1989 Blackmax compression

[Larry Tate]

Picked up a 89 Blackmax 175, the compression is as follows: 2 cylinders with 130, 2 cylinders with 120, and 2 cylinders with 110. From what I remember hearing is that they should be within 20% of each other. Now, I know this motor is tired but is this a must re build situation? It seems to run fine gets on plane and screams. Any advice? Thanks guys

 

[wired247]

Re: 1989 Blackmax compression

I'd leave it. 110 lbs is good compression no matter if the others are 120 or 130. If it were 80 you'd be looking at cylinder damage. 110 is a healthy-ish cylinder. Try decarbing it. If it runs good just let it run good until you get bucks up or bored.

 

[Larry Tate]

Re: 1989 Blackmax compression

thanks for the quick response thats for the most part how I felt...decarbing? sick boat btw

 

[Larry Tate]

Re: 1989 Blackmax compression

What is decarbing

 

[wired247]

Re: 1989 Blackmax compression

Outboards sit for years without use and the rings get stuck or just get fouled up with oil/carbon, bugs ...whatever. Everyones got their favorite was to do it but I like picking up a couple of cans of seafoam or powertune and tilting the motor up all the way with the plugs out. Pour 1/3 can in each cylinder or as much as it will take and let it sit like that for a few days. then turn the motor over a few times witht he kill switch activated so you dont ignite your blast of flammable vapor coming out of the spark plug holes. Put is back together ( plugs in ) and fire it up. When its done smoking like cheech and chong try the compression test again. Sometimes it works , sometimes it doesnt.

 

[Dukedog]

Re: 1989 Blackmax compression

Best thing ta do is just keep an eye on it. Test it a little more than normal. If it starts ta drop any lower ya might wanna take a look. Its a chrome bore that ya don't want ta let go very long when tha comp starts droppin'. It will be time ta find out why?.........jmo

 

[Texasmark]

Re: 1989 Blackmax compression

If Wired's answer doesn't suit you go to the site archives and look up decarbing. It's more involved using a dedicated fuel tank with a high concentration of SF for the process. Run let it sit, run again with adequate ventilation (smoke) and when finished new plugs sort of thing. I agree, with your low cyl at 110 you don't have any mechanical problems requiring parts replacement...my opinion.

Mark

 

[Larry Tate]

Re: 1989 Blackmax compression

Thanks guys all good stuff...so the gas in this tank ifs old and I ran it the other day sounded like it was knocking but it was definitely running on all six because of the power it was putting out. Previous owner is a good friend and said it didn't have that last season and I have been on it before with it running smooth. Some people have told me the gas could be bad and cause a knock due to pre ignition...is this true and is that what's causing the knocking sound?

 

[wired247]

Re: 1989 Blackmax compression

Try knocking the timing down to 21-22 degrees and see if it still knocks or just dump the crap gas and put some fresh gas in there. Might want to try to get to the bottom of that before you run it too hard.

 

[Larry Tate]

Re: 1989 Blackmax compression

Ok so I'm getting a timing light...how do I actually adjust the timing to what it's supposed to be? Thanks so much guys for your replies

 

[racerone]

Re: 1989 Blackmax compression

Timing normally does not change or go out of adjustment.----I would buy a new head gasket and pull a head where there is the low compression and have a look in there.

 

[Dukedog]

Re: 1989 Blackmax compression

+1. Your not gonna HEAR a knock/ping like a car motor in these things. If its gotta knock sound your need ta find out what it is..
JAN, In our neck a tha woods, 110 lbs is very borderline on tha chrome bore ta start with if its with a "known" accurate gauge. It would be watched very close if we ran it all without taking a look at it.........jmo

A leak down would be best but not many folks do that type of test.......Most rely on tha comp thing.

 

[sam am I]

Re: 1989 Blackmax compression

A leak down would be best but not many folks do that type of test.......Most rely on tha comp thing.

Concure w Duke....Leak down test!! Google that, lots of YouBoob's out there on that test, even some of 2 Strokers. Same diff thou. Can even make your own gauge if you like. Sounds like you have a few bores that are perhaps oblong and scuffed up most likely. Typically oblonging towards the top of the bores so the leak down at TDC will be crap. You'll also hear lots of air leaking by as well........it will be obvious when you get going and compare with other cylinders that aren't leaking.

Really poor leak down will let ya know if ya should be pulling the head or not. But I'd try Sea Form or whatever first, might get lucky. Take measurements as you treat it, see if it's getting better or not......If its bore and scuffing, SF won't change it a bit and you'll just need to pull the head and go from there.........but don't sweat that, "pulling the head" on 2 strokers is easy, it just a plate. Tough bolts that are sized will be the issue there, might then use a tad bit of heat. Fresh water motors typically not as bad in the issue tho.

It'll still run fine'ish, mine did too.....Merc's are bad *** tough motors, you just don't want to cause more damage if it can be helped. Judgment/money call at that point......

 

[racerone]

Re: 1989 Blackmax compression

If it is a " chrome bore " motor as stated earlier, than there will not be much wear on the chrome.

 

[Big Fish Billy]

Re: 1989 Blackmax compression

At $3.79 a gallon I would buy some new gas before doing too much else. As suggested do the decarb process a few times until there is no "Cheech & Chong smokescreen." Ethanol fuel falls apart after three months, so God only knows what you're trying to run the poor motor on.

 

[wired247]

Re: 1989 Blackmax compression

Knocking noises often come from the lower unit as well.

 

[Larry Tate]

Re: 1989 Blackmax compression

I dont think its in the lower unit, I can hear it at idle. Is it possible for me to add premium 93 octane to this tank and increase the octane rating? I will empty it if thats what u guys think I should do. But i'm pretty sure this is like a 30 gallon tank sooooo hahaha thanks guys let me know.

 

[wired247]

Re: 1989 Blackmax compression

If you can hear it at idle its not detonation.

 

[Larry Tate]

Re: 1989 Blackmax compression

So is it a rod knock then? Oh crap when I go home later in gonna do the screwdriver piston test

 

[wired247]

Re: 1989 Blackmax compression

If it were detonation it would really only do it under load. From someone who has detonated a few motors to death ...you CAN hear it and when you hear it , its too late.

Could be bearings, could be piston slap in a cold bore, could be a worn out piston or bore. rings could just be stuck. Chrome bore motors can have a bad bore or more re sleeved with an iron sleeve and saved.

I still think if it runs OK I'd just let it run if the bearings check out. Try the decarb.