. I measured this from the end of the cavitation plate to the polished rub rail at the back of the boat.

Then, trimmed out, my Port drive is only coming up about 1 inch. Higher than that and I start cavitating. My Stbd drive comes up about 2 - 2.5 inches (about an inch higher than my Port drive).

Prior to the Stbd throttling problem, I would raise the drives an equal amount. But I never realized that the Port drive is already an inch higher than Stbd to begin with.
 
you need to measure trim cylinders. measure how far the steel ram comes out of the cylinder. measure from where the steel rod comes out of the ram to where it mounts on the top of the cavitation plate. there is a bolt that goes thru the rams and the end of the drive. on alphas in the late 80's early 90's this is how we would set the trim limit switch. 6 inches was the limit of trim. i will see if i can find a pic.

found a pic but could not edit it so here goes. look where the zinc mounts to the trim ram. measure from the trim ram to where it mounts to the end of the drive. if you look close you can see a silver spot to the right. that silver spot is a pin that goes thru the end of the drive and both ends of the rams. both these are a constant. trim up your drives to where you run. i run at 2 on the smart craft and the gauges read 2 also. then measure both drives from where the zinc meets the end of the ram to the silver spot on the right. if you want, that little black plastic cap unscrews pretty easy so you can run the tape almost parallel to the ram for accuracy. if one is drive is different then the other, then you have a calibration problem from the sensor on the drive and it needs adjustment.

I hope this makes sense and is easy to understand.
 

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Seas-The-Day said:
LBI4fun said:
I also noticed that my Port drive trims a bit faster than my Starboard in either direction. I went through recalibration on SmartCraft and same results, I also checked them in the back and there is definitly a difference.

Fluids and all look fine.
Click to expand...

Interesting. My Port drive trims faster, as well. Just a little. But, I also found that my Port drive -- all the way down -- is about an inch higher at the cavitation plate than my stbd drive when it is all the way down. I measured this from the end of the cavitation plate to the polished rub rail at the back of the boat.

Then, trimmed out, my Port drive is only coming up about 1 inch. Higher than that and I start cavitating. My Stbd drive comes up about 2 - 2.5 inches (about an inch higher than my Port drive).

Prior to the Stbd throttling problem, I would raise the drives an equal amount. But I never realized that the Port drive is already an inch higher than Stbd to begin with.
Click to expand...

Hey Greg, Give me a call tomorrow I have an idea on why you tucked in measurement is different ......... 8)
Capt. Steve 573-216-4392
 
We're also experiencing problems very similar. It appears after the initial start up & run. We also have the problem of having to back off on our Port engine just to try to sync RPM's. We have as much as an inch or better between the throttles when looking at them. We can reach 4500 RPM's out of Port but only 4250 at WOT form the Starbd. ???
 
This is interesting. It is seemingly an engine issue. Yet, we all have the problem on our Stbd side and we do NOT all have the same engine.

I have seen 496 Mag HO; 8.1's and 6.2's.
Maybe others, as well.

Very interesting! ???
 
Seas-The-Day said:
This is interesting. It is seemingly an engine issue. Yet, we all have the problem on our Stbd side and we do NOT all have the same engine.

I have seen 496 Mag HO; 8.1's and 6.2's.
Maybe others, as well.

Very interesting! ???
Click to expand...

HOWEVER - we do all have Bravo III's ? Could it SOMEHOW be some type of "drag" in the GEARS of the drives that are causing the engines to have to work harder to produce the same RPM....sort of like it was two different props? Man, would I LOVE to swap drives and see what happens?

I will continue to trouble shoot...but we should all be deligent about this...and try to find a commonality..

Right now it is STBD Engine and BIII drives.

Has anyone put a computer to the STBD engine to see if different than the PORT. The computers cost a ton, wonder if we could "split" the costs as a group and somehow share amonghts us all...maybe a$50 each person, etc...would anyone go for that?
 
Ah yes, I have a similar problem but I have 5.7 Volvo's with Composite XDP outdrives on my 2007 31' PC. The problem started approx 30 days ago (after having a tune-up in early April). Went on a trip from Central New Jersey (Manasquan Inlet) down to South Jersey (Townsend Inlet - 10 Miles north of Cape May) and back. On this trip my PORT ENGINE used approx 12 gallons more each way then the STARBOARD ENGINE. The generator was not used and according to my Tachometer(s), both engines were spot-on @ cruising speed i.e., approx 3,400 RPM's.

I have not changed the Fuel Filters as suggested but this may be not only the easiest but cheapest troubleshooting method. Any other ideas?
 
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