77 Rude 140 High but uneven compression???

urugol · Dec 19, 2012

I did a couple of compression tests on my 77 140hp Evinrude today, good comp tester and all plugs out I got the following:

Bank 1,3 = 135/130psi

Bank 2,4 = 150/150psi

Is this a cause for concern? Comp seems quite high but quite a variation between banks. I'm wondering if 1,3 may have slightly larger domes, thought I would put it out there and canvass opinions from the experts thanks.

oldman570 · Dec 19, 2012

Re: 77 Rude 140 High but uneven compression???

There is something deffinatly going on with the readings on bank 1-3 as they are down quite a bit. Has the 2-4 bank had any work done on it, or the carbs? what is the SR# of the motor?
Oldman570

urugol · Dec 19, 2012

Re: 77 Rude 140 High but uneven compression???

oldman570 said:
There is something deffinatly going on with the readings on bank 1-3 as they are down quite a bit. Has the 2-4 bank had any work done on it, or the carbs? what is the SR# of the motor?
Oldman570

Not sure if any work has been done, this engine and boat are both new to me. I will update with the model number, it's definately 77' that much I do know. I read people talk about de-carb to get compression up. How does that work?

Chinewalker · Dec 20, 2012

Re: 77 Rude 140 High but uneven compression???

When compression numbers read like that, first thing I would suspect would be the thermostats. Your numbers aren't terrible and fall just outside the outside edge of the 10% variance window I like to see, but certainly cause for some investigation. One side running colder or warmer than the other can cause more wear than the other.

boobie · Dec 20, 2012

Re: 77 Rude 140 High but uneven compression???

The rings on 1 & 3 are probably carboned up. Do a real GOOD decarb on it and see what happens. If the compression improves after the first decarb, do another one.

Faztbullet · Dec 20, 2012

Re: 77 Rude 140 High but uneven compression???

Several good thoughts on compression differences... just to add a couple more:
1) that side rebuilt with low ring pistons(77-79 had high rings which failed a lot!!!)
2) different casting/cc head from possibly damage as these had a high compression heads

emdsapmgr · Dec 21, 2012

Re: 77 Rude 140 High but uneven compression???

The 130/135 compression would be for a factory stock high compression head. The other (higher compression) head is the one in question. Someone may have milled some of the face of the head off. If you are running stock sized carb jets on the high compression head carb throats, that could be a problem. Those high comp (150) cyls may be running lean on stock jets. You might scrounge around and see if you can find a used factory replacement head.

Faztbullet · Dec 21, 2012

Re: 77 Rude 140 High but uneven compression???

Someone may have milled some of the face of the head off.

Misnomer as milling heads on a crossflow will not add compression, you have to tighten the cc's in chamber. Milling head will cause pistons to contact roof of chamber...

wayneo99 · Dec 21, 2012

Re: 77 Rude 140 High but uneven compression???

FYI
my 85 and 140 have even compression across the board but it was low for what would be expected but motor is 33 yrs old. turned out one of the bottom cylinderson each block had broken a piston ring and tore everything up but they had decent compression.
would never had known it until i pulled the head off.

V153 · Dec 21, 2012

Re: 77 Rude 140 High but uneven compression???

Faztbullet said:
Milling head will cause pistons to contact roof of chamber

Agreed. There ain't a whole lotta room in there to begin with. Which is why they get beat up when a chunk of ring, or in my case connecting rod bearing gets pinched in between.

Fwiw I rebuilt my 90 from the crank up in Jan '06. Left the dinged up head as was. No problems. Ran it hard for almost exactly 6 years til it puked. Cause of death was erosion of #2 cylinder wall/water jacket up near the head. Causing the head gasket to fail. Still had 120+ in the other three holes.

phillnjack2 · Dec 22, 2012

Re: 77 Rude 140 High but uneven compression???

probably just had 2 new pistons or rings on the 150 side.

its pretty obvious a 35 year old engine wouldnt normally have 150, the 135 is more normal.

infact not many engine are reaching 135 when done with a good accurate compression tester.
Personaly i wouldnt think much is wrong.

instead of shoving a load of gunk down the carbs, you only need something like powertune in the piston area if
your looking for more compression from sticky rings and a decoke etc
shovong it through the carbs dont realy do much at all, your not trying to clean carbs or crank case,
just the piston ring grooves is what needs to be cleaned if at all..

did you test this engine hot or cold ?
with or without fuel going in the engine ?
and does the high side look like it had a new gasket or sealer on the head ?

and what does the engine run like.

phill

urugol · Jan 3, 2013

Re: 77 Rude 140 High but uneven compression???

