Motor found a new gear

[flashman252]

2008 Mercury 150 optimax,
SN:1B539103

Had the boat out yesterday and it ran great. Did all the things recommended by silverfox and I cannot thank him enough for all his knowledge. Yesterday was about the 7th or 8th time I have had the boat out in the last 6 months since purchasing. Here is the issue, I know the motor states that max RPM is 5850. Until midday yesterday, best RPM I was able to achieve was 5300-5400. Out of nowhere, the motor opened up and and I was seeing 5900-6000. Trimmed down to keep RPM pegged at 5800. No idea why this happened, but I do know that the motor only had 100 hours on it when I bought it. The 13 page computer scan of the motor checked out but the motor never was run above 5000 rpm. Worried me a little but I went for it because the price was right on everything. Any idea on why all of a sudden these RPM's just came out of nowhere? It does have a hot foot, and I did do thorough exam on it and made sure it was not sticking and everything looked good there. I did inspect the motor and made sure the idle screw and WOT adjustments had not been tinkered with because I have read they can be a burden to get tuned right again. The screws still have the factory paint/seal on the threads to verify they have not been adjusted in any way. Just seems like the motor woke up out of nowhere. I am ok with it, but once I add a JP, just worried I am going to have to buy a new prop.

Since I purchased I put a 19p prop on because of our elevation and the boat would not get on pad with the 23p the boat came with. I replaced the water impeller and am only seeing 12-14 psi at WOT and 2 at idle. I know this is within normal limits and I am not getting an alarm so I am not too worried about this yet.

I am going to add a 6" JP soon. 1)Will raising the motor reduce the amount of water PSI, and if so, how much (roughly) and will this become a worry of mine by adding a JP? 2)Will raising the motor with a JP add RPM, and if so, how much (roughly) and will I need to possibly go with a 21p prop to keep RPM around 5850 max?

[leonreno]

Did the speed increase with the extra rpms?

[flashman252]

yessir, gained like a mile or two more, not much to brag about. What scared me though was the RPM gauge was showing 6k. I trimmed down so it was pegged at 5800. I am seriously thinking about this post though and I am pretty sure there is a rev limiter on these motors. Hypothetically, this shouldnt allow the motor to redline to the point of pop. And I do not have any form of smartcraft on the boat, so I am questioning the accuracy of the gauge being a true 6k. Probably need to at least invest in the vesselview and check accuracy of this gauge on the boat.

[JOE54]

agree ^^^ slight foul on a plug or sticky injector loose wire connection are some causes

[SilverFox579]

Surely those rpm figures are from analog tach as max WOT rpm for non-ProXS 2.5L is 5750 (ProXS is 5950) and pretty sure Guardian would have activated long before you got to 6000.

Two things, water pressure specs for 2.5L are 1 - 3 at idle and 12+ at WOT so yours are spot on and see if you can locate or borrow an inductive tach to validate/compare your tach readings. Good to hear things improved for you.

[flashman252]

Silver, They are from the analog gauges that are on the dash (I am sure they are the original gauges that were installed 13 years ago). I do not have any way to verify the accuracy of this gauge, as mentioned above. These motors have a rev limiter right? Surely it would not let the motor hit above the recommended RPM's or at least sound an alarm before letting the motor blow? I am putting a 6" jp on the boat and I will be raising it an inch to start. I know it will gain even more RPM's and if I am already maxing the RPM's now, I am sure I am going to have to prop up. I will keep the 19p I have now as I do run a few lakes that are 2000 ft higher than the lake I fished this last weekend.

I will say the motor ran strong, real strong and performed very well. I could not have done it without the help from everyone here and their recommendations.

THANK YOU!

[JOE54]

and it will go above recomended , not set in stone , it ran above limlit for extended period limiter will kick in and horn will sound

[SilverFox579]

and it will go above recomended , not set in stone , it ran above limlit for extended period limiter will kick in and horn will sound

^^^+1^^^

Can't explain as well as Don but what I recall is rev limiter works in a phased reduction in power and gradually becomes more aggressive if power isn't reduced, if PCM detects a rapid rise, Guardian will activate, sound alarm, and reduce power. If your test beep works at startup and you didn't get an alarm, you probably didn't get into power reducing strategies.

Something like a TinyTach or equivalent would work. A Vessel View Mobile, though more expensive, is probably the most economical Smartcraft solution, and would provide useful data from the PCM.

[EuropeanAM]

One other thought: Before you buy a Jackplate or make further prop changes, what BOAT do you have this on?

Adding a jackplate (and raising the engine up) may not accomplish anything good... so let's get into that a bit more before you start down that path.

[flashman252]

Don,
I have a nitro z7, current Prop to pad is 4.5". On the nitro forum here, every setup I have read states 3.25-3.5" is where the best results are seen on this exact setup. The boat does not have a hooked hull, as I know these boats are prone to them. I have double and triple checked and no hooked hull and boat jumps right up on pad. JP is going on this weekend and boat is already at the shop to have this service done. I could probably do it with my tractor and a buddy but just dont have time with a new house purchase. No rooster tail when running the boat, so I know it can afford to come up a little.

