Aircooled VW South Africa

I'm also going with coil here

Due to the cold weather, rain and strong winds, its really not nice to sit in the workshop and sling cold steel.

So I did a quick compression test, and found them all around 100psi.

Now I know I am supposed to test each piston 4 times with the accelerator full open and 20 turns of the motor, but this was just to see if we have an faulty chamber.

Knowing I have to take out the motor to fix the bell housing, and to clean up the one spark plug thread that was damaged.

So the main fault finding will take place on the bench where I have more room.

So till then

Herman

Good news about the compression.
With the engine stripped, you may still have a situation where everything looks fine.

Often best to create the situation where the fault occur. Then do fault-finding.

You right Dawie,

Will 1st Do fault finding before stripping the engine but it will be easier out the bus then in, as I will be able to get to all possible areas without bumping my head.Will start it on the bench and go from there.

Herman

Now this is whats going on in my mind,

She runs normal at idle,

She starts lightly poping as the speed goes up and develops in a splutter /backfire into the carbs.

The plugs go black if higher revs are maintained, but not on idle.

It has progressively become more dominant as the time has passed.

so it was a progressive fail.

Now I think it is getting a ignition before the inlet valve closes, or it has a hot spot in the manifold area that ignites the fuel.

It must be on the intake, as there is no backfire in the exhaust.

As it gets more prominent as the revs go up, I think it is advancing the fire more and more before the valve is closed.

Now this can point to the hydraulic lifters as Dawie has said, that they do not have time to close on time.....????

Will have to think some more and test on the bench

Herman

There is indeed a condition called "pre-ignition". Often confused with knocking.

Pre ignition have nothing to do with knocking. Not affected by ignition timing, as this already happens before spark ignition time. Damage can be more severe than knocking, bent rods,etc.
Caused by carbon build up in the chamber.
The aviation people are careful to avoid this condition, so aviation engine manufacturers often specify their engine oil formula to help minimise carbon build-up.

This could be it as I think the carbon would build up faster as the 1st layer is there, and a long run would build up even faster.

So lets see ...

Herman

Is the jetting and fuel supply same as your other engines which use those Weber 34ICT's?
Do they also burn their plugs the same powdery black during loaded conditions?

Out of curiosity...
Do you still use 145 main jets as per some earlier post, Which emulsion tubes and air correction jets?

popping from the carbs is your fuel ratio running lean
black soot on your plugs is your fuel ratio going rich or not burning properly

I reckon your jetting is wrong.
Remember, even though you buy your carbs from the gurus who can jet carbs to your engine without seeing your engine, each engine needs different jetting.

What fuel do you use? Go away from Engine and use Shell or Caltex and see if it makes a difference

Dawie wrote: ↑Thu Aug 20, 2020 12:46 pm Out of curiosity...
Do you still use 145 main jets as per some earlier post, Which emulsion tubes and air correction jets?

Yes..



Has always run well and cool,

If anything its run a bit on the rich side, looking at the plugs and fuel consumption.

Herman

retrovan wrote: ↑Thu Aug 20, 2020 11:24 am She starts lightly poping as the speed goes up and develops in a splutter /backfire into the carbs.

Does this go away if you put your foot to the floor?

No She gets to about 2500rpm and if you keep your foot flat she runs out of power and stays there out of gear.

When running on the road she will start dieing due to lack of power if going up a hill, on the flat she will slowly pick up speed but the back fire stays.

Herman

retrovan wrote: ↑Thu Aug 20, 2020 2:23 pm No She gets to about 2500rpm and if you keep your foot flat she runs out of power and stays there out of gear.

I had that recently but I cant even remember whose car it was on...
pretty sure it came down to the idle jets being too small.
try to open up your idle screws 1 full turn on each carb, test drive and see if it helps (it wont fix it, but it might help - you are just looking for an indication as to which way to go.
If it doesnt help, turn it back to original and then close 1 full turn

I'm pretty sure what is happening is that your idle jet is too small and your main is too big

Any chance of a bit of strange fuel...diesel, etc? accidentally found its way into the tank during fillup? You wo'nt be the first...