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Can"t Get My 454 Mercruiser over 3000 RPM

ehlien said:
Special thanks to everyone who posted and tried to help during this process!!
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Special thanks to you for your detailed process of staying here with us keeping everyone up-to-date on the steps being taken to help come to a conclusion... It's often mentioned that a ignition module or ignition sensor typically found under the distributor cap either works or it don't, but roughly once or twice a year just on multiple boating forums alone, these two components tend to display odd behavior like you just experienced here.
Glad you found it, enjoy the rest of your boating season.
 
So here is the latest update since I got the new ignition module about a month ago. Making progress but everything is not perfect yet. I've now gone through a couple tanks of gas. The boat runs well at idle up through about 1000 RPM and over 3000 RPM. It likes to run at higher speed however it the mid range between 1500 and 2500 RPM once warmed up and I slow down off plane it misses and sometimes surges. Seems like when it climbs back on plane this goes away and it runs fine. When going out and the boat has not been run yet if it sometimes will briefly sputter or backfire when I throttle up quickly to get on plane the first time. I'm wondering now it the current problem might be fuel/carb related as when on plane the boat is using 4 barrels but at that mid RPM range and not on plane it's not using all barrels and not getting enough fuel from just the primaries. Thoughts? Thanks



boat_tech said:
Special thanks to you for your detailed process of staying here with us keeping everyone up-to-date on the steps being taken to help come to a conclusion... It's often mentioned that a ignition module or ignition sensor typically found under the distributor cap either works or it don't, but roughly once or twice a year just on multiple boating forums alone, these two components tend to display odd behavior like you just experienced here.
Glad you found it, enjoy the rest of your boating season.
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Have you tried adjusting the idle air needles out 1/4 to 1/2 turn each. Quickly accelerate from slow to fast if it hesitates turn the idle air needles out 1/8 turn each and try again. Just keep doing that until you get a clean hole shot and just leave them there. It will help on cold starts as well. Give it a shot sounds like you real close now.
 
No ..will do .. Its getting close to the end of the season so not much boating time left for debugging.. Do you think this will also help in that 1000 to 2000 range problem?
Thanks
 
When my engine starts to run funny and I have a newish (less than 2 yo) cap/rotor, the FIRST thing I do is dump the fuel filter completely into a large (peanut butter jars work... be careful not to spill any fuel in boat) glass jar and let it sit for about a half hour. First thing to look for ( immediately) is the clarity of the gas. Regardless of base tinted color, it should be clear. If cloudy... PROBLEMS. Next, after a half hour wait, I look to see if the gas is starting to stratify, i.e. break down into layers. If it does, bottom layer is water and you have water in fuel problems. In a crunch situation ( on the water once and again late on a Sunday afternoon), I had some luck just replacing the old filter ( smear a very thin film of engine oil from dip stick on gasket first) and running the engine. Once, I did this three times, got water out every time and the last time, it was OK. I added fresh fuel ( hi octane, when ethanol absorbs water and drops out, the octane of remaining fuel drops!) and was OK until I was able to get a new filter.
 
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I replaced the filter once and just bought another one but the size was too large . I will be putting on a new one this weekend and looking at the carb adjustments suggested. Thanks for the method to check the gas. I have a glass jar standing by. I checked the old filter by dumping it in a bucket but was not able to tell a thing. I would think if its a water problem though it would be evident regardless of the RPM. Also are you saying in a pinch you removed the old filter, dumped it and put it back on in a pinch to get your through the day?
 
Forgot once and ran a whole season on a "recycled" one. Don't forget the oil smear on gasket and make sure gasket is properly seated! Unless the paper element gets plugged, the water separator part should have almost unlimited life (by dumping). Since I can't tell if the paper element is plugged with fine particles, I replace filter annually. You can't "find" the cost of a fuel filter ( or oil) in your annual boat budget.
 
kimcrwbr1 said:
Have you tried adjusting the idle air needles out 1/4 to 1/2 turn each. Quickly accelerate from slow to fast if it hesitates turn the idle air needles out 1/8 turn each and try again. Just keep doing that until you get a clean hole shot and just leave them there. It will help on cold starts as well. Give it a shot sounds like you real close now.
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I tried some adjustment over the weekend. I'm now about 2 turns out on the idle screws which seems like a lot with not much difference in performance.
 
sandkicker said:
When my engine starts to run funny and I have a newish (less than 2 yo) cap/rotor, the FIRST thing I do is dump the fuel filter completely into a large (peanut butter jars work... be careful not to spill any fuel in boat) glass jar and let it sit for about a half hour. First thing to look for ( immediately) is the clarity of the gas. Regardless of base tinted color, it should be clear. If cloudy... PROBLEMS. Next, after a half hour wait, I look to see if the gas is starting to stratify, i.e. break down into layers. If it does, bottom layer is water and you have water in fuel problems. In a crunch situation ( on the water once and again late on a Sunday afternoon), I had some luck just replacing the old filter ( smear a very thin film of engine oil from dip stick on gasket first) and running the engine. Once, I did this three times, got water out every time and the last time, it was OK. I added fresh fuel ( hi octane, when ethanol absorbs water and drops out, the octane of remaining fuel drops!) and was OK until I was able to get a new filter.
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Did the glass jar test on the gas .... did not see any stratification or water
 
Gas is good

Fuel mixture screws are fine. It is what it is........preferably you want lowest idle rpm ~ 400-500 is good, adjust A/F mixture screws to get best idle quality, re-adjust main idle screw to specification. Some say adjust for best quality and then turn in (lean) until stumble then back out 1/8 turn. This is kind of called Lean drop method.

