Mercruiser 3.7L - Rattle/Knock over 2000rpm & wont go over 3000rpm - need a guru that knows these!
170/3.7l Hope for help from someone that really knows these old girls and has rebuilt over many years! Bertram 25 with twin 3.7l Mercruisers understand the limits of these engines and how some do not like them but also that those who have worked with them claim they are farily robust and great HP when looked after - Have researched a bit on these engines and know the common faults of magneto, coolant leak on shaft and coolant leak on header tank back to piston but need to find out what that terrible rattle/knock noise is once warmed up and RPM over 2000 and wont rev over 3000rpm. The other motor goes fine and revs to 4000rpm and possibly beyond.
1. Oil - changed, seems okay, not milky but maybe not normal black a little grey maybe. Oil pressure - does fluctuate but seems okay when at revs, similar to other motor and never on red line but gets close when at idle.
2. Fuel - fresh and tried 91 to 98 with no noticeable change, also tried additive but not Seafoam
3. Timing - set at 4 before TDC and seems to advance to approx 32 at rev.
4. Carby - has a kit put through it and seems to spray ok.
5. Charging - this is coming in high at revs about 16 volts but not sure if an issue as the other motor not effected, maybe it is an issue?
6. Gimbal bearing - have checked and seems okay and no change to noise with hard turn to port or starboard
7. Rockers - replaced one hydraulic lifter, maybe there are others failed would that cause the rattle/knock?
8. Push rods - have not checked all for correct gap, maybe they are a little tight and no gap?
9. Exhaust - refurbed and cleaned including drive, flappers new.
10. Bottom end - could be the issue but no noise when at low revs so could it be the bottom end bearings?
11. Carbon build up - think this motor was sitting for a long time and prior to that only used at low revs, a couple of valves where stuck a little open prior to getting going, freed up over a few weeks with lube and tapping. Would Carbon build up make this noise?
12. Points/Distributor - have set points gap, not used a dwell meter and distributor looks okay but would these components being worn cause this noise?
13. Compression - all within 10% of around 130psi.
Really not sure if we need to lift motor, strip and rebuild the bottom end or could it simply be a valve stuck, faulty lifter, push rod and a simple fix?
Appreciate any feedback that may help get to the issue.
Not sure this link will work https://photos.app.goo.gl/HCcJZjQCXV31STPt7
170/3.7l Hope for help from someone that really knows these old girls and has rebuilt over many years! Bertram 25 with twin 3.7l Mercruisers understand the limits of these engines and how some do not like them but also that those who have worked with them claim they are farily robust and great HP when looked after - Have researched a bit on these engines and know the common faults of magneto, coolant leak on shaft and coolant leak on header tank back to piston but need to find out what that terrible rattle/knock noise is once warmed up and RPM over 2000 and wont rev over 3000rpm. The other motor goes fine and revs to 4000rpm and possibly beyond.
1. Oil - changed, seems okay, not milky but maybe not normal black a little grey maybe. Oil pressure - does fluctuate but seems okay when at revs, similar to other motor and never on red line but gets close when at idle.
2. Fuel - fresh and tried 91 to 98 with no noticeable change, also tried additive but not Seafoam
3. Timing - set at 4 before TDC and seems to advance to approx 32 at rev.
4. Carby - has a kit put through it and seems to spray ok.
5. Charging - this is coming in high at revs about 16 volts but not sure if an issue as the other motor not effected, maybe it is an issue?
6. Gimbal bearing - have checked and seems okay and no change to noise with hard turn to port or starboard
7. Rockers - replaced one hydraulic lifter, maybe there are others failed would that cause the rattle/knock?
8. Push rods - have not checked all for correct gap, maybe they are a little tight and no gap?
9. Exhaust - refurbed and cleaned including drive, flappers new.
10. Bottom end - could be the issue but no noise when at low revs so could it be the bottom end bearings?
11. Carbon build up - think this motor was sitting for a long time and prior to that only used at low revs, a couple of valves where stuck a little open prior to getting going, freed up over a few weeks with lube and tapping. Would Carbon build up make this noise?
12. Points/Distributor - have set points gap, not used a dwell meter and distributor looks okay but would these components being worn cause this noise?
13. Compression - all within 10% of around 130psi.
Really not sure if we need to lift motor, strip and rebuild the bottom end or could it simply be a valve stuck, faulty lifter, push rod and a simple fix?
Appreciate any feedback that may help get to the issue.
Not sure this link will work https://photos.app.goo.gl/HCcJZjQCXV31STPt7
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