75hp mercury what is too low of compression?

[slimpickens]

First off im no boat mechanic. i need some advice. I am looking at buying what i believe is an 86 model 75hp 4cyl. mercury that I could check the top 3 cyls. 4th is too low to access and got about75-80 psi on them is this enough to make power or should i have a hard time starting and running the engine? It starts easy and only needs choking once I ran it on the lake today for about 20 minutes and got reasonable power from it it seems. it pushed a 15.5 foot kingfisher up to about 40-50 the speedo is inop too. Is my gauge bad on my tester maybe or should it run ok on this low of compression? Somebody who knows these two stroke please HELP>

 

[diaric]

Re: 75hp mercury what is too low of compression?

i would say that compression should be around 100/ cylinder, so maybe your gauge isn't reading to good. how did you figure your speed out? it looks pretty good. choke will only work to get it going, so it looks good that way

 

[Clams Canino]

Re: 75hp mercury what is too low of compression?

Ya.. operation looks OK. Suspect your readings.

Use "Mercury Power Tune" as diected (twice) to de-carb it.

-W

 

[mcdgary]

Re: 75hp mercury what is too low of compression?

A fw years ago my son's Merc 115 HP inline 6 had a scored cylinder; we got 118-120 PSI on the 5 good cylinders. The bad cylinder was, of course, shot. Incidentally, I was able to get the gauge on all 6 cylinders; I've forgotten how I had to hold my mouth to get to one. Anyway, we had the bad cylinder repaired, but it never was better than about 90 PSI. I think the main point is to have about same PSI on all, so you need to make sure of the one you are not testing. Your PSI readings seem way low. Test your gauge on your road vehicle and see if it is also low - then bad gauge if so.

 

[slimpickens]

Re: 75hp mercury what is too low of compression?

I will test my gauge today on shop air and do a compairative. I was merely guessing the speed but we were going alot faster than tow speed for skiing so i just wet my finger and put it in the wind so The holeshot was abit slow but 75hp on a pretty heavy boat plus when i loaded it iwas pretty laden with water i think the backwash was getting in when i slow up. So considering my weight i thought it preformed well. I forgot to mention it has set for a year with the 1/2 plus tank of fuel i was using in it also. I would like to do the tune up process and maybe the sea foam treatment but i am not familiar with the procedure. I guess i will buy a clymer man. but any guidance for the next few days would be appreciated. . I havent ever worked on a boat motor efore. Should i put something in the tank? Like seafoam or spray the cyl ? What to use? I think i will be premixing fuel from now on as it has the automixing system and i am leary of it working right.

 

[Silvertip]

Re: 75hp mercury what is too low of compression?

Yes you should put something in the tank -- AFTER you get rid of six month old fuel. Drain it and use it in your lawn mower, weed whacker, or your neighbors car. Use fresh fuel and add SeaFoam to the fuel. Pull the spark plugs and look into each cylinder with a pen light. If you see lots of carbon, then a decarb is necessary. That process is outlined in many threads here. Just do a search on "Decarb". If compression doesn't come up, the engine is getting tired.

 

[slimpickens]

Re: 75hp mercury what is too low of compression?

i tested my gauge and it was off at 150 but at 80 it was ok i rechecked with another tester and got 80 on the top 3 cyls and wil have to make an adapter this weekend to test #1 as it is too low for my current tester to reach. i put a tsp of 30wt in the cyls and upped it to 105. i looked in the holes with a light and saw some carbon on the pistons but not a heaping mound or anything. I will try the seafoam treatment as my local parts store keeps it. If the compression is this low will this thing hold me out the year or am i dreaming? I cant afford the whole rebuild right now and my wife wont let me buy it if its gonna puke out next week and cost me 20 hours and a grand. I have no doubt i can build it just dont want to this year or ever but i know i will.

 

[diaric]

Re: 75hp mercury what is too low of compression?

kinda hard to say. sounds like it performs well. rings could be carboned up a bit, and a decarb might smarten it right up. cruising around in the 4000 range and not screaming around at WOT too much and it might do great. depends how good the deal is

 

[billybones]

Re: 75hp mercury what is too low of compression?

As long as all of the cylinders are within 10% your engine will run smooth. It's when it gets out of this range that a motor runs rough. Now power is a different story. Compression makes power. Higher compression means a better seal at the rings and less power loss. Less compression does not mean that a failure is looming close. Just means that power is not at it's peak. Engine is getting old and not the spry thing it used to be. will use more gas also.

 

[Diamond Dave]

Re: 75hp mercury what is too low of compression?

It?s important to have the throttle plate open when you?re doing the compression test. If you?re not the motor is not drawing in enough air on the intake stroke and your readings will be low. The most important thing is that all of the cylinders are within 15% of each other. Hope this helps

 

[slimpickens]

Re: 75hp mercury what is too low of compression?

The Radings came on about three strokes even jumps of the gauge by the way. Thanks im gonna try the throttle open and a decarb as soon as i can maybe a few days its only rained like 25 inches here the past couple of weeks and the ramps are slosed cause the water is too high THAT I CANT EVER REMEMBER BEFORE. The deal is good enough that if the papers dont come through i bet one of you would pay more for the parts of the motor. So i can probably afford the rebuild a little later on as the boat only needed the seats and carpetand wiring repaired from sittling a few years. Anyway Thanks for the help and Ill be sure and update as soon as i know more or need aome more help.