1990 135 mariner wonbt remain at same rpm underway

i have a problem with my mari

i have a problem with my mariner 135 that a repair shop claims has something to do with the boat and motor combination. sea nymph 20'.at 2500 rpm it comes up on plane but the motor will not maintain the same rpm it climbs to 3500rpm by itself without adjusting the throttle. they claim its the hull and motor configuration thats causing this. Im considering getting rid of this boat because in sea swells sometimes you just can't run that fast without geting beatup. At 2500 rpm you can feel the motor hitching trying to clear itself out then all of a sudden it does but climbs to 3500.. is this normal?I have a 40 mariner on another boat and it don't care what rpm it's at or if it's getting ready to plane it stays on that rpm.. i never heard of such a thing!

"Dave,

There are probably q

"Dave,

There are probably quite a few things that can cause that to happen, so you should get plenty of input here.

I would start with the throttle/timing linkage. If the throttle plates are not opening in unison it can cause strange things like this. Likewise, if the timing is way off, or is changing because of some problem with the linkage or trigger coil it can cause the rpms to move around.

You may have an ignition problem that is causing the engine to run poorly at low speed. When the rpms pick up it may be because the engine is starting to run better. I don't know the year or model of the engine, but you may have a stator with two sets of windings; one for low speed and one for high speed. The low speed winding may be starting to go bad.

The problem might also be a flooding carb. At low rpms there is too much fuel, but as the rpms pick up the engine can handle the extra fuel so it starts to clear out and run faster. If the mixture is much too rich at idle I guess it could cause this also.

A carb that is running very lean could also cause the rpms to rise up like that. Look into the carb bores as you rev the engine and see if you can detect fuel coming up through the main horn in each carb.

What I am saying basically is that the engine may be running poorly at low speed, so that in order to run, the throttle and timing have to be set up improperly. Then when the engine starts to run better at higher speed, the improper settings cause the rpms to pick up.

Examine the spark plugs for any sign that something is wrong. Sometimes this can give you a clue about where to start.

I'm sure others will have opinions as to the source of your trouble. I looking forward to hearing their thoughts about this.

"

" thanks tony..the stator was

" thanks tony..the stator was supposibly checked.. I explained to the mechanic what was going on , he even took the boat out for a test run and hull motor configuration was his conclusion. Needless to say i was upset with the answer for it. AS far as i'm concerned that motor, 135 hp should not have any problem pushing a 20' aluminum boat around , it may even be alittle too much for it.. by the way it's a 1991 magnum...top speed is about 58 mph gps...I know alittle about gas motors in general and it sure seemed to be a fuel problem to me. I really think a new boat mechanic is needed here. i just didn't want to blow him off.. then i found this website and the waters hard around here this time of year... give me an idea as to what i'm gonna do with this boat come spring.. I've only owned it for a year as you might have figured out. thats why i'm alittle discuraged with its performance , it can't be hull and motor configuration if it is i don't want it! "

"Dave..... Mariner engines are

"Dave..... Mariner engines are not my line of expertise, and Tony has forgotten more than I'll ever know about those engines, but I was wondering......

Does the boat sort of drop to a better plane angle just before the engine climbs to that higher rpm? That is, the boat is still hanging bow high ever so slightly at 2500rpm, then as the bow drops to a normal plane, the rpms increase to 3500rpm?

If so, that would be due to either a case of improper distribution of weight within the boat, the engine is set at a wrong attack angle, or the prop has too much pitch for that boat/engine combination. Just a thought.....

Joe (30+ Years With OMC)"

"Dave,

I would seriously co

"Dave,

I would seriously consider Joe's comments here. A little experimentation with weight distribution, attack angle, etc., might pay off.

I rebuilt a Force 40 or 50 HP outboard last year. I had it bored for oversized pistons, and it ran like a sewing machine when I was finished. It was on a 16 ft. Bayliner ( I think, possibly even shorter). A nice boat for a couple of kids to have lots of fun with.

It seemed like it should have plenty of power, and it did move well, but when I took it for a test run, all by my self ( 175 pounds ), it needed almost full throttle to get up on plane. If the throttle was reduced to about 2/3 it came off plane. I tried everything to no avail. I could see that with a couple of adults and maybe one youngster, the situation would be even worse.

I told the owner my observations, and he didn't mind; the boat was basically intended for his daughter and friends anyway.

