Compression variance - '82 90HP crossflow

interalian

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Compression test shows 110 top 105 bottom STBD, 100 top 95 bottom PORT. Tested cold and warm, no difference. I've not had it on the water yet this year, and it would be the first time in a few years. Thoughts as to what it might be? This is a very low hour motor, maybe 20 per year max, and has never been rebuilt.
 

Bosunsmate

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if it runs ok i wouldnt be botheed about that. It may benefit from a decarb or it may be a ring or two down, you can pull of the bypass gaskets and check them. My mates ran ok with 6 out of the 8 rings gone. Be aware that the cooling on these is the most likely problem you will face. Renew the water deflectors and enlarge the middle hole in the thermostat hosing
 

interalian

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Not familliar with water deflectors, where are they? I just had the thermostat housing off (that's always a fun job...) to put new ones in and I don't relish taking it off again just now. How much larger should the middle holes go? They seemed to be about 1/8" as I recall.

I guess more info may be in order. This boat is a 16' aluminum sport boat (with bow deck, windscreen, padded seats etc), and when new in '83 would easily do 44mph at around 5200 and pop like a champagne cork. Turning a 17" screw. Last time I ran it a few years ago it wouldn't do much more than 4200 and felt sluggish.

I had the leg off for a new water pump impeller and rebuilt the carbs with new kits - complete teardown including the bore plugs and blow-out with compressed air. I've not run it yet but am hoping to have it on the lake in the next week.
 

Bosunsmate

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The deflectors are surrounding the cylinders you see them when the head is off. I enlarge that hole by a bit less than double its current size.
Not sure what rpm or speed should be but you could do a drop test on each cylinder, maybe one isnt sparking
 

interalian

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Right - the hard rubber/plastic bits that go in the block. Never had the heads off this one - boat and motor have been in the family forever. Do you know if the port head can be removed without taking off the lower cowling?

I suppose enlarging those holes allows more water to circulate at low RPM, before the thermostats open and before the pressure bypasses operate at higher RPM, no?

I think a test run is needed before any further teardown.
 

Bosunsmate

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Right - the hard rubber/plastic bits that go in the block. Never had the heads off this one - boat and motor have been in the family forever. Do you know if the port head can be removed without taking off the lower cowling?

I suppose enlarging those holes allows more water to circulate at low RPM, before the thermostats open and before the pressure bypasses operate at higher RPM, no?

.

Yes port head can be removed, you might need a universal joint adaptor for the ratchet to get the lower bolts.

Enlarging the holes means that it is less likely to block up. If anything in the water circulates the engine and hits that hole and then gets stuck in it then the water will not flow through the engine and the thermostats there thus wont get any hot water going past them and thus wont heat up and open up. So no more water flow will enter the engine and it will overheat and cook unless your alarm comes on and you shut off instantly. If you make that hole too big then too much water will go through and the engine will run colder longer than it should really.

In salt water that hole slowly oxides smaller too. I also take a turn and a bit clip off those springs too so that those poppet valves open a bit earlier with water pressure. I figure thats less risk for my mates running the boats carelessly the way they often do
 
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interalian

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Thanks. A bunch of years ago I drilled and tapped the head waterjacket covers right at the top for temp senders and have a switch under the temp gauge so I can monitor head temps port/starboard. The motor used to exhibit low RPM heating problems and I struggled to overcome them.

 
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ondarvr

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When you remove coils from a spring it gets stiffer, not softer. So the poppet will open later.
 

Bosunsmate

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I have a 1982 60hp and was chasing an overheat problem at idle/low rpm for about a year. I replaced the waterpump, cleaned out the heads etc and it would still overheat. It was then that i came across this service bulletin and since i moved the tell tale outlet to the top of the crankcase to evacuate all the previously trapped air it has run absolutely ideally.
http://forums.iboats.com/forum/engi...aq/228380-johnson-evinrude-adding-a-tell-tale
 

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Bosunsmate

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Heres the piston from my mates 1982 90hp that i rebuilt for him, was running lean and had a old impellor- the starboard bottom cylinder blew. Only needed a hone and rings but most of the other pistons had cracked or seized in rings. That big groove in it was caused by the broken ring squeezing against the cylinder lining, luckily the cylinder wall was only slightly grazed
Yellow paint is from a spill in my garage
 

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Bosunsmate

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When you remove coils from a spring it gets stiffer, not softer. So the poppet will open later.
if thats true, ive a sceptical mind, then thats really good to know.
i had thought that the less length would mean that theres less length needing compressing
 

interalian

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I have a 1982 60hp and was chasing an overheat problem at idle/low rpm for about a year. I replaced the waterpump, cleaned out the heads etc and it would still overheat. It was then that i came across this service bulletin and since i moved the tell tale outlet to the top of the crankcase to evacuate all the previously trapped air it has run absolutely ideally.
http://forums.iboats.com/forum/engi...aq/228380-johnson-evinrude-adding-a-tell-tale

So you drill and tap the waterjacket at the top of the STBD bank and plumb the teltale there? Makes a lot of sense.
 

Bosunsmate

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So you drill and tap the waterjacket at the top of the STBD bank and plumb the teltale there? Makes a lot of sense.


Mines a two cylinder 60hp so i just drilled into the top of the crankcase above the cylinders.
I offered to do it to my mates 90hp but he wasnt keen, his loss.
If you take the heads off to do the water deflectors you will see that theres heaps of room to carefully drill down in to the top of this water cavity around the cylinders.
The best idea in my opinion with two banks and what i would do myself is to drill into the top (crankcase not the head) of both banks and run a telltale off both sides. Id imagine joining them into a single line could create problems as one banks would normally have a bit more pressure than the other side (one side is slightly higher up the motor than the other for instance).
I think the wrapping the hose over the top bank like its written in that link is wrong as thats what my 60hp had originally and that caused issues.
 

Bosunsmate

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So you drill and tap the waterjacket at the top of the STBD bank and plumb the teltale there? Makes a lot of sense.
Starboard bank seems to run hotter on these so would be the one to pick if you do choose a side
 

emdsapmgr

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Regarding your compression readings: These crossflow engines will normally have a compression difference between the two heads. It's the way the blocks are machined at the factory. They really are not identically machined on both halves of the block. So, some variances is possible. Some blocks, the compression is almost identical, others, there might be a 5# difference between the two heads. Yours is a little outside that range, but may not be a problem. The fact that you've got compression variances within each head it interesting. That's a little less normal. I would agree with Bosunsmate that doing a decarb on the engine should be done. Get a can of Bombardier Engine Tuner and follow the instructions on the can. It may raise the compression on the whole engine slightly. In any event, it's a good process for these crossflows-eliminates carbon buildup around the ringsets. When this happens, it usually starts slightly easier and idles better. You can do a second decarb, too.
 

interalian

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I've been maintaining this engine since it was new and while it has always had a variance, I don't recall it was this much.

More info once it's been on the water.

Thanks for all the comments - very helpful.
 
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