DjMikeyC

iTrader: (1)

Anything in the drive that can break that can limit rpm? My engine runs fine but the it can no longer pull higher rpms. Have mercruiser 350/260HP alpha drive with thru hull exhaust.

Engine normally pulls to 4200 with current prop. About a month ago out of nowhere it would not pull past 3800 rpm and that is when I started checking everything. After a few test runs it would not pull past 3500 rpm. Each time I checked and replaced something engine runs even smoother then before, but now will not pull past 3300 rpm... its getting worse.

First checked and changed prop, prop was fine. I have checked and changed the water separator and the fuel filter at carb inlet, both were clean replaced both anyway. Checked carb linkage, seems to be working fine, secondaries seem to be opening (cant check while driving). Checked and changed spark plugs, dist cap, rotor. Engine does not hickup, smooth as can be even at full throttle, just wont pull past 3300 rpm now. I purchased a fuel pressure gauge to make sure the fuel pressure is not dropping off at higher rpms but I haven't installed it yet, thinking of getting the coil checked too.

The last test run out I heard a noise coming from the back of the boat, kinda sounded like a whale moaning, there may have been a slight vibration come thru the floor, hard to tell as I was focused on the noise. I plan on pulling the boat to pull the drive and see if I can find anything.

Any ideas or insight will be greatly appreciated.

Griff

With no signs of an ignition misfire, I would check fuel pressure and do a compression test.

mike tkach

iTrader: (3)

^^^^^^^i would also check ignition timing.

BUP

iTrader: (9)

MAX rpm is 4600 for this app - you have been running x amount years boarder line on the too low of WOT rpm for lugging the engine. Mercruiser sets these rpms ranges for reason.

Lugging an engine is just a bad as over reving it.

Anyways check all ignition related components then fuel pressure readings.

First and foremost is compression test and possible leak down test to determine internal engine condition. Even a good old vacuum gauge test would tell a lot about this engine. You could very easily do a power balance test as well another good test to perform. 2 very simple tests to perform that can tell a lot internally of an engine.

DjMikeyC

iTrader: (1)

I purchased the boat last year, seller stated engine is rebuilt from Quicksilver longblock (merc) part # 87971R5. This is the GM 5.7 Litre "Non-Gen.+" 350 V/8 (Standard Rotation). Seller stated it had about 50 hours on it since rebuild. It was rebuilt stock except for the Edelbrock Performer EPS intake (Part # 2701) and the Edelbrock Marine Series 600 cfm, Square-Flange, Electric Choke Carburetor, non-EGR (Part # 1409). For ignition shes running a Mallory Marine Magnetic Electronic Distributor # YLM624AV and a Mallory High output coil.

First chance I get I will be installing the fuel pressure gauge, and I will be getting the ignition coil checked and I will check timing. Compression test will also be done.

DjMikeyC

iTrader: (1)

Update: Yesterday I had the time to install the fuel pressure gauge, installed inline right before the carb. I installed an AED Performance Liquid Filled Fuel Pressure Gauge, reads from 0-15 psi. Pulled the engine hatch off the boat so my friend could keep an eye on the gauge from the back seat. Gauge at start up read about 7 to 7.5 psi and stayed pretty much right there no matter the throttle position, idle or wide open throttle it stayed right about 7 psi.

Engine sounded very solid running with the engine hatch off, no misses, no sputtering, no hickups, just wont pull the rpms, and no weird noises or vibrations. Compression check will be next.

Just a thought as I almost still think this is fuel related. I am running a 600cfm Edelbrock marine carb, model# 1409. Anyone think I have partially clogged jets? Maybe float or needle valve issues? Are there any known issues with these carbs?

Jonesyfxr

iTrader: (1)

When you were revving the engine, were you in neutral? The reason I ask is that an engine will act differently under load.

DjMikeyC

iTrader: (1)

All testing is being done in gear under load out on the water. When I'm driving the boat at WOT I had a friend sitting in the back seat facing backwards looking at psi gauge at the carb.

BUP

iTrader: (9)

So does your boat feel like it is running well and possible your tach is way off ?

vacuum gauge testing from the intake manifold (vacuum source) is really a good test to perform if you know what to look for per the readings and understand them on the vacuum gauge.

Griff

As mike t said, check the ignition timing.