70's Yachtwin Compression 4hp 2cyl

[rather_be_boatin]

Hello, I'm new to this forum. Was wondering if I could get some advice on a recently purchased 70's vintage Evinrude Yachtwin. It knocks at idle and runs well only sometimes at WOT. Did a compression check and upper cyl is 85 lbs, lower is 65 lbs and it appears as the lower plug is not getting normal carbon build up. I'm using a 50/1 mix. Spark is strong on both and fuel delivery is okay too. It is hard to start as well. Any suggestions would be appreciated.

 

[JB]

Re: 70's Yachtwin Compression 4hp 2cyl

Howdy RBB.<br /><br />Welcome to iboats. <br /><br />Try decarbonizing it (see Engine FAQs) and check compression again.

 

[BoatBuoy]

Re: 70's Yachtwin Compression 4hp 2cyl

If the lower plug is clean and that's where the compression has dropped off, you may have a blown head gasket which would allow water intrusion. <br /><br />If decarbing doesn't improve it, you may want to pull the head and havealook(that is one word isn't it).

 

[Paul Moir]

Re: 70's Yachtwin Compression 4hp 2cyl

Spark is healthy if you can draw sparks 3/8" long. What do you mean knocks?

 

[rather_be_boatin]

Re: 70's Yachtwin Compression 4hp 2cyl

Thanks JB & others. I'll try de-carb. Do they sell those products (Sea-Foam; OMC stuff) at West Marine? By knock I mean runs fine high rpm but as idles down a definate "lifter knock" (for lack of better words, I know it has no lifters) becomes apparent. I suspect its because that lower cyl is cutting out and no ignition on top like it wants.

 

[Paul Moir]

Re: 70's Yachtwin Compression 4hp 2cyl

Is the bottom plug very clean?

 

[rather_be_boatin]

Re: 70's Yachtwin Compression 4hp 2cyl

Yes, very, no regular chocolate color like it should have.

 

[Paul Moir]

Re: 70's Yachtwin Compression 4hp 2cyl

Sounds like it could be a blown head gasket then. New one is less than $3. Surface the head before you reinstall.<br /><br />While the head is off, verify that the parallel cooling passage is open and free:<br /> http://shareaproject.com/pages/imageDetail,p,182,i,2538,00.html <br /><br />Use a little more oil than the factory recommends for that particular outboard. See the oil mix FAQ in the FAQ forum for details.

 

[rather_be_boatin]

Re: 70's Yachtwin Compression 4hp 2cyl

Is it as easy as it looks for this thing? It appears to be just 6 small bolts. Even though the motor is old, is it readily availible at rude dealer?

 

[rather_be_boatin]

Re: 70's Yachtwin Compression 4hp 2cyl

Another thing possibly worth noting is the head gasket appears recently replaced and when she runs there is no white puffy smoke which blown head gaskets often exhibit. Thanks for your help.

 

[Paul Moir]

Re: 70's Yachtwin Compression 4hp 2cyl

Yes, no problem at all. Actually, this wouldn't be much of an issue if you had a 1957 engine. As a matter of fact, you have two choices in gaskets - an aftermarket one sold by Iboats and the dealer one. I suggest the dealer one since the aftermarket one seems a little thicker and lowers compression a bit.<br /><br />6 1/4" bolts. That's all there is to it.<br /><br />I have my doubts about it being the head gasket too, usually it'll have worse compression than that. The other reason is to examine the cylinder walls, etc. It's cheap, easy to do, and will explain the 65psi.<br /><br />Since you can't torque the bottom head bolts, you'll just have to make your best guess with a crecent wrench. You can torque them in place with a little math and a 7/16 box end wrench. I'll try to get my 4hp page updated to show you what to do.

 

[rather_be_boatin]

Re: 70's Yachtwin Compression 4hp 2cyl

Thanks.

 

[rather_be_boatin]

Re: 70's Yachtwin Compression 4hp 2cyl

Removed the head. Sure enough, blown head gasket. I wonder why it had even 65#'s of compression?? (and it held at 65, did not decay?) Will update when back together and test ran. Thanks for the help.

 

[rather_be_boatin]

Re: 70's Yachtwin Compression 4hp 2cyl

Seems to run okay now, going to go ahead and do a water pump for some cheap insurance against overheating and possible future blown head gasket.

 

[Scaaty]

Re: 70's Yachtwin Compression 4hp 2cyl

My 84 4HP has 120psi in both FYI