Troubleshoot engine RPM hesitation at 4600 RPM

[ORDutch1975]

I am new to the forum - I hope you guys can help:

1. I bought a 1989 SeaRay 180BR with a Mercruiser 4.3L paired to a Alpha One out drive. After driving it a while I notices a hairline crack in the hull and took it to a repair shop where they fixed this issue. While they were at it I asked them to perform a tune up on my boat.

Pre-Repair - the boat ran great on the high end coming close to 50 miles an hour with a 14" 19P prop at close to 5200 rpm or there abouts. On the low end she would let out a lot of very rich exhaust - hence the tune up.

Post-Repair :
Boat runs great on low end and up to 4600 RPM at which point she hesitates - like she's cutting back - if I feather her I can make it to 4650 or there about but have to play with the throttle, its very clear she is not happy above this RPM speed.

Prior to taken her in I replaced the distributor cap and rotor

The work they did was:
Carb rebuild
Spark Plugs and plug wires and replaced fuel filters and fuel lines.
I had them replace the impeller and change the oil in the outdrive while they were at it.

Now they want to tell me the ICM is bad but cannot tell me this is really the problem. Hence 400 bucks and 150 in labor with no certain outcome. Total for the troubleshooting and hull repair I have already spent 5300$ on a boat work 6K at best.

I don't know that this is the issue - I also find it interesting the problem appeared after the tune up / spark plug replacement. They have not checked the timing and carb 3 times and keep telling me the issue is not fuel related as with the boat running and stumbling at 4600 rpm when they shoot carb cleaner into the carb it does not improve. They say its not fuel - which I tend to believe them on but I am at a loss and worried I'm just throwing good money after bad. They did a compression test and that was fine.

Any ideas - Is the ICM the next logical item to look at? Is it a possibility that they installed the wrong plugs or cracked one? Its all I can think of? I thought maybe the timing pickup in the carb but they told me no.

Thanks all!

 

[Alumarine]

Welcome.
I thought the WOT range was 4400-4800 for that motor.
You're just about at the top if your tach is right.

 

[ORDutch1975]

The problem is not the RPM but the fact that at WOT she hesitates and wants to slow down. the engine sounds off. I can't run her at WOT as she cuts back. IE full throttle causes her to hesitate. Something is not right that previously worked. Yes with the correct prop which she has now she should be at 4800 RPM but the fact is she won't reach it and she hesitates when I give her WOT. Like she is stuck - I have to feather the throttle before WOT. This problem did not exist before they did the work.

 

[ORDutch1975]

Ok so went to the shop this morning did a sea trial with their tech. RPM miss is not only at high end - its when ever you give her WOT between 3400 and 4400. They tested fuel and don't think the problem is there. Pressure looks good - filters look good etc. They really think its ignition - they found on of the spark plug wires had been arcing - they fixed this with new wires but problem persists. today they will be testing a new coil and if that is not the solution they will be ordering an ICM. I really hope this is the issue as its costing me more money than I care to spend. I thought it might be the ignition pickup but they insists this would not be the case with a magnetic pickup and no points. If I had points they would look at this.

 

[harleyman1975]

Im confused. What are you calling an "ignition pick up"? You wrote: [I thought it might be the ignition pickup but they insists this would not be the case with a magnetic pickup and no points. If I had points they would look at this.] If it had points I don't think it would have a pick up. If it is breaking up top end some things to consider are some of the stuff you mentioned (broken plug, loose plug wire, wrong plugs, plug with a closed/bridged gap) but how about possibly a weak or binding valve spring, partially plugged fuel pick up in the tank, partially plugged fuel vent, piece of debris in one of the high speed jets (yes even if the carb has been cleaned and rebuilt).

 

[Bondo]

today they will be testing a new coil and if that is not the solution they will be ordering an ICM.

Ayuh,.... Welcome Aboard,.... You've gotta learn how to be a Mechanic, or find a different repair shop,....

These things are Diagnosed, 'n tested, rather than throwin' parts at it, hopin' to hit the problem,.....

The fact they're replacin' the coil, tells me they're hacks that shouldn't be allowed to touch wrenches,....

These things are Testable, with meters 'n such,.....

All this, 'n it's Their screw up,.... Ya said it ran Great when they got it, they tuned it up, 'n now it don't run right,...
They should be payin' the tab,...

Btw,... A brand new Delco Voyager marine ignition system, complete withthe distributor, 'n new plug wires is 'bout 1/2 the cost of replacin' yer Merc T-Bolt ignition ICM,....

