RPM's dropped from 4900 to 4600

[Mor's Lavey]

Anyone know what can cause RPM's to drop on a 496ho? The conditions were the same both times out. full tank, 90 degrees, same lake

[Daytona100]

How many hours on the motor? Any recent modifications?

[Beer-30]

Ya might check the inlet at the fuel pump. There is a small screen there. Takes a few years to collect stuff, but there is no (unless you added one) filter prior to the pump. Could be just starting to cut back your fuel. This is not something that gets looked at during services unless specifically asked.

[GofastRacer]

Damn, can't believe they would put one of them funky screens in there and no filter. Get rid of that screen and put a good filter in there and "maintain" it yourself. Another thing to check is the tank vents, believe it or not they can plug up and that will prevent fuel flow, also check for colapsed fuel lines, you won't see it but you can feel it!..

[Mor's Lavey]

How many hours on the motor? Any recent modifications?
it has about 70 hours with no modifications. I did put a 26 prop on because the 24 hit the rev limiter. I was getting 4900 rpm with the 26 except for the last couple of times out i'm only getting 4600. After reading other post I'm thinking it may be water pump impeller, which has never been changed or a clogged heat exchanger

[Beer-30]

it has about 70 hours with no modifications. I did put a 26 prop on because the 24 hit the rev limiter. I was getting 4900 rpm with the 26 except for the last couple of times out i'm only getting 4600. After reading other post I'm thinking it may be water pump impeller, which has never been changed or a clogged heat exchanger
Neither of those should cause an RPM drop - unless you are getting an ECM beep at the same time. If the water pump is not causing guardian mode, the heat exchanger dealio would only cause higher water temp as it became more and more clogged.
Is the ECM giving you a beep when the RPM drop?

[Mor's Lavey]

The ecm is not beeping thats whats wierd about losing the rpms. Everything seems normal. When I switched to the 26 prop i was getting close to 5000 rpms at havasu in 90 degrees. When i went to mead in 100 degree i only got 4600 which i expected with the higher altitude. now I only get 4600 no matter where i go. I just wanted to make sure theres nothing wrong until i get the 26 labbed.

[cougar27mtr]

I am having the same problem.. I would love to hear someone with this problem that has been solved. Please help us :confused:

[396_WAYS_TO_SPIT]

Maybe the motor is going away:eek:

[steelcomp]

Maybe the nut behind the wheel is loose...(sorry, couldn't resist) :D

[ULTRA26 # 1]

Check drive alignment. Misalignment will cause a drop in RPM's
John M

[Deano]

Have you ever had your boat serviced? 70 hours and no impeller change is alot. You need to change all fluids, filters, impeller, drive oil, and drive alignment before you go out again. Your coupler will have premature wear and fail, really fast, if you don't align your drive.

[Havasu Carrera]

Check drive alignment. Misalignment will cause a drop in RPM's
John M
27mtr!! Yours started slowing just after the new drive. NO maybe?
Misaligned stressing the U joints? Hmmmm We need to ck fuel press at wot.
Call me duface!

[centerhill condor]

ECM? hmm, do these compensate for atmospheric pressure? If so, it may need to learn the regular temp/pressure/fuel. It should return to normal the more you run it. Just a thought.
Personally, I like the fuel pump inlet filter.

[mike37]

you are just collecting more junk in your boat
so now you have more weight to cary
get rid of all that crap you stored in there for your trip and you will get your rpms back

[Infomaniac]

How many hours on the engine? 70? Valve springs going dead is the most common cause. Not sure how many hours on a 496 but 120 or so on most oter Merc's

[Beer-30]

ECM? hmm, do these compensate for atmospheric pressure? If so, it may need to learn the regular temp/pressure/fuel. It should return to normal the more you run it. Just a thought.
Personally, I like the fuel pump inlet filter.
It will compensate for alt, but only in fuel mixture. There is no way for it to add HP to compensate for alt. Only super/turbocharging can add pressure (HP) that is lost by higher altitude.