motor won't "rev up" with weight in back

[marineman16]

I have looked for a solution to this for a long time. I am not a guru with engines, and I'm pretty new to boating so I could use some help from you experts. I have an '88 Merc 150 Black Max on an 18' fish n ski. It is standard boat with 4 primary seats - 2 mid-boat console seats, and two rear seats in front of the fishing deck.

It is a strong-running motor with the exception of two issues that I'd like to clear up for the upcoming season.

1) If I seat people in the two rear seats and throttle up, the motor revs to just above idle then stays there, completely ignoring the fact that I'm increasing throttle. It won't RPM up to the point of picking up speed and planing - just enough to barely make it nose-up. And yes, I do know how to work the trim! LOL

2) With 1-2 people when I can actually reach higher speed, the motor surges causing random drops of a couple hundred RPM and a loss of a couple miles per hour.

I rebuilt the fuel pump and replaced the gas line and inline fuel filter last season thinking (or at least hoping) both problems were related, and likely a gas issue. However, the problem continued to worse slightly as the season went on.

Does anyone have any advice? Could this be a carb issue? I've had the boat for two years and I haven't had them cleaned. No idea how long it has been. Would there be some kind of a resistance sensor, breather hole, etc that may be forced under water just enough with the extra couple hundred lbs to the rear to make the motor not rev up?

Thank you very much!

 

[Bondo]

Re: motor won't "rev up" with weight in back

Ayuh,... I think both problems 1,+ 2 could be from Crap in the carbs...
Problem #1 could also have to do with to Big a prop,+ bad boat balance...

 

[marineman16]

Re: motor won't "rev up" with weight in back

Well, boat balance is an issue I'm sure, by nature of the boat's layout. I have 4 marine batteries (2 troll and 2 start), a 25 gallon gas tank, and a big motor hanging on the back of a not-so-bulky boat. However, I believe it has the standard stainless prop that comes on these, and the balance issue is the way the boat was designed.

It certainly doesn't seem like balance/prop would be the issue, but I don't know. On the water it feels/sounds like the motor is not drawing the horse to take off more so than the motor pushing but not having enough bite.

It simply doesn't respond to the throttle after the first little push out of idle. However, if there are only two passenger, it shoots out of the hole like a cannon. I don't get it.

 

[Chris1956]

Re: motor won't "rev up" with weight in back

You might heed Bond-Os advice. He know what he is speaking about.

I recommend you post the compression numbers from all cylinders, test to see that you have spark on all cylinders and post the pitch of the prop. if the compression is good, and the prop is of normal pitch, it is likely the carbs are dirty.

 

[marineman16]

Re: motor won't "rev up" with weight in back

I'm certainly not questioning anyone, but rather providing anything I can to get to the bottom of the issue. I appreciate all the feedback! Also, after thinking, I have had the boat 3 full season. 1 was a good one, 2 started the surging, and 3 was both issues.

I may be way off, but I really don't get the feeling that prop size/pitch or balance is the issue for several reasons (again, solely my observations):

1) It all started with the surging, then the bogging with passengers, and the issues are very slowly getting worse, especially early in the year (after winter takes a toll), so the sequence of events leads me to lean more to a gas issue than a configuration issue.

2) The bog down issue is pretty recent and the weight and prop configuration hasn't ever changed to my knowledge. Again steering me away from a configuration issue, solely based off observation.

3) Maybe the issues are unrelated, but is there any way the prop could cause the top-end surging?

I had the compression tested when I first bought the boat and it was fine all around. I guess it wouldn't be a terrible idea to look into it again since it was done before the issues came to be. I do know all cylinders get a spark. I had another issue last year where I replaced the plugs, tested them, etc. I'll post the prop size when I'm able to go look at it.

Aren't boats fun?

 

[j_martin]

Re: motor won't "rev up" with weight in back

Do like Bond-O says.

The easiest way to limit the cost of repair parts is systematic diagnostics.

You are obviously dropping cylinders

You may have lost compression

You may have ignition problems

You may have dirty carb/fuel problems

You have thousands of man years of experience trying to help you. Answer the questions.

hope it helps
John

 

[marineman16]

Re: motor won't "rev up" with weight in back

I definitely will. Is it natural for people looking here for help to be talked down to?

 

[fishrdan]

Re: motor won't "rev up" with weight in back

It seems to me that you may have several problems that need to be addressed. The most important is getting the outboard running properly, carbs and fuel delivery, what ever is causing the engine to be down on power and surging. (IF,,, the outboard were running fine, I would look at weight distribution.) After the outboard is running well, then look into the prop and weight distribution if there is still a problem. The prop might not be an issue once the outboard is running well, but with 4 batteries, a load of fuel and 2 adults in the stern (800-1000#?), I can see that there might be a weight distribution problem. Moving the trolling motor batteries to the bow would help a lot, along with any other heavy items, tools, anchor, etc...

