not reaching max RPMs

G

ghuskin

New Member
Jul 10, 2007
1,089
Lake St. Clair, DYC
Boat Info
SRV225 Cuddy Cabin, 1983
Engines
228 Mercruiser w/ pre-Alpha1 "R" Drive
correct prop (according to SeaRay) is 14.5 x 17 and turn 4600 RPMs. they only make 15 x 17 (on boat currently) and max out at 3800 RPMs. 30 mph on gps.

new plugs. bottom clean.

I will be replacing points, condenser, rotor, and cap.

Is there anything else that might be causing me to be 800 rpms short of max? or will this fix me up?
is the slight difference in prop size an issue?
thank you in advance.

Gregg
 
Last edited:
Hard to say... you could be in desperate need of a tune-up...

Or your engine could just be a little bit tired. The prop size you have on there is the modern day "equivalent". I went through this last year trying to get the factory advised prop for my boat.

I installed a factory spec (modern equiv) on my 160, just to see where my RPM's and performance would be... I'm within my operating range, but only by a hair; I put back on the old prop and I'm saving my Apollo until I do my rebuild. My engine is just simply tired.

As a added note, I wouldn't waste my time or money on points and condenser... for about $150 you can convert your distributor from points to magnetic pick-up; it will be MUCH more accurate and then the only maintenance required will be cap and rotor change. No more points to adjust and replace, no more condenser to replace.

You're looking for something like this (but for your distributor):
http://www.go2marine.com/product.do?no=181381F
 
nate,

there was a post on that a while back. I think the concensus was that it wasn't worth the money.

the engine only has 800 hrs. not excessive, but who knows how the previous owners took care of her?

I really don't want to re-power. I also don't need a speed demon. I'd just like to operating efficiently as possible.

Is this 800 rpm differential something to be concerned about?

I do cruise on plane @ 3300 rpm, approx. 25 mph (GPS).

Gregg
 
To tell you the truth, I'm not really sure. I'm kind of new to boating as I just started last year.

But, I kind of look at it in terms of automotive performance; you could say a prop is like tires... but, most people don't change tire size to increase/decrease acceleration and top speed; generally it's done with rear end gear ratio. So I equate the prop to gear ratio on a car... run gears that are too high and you don't transfer enough torque to the wheels; it will accelerate, but at a slower pace, and if the gears are high enough, it will have a hard time overcoming the simple gravitational and wind resistance; limiting it's top speed (also it's peak RPMS).

So by being 800RPM's shy of your advised range, you're acceleration is suffering, your top speed may be suffering and your efficiency is bound to be suffering as well. Not to mention because the engine isn't in it's peak power band, it's also incurring more stress.

Again, this is all theory, applied from the auto world to the boat world (and despite what some would say, auto engines and boat engines are like twins; almost identical but with different personalities).

I wouldn't think 100-200 RPM would be that bad, but 800 seems like a lot. On a side note, how much weight are you carrying? Full load would without a doubt prevent you from reaching spec; and likely by a decent margin.



As far as the electronic conversions... I'd have to argue about if they are worth it or not... Virtually every performance engine application today that does not run distributor-less ignition... runs either magnetic or optical electronic distributors. Mercury stopped using points in the early 90's (if points were "good enough", they'd have left "good enough" alone). Lets say your distributor tune-up kit is $30 and it takes you an hour to install. So for the price of 5 tune-up kits and 5 hours of your time (or, worse yet, 5 hours of labor; or my marina's flat $140 tune-up price), you can have a more accurate ignition that will never again require adjustment or replacement of parts other than cap and rotor. Now if this conversion kit were $300+... sure, I can see how it might not be worth it to you (me, I just install a new distributor and full ignition system because I can't seem to leave things alone!).
 
Just a thought, what do your spark plugs look like? When you remove the oil fill cover while the engine is running, does a pretty stout wind rush out like there's an air pump under the valve cover?

I would still be interested in knowing how much extra weight you're carrying; could easily be something that simple.

Bottom have any fouling on it? That will limit your top speed (and there by, decrease top RPM)
 
just had myself and my 8 y.o. son. Bottom is pretty clean. had between 1/4 - 1/2 tank fuel.

It's definately a "dog" with full fuel and 7 on board, but will still cruise 3300 rpm at 22-23 mph (gps).

Never ran the engine with the oil fill cover off. is a strong "wind" good?

the plugs were put in new last year, after i bought the boat. they only have about 50 hrs on them. I will replace them when i do the points, etc.
 
Boat motors get chronologically old, but unless they are really abused or suffer some kind of failure they don't usually wear as badly as car motors in the same time period.
Think about 800 hours on a boat motor over a 20 year period, then think about how quickly you put 800 hours on a car motor. Better yet, think about how many hours a car motor runs over a twenty year period.
I've taken apart a fair amount of boat motors over the years and there is rarely any signs of excessive wear to the cylinder walls, cranks, or bearings unless something else (usually faulty manifolds or risers) cause the engine to fail.
Start with a good old fashioned Tune Up. Check the spark plug wires while you're at it. They are relatively inexpensive. Make sure the timing is set properly also, lots of people don't check timing anymore. I loan my timing light out so much that sometimes I feel like I'm the only guy in the neighborhood who still has one.
I would re-consider the conversion from points to whatever replacement you like best. Points are cheap, but can be a real PIA and they always seem to fail at the worst possible time. The conversion will no doubt cause your motor to run better and more efficiently. If you decide to stay with the points then I would suggest that you keep an extra set aboard along with the necessary tools to change and gap them.
Back in the old days I always kept an extra set of brand new points and a condenser in the glove box of whatever car I owned at the time.
Just as a side note: I have a 68 GTO that I fully restored to 99.9% stock condition. The only changes made were minor ones that I thought would be barely noticeable and would improve safety or performance. The guideline I set for myself was to not install anything that couldn't be easily reversed back to stock condition in a few minutes. One change was a conversion from points to a Petronix set up that is hidden in the cap. There are much better options out there, but I chose this one because it only takes a few minutes to install, needs no permanent alteration, and is well hidden under the stock cap. The Petronix let me run a slightly hotter coil, and I haven't had to touch it since I first installed it about 8 or 9 years ago. The only other thing I had to do was have the stock Alternator's output boosted a little bit to compensate for the extra draw from the hotter coil at idle.
If you go with an electonic set up you might want to consider changing your Alternator. It isn't absolutely necessary, but it will help with the extra draw from the hotter ignition.
Overall, I feel that not only did the conversion make a big difference in the car's performance, but it eliminated the maintenance that points require and the possibility that they would fail on me.
I imagine the performance gains would have been even greater if I had gone with a different option that would have let me gap the plugs a little wider.
I think the conversion would be money well spent for you.
 
