Mercruser 3.0 hard throttle acceleration problem

uinthas7

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Dec 13, 2015
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I have a brand new mercruiser 3.0 I have purchased from Michagan Motorz that has about 19 hours on it, I purchased it because my first motor blew last fall (valve went through a piston) and the current motor did the same thing this spring (warranty fixed the issue) because of what Michagan motorz diagnoised as a lean conditioin (however local mechanics here said it was a manufacture default)

Anyway that is the background to my issue, and as you might expect I'm paranoid to even run the thing after 2 motors putting valves through the piston. My boat is a 2003 sea ray 176, serial# for the old motor is om628758.

My issue is when I am at around 3/4 throttle and I hit full throttle hard (push down fast) it does it hesitates for just a second, chokes and then goes.. I have posted a video to the issue, you can hear it best towards the end. If I bring the throttle up gradually it does not have the same problem. It will also sometimes do it from a say idle if you push down the throttle hard, but most of the time it's just a very slight hesitation.

Since the rebuild of my brand new motor this spring I have rebuilt the carb, (every single part but the float) checked, re-checked and triple checked it, taken the fuel line out and inspected it, replaced the fuel filter in the gas pump, pulled fuel from a separate gas tank, done a fuel pressure check at idel was 5psi. Yesterday I even bought a timing light to check the timing, used the timing shunt to put in base mode was 1 degree, and 24 degrees at 3000rpm (used a digital timing light)

At this point I'm just wondering if it is what it is, I am very paranoid about anything with this motor dont want a 3rd to go boom. Any suggestions or comments I'd love to hear. It starts up great, idle is better when it's cold seems to idle harder when warmed up, engine sounds very good when cruising it's just this one issue.
 

uinthas7

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Dec 13, 2015
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What is the WOT rpm

I have done very little full throttle runs at any lenght, both motors blew at wide open so again I'm paranoid. But the little I have taken it up for a min seems to be about 4400rpm. I have a 21" prop probably need a 19". I mostly run at 4000rpm
 

Bondo

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Ayuh,..... I'm thinkin' Detonation killed both motors,....

Detonation can be induced by several things,....

Most common in boats is a lean condition from crud in the fuel,....
Bad timin' is another,....
'n to big a prop is yet another,....

If the motor can't pull the recommended rpms at Wot, it's luggin' through-out the entire rpm band,.....
 

uinthas7

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Dec 13, 2015
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Ayuh,..... I'm thinkin' Detonation killed both motors,....

Detonation can be induced by several things,....

Most common in boats is a lean condition from crud in the fuel,....
Bad timin' is another,....
'n to big a prop is yet another,....

If the motor can't pull the recommended rpms at Wot, it's luggin' through-out the entire rpm band,.....



So you think I simply need to drop to a 19' prop?
 

Bondo

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Ayuh,.... I don't know, I'm just tellin' ya what induces detonation,....

You claim the fuel system is up to snuff,...
Yer on yer 3rd motor, 'n just bought a timin' light,....
'n to big a prop is no doubt, not helpin' any,.....

Yer description of the way it's runnin', sounds like a carb problem from here,.....
 

uinthas7

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Dec 13, 2015
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Ayuh,.... I don't know, I'm just tellin' ya what induces detonation,....

You claim the fuel system is up to snuff,...
Yer on yer 3rd motor, 'n just bought a timin' light,....
'n to big a prop is no doubt, not helpin' any,.....

Yer description of the way it's runnin', sounds like a carb problem from here,.....


I agree, does sounds like carb problem, just cant find one. Pulled the carb again just yesterday, checked both float level settings and they are dead on. Idle air/fuel screw, all jets, needle and spring, venturi gasket and parts, accelerator pump have all been replaced. I did some testing on the accelerator pump and it squirts gas to both barrels at all throttle ranges. Is there a way to tell if I still have a detonation issue? I'll be happy to buy a new prop, been planning on that anyway...
 

NHGuy

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May 21, 2009
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3,631
Your bog is normally from poor accelerator pump shot. But since you get a squirt, maybe not. I'd suggest doing a check ball reseat. At the bottom of the accelerator pump well there is a ball. Reach in there with a flat punch and give it a whack. Not enough to break things, just enough to reestaablish the ball shaped seat. Then chuck the used ball for one that's not been bashed. If I am right it will cause the well to hold a full shot of gas.
Any leaking around the carb base gasket? Intake manifold gasket? Spray some carb cleaner around there. If the motor speeds up you need to reseal.
And, get a shorter prop to allow the engine to reach full rpm without being overloaded.
 

uinthas7

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Dec 13, 2015
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Your bog is normally from poor accelerator pump shot. But since you get a squirt, maybe not. I'd suggest doing a check ball reseat. At the bottom of the accelerator pump well there is a ball. Reach in there with a flat punch and give it a whack. Not enough to break things, just enough to reestaablish the ball shaped seat. Then chuck the used ball for one that's not been bashed. If I am right it will cause the well to hold a full shot of gas.
Any leaking around the carb base gasket? Intake manifold gasket? Spray some carb cleaner around there. If the motor speeds up you need to reseal.
And, get a shorter prop to allow the engine to reach full rpm without being overloaded.

Thanks for your input. There is only a spring under the acceleration pump. There is however a small round ball looks like a bee bee in the venturi, that has been replaced. I just bought a new 19" prop, taking her out now will report the results.
 

airshot

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If you constantly run at 4000 or wot you are definitely shortening the life of your motor. Two stroke motors are designed to run at wot all day but 4 stroke motors are not. If you read thru these forums many have recommended that a constant rpm of 3400 should be maximum all day running rpm. Occasional bursts of wot for short periods of time is fine but constant wot will will implode your engine in short order. If you need to go that fast all the time perhaps you might consider a bigger engine.
 

uinthas7

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Thanks for your input, much appreciated. I did review that thread and the little ball and spring that it refers to has been replaced along with the gasket for the venturi. I read through my manual again last night and it discusses the carb seat and needle. I have the spring loaded needle, the manual states that I can swap my spring needle for the solid needle, not sure what effect that would have? When i first looked at re-building the carb as a rookie I watched a guy on youtube do it, he stated the old spring was better then the new one that came in the kit so he kept it, I lost my old one so that is not an option.

I took another video last night of the idle, after I have ran the motor and it idles a bit it begins to get rough, almost like the motor wants to die and the motor sometimes will even start shaking even though the rpm's are still around 700-800. Excuse my engine cover that the spring is broken on, need to fix that :) I think this might have something to do with the problem as well.

The 19" prop did nothing but make my rpm's raise a bit faster but still seemed to top out around 4400 rpm, and I lost top end speed, about 2-3 mph. I was told I have high altitude gears, it is a Alpha 1 gen 2.

I still suspect it's starving for fuel some how, I am planning on going out on the lake today and check the fuel pressure while I'm out running. Does anyone know the size fittings to the carb and fuel pump?
 

uinthas7

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Also, here is a pic of my plugs I pulled yesterday, cylinder #4 looks richer then the rest, dont know if that means anything or not?
 

NHGuy

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I have seen that remark on the needle valve. Basically what they are saying is, if you have trouble with one design try the other.
 

eavega

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Also, here is a pic of my plugs I pulled yesterday, cylinder #4 looks richer then the rest, dont know if that means anything or not?


Not that I am an expert or anything, but those plugs look a bit too lean.

-E
 
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