350 Merc only reaching 3000 RPM

[Biggmack77]

Hello everyone

I have a 350 Mercruiser 5.7 with an Alpha one drive in a Chaparral 27' Signature Cruiser. Unfortunately this engine has been a headache since day one. The initial problem was that it would back fire over 3000 RPMs and would not idel well. At the beginning of the season I put in a rebuilt carburetor (Rochester Quadrajet) and it ran 100 times better reaching RPMs up to 3800, which still is lower then it should be but felt nice coming off the previous situation. I have also installed an electric choke conversion kit and the boat idles much better.

After a couple times out with the new improved engine a new problem arose and this is what I need help with. The RPMs will not go above 3000 RPMs at WOT. The throttle is all the way down and it only reaches 3000 RPMs. I am not sure what to do because I feel like I have replaced enough parts to keep her happy for the season.

I am open to all suggestions and will appreciate your input in helping me resolve my problem. This is my first time posting to the forum.
Thank you!

 

[Fishermark]

Re: 350 Merc only reaching 3000 RPM

I am not sure what to do because I feel like I have replaced enough parts to keep her happy for the season.

And what parts have you replaced to make her happy?

Did you do a complete tune up?

There are a couple of things that come to mind:

1. Is your tach correct? You might double check that.

2. If your engine needs a tune up, do that. While you're at it, check the compression as well.

3. Once the engine is running properly, if it still doesn't come up to the recommended range, the other thing to do is begin experimenting with different props. Ask this question in the prop forum.

 

[Biggmack77]

Re: 350 Merc only reaching 3000 RPM

New parts this season:

Remanufacuted Rochester Quad w electric choke kit
New gas line
New coil
New Starter

Last season (towards the end):

Distributor cap and rotor
plugs and wires
adjusted timing


I believe the tach is accurate, but I will double check it. I have never checked the compression before, so I will test that. I agree that a prop change will help the boat get up to the right RPM, but at this point I know that it can reach 3800 with the current prop, there is definitely something going on with the engine.

The day that it started having the trouble was also the day I ran out of gas... I don't know if that would have affected the fuel system. The problem persisted after the fill up. (The fuel gauge didn't work after the winter, put a new sender in and that crapped out too)

Thank you Fishermark

 

[jaxnjil]

Re: 350 Merc only reaching 3000 RPM

i think i would check the fuel pump presure and fuel filters. if you had any crap in your tank one sure way to make sure it gets into the pickup and filters is to run the tank dry

 

[Bondo]

Re: 350 Merc only reaching 3000 RPM

Ayuh,....

Somewhere around here,.. Don has a Checklist for motors Not making the proper rpms,.....
I just don't have it saved,... A search might find it,...

Burnt up exhaust flappers restricting the out-flow is a possibility.....

 

[cooter2506]

Re: 350 Merc only reaching 3000 RPM

Here is a copy of Don's list.
Engine Won't Reach Operating RPM. Check

Fuel condition. Type and Octane possibly old fuel
Propeller pitch or diameter, damaged blades
Restricted fuel pickup tube or anti siphon valve Fuel System Test
Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
Marine growth on hull and outdrive
Wrong gear ratio in outdrive
Restricted carburetor air intake (clogged flame arrestor)
Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
Poor cylinder compression Compression Test
Carburetor defective, or wrong type.
Fuel pump pressure and vacuum
Boat overloaded, improperly loaded, or improperly trimmed.
Engine Overheating
Engine timing and ignition system operation
Remote control cables and linkage for proper travel to open throttle plates fully.
__________________

 

[Biggmack77]

Re: 350 Merc only reaching 3000 RPM

Thank you all for your advice.

I have a new problem though which may be related but is not allowing me to diagnose the rpm problem. The boat will not start. I went to it yesterday and the engine will crank but not fire up.

Is it possible that the starting problem may be related to the RPM problem? Any suggestions? Hopefully this can help narrow down the list of possibilities.

 

[Bondo]

Re: 350 Merc only reaching 3000 RPM

Is it possible that the starting problem may be related to the RPM problem?

Ayuh,.....

Hard to say, til you tell Us whether it's missing it's Fuel,.... Or Fire......

Look down the Carb,..?? Pull,+Check the Plugs,..?? Fire at the Coil,..??

 

[Biggmack77]

Re: 350 Merc only reaching 3000 RPM

Hi there,

I am not missing fuel in the carb or fire at the coil or plugs. I had a mechanic take a look at it and he said I was getting too much gas in the carb. We found a vacuum leak and he told me that was the problem and to replace the hose. I put a new hose on the carburetor today and it still had problems starting (it only starts about 20% of the time). Sometimes it fires right up and sometimes it just cranks. Even with the vacuum leak fixed, the boat would only reach 2900 RPM at WOT today. Any suggestions would be greatly appreciated.

 

[Fishermark]

Re: 350 Merc only reaching 3000 RPM

Remanufacuted Rochester Quad w electric choke kit...
I had a mechanic take a look at it and he said I was getting too much gas in the carb. We found a vacuum leak and he told me that was the problem and to replace the hose. I put a new hose on the carburetor today

Is this a marine carb you installed? Or a rebuilt automotive?

 

[Don S]

Re: 350 Merc only reaching 3000 RPM

Here is the list again, only this time with the links to a couple of the tests you need may need to do.

List of possible causes of low WOT.

Engine Won't Reach Operating RPM. Check

• Fuel condition. Type and Octane possibly old fuel
• Propeller pitch or diameter, damaged blades
• Restricted fuel pickup tube or anti siphon valve Fuel System Test
• Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
• Marine growth on hull and outdrive
• Wrong gear ratio in outdrive
• Restricted carburetor air intake (clogged flame arrestor)
• Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
• Poor cylinder compression Compression Test
• Carburetor defective, or wrong type.
• Fuel pump pressure and vacuum
• Boat overloaded, improperly loaded, or improperly trimmed.
• Engine Overheating
• Engine timing and ignition system operation
• Remote control cables and linkage for proper travel to open throttle plates fully.

 

[Biggmack77]

Re: 350 Merc only reaching 3000 RPM

FisherMark,

It is a marine carb, I gave them the specs for my engine so they could adjust it accordingly. When I installed it everything worked fine.

 

[Biggmack77]

Re: 350 Merc only reaching 3000 RPM

Hello everyone,

I just wanted to post an update to my situation as it may help others with a similar problem. It turns out there was some particles clogging up the square elbow fitting where the fuel pickup tube from the gas tank and the anti siphon valve connect. It was not completely restricting the flow of gas but it was preventing a good amount that would starve the engine when trying to reach higher speeds.

I checked then changed the risers and took a look down the Y pipe to check the flappers valves. There was half a flapper on the port side and no flapper on the starboard, there was no sign of them in the bottom of the Y pipe but certainly worth checking. Put new flapper valves on.

After cleaning the clogged gas fitting and changing the risers the engine starts easily and sounds great, looking forward to testing her out on the water! This should make for a better season.

I hope this is helpful to others, it took me a long time and a lot of headache to get to the bottom of this problem. Thank you to everyone who helped me in diagnosing this.

Ed