1986 175 mercury blackmax stuck at 3500 rpm

[trollin]

I just got a 87 18' baja with a 1986 175 mercury black max on the back.
The guy I bought it from said it ran perfect one day on the lake and then all of a sudden the boat will not run over 3500 rpm. My initial thought is that it may be the reeds so I replaced them with some boysen power reeds but to no avail. I checked compression and there is btw 125-122 on each cylinder. (QUESTION 1.) I talked to a mechanic and he said it may be one of three things, a high speed coil or the stator or a switch box. While I appreciate his info I don't unfortunately have the money to have him work on it. So I was hoping to find someone on here who may have had the same problem or could potentially tell me which one they think is the most likely culprit. I have taken the boat out and it idles perfect and runs perfect up to 3000-3500 rpms and will not wind out any more than that. (QUESTION 2.) I also just one a set of heads on ebay for it for 9$ that are supposed to be milled 10 thousands and I was wondering if anybody knows what the squash is suppossed to be so that I won't risk melting the sides of the piston down. (QUESTION 3.) My final question is this I was on bobs machine website and he has a item called a tuner its like 109$ for the exhaust and I was wondering if anybody had used it and if there is a noticeable difference in performance. I would not be purchasing it now just curious I would like to build it up a little over time. and am not sure what are the best and most cost effective mods to do. I understand it will never be a 2.5 but a little more power never hurt right :thumbsup: thanks for your help

[terry taylor]

HI. The electronic parts I will leave for someone with the experience . The heads at .010, will likely change nothing, Id,say a waste of time. I think the general consensus is the tuner change is also a waste of time. You would likely feel best a getting the engine running up to par for starters thanks.

[HighVelocity]

That mechanic sounds like he knows his stuff. I would hold off untill you have the funds to put him to work. Of coarse if you bring it to a Merc dealer you could ask them what they are getting for an hour of diagnostic time. I would think a good shop with a scope and modern dianostic tools could pin point it pretty easily. I would think that most shops would charge a hundred for an hour. In the mean time I would research all possible parts needed maybe you can cut cost be suppling the part needed.

The first thing I would do as a shade tree mechanic though would be to look over all the linkage, including under the fly wheel. Make sure everything is advancing properly. Secondly I would check all fuel lines as reduction in fuel flow would govern performance (Blow them Out). And lastly since the ethenol effects the older hardware the most, I would clean the carbs as best you can, and replace the diaphram. You can't hurt anything by ruling out the basics.

HV

[Wayne Cammidge]

I agree with HV above. You can spend a lot buying new parts to try. A good mechanic will be 95% right with only basic diagnostic equipement, add a scope and more advanced equipement he is worth his weight in gold. He will also be more reasonable with charging for his time if he is making something on the spares, so shopping around for the 10% discount??? does not really pay in the long run. Build up a relationship with the mech and you will be rewarded in the future

[Raceman]

If you're considering a tuner change (I WOULD NOT DO IT) I would think you could have an electrical diagnosis run on the engine for the same money. (money already spent on Boysen reeds which aren't much more than safety from breakage on a high RPM motor would probably have covered it too) Based on what you've said, I'd think high speed windings on stator would be a good place to start and somebody around here can tell you how to do it yourself with an Ohm meter. Unless somebody's screwed with the linkages a lot I doubt seriously that's your problem, but it never hurts to check for wide open throttle, and if you've got it, full timing advance.

Cutting the heads on those motors ARE a worthwhile mod, but you have to be careful with octane. Somebody like Jay Smith who does em on a daily basis can give good advice on how much, based on what your priorities are. I agree with the opinion that .010 wouldn't be worth the trouble to change em.

[HighVelocity]

If you want the motor to last, which it sounds like the budget is a bit tight right now, then leave it be. The boat should do a solid 60-65 as is, if you come in to some bucks and want to go faster, look for a used 225 Pro-max, that will put you in the 70's easily.

If you jack up the compression on the oldie but goodie 175 you will melt it eventually, and then be on the side lines watching as everyone leaves the dock to have fun.

[trollin]

Thanks for the info guys I really do appreciate it. The heads I got are milled .100 not .010 like I had said before. my question now is would you guys change them out as a worthwhile change or not, I was just curious if you guys still think I would be wasting my time. This is of course assuming after I get it running right.

[MidnightCarving]

mechanic is right, stator or switchbox. those motors use a high and low speed stator, so the high speed coil has most likely failed. However, the switchbox can cause problems like that as well. Test for spark on all cylinders. Also, i've seen high speed jet clogs cause problems like this. An effective means of testing is using a timing light aimed down the mouth of the carbs, make sure at high speed that they are all a steady spray, any irregularities will cause problems.

[HighVelocity]

I don't think you will get more than 1-3 mph change. Not a life changing experience when your doing 60.

Raceman said it too, you just need to try and keep your 22 year old motor running.!!!!!!!!!!!!!! LOL

Your trying to put a 14 pound super charger and a shot of nitrous on a model T-ford...LOL It might run great for about 2 minutes...LOL
Then K---BOOM!!!!


But at the end of the day you need to do what you need to do. Hey I blew up a lot of crap when I was younger...lol Mopeds, motorcycles, car motors, rear ends (3) LOL...

So don't take anyones word for it. Put'em on and try it. Run the piss out it and blow it up. At least you have a good story to tell right... And if anyone asks why you did it, just say the motor was 22 years old I didn't give a crap...

[hydroholic]

.100 milled heads are way to much for a for a motor in good shape to survive on pump gas,much less a 22 year old one (assuming it has not been rebuilt),so definately don't use them. I usually have .60 cut off of stock uncut 2.4 and 2.0 heads. This usually gets you around 145 to 150 psi which is the max.
for pump gas.
Hydro