1998 Merc 300 pro max

[sam hain]

Recently purchased this motor to put on my 21xd Bullet, just wanting to learn about it from you experts. I was told it was one of the only motors Merc ever made that didn't live up to the numbers on the cowling, in that its really about a 280hp motor not a true 300 but had alot of torque. Also heard they are steel sleeved? The oil injection is still hooked up and I was told to leave it in place by a motor builder. Other than spark plugs and fuel filter what other preventative maint would you guys recommend? Assume I need to keep 93* octane in it? Any fuel additives? Right before I bought it the prev owner had carson reeds put in it and had the injectors cleaned/flowed.

[johnmiffco]

my opinion is its the most dependable 300hp window motor merc ever made
285-290 is what lots of people have said dynoed next to newer 300x @325
at lower compression ,, fishing boat efi ,,,and 6200 rev limiter that goes stupid fat on fuel
its hard to blow em when maintained ,,,
300x with higher comp,,,and 6700 rev sling apart faster but do make more power
as why i think is more dependable with great torque grunt over the x
plugs,,injectors,impeller,keeping oil tank full and making sure the emergency horn is hooked up
and working is normal maintainance to make last
yes 300pm and x are steel liner motors so no nic to peal or deal with
also if want to get em more power,,,mill the heads to 150compression
as stated after 5k motors go fat so dont have to retune if do mill to get to the 300hp spot
lots of original promaxes out there still running and also lots of x motors been rebuilt a few times...lol
love em both stock promax is dependable power,,,x with more work done is than stock is a blast

[DangerNewb]

I had one on my old Seebold Ski Eagle for a season. Ran great. Don't leave home without a full tank of gas. It was a true guzzler. Was also told the oil injection was solid and to leave it alone.

[sam hain]

Anyone know if these motors can be hooked to the merc handheld thing and pull the hours on it?

[David - WI]

Yes, the handheld DDT should show total hours and hours at each 1000 rpm range if I remember right.

[Basnova]

Recently purchased this motor to put on my 21xd Bullet, just wanting to learn about it from you experts. I was told it was one of the only motors Merc ever made that didn't live up to the numbers on the cowling, in that its really about a 280hp motor not a true 300 but had alot of torque. Also heard they are steel sleeved? The oil injection is still hooked up and I was told to leave it in place by a motor builder. Other than spark plugs and fuel filter what other preventative maint would you guys recommend? Assume I need to keep 93* octane in it? Any fuel additives? Right before I bought it the prev owner had carson reeds put in it and had the injectors cleaned/flowed.

I am far from one of the experts on here, but I have an original, stock 1998 I bought new. At 460 hours now, nothing has ever been done to it, aside from me replacing plugs, and the fuel pump in the VST.

Not sure if the following two items pertain to operational longevity or dependability or not, since these are external to the motor, but the OEM driveshaft has a tendency to become pitted at the waterpump seals, and the original was replaced early in 2000 or so. A contributing factor may be that, in those days, I used to frequent the brackish waters of the California Delta quite regularly. I say that because now, 20 years later, the replacement driveshaft is now pitted, and I attribute the longer time frame to the fact that I don't hit the Delta that often anymore since I live on Folsom Lake, and have mostly been fishing freshwater lakes. I was told that even though this model uses a 3-year waterpump, it should be disassembled and lubed yearly to prevent that pitting. I wish I knew that earlier, but the point is now moot because I will be replacing it with a stainless steel driveshaft shortly.

I also now need tilt tube bushings. Other than that, it's been pretty darned dependable for 23 years.

It does indeed call for 93 octane, and if I remember correctly, it is rated to gulp a whopping 30gph at WOT. I remember it also initially drank tons of oil, too. But, at one point I installed an exhaust ring at the prop, and I remember noticing that oil consumption went WAY down. I assume gas did, too, but I very specifically remember being shocked at how much less oil I was using.

As far as additives go, I used none whatsoever for 20 years, but 3 years ago started using Marine Stabil towards late fall, in case the boat sits too long during the colder months of Northern Cal, although I do try to run it at least once a month, or every 6 weeks, to prevent any kind of fuel-related problems in the winter.

Hope that helps.

Now, if I could only figure out how to determine tilt tube bushing part numbers...