no compression

[luvthelake]

i have a 1988 evinrude 70 hp engine that came on a boat i just purchased. i just did a compression check and found that the top cyl. had 40 lbs. and the middle and bottom had like 5 pounds. could the problem be anything other than a total failure of the engine ie: reeds or something else.
thanks

 

[CATransplant]

Re: no compression

Off with its head! (cylinder head, that is) It's likely that you have a blown head gasket. It's easy to find out, and with the head off, you'll be able to see if there is major damage to anything.

With any luck, a new head gasket and maybe a manual resurfacing of the head will be all that's needed.

 

[tashasdaddy]

Re: no compression

is the paint on it scorched, discolored, peeling??

 

[samo_ott]

Re: no compression

yep, off with the head! hopefully it's only a head gasket!

 

[luvthelake]

Re: no compression

i don't see any thing that looks like it overheated. i'll pull the head today and see what it looks like.
i 'll let you know, thanks, luvthelake

 

[luvthelake]

Re: no compression

well i just finished changing the head gasket, nothing jumped out at me when i took it off, although it does look like someone at sometime replaced the middle piston. i did another compression test and now all cylinders are in the 30 lb. range. anyone have any suggestions?
thanks, luvthelake

 

[Dhadley]

Re: no compression

Detail how you did the compression test.

 

[Lakester]

Re: no compression

well i just finished changing the head gasket, nothing jumped out at me when i took it off, although it does look like someone at sometime replaced the middle piston. i did another compression test and now all cylinders are in the 30 lb. range. anyone have any suggestions?
thanks, luvthelake

hello,

sorry to hear of ur frustrations. well, i assume its frustrating...

dont seem normal for all cyls to read that low. when u did head gasket did u hand dress the block and the head? ck for any warping? did u see any areas on old head gasket with discolored blackish gray bleeding from metal compression ring.

hate to see u pull the heads after fresh headgasket, but if that was my engine, i would. then i would inspect cyl walls and clean to metal and inspect. looking for vertical grooves. also i would then pull the side covers and take a look at the ring conditions. if my memory serves me right the pistons have a pin in the ring land area to limit ring rotation, so u prob can just ck for an overall ring integrity. in and out, and not broken. u did clean the bolt holes with a tap, right? and cleaned the bolt's threads to metal?

with readings of 30 psi... u r sure ur guage is accurate? u may want to try a turn over with thumb over hole to see if u get an authoritative air squish. but in any event, anything over 30 is where u want to head. so, if it was mine, i would reinstall the head gasket. hasnt heated up so should seal well if all else is in good shape.

u may want to review the engine's last operating hours with the prev seller.

good luck, keep us posted. an interesting thread...

regards
lakester

 

[luvthelake]

Re: no compression

hi, i did hand dress the head and the block, there was a little carbon on the heads of the pistons that i removed, cleaned all bolts and ran a tap thru the block holes. the cyl walls looked ok, no scuffing or grooves that i could see or feel. i have not pulled the side cover to look at the rings. that would be the exhaust side right? maybe i'll go do that now.
thanks, luvthelake

 

[iwombat]

Re: no compression

Well if all cylinders are reading even SOMETHING is right. You just don't see all of 'em worn out like that.

I think a full description of your methods is in order.

 

[luvthelake]

Re: no compression

ok, i put the throttle wide open and rolled the engine over at least 15 times per cyl. only took out the plug for the cyl. i was testing. the compression tester is new as my other one is at the lake. i did have someone spin the engine over as i held my thumb over the plug holes and they seem to have compression. i pulled the exhaust cover off but that was a waste of time as you can not see into the crankcase from there. it appears that you must completly disassemble the crankcase in order to check the rings. at least i can't find any access plates. i'm going to the lake on friday so i'll get my other tester and try that one, and yes it is frustrating.
thanks

 

[iwombat]

Re: no compression

Take all the plugs out & ground the wires. Then test the cylinder you're interested in.
Make sure your battery has a full charge too. You need to spin the motor at at least 300rpm.

You'll get a better view of the rings by taking off the intake inspection covers.

 

[fuzzeywiggler]

Re: no compression

Have you noticed any burning oil or loss in oil? It could be something as simple as the pistion rings being lined up? Check and make sure they are staggered.

Fuzzey

 

[iwombat]

Re: no compression

Not to be dismissive, fuzzy, but it's a two-stroke.

It burns all the oil that's mixed in. No more, no less.

 

[Dhadley]

Re: no compression

When you spun it over or had someone else spin it over, were they using the key switch & electric starter? It needs to be spinning about 350 rpm by the electric starter.

 

[luvthelake]

Re: no compression

yeah we were using the electric starter and the battery is fully charged so it was spinning pretty fast. where are the intake inspection covers, i couldn't find any.
thanks,
luvthelake

 

[Benny1963]

Re: no compression

those numbers are realy low but even usualy when you have that low of compression it obvious theres damage .use your comp tester on another motor even your car to see if it comes up or iff it damaged ,[the tester that is]
good luck bennyb

 

[iwombat]

Re: no compression

I guess the bigger question is . . .

how does it run?

Honestly, I think everyone would be better off erasing the numbers from their testers and just using the hash marks to see if the cylinders match.

Could just have a bad valve on the tester. I'd run it and see how it went.

 

[CATransplant]

Re: no compression

I think I have the answer here. To start, I'll explain my reasoning:

This is a new outboard for the current owner. It has zero compression after a new head gasket, but no evidence of severe damage in the cylinders. No holed pistons, no severe scoring of cylinder walls. In the test before removing the head, it has some, but not much compression, so the guage is at least measuring compression to some degree, whether accurately or not.

In that case, any rotation of the crankshaft through the compression stroke would give some compression reading.

What's going on here is that, while the flywheel is rotating, the crankshaft is not, or at least is not rotating through a compression cycle on the cylinders. That it showed some compression earlier only means that it made it partway through a compression cycle on that cylinder.

This outboard is either missing its flywheel/crankshaft key or that key has sheared completely. I'm betting that the previous owner had the flywheel off in a repair attempt, then gave up on the outboard. He then reassembled it, possibly leaving the key out entirely. Even if he put the key back in, he didn't even tighten the nut solidly, much less torque it properly.

So, the new owner gets the thing. When he performs the compression test the crankshaft rotates until it gets partway through the compression cycle on the cylinder under test, then the flywheel slips, never allowing the crankshaft to complete the revolution. The fact that the new owner left two plugs in and only removed one means that the additional cylinders cause the flywheel to slip even more. When he replaced the head gasket and tried again, the flywheel was loose enough not to even turn the crank at all against the friction of the rings on the cylinders...so...no compression.

I thought about this a long time last night, and it's the only conclusion I could come to. I'm betting that if the new owner puts a wrench on the flywheel nut, he'll find out that it's just finger tight or maybe a little tighter. When he pulls the flywheel, I'm betting that he finds no key in there at all, or one that has sheared cleanly.

What do you guys think? It's easy enough to find out.

 

[jal]

Re: no compression

I think I have the answer here. To start, I'll explain my reasoning:

Wow!!! That is impressive.
jal