Can"t Get My 454 Mercruiser over 3000 RPM

Always something. My 1994 Baja with a 454 Mercruiser, Bravo 1 , and 21 pitch stainless prop. sat for about 3 years. Over the last month I have been debugging one thing after another. On Saturday I finally got it fueled with 50 gallons and launched however I was unable to get it to rev over 3000 RPM. It ran good up to 3000 which is maybe 1/2 throttle but then even giving it full throttle it would not increase revs over 3000. After many tries the engine would always max out at exactly at 3000 RPM. Cutting back the throttle to maintain 3000 it seemed like it was starting to bogging a bit or having a slight miss or fuel starvation. So now I'm looking for probably causes. My first thought is water in the fuel filter or system but was thinking that would create missing but not limit RPMs to exactly 3000. Any thoughts from those more experienced that me??

Possible pinched fuel line,weak fuel pump,restriction in fuel tank pickup tube, air leaks, Are throttle plates opening all the way? Is throttle cable adjusted propely to open to wot?

Will check all those potential causes.. Engine seems to run strong so its seems strange at full throttle it rev to EXACTLY 3000. After reviewing the method for mounting the prop I did notice it should be torqued to 55 PSI.. I know I did not tighten it that much. Could that be the cause??

No

How old is your gas?

Do you have a timing light?

What you need to confirm is that the secondary part of the carburetor is opening, What carb is on this motor? Rochester or Weber?

I just topped off the tank with about 65 gallons of new gas before and when I launched but there was a considerable amount of old gas in the tank, maybe 25 gallons that had been there for several years.

Ran ok when it was put away so I would think the timing is still ok but I do have a timing light so I can check timing. Not sure what the timing should be so I will have to get that.

Motor has a Rochester carb. Will check to confirm the secondaries are opening and throttle linkage is still correct also.

While one would think that water in fuel/water separator issue may not cause this problem, I'd still check to see if the separator is full of water. A cheap (no cost) test.

yep..bought a new one thats going on today ..thanks

Make sure you fill it most of the way up first. Fuel pumps cannot overcome an empty filter of that size..........

You mean pre fill the the fuel filter ?

The water separating/fuel filter.

That is the one being questioned is it not?

yep thanks.. took the old one off..does not appear to have any H2O in it

That old gas could sure be a problem. Get yourself a 6 gallon outboard tank and fill it with fresh fuel. Connect it directly to the fuel pump to isolate the cause of the problem

Map out the advance curve it should be all in around 3200 rpms between 34-38 DBTDC

Looks like tomorrow will be another boating adventure ...

It looks like the 2 rear bowl butterflies in the carb are opening full when the engine is not running but I move the controls to full throttle but there are two upper butterflies that seem to have no tension on them that stay closed so not sure how they work or if they open when the engine is running????????????? I need some carb education here. The rear butterflies are opening but how do I make sure there is fuel coming in? I see fuel coming into the fronts but not sure about the rears.

front 2 barrels should have a choke plate above the throttle plates,rear2 barrels should have no upper plate covering throttle plates,"butterflies", When engine is cold choke plate should be closed,it is thermally controlled usually. On older cars pressing gas pedal slowly to floor and releasing would set choke&high idle.Not sure on your application if throttle can be floored &brought back to neutral to set choke. Choke mechanism might need some TLC

yes the fronts have a single choke plate but the rear 2 also have 2 linked butterflies directly above the secondaries at about the choke level that are closed when the engine is not running and apparently open with the vacuum of the carb intake .. (or something like that) >>.I'mNot familiar enough with the construction and operation of the Rochester carb but I'm wondering now if old gas sitting in the carb could have gummed something up and is preventing fuel from getting to those rears and hence restricting RPMs to 3000.

You have the theory some what correct.


Johnny is a bit off,

1. No high idle on carbed marine motors...............ever. High idle is controlled manually by throttle/shift lever!!

2. Secondary base plate (bottom) butterflies open mechanically, The upper secondary butterflies (large) open on vacuum demand based on load.

They typically will NOT open unless under load. When they open this is what allows the additional fuel. If they do not open then no additional fuel.
They are controlled by a small spring (wound and round) which is adjustable. No specific method to give you as you do not know enough yet!

What I would do is,

Do another lake test. Make sure you have a second person to drive the boat while you do some testing at motor.

Get the motor warmed up, remove the flame arrestor, Have second person try to bring the boat up to full throttle and watch motor and carb linkage.

Make sure everything is working as it should

If motor stalls at (does not go over) 3000 rpms, Carefully push open the top (large) secondary butterflies open a little at a time and see if you gain rpms.

If this happens then the timing of the secondary butterflies needs to be adjusted.

If this is proven to be your issue then report back and additional instructions can be given.

Find your issues first and then fix them.

