Compression test?OOPS!

I was doing a comp test on my Yamaha 50HP 4 stroke-
Yamaha F50T
1994 or 1995



And---- the gauge?s valve stem got sucked into number one cylinder and then started to blow back out in little pieces!

I bought a new comp gauge and yup-- Cyl one reads way low?

Cyl1: 25 PSI
Cyl2: 175
Cyl3: 180
Cyl4: 175

I have never opened up an engine before. I have the pdf version of Lit18616FT50 which I purchased on the net last week. I have lots of time.

Please--- are there any things ?not in the book? which I can me made aware of?

Thanks a lot! And--- Wish me Luck!

Rick in Anchorage.

Re: Compression test?OOPS!

Hi there ,sounds like scored cylinder walls ,I am new to Yamaha engines myself so cant shed any light on strip down . The best thing to do is strip system by system ie fuel system,ignition system etc .Take lots of close up photos of connections links etc .Keep your work area clear and use fridge type plastic boxes for keeping small items in and clean as you go.Best of luck .

Jim B

PS I would advise only stripping the cyl head off as this will let you see what is causing problem ,you might get lucky and find a piece of your tester holding a valve open .good luck

Re: Compression test?OOPS!

Jim-- thanks for the reply.

Yes--- I am going by the book-- system by system. All parts are in plastic jars.

Cyl 1 is full of ground up valve stem. I am about to remove the piston today.

Later--- Rick

Re: Compression test?OOPS!

I am following the book faithfully, and have the crankcase cover almost off >>>>




but--- I can NOT able to get the bottom 2 crank case bolts out! >>>>





Plus--- the shifter rod also prevents the crankcase cover from being tilted up and removed >>>>>




Now, the best way, I think is to remove the entire power unit, but am having trouble here too. The book says there are just 4 bolts tying power unit down. Is it wrong? I see no 4 bolts.

Is it possible that I have to remove these 12 bolts? >>>>



A close up >>>



If I remove these 12 bolts will the bottom 2/3s or motor drop?

Remember--- I am just trying to get to remove the block, so I can get one piston out.

Thanks a lot!


Rick

Re: Compression test?OOPS!

A guess the simple question here is --- To remove a piston, must I remove the power head?

Re: Compression test?OOPS!

Please, do yourself a huge favor and buy a shop manual.

Re: Compression test?OOPS!

I purchased this last week>>>

and >>>


Are you referring to some other manual?

Thanks a lot >>> Rick

Re: Compression test?OOPS!

Not sure about your Yamaha model but that manual should guide you through the disassembly if it is like my Suzuki manual. Mine is really good with lots of pictures and step by step. I love it when it points out all of the special shop tools that I don't have...

A scored cylinder stinks. I had a bad one on my 96 Suzuki DT115. It was going to cost me around $700 to repair it if I did all of the work. I ended up buying a pair of DT140 engines, one blown and another working. I made a good DT140 out of three.

Good luck to you.

Re: Compression test?OOPS!

10-4 on the shop manual. You have already created way more work for yourself than necessary. Yes -- the engine needs to come off the lower to remove the crankcase which then gives you access to the connecting rod and main bearing caps.

Re: Compression test?OOPS!--- Better!

Re: Compression test?OOPS!--- Better!

I agree?this has been an expensive compression test!

I took the head to my Audi mechanic! He filled intake and exhaust with alcohol and it all poured thru the valves?not good. He then refilled and tapped the top of valve stems to allow the fluid to flush thru and voila? a 3rd fill of alcohol and -- the valves seated!

He then recommended a very mild machining of the heads to remove the very slight corrosion. But---- his machinists couldn?t handle it?they were set up for bigger heads.

I then went to the Yamaha dealer to get parts and showed him the head. He recommended a manual process. He recommended to buy a piece of FLAT thick glass, tape 150 grit sand paper and ?machine? the head. Blow out the residue.

Am gonna do this today.

All local experts believe that the pistons/ cylinders sound be OK?the particles would have been stopped by the rings. Remember--- this engine was not running with this internal debris, it was just cranking during the comp test.

I will spray WD40 in the cylinders and thoroughly wipe out the residue?and call it good.

With any luck, I will have this put together today!

Thanks to all of you for your kind support----
Especially?

Jim B-- you were spot on--- it was the head-- (I hope).

Marion M and Silvertip- thanks for verifying that I have the correct shop manual. The local Yamaha rep said so too. I paid about 8$ the electronics document. I first tried Yamaha Web site-- was out of print.

Later-- Rick

Re: Compression test?OOPS!

powerhead removal is a MUST.
you will have to replace all the gaskets and soft parts between the powerhead and adapther plate.
main bearing bolt replacement is a must.
check your manual and see if the bolts can be reused or tossed.
I belive from memory they are one time use bolts.
DO NOT shave more than .010" from the head or the timimg belt may be to loose.
remove all the bolts in your drawing plus the ones fwd and aft.
best I recall one on each side near the tiller and 2 aft under the pan.
pain to get to but have to come off.
if you remove a connecting rod the bolts should NOT be reused.

Re: Compression test?OOPS!

Thanks Rodbolt---

I decided to leave the piston alone and gamble that the heads were the only problem.

I put all back together and got 160PSI on all 4 cylinders.

I plan to change the oil a couple of times to catch all fragments I missed and--- go on with life.

Later--- Rick

Re: Compression test?OOPS!- Compression is good now

Re: Compression test?OOPS!- Compression is good now

OK---

I have the engine put back together and now I am where I began--- a very poorly running engine which (prompted the compression test in the first place).

I am looking for a rebuild kit for the carb system, which looks like >>>>


If no kit is out there, what parts would I probably need for this engine?
Specs:
Yamaha F50 TLRS, 1995. This is the first year the 4 stroke was built, I believe.

Thanks very much!

Rick