rodbolt or other Yammy expert

[billsbets]

Posted a question about a cylinder with low compression-(from a dealer '92 200 TxrQ)
(all107-112 stone cold except center right was 75)after running for 5 min low cylinder was up to 90-could this be head gasket or rings or something more sinister?--I have the feeling they pulled the head and have an answer for me-what should I be listening for in their explanation??

In addition-pulled the lower unit plug and though the oil was pure(no water or milk at all)-it was very dark-almost black and there was a black film on the magnetic plug. Ive only dealt with johnsons and merc's untill know and never really saw this.
Dealer claimed it was a graphite wear covering on gears and was normal-should I believe this guy??
thanx for the advice in advance-you guys are a huge help to us

 

[rodbolt]

Re: rodbolt or other Yammy expert

what problem of the phrase "run away!" are we having an issue with?
two strokes are vastly different in cylinder sealing than 4 strokes.
while the numbers may vary slightly with a temp change they should all rise or fall with the same temp change.
cyl sealing on a two stroke relys on a piston to wall clearence less than .002". the average human hair is .003".
if the head gasket has been leaking combustion pressure out then water has been leaking in.
cant have one without the other on that 2.6L block design.
I would never ever purchase a motor with known low compression unless I already knew a rebuild was in order before placing the motor in service.
reringing a two stroke without boring oversize is a patchem up job at best and incredibly frustrating at worst.
goes back to that piston skirt to cyl wall clearence thingy.
honing removes metal.
its a form of boring just more precise and allows a proper hone pattern to inprove cyl sealing as well as allowing a place for lubricant to hide to aid in preventing metal to metal contact.
so tke the average used bore.
its already got .001-.002" egg and taper and wear.
hone it another .002 to clean up the walls and reestablish the hone pattern and you now have a bore thats at least .001 past the service limits.
use a flexible stone or a dingle ball and all you do is hone in the egg and taper already existant in the bore.
use a rigid hone and you can hone it back to round and straight but we are back to out of service spec.
trust me I have seen it over and over in the past 30 years.
will you get away with it?
maybe for bit.
the biggest problem with a two stroke is if combustion gasses get past the rings it contaminates the incoming air/fuel charge.
not only will this cut the lubrication thats mixed with the incoming charge it will lean it out and cause piston detonation.
this a/f mix is being drawn in as the piston moves up, if as the piston moves up and the combustion gasses are blown behind the piston the incoming charge is lessened.
lean.
on the 2.6L yamaha you compare compression bank to bank.
1-2,3-4 and 5-6.
unlike mercs and jonny rudes.
so until I saw the numbers within 10 psi cold or hot, I would runaway.
toss in a saltwater use motor and I would run a bit faster.

remember thats 10 PSI bank to bank.
some, actually most yamaha V motors actually have a different compression on cylinder banks from top to bottom.
some inline 3 cylinders wil actually vary 15 PSI when new.
those we have to do a leakdown test rather than a comparison test using compression.
just my opinion based on 30 years of listening to customers cry that it ran fine on the trailer or only needs an adjustment.

 

[billsbets]

Re: rodbolt or other Yammy expert

thank you much
info is greatly appreciated!

 

[rodbolt]

Re: rodbolt or other Yammy expert

I forgot about the black oil.
years ago on the left hand gearcases,identified by the L stamped on the aft end of the case or the pad under the trim tab, a molybedenumdisulphide paste was used during manufacturing setup and turned the oil black.
there is a tech bulliten from the late 90's someplace.
the moly paste was not used on standard right hand rotation cases.
dark oil by itself may not be an issue but I have never ever heard of graphite being used nor reccomended by Yamaha.
try to go by smell.
if it smells burned the oil has been overheated and is bad, doesnt mean anything is damaged just means the maint schedule the motor saw wasnt what was needed.

 

[Dunaruna]

Re: rodbolt or other Yammy expert

Great post rodbolt, got some very nice information!