will i need a hotter coil with this set-up?

You guys are making my head spin!!!

While I can and have done a few mechanical things, timing/points/condenser are not among them.

I'm fairly certain that I can do this, I'm just nervous about going it alone.
 
Ok, I just found out that my engine does not have points! Apparently Merc used what is called "thunderbolt" ignition for approx 3-4 years, and I have one of them. thunderbolt is a breakerless ignition.

Tune up will be much easier than I thought.

Mechanic also thinks that my tach could be out of calibration. So, off I go to Sears for a shop tach to verify rpms.

Thank you for all your help.
 
Well, the wind coming from oil fill is blow by of some sort; either valve seals or compression rings. I'd have to agree with JVM in that boat engines don't "wear out" the same that auto engines do. BUT, a very real problem that boat engines can have is seal failure due to dry-rot. Seems boats have more of a tendency to sit unused for a couple years than do cars. This is where valve seal failure could be an issue. BUT, that shouldn't make for a big performance drop. Compression ring wear (or cylinder wear more likely) would. Lack of good maintenance could cause this.

BUT, what I was getting at is, look at the easier stuff first; if your bottom is clean and you're running light (which I'd say you are), it might be an issue with tune-up related items (again, agreeing with JVM). Also, your mechanic could be right about your tach just being inaccurate. Either way, plugs, wires and coil certainly won't hurt... and stand a good chance to actually help! (I noticed a marked improvement in time to plane when I did my MSD Captive Discharge ignition, Blaster coil, and Mallory wires... hopefully the my new distributor will be even more of an improvement!)

As a side note, doing ignition work isn't hard at all so long as you have a timing light. The only mildly challenging thing to do is set the timing (installing a new distributor is a little more advanced, but if you can follow directions, still within reach for just about anyone). Wires, plugs, coil... all super easy. IF I'm in the area this weekend, I could bring my timing light (actually, my neighbors; pulled mine out yesterday to find it was broken!!!!) and give you a hand.
 
NateRW21,

If you'll be in the St. Clair Shores area, let me know. A hand would be appreciated. Just let me know your favorite beverege!

do you have a shop tach? I've been looking online, and can't seem to find what i need. any advice?

my initial plan was to work on it this p.m. (only working 1/2 day).

Gregg
 
Unfortunately, no I don't. I was pretty much at the mercy of my dash tach when setting my timing. But, mine seems fairly close to accurate judging by engine sound and what the tach shows when my rev limiter kicks in (tach shows whatever I have the limiter set to).

I'd say go to napa or carquest; they should be able to get you a shop tach.

Still not sure if I'll be in your direction or not; I'll let you know!
 
ok...

I'm really at wits end. turns out i do have points/condenser. Replaced said parts, along with spark plugs.

Now the engine doesn't run!?!?

What the heck is the trick to setting points?

Sounds like I'm getting the electronic replacement...
 
Nate,

Just got your response. I already beat you to the punch.

What a great decision! If anyone tells you they want to use points/condeser, tell them they need to have their head examined! (that;s the PG version)

Anyway, the boat pulls strong out of the hole, strong at all power bands, and I got about 4400 rpm in not so calm water. I am very happy!!!
 
Awesome! Glad to hear it!

Last summer I fought with ignition problems all summer long; eventually replacing the points and condenser and pretty much solving the issue. Mine acted a bit different than yours though; it would run well for an hour to two hours straight, then just become a total dog. So bad it wouldn't get on plane (or worse yet as it did in the middle of a 5 mile long lake... not run at all)! Anyhow, I made the determination that come spring/summer this year, all of that old, out-dated ignition was going to be replaced by the best I could find. Less headaches and more time on the water are really the big benefits.

I don't know if someone else chimed in or not, but you may want to consider (provided you've not already done it) new plug wires and a new coil. The wires, look for something spiral wound so that the higher output doesn't cause RF interference or spark jump. And the new coil... well, that's just giving even more of a boost (plus, over time coils do "wear out").


ANYWHO!! It's great to hear all went well, you're back on the water and shes pulling hard!
 
Good advise from Nate re: changing the coil. If it does'nt make any differance its always a good idea to keep the old one on board as a spare. Had a 1978 Searay 26 weekender and did have a coil fail once but bought a spare only a couple of weeks after I bought the boat, was really glad I did. Glad to hear your boat is running much better, can't beat electronic rather than the points, good move.
 
well,

my son and I went out for a little ride yesterday pm, as he wanted to see what all the hoopla was. about 20 minutes into the ride (it was rather rough), we decided to cruise a local marina.

Guess what, the outdrive starts to act funny. looks like i need to rebuild the trim cylinders, as I was warned last year to keep an eye on them.

I'm hoping that this is a DIY project...

Maybe next year I'll go with a "hotter" coil, and a newer ignition.
 
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