3rd cylinder is running rough (1995 Mariner 50hp)

[Tyfab14]

Hi,

I have been trying to fix the 3rd cylinder running rough when revving.

I have:
- Changed fuel line,
- checked fuel pump - pumps fuel
- Added Water separator
- Changed filters
- Clean fuel and Used Can of Wurth to remove any water
- Removed the Carby and checked overall condition inside, No apparent gum
- Disconnected Rev Limiter
- Swapped Coils to see if the coil was burnt - Nothing
- Changed spark plugs
- checked reeds - all good, clean and closed
- Checked compression - 100 PSI on each cylinder

I removed the link arm between carbies and ran each separately. 1 and 2 cylinders run perfect but the first one cant rev.

I then saw a technique to see if the jet is clogged by vacuuming the carby with your palm.

I tested the 3rd cylinder by vacuuming it so that there is more fuel sucking into the carby. the Carby gets flooded and run really rough but suddenly works perfectly! but then goes back to running rough.

The thing that I also discovered is that the primer bulb isn't hard when running the engine

Here is a video to understand the issue and the vacuum technique.


Any thoughts?

[Noelm]

First things first, try another fuel tank, complete with a known good primer and hose, what do the plugs look like when removed, do they all look the same? Check spark on all cylinders, you cannot clean a blocked jet by putting your hand over the air intake, you might need to remove the carb and clean it, not wash it, or spray something on it, but actually clean all the jets and passageways in the carb body.

[Tyfab14]

Hi Noelm,

Plugs looks ok - Little bit oily on all plugs (means rich I guess).
I check sparks, all good.

I'll take it from there and get new tank and good primer, clean the lower carby and see how it would go.

[Noelm]

Make sure you clean it properly, just washing it so it looks nice is not cleaning, all passages need to be clean.

[stevej]

You don’t clean carbs
You rebuild them

[Steeler]

Absolutely no signs of water ingress in bottom cylinder ?. Brown spotting or beading on the plug ?.

[Tyfab14]

Absolutely no signs of water ingress in bottom cylinder ?. Brown spotting or beading on the plug ?.

Not at all the plugs are fine, just a bit oily on the 3 plugs

[Tyfab14]

Clean = rebuild for me I bought the kit just waiting for it to arrive. Will also change float needle and readjust it.

[Fed]

Sometimes you need to shake the plug into the palm of your hand to see the water.

[Steeler]

When you say you checked sparks post # 3 do you mean you performed a spark test ?. Was it a nice big blue spark ?.

[Tyfab14]

When you say you checked sparks post # 3 do you mean you performed a spark test ?. Was it a nice big blue spark ?.

I don't have a spark tester but it was a nice blue spark from the plug.

[Sore arms]

So it idles smoothly, but goes rough when you rev it?
Did the problem appear suddenly on an otherwise healthy engine or has it been sitting in the shed untouched for the last 20 odd years?

If it was running well before, my money would be on a blocked main jet.
The following is what I would be trying first if it was my engine.

Pull the fuel bowl off the carby of the bad cylinder and pull the jets from the carb and clean em out. You wont even need to pull the whole carb if you have room to get the bowl off in situ. I wouldn't bother with further disassembly just yet. If you do need to pull the carb to get the jets then you might as well pull the reed valve out and make sure its not cracked/chipped etc and there's no crap stuck anywhere.

Reassemble and run a short piece of hose from a funnel straight to the carbs, pour some fuel in and run it straight off that.
If that doesn't fix the problem then do a quick spark check.

Sometimes ignition components will bench test fine, but break down under the load of combustion pressure. Hit all three cylinders one at a time with your timing light. If they are all still firing and at the correct time while revving it then it's probably not an ignition issue.

Try that first and let me know how you get on, more often than not these problems are something very basic.

Just my 02c.
Alex.

[Tyfab14]

So it idles smoothly, but goes rough when you rev it?
Did the problem appear suddenly on an otherwise healthy engine or has it been sitting in the shed untouched for the last 20 odd years?

If it was running well before, my money would be on a blocked main jet.
The following is what I would be trying first if it was my engine.

Pull the fuel bowl off the carby of the bad cylinder and pull the jets from the carb and clean em out. You wont even need to pull the whole carb if you have room to get the bowl off in situ. I wouldn't bother with further disassembly just yet. If you do need to pull the carb to get the jets then you might as well pull the reed valve out and make sure its not cracked/chipped etc and there's no crap stuck anywhere.

Reassemble and run a short piece of hose from a funnel straight to the carbs, pour some fuel in and run it straight off that.
If that doesn't fix the problem then do a quick spark check.

Sometimes ignition components will bench test fine, but break down under the load of combustion pressure. Hit all three cylinders one at a time with your timing light. If they are all still firing and at the correct time while revving it then it's probably not an ignition issue.

Try that first and let me know how you get on, more often than not these problems are something very basic.

Just my 02c.
Alex.

Hi mate,

Thanks for taking the time to reply.

Indeed, it idles fine and run rough when revving.

I have been using the boat every weekend but last month it sat for 4 weekends in a row. When I went to the boat ramp, it started fine with a bit of choke and smoke as usual then its when I left the boat ramp that it started to play... All fine at idle and 4 knots then full throttle it sounded like the rev limiter triggered.

So I did all this diagnosis and added water separator, new filters and fuel lines just in case (the old filter was full sand). It had to be done years ago anyways!

You are probably right and I'll get a can of carby clean and rebuild the carbs as I bought the kit for the 3 carburettors.

[gazza2006au]

I have the same engine, your bottom spark plugs is breaking down and earthing the spark to the block just change your spark plugs mate should be all good

you can test spark plugs with a multimeter to see if they are earthing to the thread from the electrode it sounds like yours is working fine at idle but breaking down as u throttle up, its a common problem with petrol engines

[Tyfab14]

Hi Gazza,

I changed my spark plugs and checked the spark and it a nice and blue sparks nicely. I'l double check again when revving. thanks