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Mercruiser 4.3 acceleration frustration

K

kirkll

Regular Contributor
Good morning gentlemen,

The big camping trip we had scheduled finally arrived last Thursday, and I had this old 1990 Blue Water looking pretty good for our trip to the lake.

Thursday she ran like a top all day. We spent and hour or so cruising around at various speeds, then pulled a two man tube for awhile too. The 19” prop worked just fine getting out of the hole. We anchored and swam, fished awhile, then pulled the tube some more. I was really pleased with the boat until we headed back to the dock that evening..... on our way back we ran about a mile or so at around 4200 rpms just fine, then the water flattened out and I ran it WOT for a couple minutes. No problem, she purred like a kitten...... I had backed off to about 4000 rpms, Then it just stalled out.... just like someone shut off the gas flow....

ok.... more info needed here. This 4.3 has a 2 barrel carb that was just put on the engine before I purchased the boat earlier this summer. It’s got an electric fuel pump that is obviously auto grade and needs to be changed out to a marine grade with a fuel water separator. But for now it is functioning properly, or was until this came up. I’ve probably only got 10-15 hours run time on this motor since I bought it, and it’s fired right up every time, the idle adjustments running about 6-700 rpms.

ok.... day two I took her back out, and she didn’t fire right up like usual. I needed to use the throttle only button and give it more gas to get her started. But start she did.... ran smoothly until accelerating past 3000 rpms. Then it would stall out. If I was quick enough bringing the throttle back down, it continued to run ok. If not, it would die completely.

I tried both slow acceleration, and quickly pushing it to the stop. Both ways ended up stalling out, but I could get the rpms up to 4000 for a few seconds at faster acceleration pushing it all the way quickly. She was just about out of the hole a few times, but stalled out every time.

so we found a little lagoon to anchor in and I pulled the engine cowl. It was taking more throttle to get it started by now, but she ran just fine out of gear at all rpm levels, but was idling a bit rough at low rpms. Up at 800.... purred like a kitten.

could you guys give me some suggestions where to start trouble shooting this? I think it’s as good a time as any to upgrade the fuel pump to a marine grade electric pump and get a water separator installed. But I’m not convinced the fuel pump is the issue the way it runs so well out of gear.

id appreciate any help you could give me on this..... thanks Kirk

here is a photo of the motor. https://photos.app.goo.gl/tPsp75gwuJwVNco19
 
First and foremost....
Operating a marine engine with a non marine grade electric fuel pump is....gross negligence...IMHO. The "marine grade feature" is not just more money or non rusting fittings, its a major safety issue.... and the insurance company won't pay claims if there are non marine grade parts where they are required in the engine compartment....and... can you spell BOOM?... But hey...it's your family...

Then...

1) There should be a Wide Open Throttle RPM range on a decal on the flame arrestor.
2) Your boat, when healthy, with 3/4 tank of gas and normal load of people and gear should rev to withing that range.
3) Noting that exact maximum WOT RPM you get in #2 above, for decent engine life do not operate continuously at
more than 80% of that value.
4) Operation at maximum throttle RPM should only be a couple of times a season for only a few seconds to insure you can
still get those RPMs.

Operation of the engine higher than the above parameters shortens engine life. If money is no object, ignore the preceding.

As to your presenting issues.... operation of a marine engine with ethanol laced fuel without a water separator eventually leads to operating "issues".
 
..." operation of a marine engine with ethanol laced fuel without a water separator eventually leads to operating "issues"."

You said it! I removed a 12 oz soda bottle full of water from only 4 gallons off gas that came from my local gas station. (Yes: I showed it to the owner!)

Before added a separator I was puzzled why my engine refused to start without a shot of gas or starting fluid. Doh!

Jeff
 
I sincerely appreciate all the info I can get gentlemen.

This is my first real boat with an inboard motor. I’ve been slowly going through all the maintenance getting the outdrive in order, all the battery wiring wiring replaced, new MBSS, bilge pumps, and had to replace the throttle/shift controls. I then replaced the prop and found the right one that does not bog down the motor, or run up over 5000 rpms at WOT. I do not run this thing hard. Average cruising rpms 3500-4200 and the only time the throttle hits the stops at WOT is coming out of the hole getting up on plane.

This boat was purchased with restoration in mind from an old boy who just replaced the engine himself. It wasn’t done by a shop and there were shortcuts taken in places that I am slowly making right again. The fuel pump and water separator is next on my list, as well as making sure I have marine grade fuel lines. I’ve got her going into the shop this coming week to replace the bellows and gimble bearings, and check out the upper section of the outdrive properly.

my question is this : could a drop in fuel pump pressure, or just a clogged up fuel filter that restricts the flow of fuel give me the symptoms I’ve described above? The carburetor is brand new, and it has ran perfectly until this last trip out. AND.... it still runs perfectly when not under a load out of gear at all rpm levels. If this was a poor fuel issue, or water in the fuel, it wouldn’t purr like a kitten out of gear. It would run rough as a cob.
 
If you are "blessed" with ethanol enhanced fuel, funky things happen in carbed engines including the growth of hard deposits on the jets that no carb cleaner or additive I've ever tried will dissolve.... mechanical removal only ( very thin wire)... The "big boys" on this forum dunk the whole carb in an ultrasonic cleaner... but I'm just an "amateur". If you got that much water out of the fuel, before installing a separator, then likely there is residual water in the carb itself. A good fuel additive/treatment will help with this.

