115HP Mercury 1974 Compression Test

[kydaile]

Hello everyone! I rented a compressor tester at autozone and checked my v6 mercury outboard engine. These are the procedures that I did for the test.
1. removed all spark plugs and spark plug wires.
2. connected the tester to the port for spark plug #1
3. I cranked the engine for five times then read the PSI on the tester gauge.

So the result came out for engine #1, to engine #6 are very consistent. How ever they are all at 70 PSI. I think the PSI are very low, but I do not know anything about engines.

Please let me know if my engine still good or not.
Or did I take the right steps for the compression test?

Please help. Thank you very much.

 

[dutchdog]

Re: 115HP Mercury 1974 Compression Test

Did you have the throttle in wide open position? If not you will get low pressure readings due to the carbs limiting the amount of air being sucked into the motor.

 

[JB]

Re: 115HP Mercury 1974 Compression Test

Your compression is fine. The psi numbers are lower than usual, but there is no way six cylinders are going to go bad by the same amount at the same time. There are too many variables to say that Xpsi is minimum.

The important number is the variation among the cylinders.

At cranking speed the position of the throttle doesn't matter. The temperature of the engine and the presence of lubrication do matter, as does the condition of the battery, starter wiring and starter.

 

[hl]

Re: 115HP Mercury 1974 Compression Test

I don't know as much as the more experienced members but I am thinking the compression should be at least 100 psi. Most motors will show 120-130 psi. If its running good, your gage could be wrong.

 

[Chris1956]

Re: 115HP Mercury 1974 Compression Test

It is certain that his gauge is wrong.....

BTW - The compression should be noted when the highest reading is obtained. It may take more than 5 revolutions...

BTW - That motor is an inline 6...

 

[Boatpro]

Re: 115HP Mercury 1974 Compression Test

your engine is no good. Rebuild or trash it.

 

[kydaile]

Re: 115HP Mercury 1974 Compression Test

Thank you for all of your help. I will replace the spark plugs and test it on the lake this Wednesday.

 

[turtles11756]

Re: 115HP Mercury 1974 Compression Test

buy your own guage at maxrules.com. don't cross thread . you should get around 120 psi .ground the plug wires it's not good for the spark to be jumping around and be careful gasolene + spark = fire !

 

[crazymac-680]

Re: 115HP Mercury 1974 Compression Test

I would say that your compression is definatly low but that could be down to a poor battery or the wiring. It seems very strange to have the exact same low reading on all cylinders. This points to a much simpler fault. I have a merc 85hp 4 cylinder and I got 140-150 psi on each cylinder. That seemed very high to me as I was expecting about 120psi. My mate puts it down to the engine being very old and the large build up of carbon making the inside of my engine smaller therefor increasing compression. I also used a slightly different technique to you. I left all my spark plugs in while I did the test. I was always under the impression that you only removed 1 plug at a time while testing compression. This gives a more realistic compression reading of operating compression.

I wouldn't waste any money on spark plugs until you are certain you have compression. Does your engine run? I dont think an engine with a genuine commpression reading of 70psi would run

 

[Chris1956]

Re: 115HP Mercury 1974 Compression Test

Mac, There is no way all cylinder have worn out exactly the same. His gauge is inaccurate. His compression is fine.

 

[kydaile]

Re: 115HP Mercury 1974 Compression Test

Thanks for helping me you guys!
I bought a new compression tester and my reading is 125, 123, 126, 119,122, and 121. So I think the compression is ok.
Now the reason why I did the test was because the engine run fine on land in both neutral and rear. However, it run good only in neutral and in rear it can barely run. So the thing could be wrong is the compression or the spark plugs
I just change the spark plug 5 days ago. I took the boat to a local lake yesterday and the motor did the same thing as before. Now I do not know what to do. What should I do next? Please help.

 

[Chris1956]

Re: 115HP Mercury 1974 Compression Test

If it runs well on the fulsh device, and poor in the water, and the compression is good, it is likely carbs.

First make sure none of the spark plug wires are arcing to ground. If they are post back and I can tell you where to get new ones.

When were the carbs cleaned last? If recently, then the idle mxture is too lean. If not recently, the carbs are likely dirty and need rebuilding.