Mercruiser 4.3 (262) vibration and loss of power over 3000 rpm

Baggetorp

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Joined
Dec 4, 2014
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4
Hi!

I have a Hydrolift 23 (a 2000 lbs Norwegian deep V-hull) with a 1999 Mercruiser 4.3 / 262 MAG TBI GEN+. The outdrive is an Alpha One Gen II.

The last 2 seasons I?ve had problems with detonation above 2600 rpm until a couple of weeks ago when I changed the Shift interrupter swich, now the boat runs fine. I mention this is case these detonations could have caused this next problem I have now.

In the troubleshooting leading up the new Shift interrupter switch I have made a compression test - all cylinders within 10-13 bars pressure. I have also tested the fuel pump under load on the water - pressure is fine.

Now that the engine runs ok (no detonation) I have noticed a vibration (you can feel it standing in the boat). The vibration is probably there all the time, but its not noticable until the boat is on a plane which is above 2200 rpm.

Going on the plane I get the same sound and feeling as when the propeller cavitates in high speed and trimmed out. The outdrive is in its lowest position both on the gauge and visually, so I'm thinking actual cavitation is impossible, i.e. can't explain the loss of power when coming out of the water.

The boat should make about 50 mph at 4500 rpm and now I get 38 mph at 4000. When healthy the was a sense of massive power going from 2500 to 3500 and also from 3500 to 4500. Although the engine will go from 2500 to 3500 there is no power surge, it accelerates slowly and above 3500 there is nothing until you look at the GPS with speed increasing slowly in tenths.

The starboard riser is noticibly hotter than the larboard one but the temp gauge reads ok at 170 degrees. The is also a distinct tapping/rumbling noice coming from the rear starboard side of the engine. See link for a recording of the noice at idle (sorry for the Swedish). https://youtu.be/WxY_gv4ANNQ

Any suggestions?

BTW It was through iBoats that I found the solution with the Shift interrupter switch, my mechanic was baffled, so thanks!
 
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Bondo

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Apr 17, 2002
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70,465
Ayuh,..... Welcome Aboard,.... I can Guaranty, the answer to yer problem is in Here,....

Is this a moored boat, or trailer boat,..??
 

Baggetorp

Recruit
Joined
Dec 4, 2014
Messages
4
The boat is in the water (Baltic sea). I added my comments to the checklist in the link provided below:

1. Fuel condition. Type and Octane possibly old fuel
Have changed fuel filter, pumped out 2 litres from tank bottom, no water. Of 100 litres in tank 80 came from the pump last month.

2. Propeller pitch or diameter, damaged blades
Same prop, no visible damage

3. Restricted fuel pickup tube or anti siphon valve Fuel System Test
11. Fuel pump pressure and vacuum
Have tested fuel presure under load

4. Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
Right oil level on stick

5. Marine growth on hull and outdrive
Nothing major

7. Restricted carburetor air intake (clogged flame arrestor)
Looks ok, no massive deposits

9. Poor cylinder compression Compression Test
Tested although no leakdown test

10. Carburetor defective, or wrong type.
EFI laser measured and adjusted

13. Engine Overheating
Temp gauge reads 180

14. Engine timing and ignition system operation
Have changed distributor cap, mechanic says everything looks ok

8. Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
This is where its starts to look interesting. Went down and started the boat and put my ear against the risers. There is a distinct tapping/knocking/mechanical noice coming from the starboard riser with nothing similar coming from the larboard riser. The starboard riser is hotter as well, even when just idling.

Questions:
1) A blocked exhaust would explain loss of power, but could the vibration also come from an exhaust flap flapping?
2) Is it worth taking of the riser and checking?
3) What am I looking for and is it a DIY job?
4) Cost of replacement parts, i.e. would I have to replace the entire riser or just the flap
5) Am I damaging the engine by running it (carefully, of course, not above 2800 rpm)?
 
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