Mercruiser 7.4l Bravo 3 missfire

[Dilligaff3561]

Hi folks, I have a Mercruiser 7.4l Bravo 3 combination that runs fine for a while, then after towing a child on an inflatable tow toy, starts running rough and missfiring and wont make any decent power or run over 2,000 rpm when under load. When I disengage the drive, it revs fine to over 3,800 rpm, but under load it misses and backfires and wont run over 2,000.
Anybody have any suggestions?

 

[wrvond]

Stop towing the child on the inflatable toy?

I'd suggest checking the coil, but I don't know what kind of ignition system you have.
Also, revving an engine to 3800 rpm with no load on it doesn't seem like a good idea to me.

 

[Dilligaff3561]

The boat is a '89 Slick Craft 279SC and I'm guessing the ignition is standard Mercruiser

 

[Scott Danforth]

welcome aboard

the issue could be fuel related or ignition related.

if ignition the usual culprits are;

• failing ignition trigger
• corroded cap and rotor
• plugs that need changing
• wires that need replacing

however it wouldnt be intermittent. it is rarely the coil

the usual fuel culprits are:

• dirt in the carburetor
• water or contaminants in the fuel
• plugged fuel filter
• plugged anti-siphon valve

how long has this symptom been going on?
how long have you had the boat?
when was the last time the cap, rotor, wires and plugs were changed?
have you inspected the contents of the fuel filter and changed the filter?

 

[Dilligaff3561]

welcome aboard

the issue could be fuel related or ignition related.

if ignition the usual culprits are;

• failing ignition trigger
• corroded cap and rotor
• plugs that need changing
• wires that need replacing

however it wouldnt be intermittent. it is rarely the coil

the usual fuel culprits are:

• dirt in the carburetor
• water or contaminants in the fuel
• plugged fuel filter
• plugged anti-siphon valve

how long has this symptom been going on?
how long have you had the boat?
when was the last time the cap, rotor, wires and plugs were changed?
have you inspected the contents of the fuel filter and changed the filter?

The boat is new to me.
The distributor and leads all seem OK
It runs fine until it's put under load with towing an inflatable.
The symptoms are missing and backfiring, not making power and not revving more than about 2,000 rpm - running really rough.
If the motor is revved out of gear, everything seems fine
Fuel filter all good - seems to be only when the motor is under load.
Is it possible for the coil or ignition unit to "break down" under load?

 

[Scott Danforth]

running in neutral takes 1/1,000,000th the amount of fuel it takes to run under load the fact that it is not spinning past 2000 RPM under load is usually indicative of fuel starvation from plugged carburetor passages. read this as it takes drops of fuel to rev in neutral and gallons of fuel to run the motor under load. reving in neutral means nothing

backfiring is indicative of a lean condition or improper timing. improper timing would be all the time, however under load its a lean condition from crud in your carburetor blocking off most of the fuel flow. it is called a "lean sneeze"

no, your coil isnt the problem. there is a 1 in 500,000 chance its your coil. (read that as it is almost never the coil).

however its a 1 in 2 chance its crud in your carburetor or water in your fuel, a 1 in 8 chance its bad cap/rotor, a 1 in 8 chance its plugs past their prime, and a 1 in 8 chance its your wires. a 1 in 500 chance on a sticking anti-siphon valve and a 1 in 10000 chance its the distributor ignition trigger

you cannot visually inspect the ignition system other than corrosion on the dizzy cap contacts

did you inspect the contents of the fuel filter for contaminants and water? unscrew the fuel filter and dump its contents in a clear plastic container (1 quart zip-lock works great). if you see a visual separation in the fuel, you have water contamination. put old filter (no mater how old) in plastic bag (same 1 qt zip lock) and throw away. replace with new filter (never reuse the old filter, it contaminates the clean side)

this being a new to you boat, you need to do every maintenance item on the boat to reset the clock so to speak

• new impeller
• new cap
• new rotor
• new wires
• new plugs
• new fuel filters (both the fuel/water filter, and the small inlet filter on your carb)
• verify timing
• new fuel filter
• oil change and filter
• lower unit gear oil change
• compression test
• inspect gimble bearing
• inspect u-joints
• check alignment
• inspect and adjust trailer brakes (replace shoes/pads)
• bleed trailer brakes
• check tire pressure
• inspect winch rope/strap
• verify operation of bilge pumps
• inspect anchor lines, replace as needed.
• clean and inspect the toilet
• verify all lighting and safety signalling devices
• replace safety flares

 

[Dilligaff3561]

running in neutral takes 1/1,000,000th the amount of fuel it takes to run under load the fact that it is not spinning past 2000 RPM under load is usually indicative of fuel starvation from plugged carburetor passages. read this as it takes drops of fuel to rev in neutral and gallons of fuel to run the motor under load. reving in neutral means nothing

backfiring is indicative of a lean condition or improper timing. improper timing would be all the time, however under load its a lean condition from crud in your carburetor blocking off most of the fuel flow. it is called a "lean sneeze"

no, your coil isnt the problem. there is a 1 in 500,000 chance its your coil. (read that as it is almost never the coil).

