compression varries

[chrisdoc0608]

this is long, but try to stay with me. i have just got finished fixing a small leaking head gasket. when i bought the ski a month or two ago the compression was at 175. then i almost fried the motor, so i replaced the head gasket and the compression was at 170. then i went and found out my battery was weak, so i retested and it was at almost 180. the other day went to remove the head to reseal it bc there was a slight leak in the front cylinder. so i reinstalled the head gasket and put copper sealant on both sides. when i was retorquing the bolts, two stripped the threads (apparently cause of leak) so i tapped them out and put helicoils in. i kept the gasket on the head (didnt peal it off) and just put a little light coat of sealant around the dome area on the gasket to make 100% sure it sealed ( in addition to the existing gasket making material left on) so i torqued the bolts and let it sit. so today i went out and ran the ski on the hose to make sure that the pissers werent pulsing... it was good, so i cut it off and double checked that the bolts were torqued to spec just to make sure the helicoils were good. they were fine. i then went to test compression and the results came back as 167 in each. so i put the battery on the trickle charger and sure enough it was weak, i let it get a full charge then tried again. the results were 170 in each, with no leak. could this be caused because of the sealant on both sides making the gasket a little thicker? if so i really dont care about 5 psi, as long as it will be ok.
let me know what you guys think,
Thanks

 

[keefer]

Go buy a stock Yamaha head gasket and replace that sealed, resealed, and resealed again gasket. If the head and cyl deck are flat the new Yami head gasket will last for years. If you keep getting water leaks into the cyls. you will end up seizing and it will cost you a top end rebuild at least. Spend the $20 for the good gasket, stop wrenching and go riding with no worries.

 

[chrisdoc0608]

the sealed, resealed and resealde again gasket was brand new a month ago. i sealed it back to back with in an hour... no riding on it or anything. the thing doesnt leak down anymore either or emit smoke out of the pisser. so do you really think that i should tear it down AGAIN and replace it, then possibly have it not seal as well or something strange like stripping another head bolt? is that even worth the time and risk if it is sealed right now? why would the compression drop to 170? could the sealant make that 5psi difference?

 

[RideItLikeYouStoleIt]

If you dont have a good compression tester then they can vary a little bit as well too. You will get different compression with a hot and cold engine too. You should put in an oem head gasket without any sealer. If that sealer isnt totally flat and especially doing it 2 times over a head gasket its not going to be flat then it will probably heat up and leak ruing a day at teh lake. It will take a total of 15-20 minutes to change that head gasket. Also lets say that you added 1mm to the thickness of the gasket.

(Pi)X(r^2)X(1mm) is what you added to your dome size. If you are at an 81mm bore then you added about 5.15cm^3 to your dome size. SO even if you added a small amount it would make a dramatic difference.

 

[chrisdoc0608]

ok so what if i do replace the head gasket AGAIN and then it doesnt seal well just like it did the first time. there is a chance that it will, the two bolts obviously were not torqueing properly, and i took light grit sandpaper and layed a sheet on a peice of glass and moved the head across it in a sweeping motion to even out any high areas. but i will be very upset if i go through the trouble of doing this again and it just ends up back at square one when i had it solved right now.

and its a 650, so its not 81mm

 

[nikad58]

A gasket that is used once or 20 times is used.. used is used. head gaskets should be clean and new !

It is only a 650 and you can get away with allot on those engines but who's going to say to run it when its not right ? no one !!

Just run it if you don't like the answers you get !! I bet it will be fine !

 

[chrisdoc0608]

lol its not that i dont like the answers i am getting, but i just have a fear that if i pull it apart again that one of the helicoils will come out or it will not seal as well again. i guess ill give it a ride this weekend and see if it stays sealed, if it doesnt ill go get another. are there metal gaskets i could get so i can reuse?

 

[Mouthfulloflake]

I say run it.

dont worry about compression numbers, Ive got an engine ridden everytime we ride for the last few yeras, 185psi in one cyl and like 150 in the other.

its still strong, and im not messing with it.

most folks on here would spend LOTS of $$$ to fix that, and im sure it would make like 3 more HP if fixed.

IM lazy.

id rather ride than wrench.

if the head gasket is sealed now, and the boat is running alrigt, I wouldnt mess with it.

