1981 Yam 75AET - low compression

[JimD]

Hi - done some searching on here and it looks like a very useful forum!
I have a fletcher GTO that I've owned 5 or 6 years now and had great fun with but wanting something I can go fishing in when it's not glassy sea (rare here!) I've now bought a teal with a nice 4 stroke Yam.

I'm tidying up the fletcher to sell it and am having a few problems with the motor. It has always performed spot on once started but been tricky to start cold and I think there may be a certain amount of varnishing in the carbs perhaps. Having just had the gearbox rebuilt on it I have just started to do a tune up of the motor to ensure it's in good shape for selling and to my horror found what looks like very low compression on all cylinders!

It reads as follows -
1 - 70psi
2 - 60psi
3 - 80psi

I would assume it should be 100psi plus, and that even if not, it's a big variation from one cylinder to the next. :'(

As the boat has not been used much the last couple of years I am wondering if perhaps the rings have stuck in it maybe? I tried adding a little bit of oil to the cylinders and testing again but the results were unchanged.

What sort of things should I be trying before taking on the slightly daunting task of removing/stripping the powerhea?

Many thanks
Jim

 

[JUSTINTIME]

Re: 1981 Yam 75AET - low compression

try doing a de-carb and see if that changes it

 

[JimD]

Re: 1981 Yam 75AET - low compression

Yes that's what I had in mind first off but I'm not sure what products are evil/strong enough to do this effectively.

One problem I have is being on the Isle of Man and having limited retailers for this sort of thing, and mail order is difficult for aerosols/liquids.

 

[JUSTINTIME]

Re: 1981 Yam 75AET - low compression

well u can decarb with water, if u know what u are doing

 

[JimD]

Re: 1981 Yam 75AET - low compression

Yes, someone else mentioned that in the way that water injection keeps things nicely 'steam cleaned' as well as preventing det etc.

Not sure quite how I would go about it on the outboard though. Easy enough on the car as I have an injector tapped into the inlet manifold for the kit.

Someone else suggested removing the exhaust cover on the side of the block so I can hopefully see inside but I'm not sure if that's any use, and I'd have to go and source gaskets etc then too of course.

 

[JUSTINTIME]

Re: 1981 Yam 75AET - low compression

best way
u need a spray bottle and cup of watter
spay the water into the carbs and keep spraying do not stop

 

[JimD]

Re: 1981 Yam 75AET - low compression

Ahh, ok then, cheers.

Have just stripped and thoroughly cleaned the carbs too & rebuilt them. Will give it a run this afternoon with fresh juice instead of the older stuff, and will give this a try.

Thanks

 

[yamamarinetech40]

Re: 1981 Yam 75AET - low compression

Isn't this a low compression engine...60 to 70 psi?

 

[JimD]

Re: 1981 Yam 75AET - low compression

Yes it is low compression - apparently it's the 'Enduro' model. My local Yam man explained that it does indeed run a low 6:1 ratio (lower than turbocharged cars I have!!) so it can run on cruddy quality fuel happily around the world.

Even so, he said the factory figure for compression per cylinder should be 123.7psi. I don't doubt anything he tells me (unlike some of his predecessors over here!) as he is an extremely helpful and knowledgeable chap and can normally provide information to the nth degree.

I tried spraying lots of water into the carbs with the engine running and some muck came out but not that much, have replaced a grotty looking pilot air screw on one carb (and set them all to the correct settings), bought new plugs, and run fresh petrol through but the thing still will not fire from cold without pouring some juice into the bores. Once it's running and warm it will idle (not specially smoothly), and will easily start from the key but not at all from cold.

I'm thinking my only option now is to remove the powerhead and check/hone the bores & if necessary free off/replace the rings. Another suggestion that has been made to me is that the crankcase seals may have gone, as the engine has not really been used over the last 2 or 3 years.

If I have to do this am I likely to encounter loads of snappage of the bolts that hold the powerhead in place or are they stainless? I'd like to get a PDF or something of a workshop manual if I can but I'm told Yam manuals are only stored electronically from I believe 84 onwards so do I have to rely on the Seloc & Clymer manuals that I have (which are for 84 onwards anyway)?

Many thanks for any advice!

 

[Ray Neudecker]

Re: 1981 Yam 75AET - low compression

I would suspect a leakdown test would have you pulling the powerhead down and likely replacing the rings. First try some ring free in the fuel and tilting the motor and let some penetrting oil run down into and soak in the cylinder though. If the rings are sticking it might free them. The motor is old and I have no clue what problems you might run into tearing down.
Model changes for those came in the late 80's in the states and most weren't that drastic. Your manual should be close enough.

 

[JimD]

Re: 1981 Yam 75AET - low compression

Thanks Ray

If I can find some product similar to ring free then I will certainly try it before pulling the powerhead apart (I already have a stripped Nissan engine in my garage and a Subaru that needs a rebuild so any way of avoiding this job would be nice!).

Problem is getting products like this as I live on a small island with limited retailers for this sort of thing and the post office don't like sending liquids.....

In reference to tilting the engine and leaving a substance in the bores to soak in, how can you get the upper sides of the pistons/rings to be helped by it? Obviously the engine only tilts part way up and short of filling the bores right up it'll only be the lower part that's in contact (depending on how much stuff you are meant to put in of course).

Thanks!
Jim