Low compression on recently rebuilt 1990 90hp

[rodbolt17]

i may have seen one or two blocks.

[Capt. Barry]

Lot of corrosion in the water jacket around the cylinder. The rebuilder did not have the block dunked (cleaned) before assembly?

[rodbolt17]

not much to dunk them in.
most likly typical salt water.

[SaltDrum]

The motors ran fine while on the previous boat, now that i have them running on another boat they idle good and rev up freely but under load one was breaking down allot.

Per Yamaha FSM:
New engines require a period of break-in to allow the mating surfaces of moving parts to mate evenly. Correct break-in, as described in your Owner’s Manual’s “Operation” chapter, will help your outboard provide proper performance and longer engine life. Yamaha’s two-stroke outboards have a 10-hour break-in. All carbureted (non-HPDI) engines require a 50:1 gas to oil ratio during the ten hour break-in period. The break-in steps are as follows:


First 10 minutes: run the engine at the lowest possible speed (idle). A fast in-gear idle is best

Next 50 minutes: engine speed should not exceed half throttle or approximately 3000 rpm. Vary engine speed occasionally. If you have a boat that is easy to plane, put the boat on plane (but avoid full-throttle operation), and then immediately reduce the throttle to 3000 rpm or less. Do not go wide open throttle.

2nd hour: go onto plane and reduce the engine speed to 4000 rpm or approximately three-quarter throttle. Vary engine speed occasionally.

Next 8 hours: Take the engine to wide-open throttle but for no longer than five minutes at a time. Let the engine cool between full throttle runs by reducing the rpm to idle or cruising (half throttle).

After the first 10 hours, operate the engine normally.


Just how freely were you reving it? Upon a fresh rebuild one might give the rings ample time to seat at low to mid rpm's under minimal load so as not to cause a ring to potentially grab a transfer port as Rodbolt suggested... I'd say a ring hung and broke from free revving initially.
The trash in the carb bowls... Isolate the cause of the trash,
I.E. deteriorated fuel line/bulb, fuel/water seperator, and or tank.

[Double Trouble]

SaltDrum thanks for the info on how to run in the motor but thats the exact same procedure I used to run the motors in the last time round, One would expect to run the motors in correctly after spending 7k rebuilding them.

The motors were not excessively over revved in the nuetral position and the motors had been run in , Doubled the oil doseage for the first 20hrs use.

I agree with what you and rodbolt say about bedding the rings in correctly or that the rings may have nicked the port, Highly Possible This may have also been due to poor machining of the bore, Bore being out of round, some of the other bores on the port engine still had hone marks in the bore from where the rings wernt even touching causing blow bye which had the low compression readings.

My current engine builder put the cause down to getting to hot at some stage during the running in proedure which he noticed by looking at the skirts on the pistons.

Capt Barry, My mechanic said that the water jackets looked good considering they have been in salt water use for the last 22yrs so you would expext to see some corrosion, None of the water passages were blocked at all, upon inspection of the thermostats, they looked to be in average condition, tested fine in a cup of boiling hot water.
I have since bought all new replacement parts for the cooling system, including getting the blocks dunked just to ensure that they are fully clean prior to re-assembly.
Next job is to test the overheat sensors to ensure they are working properly.


I will update the thread once the motors have been rebuilt once again!!

[rodbolt17]

I would like to see pictures of the skirts.
the crowns show erosion thats typically due to detonation.

[Double Trouble]

Rodbolt17 here is some pics of the pistons, some of the pics maybe of the same piston just the opposite side. Note only 1 piston broke the rings.

[Double Trouble]

here is a few more pics

[Double Trouble]

This pic is the starboard engine. compression was 30psi, 125 and 120

The single pic shots are of the bores are from the port engine which had poor compression, 115psi, 70psi and 90psi.

[rodbolt17]

I am not seeing overheat damage.
lots of mechanical damage.
look for brokek wristpin bearings,broken rings or missing ring locating pins.
also look to make sure all the carb shutter screws and reed screws are still retained.
the one piston had stuck rings for a long time judging by the carbon blowby.

[Bilge Rat]

When rebuilding any motor we would always check and beburr the pistons,grooves, and check rings for burrs.

Ring tension and proper end gap need checking before final assembly.

Proper crosshatch in the bores.

Any of these done wrong will cause ring issues during break in.

A light load on motor helps make the parts happy as well.

What does the ports edges look like?

Something was not right somewhere.

[Double Trouble]

@Rodbolt17 The wrist pins and bearings were all good and the circlips are in intact along with the ring locating pins on the piston, The damage on the 1 piston was from the broken ring. thats all I could find damaged other than the piston.
Reed valves still look brand new so it didnt suck nothing thru the carbies.

Ive just got the blocks back from the machine shop now, They look nice and new again,They said the bores machined up good considering the score marks in the bore was quite deep on the starboard motor.
Water jackets look like new again inside.

@Bilge Rat the engine builder will check all of the above mentioned items during the rebuild process next week.

Tomoro"s job is to remove the port motor partially so i can change over the complete hydraulic trim assembly, I just love pulling these $%$%$ motors apart
Next will be fit the powerheads back up and start the run in process.
Thx for the feedback