Also like said you need to confirm your timing is advancing.

MCM 7.4L/MCM 454 MAGNUM/MIE 7.4L
Identification Mark: V8-24 on timing module.
Module Advance: 24 Degrees (this is how much the module will advance the timing under wide open throttle)
Initial Timing: 8 Degrees BTDC (this is what you need to set with your timing light)
Total Advance: 32 Degrees (this is the total timing advance [initial + module advance] that you should reach by ~3400 rpms)

You should be at ~ 15 degrees before top dead center at 2400 rpms.

if you are not achieving any more than the 8-10 degrees BTDC at full throttle then the timing module may be bad.

thanks much .. will investigate further as recommended..

So today a freind and I sea trialed the boat for about an hour but we did not find or cure the problem. We changed the fuel filter today and temporarily removed the fuel line check valve to insure free fuel flow from the tank. Then we went to sea. The boat ran strong up to 1/2 throttle which again is just at 3000 RPM . We have pretty much ruled out water in the fuel as an unlikely cause as the engine runs to smooth. Opening to full throttle did not increase RPMs. On occasion it seemed to create a slight stumble, miss or pop. Below 3000 it ran good. I checked before starting the engine and the secondary butterflies do open at full throttle but while at sea and 3000 RPM the upper secondary butterflies, while very loose and free, did not open. I used a small wire to open the butterflies manually but nothing happened. (not sure if the lower butterflies were open but if they are mechanical and open when engine is off at full throttle, they must also open at speed and full throttle) Next I sprayed raw fuel into the rear secondaries several times but nothing happened, no change in engine and still no increase in RPMs. Finally we turned our attention to he possibility of timing. At idle the engine appeared to be timing at or about 8 to 10 degrees BTDC. We again went to sea and experimented with altering the timing at speed but nothing we did could improve engine performance or get us over the 3000 RPM hurdle. Right now I'm kind of stumped.

Did you check total timing at WOT. Total timing or (all in) should happen around 2600rpms from what I have been reading. It should never go over 38degrees total advance. There is a bunch of different specs floating around but the most common is 32-34 degrees at 2400-2800rpms. Alot of guys also say you dont want to run those motors much over 3200rpms WOT anyways. Have you checked the timing chain? Pull the distributor cap and with a socket on the crank turn it clockwise to TDC then watch the rotor and turn the crank counterclockwise until the rotor just begins to move look at the timing mark and see how far it moved. Anything over 3-4 degrees you should consider putting in a timing gear set. Make a graph and map out you timing advance. On the bottom mark 500rpm segments and on the left side use 5 degree segments

Any possibility of an accidentally crossed spark plug wire? Exhaust restriction?

Round Camshaft???? Easily checked after removing valve covers.

If he had a ronded cam lobe he would have a skip and or a backfire... same with crossed wires...

he needs to confirm full timing advance first.
Also make sure he has the correct prop pitch..

We did try to adjust timing at WOT but have to admit it is pretty hard with the basic timing light I have. At thi point its getting a little over my head and loosing a lot of boat season. Will be reassessing next step today.

You do not adjust timing at wot....

You set initial as you confirmed...8-10 btdc.

Rev enginge to 2500-3000 rpms..with timing light pointing at timing tab. Mark on harmonic balancer should move counter clockwise and reach ( not acurate just for informational purposes) the timing mark should move c'clock wise to about the one oclock position. The mark at base timing is at about ~4 oclock...

Of course i am only making a reference since you do not have a advancing timing light or timing tape on balancer...so in other words the timing mark will move as rpms increase counter clock wise about 1 to 2 inches so. This will at least show the timing is advancing. Timing advance is controlled by the timing module NOT you...

Never adjust the timing using your earball. Detonation is a big killer and most of the time you cannot hear it until its too late. Your local auto parts store will likely let you borrow a timing light/tach. Base timing is not so important but max timing must be confirmed.

Just though of something,

Your year motor should be a TBV ignition with the timing module attached at the distributor, Correct?
You may also have a KNOCK sensor. It would be located justg forward of the starter and screwed into the block most likely where the block drain is.
A single blue wire will be going to it.
Do you have this?

If you do then you would have a second module piggy backed at the distributor with the timing module...

Please confirm questions.

Yes it is a TBV ..Thunderbolt Ignition. There appears to be an ignition module attached to the back of the distributor. There is also a knock sensor as described attached to the block . Not sure if there is a secondary module piggy backed . Attached are some pictures ..if I can figure out how to attach them...

Yes it is a TBV ..Thunderbolt Ignition. There appears to be an ignition module attached to the back of the distributor. There is also a knock sensor as described attached to the block . Not sure if there is a secondary module piggy backed . Attached are some pictures ..if I can figure out how to attach them...

(Trying to add pics here but error code 413)...