For the outboard, get it fixed before it wipes out something. A (1) dirty carb could lean out a cylinder (or 2) and burn them up. Heck, the surging problem could be as easy as a loose fuel line or fuel fitting sucking air, causing fuel starvation.

I don't see how the prop could cause the engine to surge/stumble, that sounds like a fuel delivery problem. Do you have a water separating fuel filter, how old is the fuel, have you sampled the fuel to see if there is any water present?

Also, the problem you are experiencing has probably been discussed in an old post, might be worth some time to search old posts for similar problems.

 

[sschefer]

Re: motor won't "rev up" with weight in back

Your boat is pretty well loaded. My 18' F/S is simillarly load minus about 150lbs of battery and gas. Add a couple of 100lbs of passengers in the back and your engine is going to be down pretty far in the water. The symptoms your describing are what happens when an already weak engine encounters excessive back pressure. The cylinders are simply flooding with fuel and more throttle won't be of any help.

The compression test is the first thing you need to do. I believe your engine has releif ports so anything over 105 lbs and fairly even accross all 6 cylinders is good. If it doesn't have the relief ports then 125 would be about normal for your engine and its age.

Next, do a spark test using a spark tester. The best way is to get a CDI multi spark tester (I have one that will test 8). If the spark can jump a 1/4" gap and be bright blue then your good. If it's yellow or doesn't jump and all of them look the same then replace the stator, the low speed coils are going out. Remember the key is if they all look the same. If only one or two or three out of three look bad you need to look into the coils, switch boxes or trigger. Get back to us before you go there and we'll take you through that process.

Do those simple checks and get back to us and we'll guide you the rest of the way.

Steve

 

[j_martin]

Re: motor won't "rev up" with weight in back

I definitely will. Is it natural for people looking here for help to be talked down to?

It is natural (it seems) for folks with very low post counts to ask questions, and ignore the answers.

You have a problem, and have had for some time. Folks are giving you ideas and asking questions to help you. If you express a line of thinking that is incorrect, (like a 3 year old compression test has anything to do with the price of beans) you are likely to be corrected. How is that "talking down" to you.

If you want lots of very valuable help, this board is the place. If you have a thin skin, then maybe not.

hope it helps
john

 

[marineman16]

Re: motor won't "rev up" with weight in back

Hooked her up to the hose when I got home from work. Compression checks out, per my manual and the info on here. All cylinders run between 112 and 117, which is oddly consistent too, it seems. I guess that is good? I also checked the sparks the old fashioned way (ground each individually it and watch the spark) - hope that works? I'm not a mechanic, so I don't have the fancy tools. Luckily the engineer up the street had a compression tester and couple minutes to help a brother out!

 

[sschefer]

Re: motor won't "rev up" with weight in back

O.K. if you warmed up the engine first, then pulled all the plugs and grounded them and with the throttle wide open, cranked the engine and captured the compression then your numbers should be even with 10% and no more than 15% loss from new.

Since compression is fair the next step is to do a leakdown test. If you don't know how to do one ask your engineer friend to help. You'll need a compressor and the correct gauges. 2% is tops, 6% is rebuild, 10% is you waited too long and now it's gonna cost you.

Have a shop do the leak down test for you.. It's not hard, might cost you a hours shoptime but will tell you everything you need to know to move to the next level of troubleshooting.

After all, what's the point in throwing money at it if it's shot.

Steve

 

[j_martin]

Re: motor won't "rev up" with weight in back

I don't know that leakdown is that important right now. It's just a more sophisticated compression test that helps to measure engine wear, and give you a heads up when it's getting near rebuild time.

A thought came to mind. Is it possible that your boat is picking up water weight. You'd be amazed how much wet plywood or balsa core weighs.

As far as engine testing goes, a spark jumping the plug gap in open air is nothing like jumping the same gap under compression. That's why we use a spark tester where the free air gap can be adjusted more like 1/2 inch. Then you look for a sharp blue spark.

How do the plugs look? Each should have a brown paper bag color to the insulator. If it's black it's running rich. If it's light, it's running lean. If it's like new, you probably have water getting into that cylinder.

Next would be checking the stator and trigger outputs with a DVA, or a DVA adapter and a regular meter.

All good, then go after the carburetors. They aren't real complicated, but they are fussy about adjustment.

hope it helps
John

 

[Lone Duck]

Re: motor won't "rev up" with weight in back

I have looked for a solution to this for a long time. I am not a guru with engines, and I'm pretty new to boating so I could use some help from you experts. I have an '88 Merc 150 Black Max on an 18' fish n ski. It is standard boat with 4 primary seats - 2 mid-boat console seats, and two rear seats in front of the fishing deck.

It is a strong-running motor with the exception of two issues that I'd like to clear up for the upcoming season.

