engine rebuild low compression

[moey]

Hey guys i recently rebuilt my 1975 evinrude 70hp
It was my brother and had a ceased top piston
Basically we changed all the bearings
linished the crankshaft
Honed the cylinders
Changed all 3 pistons completely
All new gaskets
All electrics

Once the powerhead was put together my brother done a compression test which gave
Top to bottom
#1 115
#2 120
#3 117

The engine was never water tested as the boats is a project so it has just been on the muffs and would turn on very easily and rev very nicely
Now all of a sudden has been hard to turn on and today just did not want to turn on has spark and fuel

I did a compression test on 2 different gauges as my brother lost his compression gauge we did the first test on
It is now #1 87
#2 92
#3 90
I just dont get it and its very frustrating i havnt even taken it on the water and its giving low compression
Suggestion would be much apreciated
Thanks guys

 

[F_R]

No Title

Does "all new gaskets" include the exhaust cover? Did you check the inner exhaust cover for warpage? Is there any sign of water getting into the cylinders? What happened to it that caused the need for such drastic rebuilding in the first place? The whole story strongly suggests that it was trashed by a warped inner exhaust cover---a common problem.

 

[oldboat1]

Pattern between cylinders is similar -- so in addition, think brother needs to find the original compression tester, and try it again. Test spark with a tester.

I get antsy when I hear about revving on muffs.

 

[emdsapmgr]

The 115-120 compression would be "normal" for that engine. Agree, re-check with original compression tester.

 

[moey]

Hey guys thanks for the replies
I get ansy with reving on muff but when i say i was reving it was literali wot for a second thats all
Never prolonged high revs

To be onest i changed all gaskets and bearing brand new
Except the exhaust housing
Could the exhaust housing cause low compression?
The original problem was a ceased top piston which was left for couple years but everything was cleaned and honed liked brand new

Just a question with causes of low compression
I know bad cylinder walls can cause low compression
Piston rings
Head gasket
What else can cause low compression if i was to rule those things out above

Exhaust housing?
Carbies?
Reed valves?
Crankcase?
Carbies plate that sits ontop of the reed valve?

Im just trying to eliminate all posibilities besides of the cause of low compression besides the usual piston rings cylinder walls head gasket

Thanks

 

[racerone]

Bad testers will confuse folks.------On my tester I would like to see 140 / 150 PSI on those motors.--------And the reed valves have nothing to do with compression in the cylinder.

 

[moey]

I know compression tester arnt all 100%
But a ebay compression tester was giving me around
#1 85-90
#2 90
#3 90
Stanley branded compression tester
#1 92
#2 95
#95
Which for a rebuild is not good at all
I have a video of the compression test being done with 120 even on all cylinders which turns out was also a stanley branded compression tester

But if you guys are telling me exhaust housing could affect compression reading maybe thats my problem as i never touched the exhaust casing at all

 

[moey]

This motor was a project to be done and i have learned alot as im mechanically minded

I could of changed 1 piston would of been all good but thats not good enough for me so i got a full rebuild kit so i could have a reliable engine for years to come but that isnt the case at the moment

 

[Faztbullet]

Did you "mike" the cylinders to see if they where within wear limit or out of round???New parts in worn holes=low compression

 

[racerone]

In your motor the compression starts to build when the piston comes up and closes off the exhaust ports,---So compression is determined by condition of -----rings / piston grooves / condition of cylinder walls / piston side clearance / head gasket.----And how fast it is cranking.----Did you make sure that the cylinder head was flat.----Every head that I have taken off older motors has been warped !

 

[jimmyebay]

You holding throttle wide open during compression test?!

 

[racerone]

Throttle position does not matter on this 2 stroke engine !

 

[moey]

The head was in was good condition
The things thats got me is that it had 120 psi all round

Is there something i can do without ripping this thing apart again

 

[moey]

Would it help to replace all the exhaust housing baffels?
Also the only thing i touched after i put the head on was to put in a thermostat so that included obviusly taking off the bolts only on the thermostat housing ovcourse but could that somehow effect the head gasket

 

[racerone]

No it does not !----Did you check the head for being flat ??---------Or did you just look and say --" I think it is flat " in this case.-----Did you retork the head bolts after the motor had run ??

 

[moey]

No i havnt retorqued the head yet maybe ill try that this afternoon would you know how many lbs i should retorque at?
As for the head being flat i had my mechanic clean the surface and he was happy with the straightness of it

 

[moey]

Hey guys just a update
I i loosend each bolt in sequence slightly then retorqued to spec
im not sure if this helped the psi but
Cylinder #1 95
#2 110
#3 110
Are these compression results resonable
Someone was telling me once i run in the motor the pistons will seat themselves better

Shouldnt these readings atleast get my boat to start
I tried to start after i retorqued
It hestitates like a cough but wont start
a week ago before i compressioned tested it wouldnt start so i took carbies off cleaned put them back on and i started first kick
I put the airbox and silencer box on the next day and wouldnt start
Im going to clean carbues again and see what happens

Also with my carbies i noticed the float needles didnt have a clip attached neither did the needles have the groove for the clip to hang off

 

[emdsapmgr]

Check to see if the flywheel key is broken. If the key breaks, the engine will immediately get out of time and won't start. A simple replacement item. Just make sure you lap the crank and flywheel mating surfaces, clean off the lapping compound and retorque to factory spec.

 

[moey]

Hey guys
Late update but ive been busy with work
Unfortunately i pulled the head off today to see if i could find the problem and i think ive found why the low compression
All three cylinders on the bottom side of the piston have scoring on them
But what has me thinking is how or what could cause this i havnt even taken out the boat to the water yet just been on muffs

My question is what can cause scoring
Could the crank have anything to do with this or just from piston rings or piston?
Thanks

 

[racerone]

????---------What honing tool / method did you use.----Pictures of the honing equipment ?--------What oil mixture are you running ?