Mercruiser 3.0 won't rev past 2000 rpm

Hello, I have a 1989 wellcraft 190 classic. It has a mercruiser 3.0 that I have replaced basically every single part external to the engine on. It has good compression on cylinders 1,2,3 for its age(going front of boat to back). The fourth cylinder closest to transmission has only 75psi. This is the one that fails in most cars in my experience since it gets the hottest being in that location. But anyways I get 75psi repeatedly. I have only ever done leak down tests on two strokes so this is new to me. But pressurizing the cylinder at various points of the stroke got results such as the whole motor turning. At the point I believe was top dead center I was able to hear air coming out the carb (probably from the valve cover breather? When we took the boat put anyways since it was so nice it did its normal 2000 rpm under load revving. We ran it with motor uncovered and I could hear a high pitch almost chirping from the back of the motor (only when under load). Am I right to assume the valve is either out of adjustment or the seals are bad? The motor starts and runs great, and does not overheat at all, or burn any concerning amounts of oil (dipstick hasn't changed in eight trips out or roughly 25hours running). Thanks for your help and time consideration on the matter. Enjoy your memorial day weekend everyone

Re: Mercruiser 3.0 won't rev past 2000 rpm

i would check for the throttle cable binding make sure it is opening the carb all the way. if it runs good and sounds good up to 2000 rpm under load i dont think its an internal engine problem

Re: Mercruiser 3.0 won't rev past 2000 rpm

Throttle works fine. Burns gas great also. I can use a tank of gas in a couple hours. I am sure it is internal because cylinder 4 only has 75psi compression and under load the back valves chatter so I'm thinking that's te problem

Re: Mercruiser 3.0 won't rev past 2000 rpm

is good compression over 150. 75 and theres a problem.pop some oil in that cylinder and try again.set throttle fully open with centre coil wire removed and lets see the figures atained.could see if the valves r operating correctly by removing rocker.someone may have set no 4 wrong but you can reset whilst cover is off whilst running if you wish.we can tell you how to do it if you dont have a manual.you could have a tuliped valve or many other things which you have to try and eliminate first.

Re: Mercruiser 3.0 won't rev past 2000 rpm

The 75 psi is what I get each time I do it. The others are all around 120 which I was told is fine since the motor is all original. Only thing ever done was a water pump, I have all the service records. The boat sat unused for eight years before I bought it.

Re: Mercruiser 3.0 won't rev past 2000 rpm

So I took te boat out anyways. 8 mph is better than nothing. Anyways it ran perfect sounded great just not revving so in taking the motor apar today after work. Any tips? I'm used to 2 strokes not 4 strokes

Re: Mercruiser 3.0 won't rev past 2000 rpm

To explain a bit further what dingdong said about putting oil in the cylinder. Ya squirt a little oil in the bad cylinder, and do another comp test. If the comp rises significantly, ya can assume you have a problem with the piston ring to cylinder wall seal as the oil will temporarily create a better seal. If the comp stays bad/low ya can assume its a valve problem as the oil doesnt seal valve problem as well. If ya hear air thru the carb, I would suspect a bad intake valve. The low rev may be unrelated... fuel sys problem restriction. Has it had a basic tune up recently? Timing, vacuum advance checked ?

Re: Mercruiser 3.0 won't rev past 2000 rpm

N01290087 said:

I have a 1989 wellcraft 190 classic. It has a mercruiser 3.0

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Wellcraft 190 =19' long? That's a lot of boat for a 3.0. What prop is on the boat, pitch and diameter? Have you weighed the boat at truck scales, (boat unloaded) to see how much it weighs. Waterlogged foam could make it run doggy.

Run the wet compression test as suggested. For the leak down test and hearing air going through the intake/carb, and not knowing if the piston was on it's compression stroke, it's quite possible to hear air through the intake. You need the piston coming up on the compression stroke, both valves closed, for the leak down test. Try the test again with both valves closed, remove rocker cover to verify valves are closed if needed. Also, use a tube (heater hose) as a stethoscope, to see if the air leak is coming through the carb or valve cover breather.

At 75PSI on #4 you certainly have an engine problem, but other things could be contributing to the performance issues, weight, prop....

paultjohnson said:

vacuum advance checked ?

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No vacuum advance on a boat...

Re: Mercruiser 3.0 won't rev past 2000 rpm

fishrdan said:

No vacuum advance on a boat...

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Ah yes... Its not vacuum but centrifugal ???

Re: Mercruiser 3.0 won't rev past 2000 rpm

The boat on trailer is something like 3400 pounds with full gas and everything except people(had to check my Tacoma which can pull 3500 pounds is ok. I redid the compression after running it. It went to 108, 108, 108, 95. I took apart whole motor after that. Turned the valves upside down and put gas in them. I found cylinder 1 they don't leak, cylinder 2 and 3 both leak and cylinder 4 intake leaks. Now onto the problems. Cylinder 3 the new plug was clean and shiny after 3 hours of running. It is getting no fuel at all for some reason I would assume the intake valve is not opening enough even though I saw the rocker arm actuating? I also believe that cylinder 4 needs new rings since the cylinder looks like every other motor I've had to put new rings into.
A recommended engine and machine shop will do the valves for 250 is that a good price? I've used them for honing and boring before they are very good

Re: Mercruiser 3.0 won't rev past 2000 rpm

if you have the complete engine in bits by yourself that means your quite capable of hand lapping a few valves in with some paste and some elbow grease.can you see any cross hatching ,e.g honing marks in the bores.save your money and spend it on a honing tool and new shells mains and rings and a full gasket set.check bore diameters with a ring at different depths by measuring end gap to see if bores are pretty parallel.take some good close up photos and post if your not sure.

Re: Mercruiser 3.0 won't rev past 2000 rpm

#3 plug being completely clean sounds like the plug isn't firing or the cam's intake lobe is totally wiped out for that cylinder.

I had my 140HP head rebuilt a couple years ago and it was around $225, everything new except the valves.

I'd mic the bores and and see if it can be re-ringed, or if you need to bore it and put in oversized pistons. 108# compression is a tired engine (if the compression test was done correctly)...

Re: Mercruiser 3.0 won't rev past 2000 rpm

wow idk what your shops are charging you guys but i had my 140 head remaned everything new besides springs and its only cost me 150

Re: Mercruiser 3.0 won't rev past 2000 rpm

I wish it was cheaper. The compression test was done right. It's a 1989 that was never worked on. It makes sense that it's tired. The cylinders look perfect and smooth so I'll probably have it just honed

Re: Mercruiser 3.0 won't rev past 2000 rpm

that should be good i replaced almost everything except the crank and head for about 880 lol maybe you should be less

Re: Mercruiser 3.0 won't rev past 2000 rpm

Anyone know a good source of piston and rings that are not way over priced? I can't afford mercruiser parts and honestly have never had problems in the aftermarket with other brands

Re: Mercruiser 3.0 won't rev past 2000 rpm

contact a local machine shop they should be able to get you the after market equivalents that replace merc parts i paid 8 for a SET of rings and 21.92 for each piston or something like that lol

Re: Mercruiser 3.0 won't rev past 2000 rpm

Ryanlewis2010 said:

contact a local machine shop

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^^^ Yup.

They should be able to get you what you need for a decent price. I bought a complete rebuild for my 140 (3.0) off Ebay for around $400 and was happy with everything, but wished I had gone through the machine shop as their prices weren't that much higher than the internet. Greenbaypartsworld is another source for engine parts. Do yourself a favor and get the 1 piece oil pan gasket, better than the 4 piece set.