Most setups I have found on the Nitro forum are running a 23p prop and some a 21p, but most are around sea level. Our lowest lake here is 4000 feet and another lake I run is at 7200 feet, so having to prop down is not uncommon here. I went with a 19p based on how low the RPM range was when I bought the boat. I was only able to get it on pad maybe 3 times the first time I took it to the lake and it was only hitting 4800 RPM. Put a 19p prop on it and 5000 RPM is all I saw but the hole shot was incredible. This lead me to dig into the motor and follow recommendations from the great resources here. I followed the full list on 100 hr maintenance recommended on the FAQ tab. Next time out hit 5400 RPM and motor felt MUCH stronger and performed flawlessly. The last 4-5 outings, 5400 RPM was max I saw and just figured this was where it was maxing out and that adding a JP would increase the RPM a little get me closer to the 5750 max RPM recommended for this motor. All of a sudden, last outing, midday I looked down at the RPM gauge, when I was making a good run, and saw 6000 pegged. I trimmed down a little and pegged it at 5800. Out of nowhere 600 more RPMS. Again, no alarm and the motor ran completely fine and strong despite the extra RPM's seen on the gauge, so I am questioning the accuracy of the gauge on the boat. I do plan on adding a vesselview at some point just to have a little more data on the motor.

Forgot to mention this on the original post and now it has me thinking. I did replace all batteries before this last trip. Went with 2, 31 series deep cycle for TM and a 27 series with 1000cca (as recommended). I do not think this would have any influence on RPMS but never know since spark comes from electricity. The battery I replaced came with the boat and it was a 24 series, not sure what the cca was but it started the motor up with absolutely 0 issues, but it was 4 years old.

[EuropeanAM]

Wow... 27 series with 1000 CCA? Would be good to know which battery that is for future reference (make sure that's not MCA or "CA").

You're on the right track on all accounts, I believe. Tach is questionable, significant pitch drop will be needed up at those elevations. Also do strongly recommend some form of Smartcraft Instrument as a top-priority.

Keep in mind that lower speeds will result in lower water pressure (also). Be careful with the jackplate... I am leaning towards the "it's probably not gonna help" side, based on where you are operating. You'll soon know, though.

[flashman252]

Don,
Sorry you are correct, 800 cca and 1000 ca on the battery. Lol so now I am questioning if I need a bigger battery or a different battery. This might be why I get a low voltage alarm after fishing for a few hours and go to start big motor. Motor turns right over and starts and within seconds voltage goes right back up and fishfinders dont power off but I was not impressed that I got the low voltage alert on fishfinders on a new battery. BETCHA I NEED A DUAL PURPOSE BATTERY.... HMMMM THANKS DON!

I agree that tach is in question here since no alarm and after reading what multiple people have said on this thread. I do plan on getting the vessel view soonish. May also do smartcraft gauges as well just haven't due to laziness and other priorities at this time.

I do know that lower speeds result in lower water PSI. When I am half throttling around (been doing it a lot lately due to how low our lakes are here. Watch depths and lay down a trail line on GPS so I know I won't hit anything next time I go blazing through there at WOT), the water PSI is around 7-8 PSI. The telltale is strong though and it has never dribbled and always has had good pressure coming out, even at idle. Again, no alarms so I assume all is well still.

Once JP is on, I will go test at 3.5" prop to pad and see if there is any difference and may go to a 21p prop if I am exceeding RPMS, which I assume will be an issue since I possibly am close to exceeding at this point. Will keep 19p for the higher elevation lakes though and as a backup.

Thanks Don, great info as always

[leonreno]

Battery recommendations from faq:
http://www.bbcboards.net/showthread....batteries+meet

[flashman252]

Old thread but update to situation.......

Tested this weekend after the JP install (Set to 3.25" PTP) and Vesselview installed on motor

Went to the highest elevation lake in New Mexico (7200 ft) because smallmouth bite was ON!!

50 MPH and 5000 RPM was the best performance I saw, 19P prop, I stared at my phone instead of verifying RPM gauge when running so could not verify accuracy of RPM Gauge..... BOZO move on my part, will do this next time out, focused on all the boats out this weekend instead of gauge verification.

With JP this is an increase in speed for sure. Motor ran strong, no issues, but I could not find the water pressure feature within vesselview. No overheating alarms and good telltale, new water pump install in February (not too worried)

I have read that rule of thumb is 300 RPM for every 1000 ft elevation decline, does this sound accurate for most here? If so, the other few lakes I fish are 3800 ft, 4200 ft, and 6000 ft in elevation. The lowest lake, I fish, would be about 1000 RPM gain and this is the lake I saw the 6000 RPM on the dash gauge. So it might not be that far off and may need a prop change to run the lower elevated lakes? Am I thinking correctly here?

[JOE54]

yes very close need some more props