Thanks everyone for your insightful responses! I was off during the xmas break but I had a chance to get the boat out yesterday and give it a real day out in fairly choppy conditions around the bay. Performance seemed ok but I did notice the engine did not scream as hard by the end of the day. I got it back home and did a revised comp test, 150/150 and 130/110! I decided as suggested to remove the heads for further inspection - pics tell the story. Lean condition burning up this piston? Or something else? I noticed a bit of water in the bottom cylinder as well not sure if this is just excess from the cooling jackets or something that requires further attention. The cyl is scored a little (may not be that visible in the pics) so it seems I need at least a new piston and rebore probably as well. Rings are definately not sticking and the engine turns fine, I'm thinking I should address this before I do further damage I don't want to do a crank. I will also need to attend to the source of the lean condition. Carbs or air leak? This sucks we are only getting into summer over here.

carholme · Jan 3, 2013

Re: 77 Rude 140 High but uneven compression???

I'm sure that when you pull that piston you are going to find a broken ring and that is what has caused the damage. Do you see the same kind of damage inside the head?

Gerry

urugol · Jan 3, 2013

Re: 77 Rude 140 High but uneven compression???

carholme said:
I'm sure that when you pull that piston you are going to find a broken ring and that is what has caused the damage. Do you see the same kind of damage inside the head?

Gerry

There's 2 pics of the head in the set, definately pitting on the head but I can't find any loose metal fragments.

carholme · Jan 3, 2013

Re: 77 Rude 140 High but uneven compression???

Yes, I see the head pics, same damage. interesting to see what you find when you pull the piston. Any fragments that weren't chewed up and blown out, will be imbedded.

Gerry

emdsapmgr · Jan 4, 2013

Re: 77 Rude 140 High but uneven compression???

That bad cyl got hot, causing the ring to disintegrate and cause the visible damage you see. A couple of things to check: it's hard to see, but veryfy that the rubber water diverters in pic 1,3 are in their proper orientation in the powerhead. If out of place, the will cutoff water flow around a head. That can cause that cyl to overheat, yet probably not set of the overheat warning horn. Also, pull the carb apart that feeds that cyl, and specifically, remove the high speed jet for just that cyl. Visually inspect if for any debris in the jet. The original hoses on that engine were not made to handle today's ethanol fuels. Possible some of the black bits of disintegrated hose have lodged in the jet. If the jet is restricted in any way that cyl will run lean and excess heat will cook the outer ring.

urugol · Jan 5, 2013

Re: 77 Rude 140 High but uneven compression???

Thank you for the suggestions. I checked the water diverters, pulled them and they do not appear to be clogged (tiny hole). I'm not sure what the proper orientation should be, any help would be appreciated. What I have determined upon closer inspection is that the ring most definately got caught on one of the ports. I suspect it may have been rebuilt not long ago given the high compression. Incidentally the 150/150 cylinder head has most definately been shaved even with the naked eye which would explain the high comp. So when I change this piston and get a rebore, should I get the head shaved so that even 150 across the board is seen, not worry about it or go back to a stock head for the 150/150? In terms of detonation, I haven't heard any signs at wot and I use premium fuel premixed so I assume it will be ok. I checked the high speed jet, no clogging to speak of. Maybe I had an air leak coming up through the head gasket that caused this leanout? In terms of the rebuild, I have never wrenched an outboard but I have done plenty of rebuilds on 2 smoke jetskis in my time so I'm confident of the job. Unlike the jetski motors the cylinder bores on these engines appear to be the crankcase as well is that right? I assume the flywheel will need to be pulled as well that being the case.

carholme · Jan 5, 2013

Re: 77 Rude 140 High but uneven compression???

Do you know the model number of your engine? If it is a '77 it should be one of these and we can send you the parts list.

140740S
140743S
140783S

Though the OEM manual is the best, here is a link to the condensed version of the Clymer manual which will give you a start. Download it soonest to your computer as the link is short lived.

Small Engine Repair Reference Center Home

Gerry

urugol · Jan 5, 2013

Re: 77 Rude 140 High but uneven compression???

Thanks Gerry,

I have this entire manual so I will reference it as I go. I find it a bit big though, its like they tried to pack so many models into a single manual! And mine is a 140783S model.

cheers,

Al.

carholme said:
Do you know the model number of your engine? If it is a '77 it should be one of these and we can send you the parts list.

140740S
140743S
140783S

Though the OEM manual is the best, here is a link to the condensed version of the Clymer manual which will give you a start. Download it soonest to your computer as the link is short lived.

Small Engine Repair Reference Center Home

Gerry

carholme · Jan 5, 2013

Re: 77 Rude 140 High but uneven compression???

Al:

1977 Johnson/Evinrude 140 - OMC, Evinrude & Johnson Parts Lookup

Gerry

77 Rude 140 High but uneven compression???

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