When you reach you max rpms/speed. Try to move throttle back and forth quickly kind of like pressing a gas peddle quickly. This will add additional fuel thru the accelerator circuit and if this increases your rpms and speed the you have a lean fuel condition. Or close choke slightly while at max speed but you will need a second person to drive.

It has been a while but what ignition is in the boat???
 
FYI ..ran Monday night . Tried the "Try to move throttle back and forth quickly kind of like pressing a gas peddle quickly." procedure. The RPMs came right back to exactly where they were. No increase in RPMs. Thinking and off season carb rebuild may be in order. x
 
What carburetor are you running?

If it is a Rochester maybe need to adjust secondary top butterfly spring tension.
 
Now you say it will go over 3000 rpms but at mid speed not good performance.

So there is a small socket head cap screw (Allen head) upside down shown by Yellow arrow.

Behind the choke pull off rod, (move out of way by hand) there is a slotted head (round and flush with casting) RED arrow

Use a small screw driver to twist this.

If you look close you will see a steel rod hanging down Blue arrow.

now there is a spring wrapped around the slotted shaft that you will be turning. ONE end of the spring wraps around the rod (small U shaped wrap)(blue arrow).

The way to adjust is, loosen allen head screw (CCW),

turn slotted shaft CCW until spring end wrapped around rod moves away from rod a small amount.

Now turn slotted shaft CW to increase tension on spring and watch spring end and as soon as it makes contact with rod and is engaged that is ZERO.

Now turn slotted shaft clockwise 3/4 turns to increase tension on spring, This is what controls when the top butterfly opens. The vacuum generated by carb over comes the spring pressure ( for no better way of explaining).

If the top butterfly opens to soon you will have a serious bog
If the spring pressure is too great then the top butterfly will open too late and the condition you are having may result.


So use the 3/4 turn CW as a starting point. see if you can get better performance with either greater spring pressure or less. So adjust CW or CCW after initial by 1/8 turn at a time to see if it makes any significant changes in mid speed performance.

My thought is the secondary top butterfly is opening too late so you are stumbling on the primaries at mid speed.

Hope pic helps........
 
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After reading your directions do you think its best to start by adding a little more tension or taking a little off?
 
agreed ..but after finding the start point and making that adjustment do you think its better to start by going tighter or looser 1/8 if needed to start ...
 
If you throttle up and it bogs down like it is flodding then tighten the spring pressure.
If your condition does not change then lossen the spring pressure.
Remember this may not be your issue... it is just another suggestion.

The fact it idles well. Good initial speed and rpms up to a certain point and then a gap of performance then once it reaches a certain rpm it runs fine again.....says to me the timimg of the butteryfly opening is not happening when it should. Too much spring pressure on butterfly. The result is, with no butterfly opening no additional air thus no additional fuel. The movement of that butterfly (open) lifts the secondary metering rods up and thus allows additional fuel. No butterfly no additional fuel.

Your motor will function on just two barrels...as if you had a 2 bbl carb. Performance may not be as good but it will work.

But you need a place to start... so i suggested a starting point.

You can actually watch this with the flame arrestor removed before adjusting anything. But it must be watched under load...not on the trailer...
 
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ehlien said:
Guess ..what ..turns out its NOT a quadrature but a webber carb.
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Hello there, I am new to the thread but I have the same issue with my 1994 454 MercCruiser (rebuilt from bottom up)
i took it out after rebuild and it ran great. Second outing was the same. The third time I began to have the same issues that you described but at 3300 RPMS. Did you have any resolve? If so can you share? Thanks a bunch!!
 
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NordicJG said:
Hello there, I am new to the thread but I have the same issue with my 1994 454 MercCruiser (rebuilt from bottom up)
i took it out after rebuild and it ran great. Second outing was the same. The third time I began to have the same issues that you described but at 3300 RPMS. Did you have any resolve? If so can you share? Thanks a bunch!!
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I haven't got the problem resolved yet ...that will be this years project! (one of) Will keep you updated if you do the same.
Thanks
John
 
Hello there, I’ve got great news!!! I’ve got my issues resolved on my 1994 Nordic 25ft 454 Merc Cruise engine!!! It took a while to trouble shoot. The bottom line is there is a fuel filter at the rear of the engine before it goes to the fuel rails. That filter was clogged. It would run fine too a certain rpm (3300 for me) and then sputter. The mechanic at Galeys Marine in Bakersfield Ca. changed the filter and it is now running like a champ! I will take it out myself this weekend and see for myself! If you need more specific details let me know and I’ll try to get it to you. Good luck and let me know if this helps!
NordicJG
 
NordicJG said:
Hello there, I’ve got great news!!! I’ve got my issues resolved on my 1994 Nordic 25ft 454 Merc Cruise engine!!! It took a while to trouble shoot. The bottom line is there is a fuel filter at the rear of the engine before it goes to the fuel rails. That filter was clogged. It would run fine too a certain rpm (3300 for me) and then sputter. The mechanic at Galeys Marine in Bakersfield Ca. changed the filter and it is now running like a champ! I will take it out myself this weekend and see for myself! If you need more specific details let me know and I’ll try to get it to you. Good luck and let me know if this helps!
NordicJG
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That is great news ..unfortunately my Baja is sitting under a tarp and a foot of snow right with a ft of ice on the lake and snow mobiles and ice fishing shacks a plenty out front but when spring breaks I'll be sure to check for that . One question though .. no fuel rails on my floater ..sounds like you might have and injected fuel system while I'm carbed??..
 
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