This engine is doing my head in

[nightdive]

I have a 1984 Yamaha 140hp v4 2 stroke with a dead cylinder.

I have resurfaced the head, installed a new head gasket, a new water manifold gasket, new t-stat gaskets, and new exhaust manifold gaskets.

The compression is 10bar or 145psi in all 4 cylinders and I am still pulling water into #2 (port side top) cylinder.

It will run for 5 seconds or so when first started, then you can hear #2 drop out and the plug will be coated in milky white emulsion.

I cant sort out where it is coming from. With that compression she block is not cracked right? (tell me its not cracked) I looked hard when I had the head off and everything looked sweet.

Any ideas where to go next??

Thanks
Forest

[robert graham]

Maybe very fine and almost impossible to see pin-hole leak in one of the castings. The cylinder has a steel sleeve, so that should eliminate that for a pin hole leak, but maybe in the cylinder head or back inside one of the ports. If you can find it and it's in an accessible location, it Might can be welded up. Keep looking and good luck!

[nightdive]

How would i go about finding something like that? Is there another way besides just looking, because I cant see anything like that. And ive looked

[99yam40]

Hopefully Rodbolt17 will come on line and help you if he can.

[rodbolt17]

you can try a leakdown test.
however my bet is a pin hole corroded in the V part of the block,a pin hole in the ex divider or a pin hole in the head.

[nightdive]

Thanks for the info. This will be the first engine i have taken to a mechanic, she is going in on Thursday. Hopefully they can find the problen.

[200090tlry]

I know on mercurys and stuff exhaust port gaskets

Near water jacket areas can be problem, have you looked at those. There are only few spots water could get in.

[nightdive]

So there is a pinhole in the exhaust manifold that is pointing directly into the exhaust port of the #2 cylinder, which is why there was no water in #3. $100 to weld her up and we are back in the water early next week.

Thanks to everyone for the help

FS