1994 Mercruiser 5.0 carb'd/thunderbolt ignition, popping and banging after a while

Marko_f1

Cadet
Joined
Apr 15, 2020
Messages
11
Hello everyone,

New to the forum here. Purchased a 1994 Bayliner 2452 which has a mercruiser 5.0/alpha1 last October. The boat sat for about 3 years before that. I changed the engine oil (Mercury 25W40), oil filter, fuel filter & spark plugs and it ran fine on the first couple outings. Last time I took it out, I probably had too many people on it (6 x 200+lb guys) and after getting on plane and cruising for a while, the engine starts getting louder and then it starts banging and popping and loosing power. If I shut it off for about 10 minutes, fire it back up, get it on plane and cruise at around 3,500RPM its fine for about 10-15 minutes before the popping and banging comes back as well as what sounds like a valve clicking. This problem occurs even when I am alone on the boat.

I got an older boat mechanic to take a quick look and one of the things he noticed was cylinder 1 & 8 spark plug wires were reversed on the distributor cap so we corrected that and it definitely starts and idles much better now. The two of us went for a little cruise and it ran normally for longer than before, but the engine getting louder and then popping/banging/losing power came back. The other thing I should mention is that the oil pressure drops slightly when this happens.

His theory was that it runs fine until the engine oil gets hot and that I may have the wrong engine oil in there ( he said to get 10W40) and also that I may need a valve adjustment. We also removed the distributor cap and what we found below was quite a bit of corrosion on the bottom as well as on the rotor. I got a distributor cap & rotor to replace and see how it goes from there.

Any ideas as to what might be causing this? Could just a corroded distributor rotor/cap cause all of these issues? We don't think its fuel related as there was no water in the fuel filter after we dumped it in a jar and we also ran it with the flame arrestor removed and there was no change in fuel amount being delivered in the period where the engine goes from normal to popping and banging.

Any and all tips and information will be greatly appreciated, thanks in advance!
 

TunaFish389

Petty Officer 2nd Class
Joined
Apr 26, 2018
Messages
184
First the 10w40 will be the same as 25w40 when warmed up, 40 being the hot weight. Plugs, cap, rotor and wires wouldnt be a bad idea. If that doesnt fix then you have to go thru make sure you are getting spark, fuel and air.. Might be a sticky valve but who knows.
 

alldodge

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Staff member
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Mar 8, 2009
Messages
43,145
Don't know what banging is but if it starts backfiring then its starts to run lean. Since it takes a bit of time, this is either a weak fuel pump, restriction in the fuel system feeding the pump (tank to carb), fuel tank vent line.

Distributor cap gets corroded over time from moisture, cleaning and filing can get it going again.

When it starts popping, stop and open the gas cap and listen closely to hear if air rushes in

If no air, then connect fuel pressure gauge to the carb using a Tee fitting. Pressure should be 3-7 psi from idle to WOT

Having to many folks on the boat will not be the issue
 

Marko_f1

Cadet
Joined
Apr 15, 2020
Messages
11
First the 10w40 will be the same as 25w40 when warmed up, 40 being the hot weight. Plugs, cap, rotor and wires wouldnt be a bad idea. If that doesnt fix then you have to go thru make sure you are getting spark, fuel and air.. Might be a sticky valve but who knows.

That's what I thought as well in regards to the oil, since its the hot weight that might supposedly be causing the issue he mentioned and not the cold weight. Plugs have been already changed, rotor and cap I purchased and just need to replace. Any tips in regards to removing a stuck rotor? On youtube I saw a couple guys breaking them apart instead of trying to pry them off, does that sound about right? The existing wires seemed fine and there was no corrosion at all on either end of them but might as well rule out the basics/easy stuff and replace those anyways.
 

Marko_f1

Cadet
Joined
Apr 15, 2020
Messages
11
Don't know what banging is but if it starts backfiring then its starts to run lean. Since it takes a bit of time, this is either a weak fuel pump, restriction in the fuel system feeding the pump (tank to carb), fuel tank vent line.

Distributor cap gets corroded over time from moisture, cleaning and filing can get it going again.

When it starts popping, stop and open the gas cap and listen closely to hear if air rushes in

If no air, then connect fuel pressure gauge to the carb using a Tee fitting. Pressure should be 3-7 psi from idle to WOT

Having to many folks on the boat will not be the issue

My buddy thats much more mechanically inclined than I am also mentioned a potentially weak/bad fuel pump. But what we did to check if its a fuel delivery issue, we removed the flame arrestor so you can see inside of the carb as fuel is being delivered in there and while I was driving the boat, he was looking to see if the amount of fuel delivered in the carb decreases once the engine starts acting up but it does not. The same amount of fuel was delivered during those 10-15 minutes of running fine, as it did once the engine started acting up.

The way it happens is it runs fine, and then the engine starts getting louder for about 20 seconds (almost like a bad exhaust leak) before it starts losing power and starts popping and bogging out, but does not die, I just have to keep it at full throttle for it to barely stay on plane.

Considering this, do you still think it may be a fuel pump issue?
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
43,145
If the popping is backfire then yes it could be fuel pump
If the popping is loosing spark (no backfire) then no
 
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