97 Merc 3.0LX Low RPM at WOT

[maxumized]

I just bought my first boat, a 1997 Maxum 1750SR with a 3.0LX / Alpha One. I've read that RPM at WOT should be 4400-4800rpm. However, I can't seem to get over 3000rpm and 30mph trimmed up. I should also mention that the boat lacks low end power, also. I know it's only a 4cyl, but it struggles pretty bad. Takes quite a while to plane.

I've pulled the plugs and they look OK.

I read somewhere that my exhaust baffle may be broken off causing too much back pressure.

Anyone have any suggestions?

 

[Don S]

Re: 97 Merc 3.0LX Low RPM at WOT

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.

 

[maxumized]

Re: 97 Merc 3.0LX Low RPM at WOT

Well, got the exhaust apart and the flapper is no where to be found except for the crossbar of part #6 and the #8 piece on each side of the tube. Guess I'm going hunting in the outdrive. I've removed the prop and don't really see anything in the passages behind it. Where do I begin looking for these things... is it even possible they've blown out completely?

If I need to get into the lower drive, I'm sure I'll need replacement seals/gaskets... correct? Any lower drive disassembly tutorials?

 

[Maclin]

Re: 97 Merc 3.0LX Low RPM at WOT

You will probably need to pull the drive to find it, someone here has a pic of where the flapper usually winds up which is at the junction between the drive and the bottom of the #7 piece from your drawing.

 

[maxumized]

Re: 97 Merc 3.0LX Low RPM at WOT

Maclin,

I could be wrong, but from the looks of it, it almost seems I could remove #7 from drive inside the bilge. Anyone tried that?

Failing that, I wonder if I could vacuum the pieces out with a shop vac? May sound ridiculous, but I really don't want to tear into the drive.

 

[Maclin]

Re: 97 Merc 3.0LX Low RPM at WOT

I think some would consider removing the drive to be easier than all that. Merc actually recommends to do it yearly anyway. It will also give you a chance to replace the drive bellows and inspect drive shaft ujoints, etc. The drive bellows is an important item to have in good condition and if it is more than 3 years old it needs to be replaced anyway. Since this boat is new to you then you may not know the exact conditon of everything and this could be well spent time and dollars on preventative maintenance, just my take on your situation.

 

[chiefalen]

Re: 97 Merc 3.0LX Low RPM at WOT

Probably gonna have to take off the riser, and #1 than try your shop vac and pull it up #4. If not take off #4. Stick the hose of the vac all the way down. Maybe if your lucky you'll get it. If not #7 has to come off.

good luck

 

[Maclin]

Re: 97 Merc 3.0LX Low RPM at WOT

The drive comes off as a unit, no need to tear into it. You may think about impeller replacement though while it is off, that does require removing the lower unit. Just some thoughts to confuse you even more...

 

[maxumized]

Re: 97 Merc 3.0LX Low RPM at WOT

Hey chiefalen, already got it torn down to where #7 is all that's left... that's when I found the flappers missing. So, maybe I'll get lucky.

Maclin, I consider myself fairly handy on cars and trucks... and now even boat motors. But, outdrives are still new to me. I really don't want to cause more problems than I fix by causing leaks, etc... I guess I need to get a shop manual for the engine/outdrive. I'd love to learn outdrives, but I just need a little bit of guidance to get started.

Also - just to provide a bit of history... the reason the guy I bought it from sold it is because every time he took it to the shop it came back with new problems. I called BS several times on issues the shop "found." Needless to say, I bought it off him and do my own work now. If I need some really involved work done, I know another reputable marine shop I'll be going to. The drive, exhaust, and shift bellows are new. Now I'm storing the boat at a neutral trim and keep the bellows new with silicone spray. Previously, they cracked up from getting dry and being stretched at trailer trim while in storage.


Will dig in tomorrow morning on the case of the missing exhaust flappers as I have the day off work!

Thanks!

 

[maxumized]

Re: 97 Merc 3.0LX Low RPM at WOT

Here goes...

First... Fished all through the exhaust to find the broken water stopper without any luck. I removed the exhaust bellows and worked my way up and then back down from the bilge. Nothing. When I went to the shop to pick up a new stopper and grommets the shop guy told me they must have been removed because there's no way the stopper made it into the drive because the rubber flaps have a metallic core that wouldn't fit through the oval exhaust port where the bellows attach. Is this true? If it didn't make it past the bellows, was it removed during a previous service?

Second... I ran a compression check on the motor. I got a consistent reading of a little over 100 for each cylinder. However, it didn't reach 100 on first stroke... it took about 3-4 compression strokes for each cylinder to reach its max of ~100. I'm guessing this isn't too bad, but not great either.

Third... put the hose on today and ran it to get it warmed up for the compression test. After it was at operating temp, I did a few throttle only runs through the RPM range and it will reach WOT around 4500-4800. So, I'm guessing I'm not limited by some type of ignition issue.

Something is keeping me from surpassing around 3000rpm when underway, regardless of trim.

Thinking of rebuilding carb...

 

[maxumized]

Re: 97 Merc 3.0LX Low RPM at WOT

Anyone?

 

[cooter2506]

Re: 97 Merc 3.0LX Low RPM at WOT

Ran it on hose and got to 4500-4800 rpm. Not a good idea on muffs. Engine will reach massive rpms out of the water with no load. Go over the list that Don gave you with a fine tooth comb. Trust me it will give you the answer to your low rpm at WOT

 

[maxumized]

Re: 97 Merc 3.0LX Low RPM at WOT

Oh well, live and learn. I figured the Delco EST had a rev limiting capability which is why it topped out around 4500-4800. I didn't run it at high rpm but for a brief moment and it never ran hot.

Anyone think those compression numbers of ~100 are low enough to be causing the issue? Anyone think a carb rebuild would help?

 

[maxumized]

Re: 97 Merc 3.0LX Low RPM at WOT