Both pistons simulataneously at TDC on Yamaha 2000 15 hp 4-stroke???

Joined
May 26, 2012
Messages
18
Hi all,

I have a 2000 Yamaha F15 4-stroke that I have been knuckle deep in for quite sometime. The problems first began when my head gasket blew. So I replaced it and when I went to go fire it back up...nothing. It would turn over, but it wouldn't actually fire, not even a pop. I had fuel and spark, but couldn't even get a backfire. So I tried adjusting the timing on the cam wheel and still got nothing. Finally, I decided to just pull the cam off again and remount the head gasket again from scratch (just to make sure that I didn't mess up the headbolt torquing sequence or something). When I pulled the cam and went to check my timing marks against my #1 cylinder's TDC, I noticed that both pistons are now moving simulatenously (both were reaching TDC at the same time). What could have caused this? And the ultimate question; Is this really worth fixing on a motor this old? Thanks in advance!!
 

JB

Honorary Moderator Emeritus
Joined
Mar 25, 2001
Messages
45,907
Re: Both pistons simulataneously at TDC on Yamaha 2000 15 hp 4-stroke???

That is normal for a 2 cylinder 4 stroke, Johnny (Except for rare engines like the Honda 250cc and 305cc 180 degree bikes of the 60s). One is at the top of the exhaust stroke and the other is at the top of the compression stroke. They fire 360 degrees apart.

I suspect cam timing is at the root of your trouble, but there is not enough info to tell for sure.

Are you using a Yamaha factory Service Manual?
 

99yam40

Fleet Admiral
Joined
Sep 7, 2008
Messages
8,851
Re: Both pistons simulataneously at TDC on Yamaha 2000 15 hp 4-stroke???

Blown head gasket usually comes from overheated motor, and you may have damages more that just gasket.

Did you check compression or do a leak down test of the motor before and after your work to see if you have cylinders sealing?
 
Joined
May 26, 2012
Messages
18
Re: Both pistons simulataneously at TDC on Yamaha 2000 15 hp 4-stroke???

Thanks for the replies!

JB: Thanks for the info, I had no idea that they cycled that way. I'm using a PDF manual that I found online but I'm seeing now that I definitely need to get a hold of the real deal. All of the timing marks appear to be accurate, is there something that I'm missing when setting the timing on this motor?

99yam40: Before i removed the gasket again, I did a compression test (I don't have access to any leak down equipment). I got 80 on my #1 cylinder and 75 on #2.
 

99yam40

Fleet Admiral
Joined
Sep 7, 2008
Messages
8,851
Re: Both pistons simulataneously at TDC on Yamaha 2000 15 hp 4-stroke???

I have no idea what that motor normally runs in compression,
but I thought the 4 strokes were a lot higher compression than the 2 strokes and 80 is even low on a 2
 

JB

Honorary Moderator Emeritus
Joined
Mar 25, 2001
Messages
45,907
Re: Both pistons simulataneously at TDC on Yamaha 2000 15 hp 4-stroke???

75-80psi seems low, but the engine is cold and rope start. I would proceed as though it is okay for compression.
 
Joined
May 26, 2012
Messages
18
Re: Both pistons simulataneously at TDC on Yamaha 2000 15 hp 4-stroke???

Thanks again for the input, guys. I'm hoping that not replacing the headbolts in the right order was the cause of my compression lost. I'll keep you posted.
 
Top