2012 3.0L TKS runs great with muffs, but slow on lake...

GeorgioP07

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Hello,
My Family and I bought this 2012 Bayliner 185 with Mercruiser 3.0L in later Summer 2024, ran it a few times, did a tune up, new plugs, wires, cap, rotor, water pump impeller, anodes, thermostat etc.. no problems, and winterized it well. Starting in 2025, w/fresh pump gas, -we noticed that the tach won't reach over 2900 RPM's on the lake at wide open throttle. It runs fine in the garage with muffs, and reaches over 4500 RPM's - no problem as long is it's not in the lake. So I went down this rabbit hole, new fuel pump, rebuilt the TKS carb to specs in service manual, got a timing light, checked timing and was dead on, bought the special slotted screw and fuel/air mixture screw then adjusted (she preferred 4 turns out when warm), re-gapped plugs, tightened throttle cable, oil level not over, but full on dipstick.
I didn't replace condenser, since engine starts well, runs well enough (has a slight stutter from 2400 RPM- 2600RPM), but smooths out. On the lake at wide open throttle, it only climbs to a max of 2900 RPM, and goes no more which equates to going 15MPH at best. Oil pressure at 65psi at wide open throttle. Temp at 180F or so. In garage, at wide open throttle, it's fine, over 5K RPM's on tach with no load on prop. Engine wants to idle at 900RPM, not so much at 750RPM, like the manual states, or a bit of shaking takes place.
Using same propeller as last year, with it went 35+ MPH on the lake. It is a 3-Blade MerCruiser 135-300HP: 14 1/4 x 21 with hub kit. Boat not overloaded, and no growth on hull, it's on trailer 98% of the time, no drag on hull. I've not adjusted the shift cables yet, but I am leaning from lack of speed on the lake not being an engine issue to more of a drive issue, or power loss from engine to prop. Kids want to go fast on the tube and Dad is letting them down.. Please help.
 
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alldodge

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Welcome
Reving in neutral with no load means nothing

Engine wants to idle at 900RPM, not so much at 750RPM, like the manual states, or a bit of shaking takes place.
The lower RPM under load and rough idle where it should idle smooth leans me in the direction of motor is not firing on all cylinders.

With motor running around 1000 RPM, pull each plug one at a time and listen if it effects RPM. Each plug should have an effect

Do a compression test and see if all cylinders are in the 140 to 150 psi range. Make sure throttle is wide open and you get at least 3 power strokes (6 revs) for each cylinder, more is even better.
 

GeorgioP07

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The compression test and fuel pressure test were not done, but I can go down that route, as I will need to purchase the compression test kit.
I did so many other small tests that I forgot to mention so far , - I forgot to mention that I bought a spark tester, the one that's clear and goes inline from the cap to the plug and all 4 were firing (glowing red on the unit have). I performed that one at a time of course. I didn't suspect compression, since it ran well before winterizing in late 2024.
 
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GeorgioP07

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Hello, I am revving with the throttle in Forward, not Neutral. Yes no load. Would it still rev to 5K RPM's on the tachometer with compression issues?
When you say pull the plug, you mean the wire right? I will do that, I think I did that before, and you can hear / feel the difference. I recall that the #4 plug was not as brown as the others, but only a very slight difference. It had combustion wear as expected.
 

ESGWheel

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A note of caution: do not run your motor past a high idle when on the muffs. While a burp up to say 2000 is ok you run the risk of overheating your engine and/or ruining your impeller running it up to 4500.
 

alldodge

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Go to a local auto parts store that has a loaner tool program, most are free

Yes I mean pull the wire off plug. Where rubber gloves or at least some for some added insulation
 

GeorgioP07

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A note of caution: do not run your motor past a high idle when on the muffs. While a burp up to say 2000 is ok you run the risk of overheating your engine and/or ruining your impeller running it up to 4500.
Thanks and Noted, I do that infrequently, as little as possible. I keep my eye on the temp gauge as well when I did that.
 

ESGWheel

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While good to keep watch on the temp, the impeller is probably cavitating like heck and thus may burn up / become less effective. Also, the temp rise on the risers and the downstream rubber connections, etc., will see a spike in temp long before the motor temp sensor. Recall the water flow is keeping more than just the motor cool. The way the risers work is the exhaust mixes with the water to keep things from melting. And no way you have enough water flow on the muffs at 4500 RPM. So something to consider.

And another note of caution if I may: the manuals I have read tell you not to run on the muffs with the drive in gear. > it may relate to no load on the prop and thus setting up a hormonic on the gears (play, no play, play > a sort of ‘hunting’ that could damage the gears). Others may know as I am not well versed in sterndrives,

I also suspect the concern is someone coming in contact with the spinning prop. I am not saying you were unsafe but wanted to highlight this as well.
 

GeorgioP07

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I know, I said I am sorry, in a way & yet no closer to solving my problem, Nobody is allowed to be near the prop when running of course. Compression Test kit arrives tomorrow. To find out that the compression is off at this point would be a real kick to the groin. Fingers Crossed and Thanks.
 
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