2000 mercury 50hp rebuilt no compression in cylinder 3 prior to starting up

[thabel]

Bought a 2000 mercury 50 hp 3 cyl rebuilt from ebay. Mounted it on the pontoon and before it was started the mechanic advised that cylinder 1&2 had compression but cylinder 3 was either low or no compression (bottom cyl). Question: Is the no to low compression an issue if the motor has not been run since the rebuild. And does the engine need to have a break in period until the pistons and cylinders can seat with each other to get a true compression? Please respond. Thanks

 

[Ride.race.live]

Rings seat in the motor pretty quickly.. more quickly then most people think.... having no compression even on a new build would show signs of an issue... I am not all that familiar with a boat motor, but work consistently on 2 stroke and 4 stroke atv's and snowmobiles and I have yet to see a motor have 0 compression once assembled.

a break in period is brief, in 4 strokes most break in periods are for the cam / valve train.

2 strokes I would do a couple heat cycles and RE-TQ the major points on the motor.


I would ensure that cyl 3 has proper Cyl to piston wall clearance as well as ring gap. I would also advise to make sure there is no leak somewhere where the compression is blowing out.

 

[Faztbullet]

Contact seller as there is a problem.....all 3 cylinder should have close to the same cranking compression

 

[thabel]

Thanks for the reponse gentlemen. I have no recourse with eBay (past 30 days). If all the tolerances are correct, the pistons are new and the cylinders are all honed correctly, where could the compressed air be escaping to? Where should I look next?

 

[Chinewalker]

Well over 100 psi in each hole and within 10% of each other is a good rule of thumb. Fresh rebuild, make that 5% after a little break in time. Any rebuilder worth his salt will have oiled the bores prior to installing the pistons, so they should not be even remotely dry. They should also have run it prior to attempting to sell it to get it dialed in, check for issues, leaks, etc.

 

[Faztbullet]

If all the tolerances are correct, the pistons are new and the cylinders are all honed correctly, where could the compressed air be escaping to? Where should I look next?

The only way you will know if the tolerances/parts are in spec/new is to tear it down. Sounds like you bought a used powerhead not a rebuilt one...

 

[Ride.race.live]

yup.... in my honest opinion... if this motor WAS rebuilt... I still wouldnt trust it knowing something went wrong in it and hasnt even been started.... I would have it re assembled a-z to ensure nothing else was missed or over looked.


You can always hope while dissembling it that everything IS new and within spec so you dont have to spend much money minus new gaskets at best.....



If the pistons and rings are within spec in cyl 3.... there is only 1 other place the compression can escape and thats between the head and cylinder... so torn gasket or something in that line may be a possibility.

 

[DavidMoore]

Could you test it yourself / have a friend test it and post the results here.

Remove all plugs and ground the wires.
Hold the carbs to wide open throttle position, (I use a squeezed up roll of painter tape carefully jammed in the linkage to hold the carbs open, there may be a more professional way )
Install the gauge.
Crank the engine until the gauge stops moving, note the reading
Release the pressure in the gauge and repeat on to the 2 cylinders.
After all 3 cylinders are done, remove what ever you used to hold the carbs open and re-install the plugs and wires

As others have said, more that 100 PSI and all 3 readings close to each other. Don't worry about the actual values as most gauges are inaccurate, it's the closeness of the values to each other that's important.

 

[thabel]

Thanks for all of your suggestions gentlemen. Currently the engine has been taken apart and my mechanic advised that the pistons and rings are new, undamaged and the cylinders are bored and honed with proper tolerances. My mechanic is checking for any cracks In the head. Do you have any other suggestions of where the lost compression might be going? Thanks again

 

[Ride.race.live]

sometimes easiest way is spray it down with soapy water and get it cranking see if you see any bublbes build up around that cyl matting surface and or crack.. esp around studs ect

 

[thabel]

Further update. My mechanic reassembled the block and did an air test . Cyl 3 leaked air through the cylinder sleeve and out the water jacket. Thinking it's time to get a used block. Anyone know where I can get a used mercury block for a 50 hp 3 cyl 2 stroke (50ELPTO) for a reasonable price?