weird A4 question

[steven]

Laying A4 up for the winter as usual with raw water cooling intake from a bucket of antifreeze.
Running very smooth when . . . piston#4 (furthest aft) stops firing and just clacks.
Other 3 cylinders run fine without #4 firing.

Anyone encounter this ? Most likely reason and how to firgure out whats wrong? (spark plug wire, coil, blocked fuel jet, . . . )
Tried changing spark plug but that didn't work.

thanks

--Steve

 

[AlaskanPuffin]

The first thing I would do is pull the plug and turn it over and see if it will spark while grounded to the block. If you are getting spark then check the fuel system and intake and exhaust valve function. If no spark then it's probably the coil or plug wire or possible distributor issue. I hope that helps.

 

[steven]

thanks. that helpful. --steve

 

[tenders]

The valves in that cylinder have a tendency to stick. I run a few ounces of Marvel Mystery Oil in my fuel all season to keep ahead of that tendency and it’s worked for decades. This has been discussed at the Moyer Marine forums, which are an incredible source of A4 expertise.

 

[steven]

Thanks for te MMOtip. De-winterizing this weekend so I'll try it.
Just to be clear, dumb question, do you add the oil to the fuel tank ?

--Steve

 

[gabriel]

The valves in that cylinder have a tendency to stick. I run a few ounces of Marvel Mystery Oil in my fuel all season to keep ahead of that tendency and it’s worked for decades. This has been discussed at the Moyer Marine forums, which are an incredible source of A4 expertise.

Theres been some talk about what valvoline claims is a new category of oil, their restore and protect, how it’s able to clean engine internals to factory over time. it might help. I would start by looking at the ignition system first.

 

[tenders]

Thanks for te MMOtip. De-winterizing this weekend so I'll try it.
Just to be clear, dumb question, do you add the oil to the fuel tank ?

--Steve

Not a dumb question at all. Yes, you add the MMO to the fuel, same as you would with an old two-stroke engine. I use 2oz of MMO per 5 gallons of fuel. It adds just a little more lubricity to the valve stem so that it moves more readily.

Going back to your original post, if you aren’t getting spark out of that spark plug when you take it out and ground it to the block while cranking the engine, you should check all the way back to the distributor to make sure all the components are intact. If the other plugs are firing, it isn’t the ignition, but you could have a bad contact at the rotor (misaligned), the cap (bad contact/corrosion), the plug wire at the cap (bad contact), the plug wire at the plug (bad contact), or the plug itself.

 

[gabriel]

There shouldn’t be any fuel getting past the valve seats and into the valve stem in a properly functioning four stroke engine. The valve stems are oiled by engine oil from underneath in a side valve engine. If valve suspected of sticking prob better to put the MMO directly into the bore.

 

[alcodiesel]

Gabriel- good point I had not thought of. I have been adding MMO per Tenders for a decade. Does it make me feel better? I don't know. I guess so. My A4 has been starting and running great. Absolutely reliable. Do I want to change anything that isn't broken?

Also, there is a site dedicated to the A4, dare I say "cult?". Great source of experiential knowledge: https://www.moyermarineforum.com/forums/index.php