romie

Triples (525 SC carbureted) consistently ran 5200 5300 5200. Today i went out and the Port engine would only hit 48-4900. My first thought was fuel starvation (filter,pump or pick up) but then i assumed it would lean pop. It will run for miles at 4900 and not stutter or pop. Compression test and leak down was done mid season last year to determine if heads needed to freshened up. Everything was still good.
The port outdrive was apart to fix an oil leak, could there have been anything over looked that would cause this issue? if not I am thinking some junk may have made it into the carburetor from running a saddle tank empty last season. I have found the odd post with a similar symptom here and all around internet, but unfortunately either the issues were never solved or people never posted their solution. Any ideas, or direction from experience would be appreciated.

dereknkathy

nice thing about 1 engine, you can hear a miss. if it holds that rpm consistently, it probly isn't fuel which would most likely cause a fluctuating rpm. plugs-wires-cap-timing. it can't hurt to pull pickup tubes outta the tank and clean screens on them, but that also causes rpms to really drop. fuel starvation with a carb. engine runs perfect for the couple of seconds that float bowls have fuel and then practically shut off until bowl refills. not hold a constant rpm a lil lower than before.

1BIGJIM

iTrader: (1)

Do a Compression test and see if you lost a cylinder. I lost a ring landing last year from running lean and both engines still spun over 5,000 rpms. I did not notice the problem until the end of the boating season.

SB

iTrader: (1)

Believe it or not, most fuel supply issues (pump/filter/etc) will usually make the engine 'lay down' and not get to top rpm and without popping/banging/etc.

Crap in the carburetor blocking a passage usually will...usually.

Go to basics. Check compression and fuel pressure. Fuel psi if good at idle will not tell the story at higher rpm. So if good at idle, check at higher rpm.

Griff

Is the boost pressure down a little????? I'd replace the blower belt before anything else.
The blower belt on my 525SC needed replaced about every 50-75hrs because it would start slipping.

mcollinstn

A) Swap to another tach to see if there is a tach problem (too easy not to try this).
B) Steady but low revs is usually a plug wire.

Do you have mirrors on the bottom of your hatches? If so, go out at night, have somebody sit in the engine compartment in a safe place (in front of a non-running engine is a safer place) and push the power up on the suspect motor. Chances are, they will see dancing sparks happening somewhere and a lot of the time, the reflection off of the hatch mirror gives it away. You don't have to go WOT with it, but push it up to at least 2000 rpm and if it is spark breakdown chances are it will be visible.

romie

Tried a few more things today after work. Checked linkages .New water separators. Tach is right on, boost is right on. I do notice it does not sound quite as throaty as the other two. So I am convinced it will be junk in the carburetor. Going to double check the compressions and leak downs. Only has 4 hours on them since last test, but may as well be sure.

romie

It was the belt slipping. I noticed the anodize power on the pulleys. After i put the new belt on it it picked up more RPM but is about 50-100 off and will break free if i tramp on the sticks. Could the pulleys need attention?

Sydwayz

iTrader: (1)

Good deal that you found it. I went around and around with this issue on computer controlled 496HOs, and ended up being a bad Crank Position Sensor best I could tell.

Are you certain your props are all status quo? 50-100 could easily be a dinged blade. Also, were your blowers rebuilt too?