As far as I could see, there was no way to remedy the situation. The boat was too short for the Bayliner hull form.

Tony"

ya i know what your getting a

ya i know what your getting at joe.It's hard to explain in words as you might know!The boat will come on plane at 2500rpm but right about in that area you can feel it lurching or in better words you can feel the motor trying to clear itself out but when it does it jumps to 3500 rpms which would be too fast for conditions in most cases.If i pull back on it alittle it will drop below planing speed and come off plane. There's no happy medium here.. Even if i was running at 3000 rpm's it changes rpm on me without adjusting the throttle.. it's like theres no fine adjustment on the throttle.I'm gonna take it to another place this spring for a lookover i'd just like to point this guy in the right direction... it sure seems like a fuel problem

" Dave,

I'm wonderi

" Dave,

I'm wondering what type of boat that is - an aluminum 20 footer that takes a 135 could only be a few different makes & models - is it a starcraft by any chance?.. If not and it's still a factory (not custom) make they all are pretty much the same shape and similar weight. I have a merc 115 L6 on a 21 foot starcraft cheiftan - everyone told me it wouldn't plane right, but it jumps right up on a plane and then to speed amazingly well. I've even got an extra 140 lbs of kicker and bracket w/ a retro-fit under deck type fuel tank - no problems. There's also another 21 foot cheiftan in my area w/ one of those 135 HP merc's, as far as I know it performs tip top.

Mine planes at about 15-20 MPH, and that's about 2500-3000 RPM's. It then will stay on a plane down to 2500 at close to 15 MPH, until I let it fall and then it goes back to displacement mode. I rebuilt the engine recently, and breaking it in at mid-planing RPM's sucked, to make it easier to keep it planed I picked up a hydrofoil. The planing increase with the foil was just enough to keep it planed easily in that 2500-3000 range. The other nice benifit of the hydrofoil was I was able to get more bite out of the power trim - this helps to push the bow down in rough ocean, allowing for a nicer ride and more control over the boat in general. Be sure your WOT RPM's are in the right range, as Joe stated. Mabye a hydro-foil and the right prop might fix it - what is the prop you are currently running?

Jon "

" not sure of the prop size ri

" not sure of the prop size right at the moment i think 15...? the boat a sea nymph 1995 fish and ski{i fish} no one skis!i also have akicker on it and a 20 gallon live well in the rear. i'm looking to get a 15hp kicker instaed of a 5 i've used last summer... but even without this xtra weight the problem still exists.. i'm sure not use to the gas this thing uses though. I also have a starcraft mariner with a 40 hp mariner magnum on it... i'm not new to boating thats why i'm alittle frustrated with this setup.. oh this thing will jump right up there on plane not a problem and i have installed one of those planer things on the lower unit just to try it out{it helped} but not as far as the hitching at 2500rpm's .. ever try one of those 150 dollar motor connectors for steering, whata peice of crap that thing is! anybody like striper fishing how about sharks!...we got a 9ft mako last summer next time were bringing a shotgun.. coast of maine "

" Dave,

As for the boat, my

" Dave,

As for the boat, my merc 115 has the standard sized (I think 13.25) prop from michigan with a 17 pitch. The engine is an 86 and it's the L6 design - I don't know how much the gearing is different in these two. For whatever good it does, I've got the standard aluminum (I think 13.25) sized 17 pitch from michigan. I get a top speed of 40 MPH on the GPS, and that's only when the water is glass - at 40 MPH it's about 5500 RPM's.

For the hell of it, you might want to try decarbing the engine. My merc (probably out of tune by my own fault) requires a de-carb every 20 hours - it tends to hesitate and then uncontrollably slam out of the hole around 3000 RPM's when it needs it. You might be experiencing something like this.

As for the weight of the kicker; move your batteries up to the bow (or in cabin or what-not); decent sized batteries weight about 65 lbs a piece - moving two of them is quite a weight distribution change. It will require better battery cable to get the power back to the engine (I used 2 guage anchor), but really can help lighten the stern and neaten things up. I hooked up my kicker with a second set of controls and it's own steering on the port side; a 25 HP Johson with electric start - gets me going 8-10 MPH. I've got a picture that sorta shows the setup, will post it below this message.