 

[alldodge]

Agree with Bondo find another mechanic.

Carb rebuild

My opinion is your carb is messed up, the float isn't adjusted correctly or something else done wrong.

 

[Rick Stephens]

Agree with Bondo find another mechanic.

My opinion is your carb is messed up, the float isn't adjusted correctly or something else done wrong.

Agreed. The most common poor repair job is a carb rebuild.

 

[ORDutch1975]

I am sick of all of this to be honest - I have yet to enjoy my boat. Here is where we are at - one of their employees bought a identical boat and thus they could swap parts. they finally figured out what it is - the "plunger" in the carburetor. I am not sure what they mean by plunger - my guess would be the accelerator pump plunger. Is this not included in a rebuild kit? For some reason I thought it was? They are going to "look" for a replacement and "hope" that I don't need to buy a new carb?

I feel like a lame duck working with people who are pulling the wool over my head. My guess is they are going to charge me for all of this even though they did the rebuild and prior no issues existed.

- thoughts?

 

[thumpar]

The "plunger" would most likely be the accelerator pump. It is part of a rebuild kit. It has nothing to do with WOT though. It is used on throttle up only. Once you are at full throttle it doesn't do anything.

 

[airdvr1227]

Welcome to the wonderful world of marina service departments. Some are good and some...not so much. Just remember you catch more flys with honey than with vinegar. If you can state your case in a non-inflammatory tone it will be better for everyone involved. Seems to me they screwed up the carb rebuild.

 

[Rick Stephens]

Scares bejeezus out of me to take things to a shop unless I know someone well. Hence, my policy is to learn to do it myself. Realizing lots of folks don't want to do their own repairs, it still is one of the best ways to not get screwed over. This forum is jammed packed full of do it yourselfers who would love to help.

Rick

 

[ORDutch1975]

So when they say plunger - is this the power valve ? Or is this the accelerator pump or are both of these the same ? They said the ominous plunger was not part of the rebuild kit. It's a 2bbl on a mercruiser 4.3. I just want to know what a 'plunger' is so I can talk intelligently when I go in to discuss the bill. Bottom line is that I told them multiple times the problem did not exist before they did the rebuild. If I got WOT without issue prior to taking it in and the only thing they did was a carb rebuild and plug replacement before the problem started I told them multiple times I thought it was carb related.. It had to be! They told me they checked 3 times and they verified they did everything 'perfect' lol. All troubleshooting imo from the carb rebuild on should be written off right? And yes I am in the service business myself so I know to be logical calm and kind when presenting my case as well as praising them for sticking with it etc... Just need to know what this ominous 'plunger' could be...

 

[DeepBlue2010]

Simply put, there is a time delay between the moment you push your gas ( open up some plates on your carb to deliver more gas and air) asking for more speed and the fuel system being able to deliver the required amout of gas to the engine.


Engineers compensated for this lag by adding something called accelerator pump to the carb design. When you push your controller forward (or push the gas on a carbed car), the movement atuate a pump momentarly to deliver the needed extra gas for the acceleration so the car/boat don't hasitate wating for the correct amount of fuel to arrive.


Once you stop pushing the gas or the controller, the pump is no longer actuated so it does nothing. Some people call this pump a plunger because of the way it looks. Old carbs were built with part that were desinged for the old fuel leaded/no ethanol including this pump. These days they make these pump from materials suitable for our current fuel. The pump right now has a blue band to identify it as ethanol friendly part. The pump is a part of almost all rebuild kits. Actually, some people rebuild the whole thing just for this new ethanol ready pump. Claiming the pump was it in the kit is PS like the shop itself and the team in it.

You made few mistakes in dealing/trusting this shop. Allowing them to charge you for their own screwups will be another mistake.

 

[JustJason]

1 Hook up a fuel pressure gauge and go run the boat. At full throttle you should be seeing at least 3 psi of fuel pressure. If you do not, either the fuel pump is weak or there is a restriction issue between the pump and the tank. Run it off of a portable tank filled with brand new gas next to isolate the tank and pump.

2. If fuel pressure is good. Run it on the portable tank anyways, if the gas is starting to go bad that will cause you to loose rpms as well. And there is no sense of rebuilding a carb over old gas.

3. If fuel pressure is good and still no change on running on a portable tank with new gas. Now it's a safe bet to rebuild the carb.