results of compression test GM305 1978(not running WOT)

[sarantis]

I give you the results of the compression test.
1-150,3-150,5-150,7-145
2-147,4-145,6-135,8-125
According to the specs everything is ok but i don't know whether the gradual decrease of compression at the starboard side is a mater of coincidence or indicates something.What is your opinion?

 

[JCF350]

Re: results of compression test GM305 1978(not running WOT)

With over a 15% difference? I wouldn't call that "within specs".
I believe it should be within 10%

 

[MikDee]

Re: results of compression test GM305 1978(not running WOT)

It could be a bad head gasket between those 2 rear cylinders 6, & 8.

 

[sarantis]

Re: results of compression test GM305 1978(not running WOT)

Have you ever read the relative Mercruiser service buletin ?
It states that
a) No cylinder should read less than 100 psi (690 kPa).
b) Compression recorded on any one cylinder should not be less than 70% of the highest cylinder.
Example: If the highest cylinder has 150 psi (1035 kPa), the lowest allowable pressure for any
other cylinder would be 105 psi (725 kPa). [150 X 70% = 105 psi (1035 X 70% = 725 kPa)].

 

[John_S]

Re: results of compression test GM305 1978(not running WOT)

Run it, and recheck latter.

 

[JCF350]

Re: results of compression test GM305 1978(not running WOT)

Run it, and recheck latter.

Good idea.

 

[JustJason]

Re: results of compression test GM305 1978(not running WOT)

it's within spec... but if you have any question by all means do a leakdown test.

 

[Bondo]

Re: results of compression test GM305 1978(not running WOT)

Ayuh,......

While your motor is Not New by any means,......
I don't see any driveability issues with those #s,......

Maybe it got alittle damp in those 2 holes one day,+ possibly abit of rust set in,.... I'd have to believe it's long gone now, but it's effects might be the reason for the slightly lower #s.....

In Other Words,...... I Don't see any Problem...................

 

[Don S]

Re: results of compression test GM305 1978(not running WOT)

Did you try putting any oil in the low one to see if it would come up?

I'm with Bondo, those compression readings are not going to cause you to not achive WOT rpm.

Did I ever post this list of possible reasons for low WOT rpm?


Engine Won't Reach Operating RPM. Check

• Fuel condition. Type and Octane possibly old fuel
• Propeller pitch or diameter, damaged blades
• Restricted fuel pickup tube or anti siphon valve Fuel System Test
• Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
• Marine growth on hull and outdrive
• Wrong gear ratio in outdrive
• Restricted carburetor air intake (clogged flame arrestor)
• Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
• Poor cylinder compression Compression Test
• Carburetor defective, or wrong type.
• Fuel pump pressure and vacuum
• Boat overloaded, improperly loaded, or improperly trimmed.
• Engine Overheating
• Engine timing and ignition system operation
• Remote control cables and linkage for proper travel to open throttle plates fully.

 

[TilliamWe]

Re: results of compression test GM305 1978(not running WOT)

Excellent List Don. Is that carved on a Tablet somewhere? It should be!

 

[JCF350]

Re: results of compression test GM305 1978(not running WOT)

Have you ever read the relative Mercruiser service buletin ?
It states that
a) No cylinder should read less than 100 psi (690 kPa).
b) Compression recorded on any one cylinder should not be less than 70% of the highest cylinder.
Example: If the highest cylinder has 150 psi (1035 kPa), the lowest allowable pressure for any
other cylinder would be 105 psi (725 kPa). [150 X 70% = 105 psi (1035 X 70% = 725 kPa)].

I know what GM/Mercruiser says. Everybody else specs are tighter though. Anything more than 10% indicates something is not right even if it is a GM. Could be nothing except sitting to long or other regardless it does need to be investigated.

 

[sarantis]

Re: results of compression test GM305 1978(not running WOT)

Since at one side the numbers are slightly lower than the other do you think that it may mean a bad timing chain (i can get the 8* advance and it goes higher when i increase RPM)?Today i noticed a thin noise at the place of the timing noise a chick,chick,chick but i can distinguish if it's the chain or a bearing
Don S i'm aware of the right way to find the problem but i don't have all the necessary equipment.Your sugestions may be very helpful and i understand that you are not magicians.