If an engine runs poorly in neutral that surely is an indication of a problem, however the fact that it runs fine in neutral is not an indication of .. no problems. Fuel ( and air) flow rate increases drastically with load and RPM. In neutral, all most IC engines need to develop full RPM, is a relatively few HP to overcome system friction ( pumping and accessory drive) losses... waaay different than the full rated HP under load.

Which carb is on this engine???.... and REGARDLESS, you need a marine rated , i.e. ignition protected a.k.a. "explosion proof" fuel pump, a good fuel additive, and frequent dumps of the water separator until no more water appears ( after every 10 ish gallons of fuel run thru the system).

..... and if you were to put 1 gallon of fuel (with or without ethanol) in a sealed 5 gallon container that is vented to the atmosphere, in time almost all of the gasoline will be replaced with.... water. This will happen faster as the humidity increases
and day/night temperature extremes increase...
The process: gas forms a vapor, temperature increases, vapor expands and escapes... temperature drops, vapor in container condenses, sucking in moisture laden air, moisture precipitates out, temperature rises, vapor forms and escapes.... cycle repeats until so much gas is transformed to vapor that there isn't enough gas to turn to vapor anymore.
 
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Well I don’t believe I said anything about water in the fuel Capt Bob. What I did say was that with contaminated fuel, it wouldn’t run smooth at all. And this thing runs excellent out of gear at all rpm levels...

now I don’t claim to be a boat mechanic at all, but I’ve dealt with water in the fuel working on auto engines over the years, and this definitely is not the issue here.

It seems to be not getting enough fuel at higher rpms, and this happened rather suddenly. What I mean by that is....This isn’t and on an off situation where sometimes it runs ok In gear at higher rpms and sputters and craps out.... it runs smooth up until it hits about 3000, and stalls. If I push the throttle quickly to the stop it might smoothly reach 4000 rpms before it stalls. but a using a quicker acceleration it definitely reaches higher rpm levels before it stalls.


I realize that there are safety factors and features in marine inboard motors not found in autos. But these dad burn things are still basically internal combustion engines. I’m thinking a brand new carburetor with less than 25 hours on it isn’t going to have gumming issues do to poor fuel. Am I wrong about that?
 
With thirty year old tanks and plumbing, I think you were on borrowed time and thankfully you are able to report on this problem from dry land and not the middle of the big wet.
Do yourself a favor. Rip out all the old stuff. Have the tanks professionally cleaned. Replace all the supply lines with Shields Flex-acol, install new water sep filters, new fuel pump, new carb and then see what that gets you.
 
Well the fuel tank was replaced and a new 2 barrel carb installed at the same time the previous owner swapped out the motor. I bought it with about 10 hours on the “New motor” ..... all I was told about this “new motor” was “ I can’t remember the guys name I bought it from, but it’s a 1990 motor, and it runs good!” So with that outstanding information, I’ll be double checking everything that was replaced. I will be replacing the fuel pump, filters , and adding a water separator. All marine grade parts. The fuel tank does look new, but I’ll check that too.

Im taking it to the shop today to have the upper portion of the outdrive serviced, bellows, gimble bearings, and the lower shift cable replaced. the water pump impeller was replaced already.

I think I’ll have a professional take a good look at the motor for any other auto parts that may have been used on this so called new motor, and check out the throttle/ shift controls why it’s in there. I replaced those controls myself, and wouldn’t mind having them checked by a pro.

Is it possible that the lower shifting cable with the leaky bellows could be dragging and causing the shift interrupter switch to trigger while pushing the throttle forward? Thanks for you help guys! Kirk
 
kirkll said:
Well the fuel tank was replaced and a new 2 barrel carb installed at the same time the previous owner swapped out the motor. I bought it with about 10 hours on the “New motor” ..... all I was told about this “new motor” was “ I can’t remember the guys name I bought it from, but it’s a 1990 motor, and it runs good!” So with that outstanding information, I’ll be double checking everything that was replaced. I will be replacing the fuel pump, filters , and adding a water separator. All marine grade parts. The fuel tank does look new, but I’ll check that too.

Im taking it to the shop today to have the upper portion of the outdrive serviced, bellows, gimble bearings, and the lower shift cable replaced. the water pump impeller was replaced already.

I think I’ll have a professional take a good look at the motor for any other auto parts that may have been used on this so called new motor, and check out the throttle/ shift controls why it’s in there. I replaced those controls myself, and wouldn’t mind having them checked by a pro.

Is it possible that the lower shifting cable with the leaky bellows could be dragging and causing the shift interrupter switch to trigger while pushing the throttle forward? Thanks for you help guys! Kirk
Click to expand...
did you ever find out the problem? im experiencing the exact same problem. also running electric car pump, starting to think its the problem
 
hi. did you ever find the problem? im having exact same problem and also running electric car pump. starting to think the pump is the problem
 
On MERC engines, the water separtor in part of the standard setup fuel filter. Unscrew a nd dump into a c lear wide mouth glass jar and let it sit for 10 to 20 mins. Any water will settle to the bottom and be an indication of the quality of the fuel in the tank. Some carbed MERC engines have mechanical fuel pumps, some have an electric fuel pump.
 
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