Mercruiser 3.0 low wot rpm... Yes another thread

[bfeils]

Hi everybody. I bought a non running sea ray 185 in the middle of the summer and partially rebuilt the motor due to two stuck piston rings. I replaced the crank and lower end bearings as well as the pistons and rings. No machining done because it was in good shape with low hours. Anyways, it runs really good but I'm not getting as much rpm as I should be. With the stock 14.25x19 prop I can only get 4200rpm when most people with this boat are able to get 4700-4800. I've gone through almost the entire Low rpm checklist including weighing the boat to check for water logged foam. The only thing I haven't checked is fuel pressure and it's not easy to check on this motor but I'm going to get to it. Anyways, I opened the carb up after running it and the fuel level in the bowl seems low to me. Even though the float levels have been checked and rechecked by both myself and a fuel system specialist shop. Can low fuel pressure leave the fuel level low even with properly setup float? And, can low fuel level in the carb cause a lack of rpm at full throttle?

 

[bfeils]

Here's a pic of the carb with the fuel level visable after running.

 

[achris]

... as well as the pistons and rings. No machining done because it was in good shape with low hours. ...

Right there...

Each bores is machined to match the piston going into that hole. That's why when you take a block to a shop they ask for the pistons too. And why each piston is mark with its cylinder number when you get it all back.... I would be doing a leak-down test to see just how much you're losing passed those pistons and rings....

Chris.......

 

[bfeils]

Compression is 185 in all cylinders. The bores still had the factory cross hatch visable and checked round and right at
Original diameter.

 

[fishrdan]

What year is the engine, better yet, serial number. Also, points or electronic ignition (DDIS or EST)?

What is the timing set to and what is the max advance at 2800-3000RPM? You'll need an advance timing light to find the max advance. If the ignition isn't getting max advance the engine will be down on power/RPM.

If the carb was running low on fuel, I'd expect surging or lean pop at high RPM.

 

[Scott Danforth]

Compression is 185 in all cylinders. The bores still had the factory cross hatch visable and checked round and right at
Original diameter.

you better check your gauge.

the stock 3.0 compression will give you numbers of about 140-150 psi per cylinder. that is even what is printed in the manual.

my fresh big block running 9.6:1 has just over 160psi on each cylinder. there is no way you are getting 185# per cylinder unless you have water in the cylinders taking up volume.

 

[aimlow]

Timing advancing properly? Have you checked total advance? Dirty/plugged flame arrestor? What do the plugs look like? Frankly, the plugs are an important tell.

 

[alldodge]

Can low fuel pressure leave the fuel level low even with properly setup float? And, can low fuel level in the carb cause a lack of rpm at full throttle?

Yes and Yes

 

[bfeils]

What year is the engine, better yet, serial number. Also, points or electronic ignition (DDIS or EST)?

What is the timing set to and what is the max advance at 2800-3000RPM? You'll need an advance timing light to find the max advance. If the ignition isn't getting max advance the engine will be down on power/RPM.

If the carb was running low on fuel, I'd expect surging or lean pop at high RPM.

I have checked it and estimated the advance to be about right. I don't have an advance timing light but it looks to be right about 25 degrees which is in the ballpark.

 

[bfeils]

you better check your gauge.

the stock 3.0 compression will give you numbers of about 140-150 psi per cylinder. that is even what is printed in the manual.

my fresh big block running 9.6:1 has just over 160psi on each cylinder. there is no way you are getting 185# per cylinder unless you have water in the cylinders taking up volume.

The gauge very well could be reading a bit high but the point is it's certainly not low. The compression ratio is supposed to be 9.25:1 so it should be somewhere around your big bock build. Also I think it could be a little different depending on your cam profile.

 

[bfeils]

Timing advancing properly? Have you checked total advance? Dirty/plugged flame arrestor? What do the plugs look like? Frankly, the plugs are an important tell.

I have checked all that. I Cleaned the flame arrested. I haven't looked at the plugs in a while but I've heard unless toy run it at high rpm and immediately kill the motor it doesn't tell you much. I'll post some pics I took off the plugs about ten motor hours ago and maybe someone can read that.

 

[bfeils]

Here's the spark plug pics.

 

[alldodge]

Your running real lean and if that keeps up your going to burn a hole through a piston

Check your fuel pressure

 

[Scott Danforth]

your 4th plug is almost dark enough

 

[bfeils]

Your running real lean and if that keeps up your going to burn a hole through a piston

Check your fuel pressure

Geesh! Well I am getting surprising good fuel mileage. I'll check that out as soon as I can.

 

[QBhoy]

Hi
just thinking simply and in the off chance. Is the tachometer accurate ? Worth checking the 3 settings on the back of gauge to make sure it’s in the right setting for 4 cylinder 3.0.
unlrss you’re sure enough by maybe using a tiny tach or similar. You’re not a million miles off the norm, but maybe a bit low as you say.
Youre searay is a heavier boat than a comparable maxum or bayliner etc, but should certainly turn that 19” almost to its max or further trimmed out.

 

[bfeils]

Hi
just thinking simply and in the off chance. Is the tachometer accurate ? Worth checking the 3 settings on the back of gauge to make sure it’s in the right setting for 4 cylinder 3.0.
unlrss you’re sure enough by maybe using a tiny tach or similar. You’re not a million miles off the norm, but maybe a bit low as you say.
Youre searay is a heavier boat than a comparable maxum or bayliner etc, but should certainly turn that 19” almost to its max or further trimmed out.

Thanks, I looked into that before but it is set correctly. My top speed is lower than it should be also.

 

[aimlow]

Seems when I had one of those little motors decades ago, it ran somewhat lean. Float level has little effect on a/f mixture on the main metering circuit, btw. Float level mainly affects the "crossover" point between the idle circuit and the main metering circuit. Quick, cheap tach checking method..a120v/ 60 cycle fluorescent light will "strobe" the timing marks at EXACTLY 1800 and 3600 rpms. compare the engine rpm with the tach rpm.

 

[achris]

.... Quick, cheap tach checking method..a120v/ 60 cycle fluorescent light will "strobe" the timing marks at EXACTLY 1800 and 3600 rpms. compare the engine rpm with the tach rpm.

That's a nice 'quick check'... I was going to say that the dash tacho is not reliable to check revs. Use a calibrated tacho, or one in a timing light...

 

[alldodge]

Don't remember how many times I have heard "calibrated" but suggest need to remove the term. Next to no regular user will use a calibrated unit. They will use another unit and compare it to the one at the helm. A timing light is no more calibrated then any other unit once it leaves the factory. Only way to get calibrated units is to have them on a predetermined re-calibration testing time line