Help! Engine wont build rpm
 

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.

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

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

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.

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.

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?

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

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.

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.

As mike t said, check the ignition timing.

- Tach should be reading correctly because MPH is way down also.

- Going to pick up my timing light from a friend that borrowed it. Will check that asap.

- Compression test will be done at same time I check ignition timing.

- Boat appears to have original fuel hose from tank to fuel pump, boat is a 1981, time to change that too just as a precaution.

When you replaced dist' cap and rotor did you replace the ignition module? Can't these go bad and be dropping rpm as it gets worse or do the modules go from good to junk in a single step?

Update: I found out this past week that you can test the ignition module using a volt meter (I didn't know you can do this), and guess what... it didn't pass the test, so it seems to be bad. I ordered one Friday and it should be here by Tuesday.

Ok, I need Timing Specs for this engine if any one knows them...My engine has old vette valve covers on it, so I have no sticker with spark plug gap, timing, max engine rpm, etc..., its a Quicksilver longblock (merc) part # 87971R5, basically a stock 1981 350/260 with a Mallory Marine Magnetic Electronic Distributor # YLM624AV. Anyone have the correct timing specs for this thing?

Set it to 34* total

I just had a similar issue on a friend's boat with a vac sec Holley. Ended up the rear bowl wasn't filling due to a stuck float. I rebuilt the carb and now it runs better than it ever did.

What about initial static timing? I've seen people say it should be 8*, some say 10*... just want to do this right.

Also I'm having difficulty on finding out if its possible to set the dwell angle on my Mallory Marine YLM Series Magnetic trigger distributor, model YLM624AV. When I check the dwell its coming up as 22*, when everyone tells me it should be 30*. I know how to set it on older points distributors, but with this magnetic trigger setup I have no idea if you can even adjust it.

What about initial static timing? I've seen people say it should be 8*, some say 10*... just want to do this right.

Also I'm having difficulty on finding out if its possible to set the dwell angle on my Mallory Marine YLM Series Magnetic trigger distributor, model YLM624AV. When I check the dwell its coming up as 22*, when everyone tells me it should be 30*. I know how to set it on older points distributors, but with this magnetic trigger setup I have no idea if you can even adjust it.

Already rebuilt the carb, no stuck float, had a little dirt in the float bowls but no stuck float. But thanks for the idea. I basically went through the entire fuel system first, new filters, new fuel line, added fuel pressure gauge (pressure stays at about 8-8.5 psi). New dist. cap and new rotor, new plugs, new module in dist. Need to set timing, check advance... but this dwell thing has me all messed up as I think you can't adjust it. should it be only 22* with the magnetic breakerless trigger? I cannot find an answer.

Your initial timing really doesn't matter, without typing 3 pages of timing theory etc (I'll leave that to mild thunder he's better at it than me) rev the engine in neutral until the timing doesn't climb any higher, set it at 34* at that rpm.

If it was fuel related with that much RPM loss you would get a lean pop at full throttle. When was the last time you pulled the drive and checked the gimbal bearing. ANYTIME YOU BUY A NEW BOAT YOU MUST CHECK IT OVER. Either you do it yourself IF you know what you are doing or you pay YOUR mechanic to look it over. DON'T trust the seller or THERE mechanic. Static timing doesn't matter with a stock engine only MAX advance set to 34*.

Take off flame arrestor and have your buddy see if the secondaries open.

I've run into this with all the big three - Holley - Quadrajet - Weber/Edelbrock

Few of the boats wouldn't even get on plane . Ran fine though. LOL.

You posted while I was typing.

Quite the opposite. LOL.

Your advice was great !

Boat ran fine all last year since I bought it, ran fine this spring, then it started acting up... so gimbal wasn't checked since it started giving me problems as I leave the boat in the water. Can that really cause that much rpm loss?

"Take off flame arrestor and have your buddy see if the secondaries open." - Planned on doing this the next time I can get someone in the boat with me.

Thanks again guys for all the advice! Just got a compression gauge, so I'll be checking that also.

You say that you leave it in the water. do you get water in the engine compartment? Is there a trickle of water running down the transom under the engine? You may have a bad bellows and the gimbal bearing is under water and binding. The water would cool the bearing. You said you felt a vibration.

Believe it or not I do have a trickle of water running down the inner transom plate and thought I have a slight leak in the bellows, I planned on upgrading to a bravo 1 setup this fall. Didn't think the gimbal bearing could bind from a trickle of water... maybe that could be the problem?

I would think if a gimbal bearing was seizing enough to slow the motor down
A. You have 15 hp turning the drive and any extra resistance will slow it down
B. It would explode within 5 min

I would think if a gimbal bearing was seizing enough to slow the motor down
A. You have 15 hp turning the drive and any extra resistance will slow it down
B. It would explode within 5 min

Yeah I figured with all the WOT testing I've been doing I think the gimbal bearing would have let loose already if it was the seizing.

Checked timing today, I let it warm up to normal operating temp then checked it. Initial timing was about 2 degrees before TDC, advancing to about 24 degrees before TDC. so I set the initial at 8 degrees before TDC and the idle went up about 200 rpm, so I reset it. Total advance was about 30 degrees. I was worried to advance it any more because the initial would then be at about 12 degrees, wouldn't that be too high? The mechanical advance arms move freely and do not bind. Engine sounded like it had a bit more pep but did nothing to fix my issue.

Oh and the dwell climbed to about 27-28 degrees when I changed the timing, weird?

I let the engine sit for about 45 minutes to cool a bit so I didn't cook my hands taking out the spark plugs. Then I finally got the compression test done. 150lbs pressure on all 8 cylinders.

Now the only things I have not touched are the coil and the spark plug wires, guess I'm doing those next.

Did you check the oil for metal? It sounds like you could of wiped out a cam lobe or mushroomed a lifter. I'd pull the valve covers and see if all the rockers are opening the same.

If he lost a cam lobe you would here a lifter ticking. Increase the timing to 34* total advance. 16* initial will make it idle better.

as already stated,set total timing at 34.do not worry about initial timing.

Some Edelbrock 600cfm secondaries not working info:
http://www.gearheadtalk.com/forums/s...ies-won-t-open

Not always, a mushroomed lifter, or bent push rod will make noise but not a wiped lobe. He's checked almost everything else.

Not the secondaries.

Yes.....I'll continue to mention these until he checks them. LOL.

Yep - since he mentioned that it is a 600 cfm Holley, I am guessing it has vacuum secondaries. A vacuum leak at the diaphragm housing would not pull enough vacuum to overcome the spring and open the secondaries. And this is something that could possibly get progressively worse. Only way to check it is to put it under load and look down the secondaries. You could do that with the boat lashed to the trailer on the ramp, or run it and if a buddy is not around to watch it, mount a GoPro over the carb and view the video. That's what I did to check my fuel pressure by myself.

duplicate post

DJ..sounds like your motor mount might have loosened up and knocked the drive aleignment off..ck the lower nuts on the mounts..i have seen where they come loose and will bind up the coupler then the engine wont rev..

If the engine was that far out of alignment the coupler would stink of burning rubber. AND it wouldn't live long.