As for the fishing - I'm from Newton, NH. I fish mostly a little south of maine, Merrimac river/ Plum Island area and the coast up around Hampton, NH for stripers and blues. I shoot out to Jeff's ledge when the weather permits for cod/haddock/polluck, tried for sharks for the first time this season unsuccesfully. Hopefully a 9ft mako will come by my way, I'll settle for a 5 foot blue though. If you don't check them out allready, www.nesportsman.com and www.noreast.com are great local fishing sites.

Jon "

"My boat with the kicker setup

"My boat with the kicker setup:

"

looks alot like my brothers h

looks alot like my brothers he has a sylvan off shore with a honda kicker he hates his 150 dollar steering connector also.. those cabins are nice in the spring around here but imposible to bring in a big fish that likes to circle the boat a dozen times.. we run planer boards off it and get a few touge in the spring.. we mostly use the cabin to hold all our tackle our wives think we need another boat just to haul around with the tackle boxes and poles

" I actually never spend any t

" I actually never spend any time in the cabin, it's just pure storage - the bins/seats on the inside are covered with horizontal rod storage, the batteries and electrical stuff are all mounted there, along with anchors, buckets, life jackets, nets, gaffs, tools, 6 tackle boxes - you name it. At some point I'll put a porta-poti in there too. I've got a set of downriggers & outriggers I use for trolling - installed after this pic, brute force - 6 lines easy.

It does OK, the cabin makes them much safer for the occasional one over the bow, but these are just not offshore boats. Mine looks good in this pic, but it's pretty beat - had a 2 foot stress crack repaired last season, and have replaced a bunch of rivots too. I'm not sure if they just cannot handle the beating or this is normal condition for a 1974 .. It'll get me by until I can afford a Parket Deep-V or a custom aluminum ..

Jon "

" Jon,

Did you or someone e

" Jon,

Did you or someone else cut that transom down, or is that the original transom?

Just curious.

Tony "

" That's actually the orig

" That's actually the original sized transom - 21 inches. It's the second transom that the boats seen; when I first got it, I completely gutted the wood and started from scratch, but didn't make any modifications there. With the added weight in the stern, a 25 inch transom seems more fit though - the water comes up level with the bailing holes and waves often swamp the engine well.

Jon "

" Jon,

I thought the transo

" Jon,

I thought the transom height was even lower than that, but it is hard to tell from pictures. If water does swamp the well from time to time, you have to be careful. If the engine is in the down position, and you are not moving, the water can get into the lower cylinders through the exhaust, especially if the engine is not running. You probably know this already, but quite a few people are unaware that this can happen.

Tony "

" Tony,

Thanks, and y

" Tony,

Thanks, and yes, I am aware of that. The water is just a bit below the exhaust ports, when in reverse though it comes over them - lowering the RPM's, and sometimes stalling it out. There doesn't seem to be any easy solution though. I have put some thought into adding an engine mount / swim platform that could float things up a bit; have also thought about just adding a transom jack and using a long shaft outboard instead. The boat is actually rated for a 175 - definitly not enough beam there to hold one.

At this point though, I have some concerns about the structural integrity of the hull. When that stress crack appeared, it was heli-arc welded shut... a couple trips later it cracked along the outside of the weld. Reinforcing plates were then added to the inside to prevent flexing in that area, and it was welded shut again (so far so good). I've also replaced a abut 10 rivots. The issues seem to be in the forward 1/3, and mostly near the edges of the bottom side. From the inside, the aluminum looks new (after 30 years), but I wonder if the entire hull may just be fatigued and a lost cause.

I've begun putting together a 16 footer for small excursions this season and also so I'll have another boat to get me through, incase things really go to hell. It's sad though, because the cheiftan has a new floor, transom, custom livewell, electrical system, windows - a chunk of change tied up in materials/time that I won't get back if it's parted out - on the other hand, selling it as a package in it's current condition might leave me with less than I'd get parting it out.

Mabye it's possible to have some proffesional aluminum work done to make things sound again? Have you ever heard of any of these type issues and/or solutions (or knowledge of the opposite)?

Jon "

" Jon,

Most of the aluminum

" Jon,

Most of the aluminum boats I have seen were small skiffs up to about 16 feet. Problems always seemed to start with electrolysis; usuallly due to using the wrong anti-fouling paint.

There was a talented welder here who would fix things up at a reasonable price, but someone like that is tough to find these days.

Maybe it is time you started welding yourself.

Tony "