 

[SUPERJET-113]

id rather ride than wrench.

Is that why you dont shave your back?:biggrin:

 

[Mouthfulloflake]

im too lazy to do that too.

:biggthumpup:

Is that why you dont shave your back?:biggrin:

 

[SurferSmurf]

just an fyi. your not going to mess up your heli coils. thats the whole point of them is to actually make a stronger thread. i had 2 holes on my ski that needed coiled, so i did them all. do what you want to do but i agree with everyone else. put in a new gasket and while your at it heli coil the rest of the holes. you should never have another problem.

 

[chrisdoc0608]

i rode it at the beach yesterday, gasket is still sealed. but now im getting carbon build up on the tips of my plugs. dont know if it is too rich or its the oil im using. but they normally dont gunk up like they did. i switched the plugs out after i flushed and cleaned it up, but it runs great right now, so im just gonna leave it rich or whatever until i see bad throttle response or something.

 

[chrisdoc0608]

lol i spoke too soon. i went to run it in the lake to give it a good flush, and after abt 30 min of riding i went to go back in and i cut it off for a sec then cut it back on and unfortunately smoke started to come out the pisseers again ... and the bendix i put in last month started to go out :banghead:
so i am gonna go to the yamaha store and get a new gasket i supose. ive got a spare bendix laying around somewhere. but what do i need to do to make sure that i fix this. i have layed the head on a peice of glass and it seems very flat to me.

how do i make sure the area that seals the cylinders is flat also ( i can only check the outside when on the glass)?
im gonna make sure everything is right so i dont have to do this ******** again.

 

[Mouthfulloflake]

smoke comes out of the pisser anytime I run my boats on the trailer, or out of the water.

otherwise, use a 0.001" thick feeler guage and work all around that head on teh glass and see if the feeler fits under.

OR, put some three bond or copper goop on that gasket and go ride.

if your engine overheated badly ( ever in the past) its likely a warped deck or warped head, Id goop and ride before I tore down and spent $$$ fixing a damn water leak.

 

[chrisdoc0608]

i did over heat badly about a month ago. the pissers didnt smoke at all for the tank of gas i ran through it at the beach yesterday, and it didnt smoke when i rode at the lake until after abt 15 minutes. like i said, the gasket that is on there now has copper gasket maker on both sides. if my compression stays constant and doesnt leak down for a min or so would it be safe to say that it is sealed well enough? seeing the smoke comeout of both pissers just worrys me bc even though im abt to build a 701 i dont want to fry the motor i have in there now.

 

[Mouthfulloflake]

was the boat in the water? or on the trailer when it was smoking?

if you overheated it badly enough to warp the head, you are likely NOT looking at a long term engine there.

 

[chrisdoc0608]

it was smoking on the water after i had cut it off, so no water was going throught the cooling system. why would the motor not be a long term engine? when i pull the head again i will take pics, but the cylinders look like they have just been honed, the pistons look brand spanking new, and the compression is good. i will have to replace the crank seals at the end of the season bc they only hold up to 6.5 psi but other than that arent those all factors of a great motor?

 

[Mouthfulloflake]

mine all smoke some when I turn them off in the water, and when initially starting in the water, i wouldnt worry about that at all.

If its running good go with it, dont make problems where there arent any.

If an engine gets bot enough to warp the aluminum peices that make it, its generally a hard time on the bearings and everythng else.

It might last hundreeds of more hours, it might last another 40 seconds.
who knows.

 

[chrisdoc0608]

haha alright thanks for the help man. ill prob just run it, if it blows up then thats just more excuse to hurry up and build a 701.

is it true that the if the piston top remians nice and shiny that you are injesting water into the cylinder?

 

[Mouthfulloflake]

do the plugs look steam cleaned?

you can usually feel it bogging as well.

also, run it in the water.
put it on the trailer.

wait an hour or so, pull the plugs, if they have droplets of water on them, then well, you have some leakage.

Mine was doing that before SFF.
we sealed it up with that copper stuff and its still holding, im sure its borrowed time, but ive been riding it instead of worrying about it.

ive got a spare engine to swap in, but im too lazy to do that in any sort of ralistic time frame.