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Does anyone know why BOTH of my trucks have the habit of running on three cylinders after a steep eight mile descent?
It's easy enough to fix (pulling and cleaning the plugs) but this is becoming quite annoying.
Ideas?

Upper gasket on carb allowing fuel from the bowl to leak into venturi?

How are the plugs fouled?

Oil I would suspect. With the engine on the over-run and little to no fuel getting in due to being off the gas there is not enough combustion to burn the oil that blows down between the valves and valve guides. To add to the problem the engine will be on the over-run and with the throttle closed the air flow is restricted, the revs are high due to the over-run and these factors cause a significant vacuum in the cylinder on the intake stroke. Without oil seals on the valves the vacuum will suck oil into the cylinders at a higher than the usual blow by rate. Welcome to the nature of the beast.

As for solutions, some will recommend valve seals. I would not recommend this as the oil loss is insignificant and keeps the valves well lubricated and limits overheating in the head. In your case I would suggest trying a down and back approach. Try 2km downhill and then turn back up for a half a km to burn the plugs clean. If it works try playing with the distance downhill and uphill to keep your plugs clean. It will take time and increase the distance but it may trade off the time you will save not taking out seats, removing the dog house and cleaning plugs.

Good luck. Let us know how you make out.

Cheers, Clive

63rover wrote:How are the plugs fouled?

Oil I would suspect. With the engine on the over-run and little to no fuel getting in due to being off the gas there is not enough combustion to burn the oil that blows down between the valves and valve guides. To add to the problem the engine will be on the over-run and with the throttle closed the air flow is restricted, the revs are high due to the over-run and these factors cause a significant vacuum in the cylinder on the intake stroke. Without oil seals on the valves the vacuum will suck oil into the cylinders at a higher than the usual blow by rate. Welcome to the nature of the beast.

As for solutions, some will recommend valve seals. I would not recommend this as the oil loss is insignificant and keeps the valves well lubricated and limits overheating in the head. In your case I would suggest trying a down and back approach. Try 2km downhill and then turn back up for a half a km to burn the plugs clean. If it works try playing with the distance downhill and uphill to keep your plugs clean. It will take time and increase the distance but it may trade off the time you will save not taking out seats, removing the dog house and cleaning plugs.

Good luck. Let us know how you make out.

Cheers, Clive

You sound like you understand my issue pretty well. Makes sense.
The problem is, these are commercial 4x4 tour trucks full of guests on schedules. Soooo... unfortunately turning around on my way down and going back up again just isn't going to happen.
Sometimes after I get down to the (flat) valley and drive around for a bit, they will fix themselves and start running strong again... but not always. And cleaning the plugs is the only thing that I can do to make them run right again.
Any other suggestions or preventative maintenance that I can do?

Explain the quirk of the old tech to your passengers and tell them you don't want them to miss out on the view from the other side.

If you experiment you may find that only one double back may be needed as you say the trucks often clear themselves after the full descent. One loop should cost less time than wrenching and hopefully will keep you on schedule.

As for other options, I'm afraid it is what it is. However, I have found that I do use less oil by running 15-40 Rotella with 1 Lucas additive. It might be worth a try. Is this a new problem with different oil after a change?

This has pretty much always happened (11 yrs now). I have always used 20W50 and noticed that it happens less when I only fill the oil level to half way on the dipstick. But it still happens.
I guess I'll try a different oil grade.
Thanks again.

20w50 is heavier and should be fine. Try adding a quart of Lucas additive.

I cannot comment on the reasons but as you know of the issue, just give the engine a squirt of revs every now and then to keep the plugs clear. I have a similar issue with my FC 101 on gas (LPG) - on extended trailing throttle like the situation you describe the metering is not accurate enough to provide correct mixture and is too rich and at the bottom it misses etc for a while but thankfully not because the plugs are fouled as LPG is too clean - once the rich gas mixture is cleared all is OK. I just give the throttle a blip every now and then and it seems to keep things all clear.

garry

Garrycol wrote:I cannot comment on the reasons but as you know of the issue, just give the engine a squirt of revs every now and then to keep the plugs clear. I have a similar issue with my FC 101 on gas (LPG) - on extended trailing throttle like the situation you describe the metering is not accurate enough to provide correct mixture and is too rich and at the bottom it misses etc for a while but thankfully not because the plugs are fouled as LPG is too clean - once the rich gas mixture is cleared all is OK. I just give the throttle a blip every now and then and it seems to keep things all clear.

garry

I tried this yesterday and it seemed to work. On my eight mile descent, I stopped and revved the engine a few times about every two miles. Whenever I did, It blew out some noticeable blue smoke (burnt oil).
When I reached the valley It didn't stumble like usual and blow out black smoke (unburned gas) this time.

Thanks again guys!

You might try hotter spark plugs. The tips do not conduct heat away so well so they do not foul so easily.

That's true for carbon but not much help with oil.

What are some hotter spark plug models then? I have the civi electronic ignition.

The champion spark plugs use to have a pattern in the number so that if you went one number higher it was the next hotter, or maybe the next colder. probably in their catalogue.

Here is the Australian catalog. Numbers look the same. Smaller numbers are colder, bigger numbers are hotter.

http://www.google.com/url?sa=t&rct=j&q= ... 2980,d.cWw

NGK chart
http://www.ngksparkplugs.com/pdf/DYK/DY ... Ranges.pdf