 

[Bondo]

Re: results of compression test GM305 1978(not running WOT)

Since at one side the numbers are slightly lower than the other do you think that it may mean a bad timing chain

Nope,... Not Really.....

(i can get the 8* advance and it goes higher when i increase RPM)?

As it Should,......

Today i noticed a thin noise at the place of the timing noise a chick,chick,chick but i can distinguish if it's the chain or a bearing

Ayuh,.... More likely a lifter or the fuel pump....

Don S i'm aware of the right way to find the problem but i don't have all the necessary equipment.

Ayuh,.... 99% of that can be checked with a set of end-wrenches,+ a set of sockets....

and i understand that you are not magicians.

Ayuh,.... Ya got That Right.....

 

[Don S]

Re: results of compression test GM305 1978(not running WOT)

Since at one side the numbers are slightly lower than the other

In older engines, especially ones that may have had a head replaced or valves ground on one side and not the other, it's very common to see one side lower than the other buy 5 - 10 psi.

Don S i'm aware of the right way to find the problem but i don't have all the necessary equipment.

What "equipment" are you refering to. In the fuel system test, the gauge is just a fuel pressure guage found at almost any automotive store and some hose and plastic fittings (also from an automotive store) You already have a compression tester, since you have compression numbers.

and i understand that you are not magicians.

Well, maybe Bondo isn't but when my crystal ball gets back from the CB shop, I'll be set to answer all questions. However, they have had it for years
seems a part is NLA for it, and is what caused the crack to begin with.
Here is a picture of me at work a few years ago when it worked.

 

[Bondo]

Re: results of compression test GM305 1978(not running WOT)

seems a part is NLA for it, and is what caused the crack to begin with.

Ayuh,........ I Told ya not to buy that OMCrystal ball..........

 

[Don S]

Re: results of compression test GM305 1978(not running WOT)

Ayuh,........ I Told ya not to buy that OMCrystal ball..........

I know, I know, but it seemed like such a sweet deal at the time. I should have listened, and didn't. and Now, I'm paying dearly ..........

 

[Bondo]

Re: results of compression test GM305 1978(not running WOT)

Ayuh,...
Well,.... Atleast you had 1 Once,......

The Only Crystal Balls around here are the Ice Crystals on My,.... Oh Never Mind.....

 

[John_S]

Re: results of compression test GM305 1978(not running WOT)

I thought Bondo's crystal ball was pretty good:

"Maybe it got alittle damp in those 2 holes one day,+ possibly abit of rust set in,.... I'd have to believe it's long gone now, but it's effects might be the reason for the slightly lower #s....."

Didn't they have log type exhaust manifolds back then? and if there was a small leak, the back cylinders might be more effected.

 

[JCF350]

Re: results of compression test GM305 1978(not running WOT)

I know, I know, but it seemed like such a sweet deal at the time. I should have listened, and didn't. and Now, I'm paying dearly ..........

Crystal Balls have been replaced with "Diagnostic Dice".

"http://www.brandsplace.com/0384-credice.html"

 

[sarantis]

Re: results of compression test GM305 1978(not running WOT)

Hi all
Well, today i went for a run to check some more things.I warmed up the engine
and i caught 3000RPM.I was traveling for two minutes until the engine started loosing power and then died.Now i thought is a good moment to see the results of a warm compression test.And to my surprise the starboard side head that counted 145-140
-133-127 with the engine cold, now counted perfect(all of them 150-155).What kind of SURPRISE is this????
How do you explain that?All the valves of that side had problem and now dissapeard? It doesn?t sound logical.(The port side when the engine was cold had the three cylinders 150-155 and the last aft 145 and with the warm test they remained the same).Do these numbers may be affected by a leak to the intake manifold which is the most possible reason for my troubles?I think now i have to take out that weer square bolt of the intake manifold and connect the vacuum gauge somethin that i should have done many months ago.(Why there is never enough time the first time.........Don S says!!!!)
Anyway i need your opinions
(Carb, fuel pump,distibutor exept rotor ,gas tank, filters,coil checked or replaced.Ater dying the engine starts up again if you wait for a while and then the next dying will come sooner.)