1) If I seat people in the two rear seats and throttle up, the motor revs to just above idle then stays there, completely ignoring the fact that I'm increasing throttle. It won't RPM up to the point of picking up speed and planing - just enough to barely make it nose-up. And yes, I do know how to work the trim! LOL

2) With 1-2 people when I can actually reach higher speed, the motor surges causing random drops of a couple hundred RPM and a loss of a couple miles per hour.

I rebuilt the fuel pump and replaced the gas line and inline fuel filter last season thinking (or at least hoping) both problems were related, and likely a gas issue. However, the problem continued to worse slightly as the season went on.

Does anyone have any advice? Could this be a carb issue? I've had the boat for two years and I haven't had them cleaned. No idea how long it has been. Would there be some kind of a resistance sensor, breather hole, etc that may be forced under water just enough with the extra couple hundred lbs to the rear to make the motor not rev up?

Thank you very much!

Bondo and some of the others have given you sound advice you should systematically check these suggestions . However! The part about no rev over idle when 2 people are in the back seat is bothering me also. Is there a chance that your fuel and control lines are running through the frame or under that seat causing lines to be pinched or filters to become slightly dislodged from fuel lines causing an air leak ? Or anywhere under the floor boards ? Or the fiberglass squashing them ? Sounds nuts I know , but stranger things have happened .

 

[Bondo]

Re: motor won't "rev up" with weight in back

Ayuh,... It's Good to hear you have pretty solid compression,...
I still think you're fighting,...
A) a Fuel Delivery Issue,.. The Surging...
B) a Over-weight/ Bad balance Issue, at launch...

Pull the fuel filters,+ dump 'em, inspecting what's in 'em...
Testing the fuel pressure, at WOT would help as well....

On launch,.. Have the passengers move forward, overloading the back of the boat is no doubt having a serious effect...
Also,.. Why so many batteries,..??
I can see the trolling motor batteries, but a single starting battery should do, especially when if it Died, you could still start off the trolling motor batteries...

Lots of, or more likely, All boats will struggle, or Not plane at All when too much weight is too far aft...

Btw,... How much Extra Junk has collected aboard over the last 3 years,..??

 

[marineman16]

Re: motor won't "rev up" with weight in back

Thanks again, everyone!

I'm going to try to touch on everything here. LOL

1) Water weight - I put in a new floor right after I bought the boat. Following much advice from this forum, I coated the ply with resin for longevity, so it wouldn't be taking in water yet. I also cut out the foam toward the rear of the boat cause it was soggy, obviously. Basically, there isn't much back there to get wet anymore, and the boat isn't known for taking on much water anyways. I can leave it int he lake for a full day and barely have anything to bilge.

2) Weight - I don't always have the 2nd start battery - only on outings where I know it will be in the water for days at a time. I should have clarified that. As for extra junk, I lack storage on the boat as is, so there is literally nothing but enough to keep my legal, and fishing and watersport stuff when needed. That's it. I should also add that I have tested with lighter people. I'm not putting 400-500lbs of beef in the back, but rather 300lbs or so between both rear passengers. I also only get as much gas as I plan to use so it doesn't sit in the tank, so 5-10 gallos is probably pretty standard. Again, not a ton of weight...

3) Gas flow - when I rebuilt the fuel pump, I did extensively test the pressure. It was spewing it out pretty well, but that wouldn't mean much if my carbs are trashed, and I don't have the tools to get a flow measure. I know the mid-size pitcher I was using to catch the gas wasn't lasting real long... LOL

4) Plugs - these are new and have only been used a few times at best, so it might not be best to use them for testing yet, but the seals still look pretty much brand new. There wasn't much crud on the threads or anything.

So if I am understanding, it sounds like my gut feel about fuel delivery may very well be the case? Are carbs the next step? Would the loss of power from them being fouled up be enough to cause both problems?

 

[sschefer]

Re: motor won't "rev up" with weight in back

J_Martin - Re the leakdown recommendation. Just read between the lines, you'll see the method to my madness

 

[j_martin]

Re: motor won't "rev up" with weight in back

J_Martin - Re the leakdown recommendation. Just read between the lines, you'll see the method to my madness

Musta been past my bedtime.

 

[marineman16]

Re: motor won't "rev up" with weight in back

So I should perform the leakdown test before having the carbs cleaned? I don't even know what a leakdown test is, so my thoughts are that I may just take the boat to the shop and let them have at it.

However, any advice I get beforehand would be great for discussion with them!

 

[sschefer]

Re: motor won't "rev up" with weight in back

I think you're making the correct choice by taking it in. From this test you'll learn the true condition of your engine. Hopefully the tests will show no problems and when you explain the problem you're having to the shop they should be able to evaluate your setup and give you the best possible advice. They may see the problem right away and the leak down test might not even be needed.