however its a 1 in 2 chance its crud in your carburetor or water in your fuel, a 1 in 8 chance its bad cap/rotor, a 1 in 8 chance its plugs past their prime, and a 1 in 8 chance its your wires. a 1 in 500 chance on a sticking anti-siphon valve and a 1 in 10000 chance its the distributor ignition trigger

you cannot visually inspect the ignition system other than corrosion on the dizzy cap contacts

did you inspect the contents of the fuel filter for contaminants and water? unscrew the fuel filter and dump its contents in a clear plastic container (1 quart zip-lock works great). if you see a visual separation in the fuel, you have water contamination. put old filter (no mater how old) in plastic bag (same 1 qt zip lock) and throw away. replace with new filter (never reuse the old filter, it contaminates the clean side)

this being a new to you boat, you need to do every maintenance item on the boat to reset the clock so to speak

• new impeller
• new cap
• new rotor
• new wires
• new plugs
• new fuel filters (both the fuel/water filter, and the small inlet filter on your carb)
• verify timing
• new fuel filter
• oil change and filter
• lower unit gear oil change
• compression test
• inspect gimble bearing
• inspect u-joints
• check alignment
• inspect and adjust trailer brakes (replace shoes/pads)
• bleed trailer brakes
• check tire pressure
• inspect winch rope/strap
• verify operation of bilge pumps
• inspect anchor lines, replace as needed.
• clean and inspect the toilet
• verify all lighting and safety signalling devices
• replace safety flares

Thanks for the comprehensive response.
The boat has a great service history and the fuel seems unlikely to be the cause because it will run fine under normal operation and the problem only arises after towing the inflatable.
Yesterday we ran the boat for about 25 minutes at 3,200 - 3,600 r[pm while underway and all was good.
Problem only seems to surface when towing the inflatable.
After the issue arrives, there doesn't seem to be any way of clearing the issue.
It seems to get worse and worse as you run the boat - revs drop and missing gets worse.
You can rev it easily at 2,800 and hold it there in neutral fine, but when under load problem is immediately there.

I will take your advice and check the fuel filter and double check fuel pressure etc.

 

[alldodge]

I think your running out of gas. Your under heavier load when pulling toys, so it needs more gas then even when just running. I would suspect, a weak fuel pump or anti-siphon valve.

Check your oil and see if it's showing higher then normal to check for a diaphragm problem.

 

[ottawamerc]

Possibly the wrong heat range on the plugs too, Does the engine heat up at all when towing? If it was me I would start with the ignition, a weak spark under load will cause all sorts of issues. I had a old 165 Merc that would run great then after a long run it wouldn't start. I eventually found the 12V lead to the coil had excessive resistance and was causing a weak spark that would not let the engine start. An engine under load will operate differently than one under no load if there is an ignition issue. You could pull your high tension coil wire and insert a screwdriver or similar (insulated handle) and start with a 1/8" gap have someone crank the engine and observe the spark, it should be crisp and blue if its orange there's a problem somewhere in your ignition system then try to increase the gap it should be strong enough to get to 1/4"+.

Scott

 

[tpenfield]

What happens if you don't tow the child around on the inflatable toy?

 

[Dilligaff3561]

Possibly the wrong heat range on the plugs too, Does the engine heat up at all when towing? If it was me I would start with the ignition, a weak spark under load will cause all sorts of issues. I had a old 165 Merc that would run great then after a long run it wouldn't start. I eventually found the 12V lead to the coil had excessive resistance and was causing a weak spark that would not let the engine start. An engine under load will operate differently than one under no load if there is an ignition issue. You could pull your high tension coil wire and insert a screwdriver or similar (insulated handle) and start with a 1/8" gap have someone crank the engine and observe the spark, it should be crisp and blue if its orange there's a problem somewhere in your ignition system then try to increase the gap it should be strong enough to get to 1/4"+.

Scott

I have tried this also with the trim tabs down and same result.
Seems to happen when the motor is loaded.
The fault wont clear when in idle for a few minutes as I checked for anything obvious, so would have thought fuel would have filled the carb at that stage.
Would an ignition unit breaking down give this result under load?

 

[Fun Times]

A 1989 Slick Craft 279 SC is a big heavy boat.. At what RPM do you tow the child?
If it's lower RPM to where the boat is hardly planning, you are creating the heaviest load which in my past experience sometimes has an tendency to affect a weaker ignition system first before having a clogged fuel system that unless the bow of the boat is up high in-turn allowing any debris inside the fuel tank to catch on to/clog the fuel pickup tube screen.

Not really knowing the history of the boat, performing a complete tune up with adding new Mercruiser OEM plug wires & inspecting the ignition coil would probably be a good starting point for you.

Also while in there, ensure that all the electrical wiring connections are tight and clean of corrosion and inspect/change the fuel filter too then go from there should the issues continue especially since it appears that you have to buy dock fuel which could be more contaminated vs buying fuel on land.

http://www.bing.com/videos/search?q=...E9&FORM=VRDGAR

 

[HT32BSX115]

unless the bow of the boat is up high in-turn allowing any debris inside the fuel tank to catch on to/clog the fuel pickup tube screen.

I would also inspect the fuel vent system. The deck angle may result in a partially or fully clogged vent and cause a vacuum in the fuel system that could overcome the fuel pumps ability to work. When the problem happens, stop the boat, remove the fuel cap and see if you hear a "sucking" sound of air rushing into the tank.

 

[Dilligaff3561]

Thank you everybody for your responses.
The issues have been uncovered - carb out of tune, ignition breaking down, leads and plugs too, compression's only 80 - 125psi, fuming / blow by - essentially requires a full rebuild.
Thanks again for the suggestions.