View Full Version : 1991 merc black max

15-09-2008, 09:10 PM
hi guy im thinking of buying a 1991 175 black max the guy says its only done 300 hrs what should the compression should it have? and any other things to look out for any ideas would be great thanks baz.

16-09-2008, 07:23 AM
Is that the tower of terror!

If it is, don't worry about the compressions too much, it's the dreaded Merc disease to look out for.
And that is the reed valves are loose and lost there flex, and can't hold the comps in the crank. Therefore one lousy motor and very Exxy to fix.

16-09-2008, 08:32 AM
I dunno the factory spec compression figure, although if you do not know, due to the variance in readings between testers look for consistency through all cylinders, for me under 10% without doubt <5% is a nice place to be when purchasing although the industry wouldn't like it.

cheers fnq

16-09-2008, 05:02 PM
Mate dont touch it if your not 100% sure of the history, theres better buys around that that one.

16-09-2008, 06:34 PM
attach a piece of rope n chain and u got ur self a nice expensive anchor. how much is he asking??

16-09-2008, 07:51 PM
Hey, back off the Black Max, they aint that bad :)

They aren't the "Tower of Power either, I'm pretty sure thats the straight six you're talking about. The Black Max would be a V6. I've got a 1990 135hp Black Max thats been great so far, besides a troubling cooling issue that turned out to be a simple fix. From what I've been able to find out, they are a pretty bulletproof motor mechanicaly, let down a bit by dodgey electrics.

Can the owner validate the hours? Check the hour meter works too, mine supposedly only had 300 hours on it when I bought it but the hour meter hasn't budged since I've had it.


16-09-2008, 08:16 PM
All V6's run low comps. Typically between about 100-110psi. Consistency is what you are looking for however. 10% variation at max.

By 1987 Merc had more or less perfected there V6's and from then on they were excellent and remain such.

If its a 1991 model it will have a 1 piece top cowl. Pre 89 and its a 2 piece "clamshell" cowl, which hinges at the rear.


16-09-2008, 10:56 PM
thanks bm how do i get hold of boat inspections made easy::)

17-09-2008, 08:11 AM
i agree, the black max's are awesome donks, don't listen to any of the old "i heard it from a bloke at the pub" tales. pretty agricultural but dependable nonetheless. they love petrol, make sure you've got lots. check the wiring loom under the cowl, the loom is cable tied around the connectors in it and they rub/heat up and will cause a short. simply cut cable tie and seperate all connectors, replace them as necessary. my 135 had compressions around the 110 mark in all pots. there is a starting knack because the choke is a fuel solenoid that trickles fuel into the throat below, although once you figure it out it'll always start easy. my one used to trickle oil back down the fuel line during periods of non-use, either disconnect the fuel line or pump a litre or so of fuel out through the line before using it after a period of time or you'll get hard starting and a rather large smoke screen at the ramp lol.

17-09-2008, 07:03 PM
Yeah, there can be a bit of a knack to starting them, they're not bad though. I just prime mine, turn the key on and push it in for 8 seconds then crank, it usually starts first hit. If it doesn't, hold it in for 2 more seconds and she's away. Once she's warm it's first hit of the key every time.


18-09-2008, 07:59 AM
yep, mine was push key in, count to 5, let key out and crank the furker. it'll start. never push key in whilst cranking, they load up way too easy when cold.