Carbon buildup and expectations and maintenance

Hi, I put my engine back together and tuned it about a month and 1k miles ago. I’m rather satisfied with the results (thanks HPA!). Got to 186 on a Dynapack with my NA b18c1.

Out of curiosity I pulled out a spark plug and took a look into the cylinder the other day. There is a film of carbon on the piston top. It’s soft, I stuck a long screwdriver in there and gently wiped across it and it easily wiped off and made a nice clean spot.

I know carbon build-up is normal, I don’t really know how much is normal and if I should be doing something at some mileage interval to keep that cleaner. I was also a little surprised how easy it was to wipe it off, the stuff I cleaned off the pistons when the head was off was stuck on there pretty good. I’m rather sure it’s carbon and not any sort of oil situation as the leakdown tester is near 0% leak and the valves and valve seals are new along with the valvejob done by PortFlow.

Other thoughts / questions: It’s tuned for stoich idle and cruise, maybe I should do a slightly leaner idle to help keep the chamber clean? Also I think I I had checked it after it had been idling for a while in the garage the night before. Perhaps the softer buildup varies in that if I checked it right after doing a hard pull it would be cleaner looking than after a period of idling? Some internet searching tells me that hard pulls clean it off?

Sorry, pretty hard call to comment on this without seeing pictures, but if you give it a hard run and recheck it without letting it idle you may get a totally different look in which case it could just be unburnt fuel. is it sooty-looking?

I’d say it was probably a damp sooty type substance. It did not stick to the screwdriver when I wiped some of it off / screw driver came up clean and basically dry. I think it must have been a little damp by the way it sorta brushed out of the way with the screwdriver. It was relatively thin in that you could see the piston through it though it and gave the piston sort of a textured appearance which is why I brushed it with the flat head screwdriver to make sure it was something on the piston and not the piston itself being textured. Sure enough it wiped right off with the flat head leaving behind a nice smooth shiny piston.

Would you have expected the piston to still be clean and shiny like new at 1k miles? How much carbon is typical at 1k miles? 10k miles, 30k and so on? I know by 100k miles there is typically a considerable amount of carbon on there but don’t know what the typical progression is.

do you have any pictures? it is always hard to comment on this sort of thing your interpretation of it may be way different from what I'm thinking

It sounds like unburned carbon, from low temperature and/or over rich combustion, that has been deposited on the piston and chamber. It would be compounded by the ignition being retarded from optimum.

For the most part, I'd suggest running light throttle at around 1.1-1.15 lambda, with more light throttle timing - but this will depnd on application and what you currently have.

Different engines, used different ways, will have different deposits. My "shopping basket", which has 98% town commuting and 2% highway driving and is set up for economy, had the very lightest of deposits after 170k km - so light I didn't bother cleaning it, as the piston, chamber and even the valves and ports had just a light semi-transparent film. Guides and seat faces were also VGC, so replace the leaking core plug and reassembled. Cranking compression is still top of OEM spec'n. I still get around 6k kms/litre of oil, which may be a factor as I believe it's mostly oil being oxidised and burned that causes most of the deposits.

On the other hand, back in the day, it wasn't unusual to see very thick deposits even on some relatively low mileage engines - but engine design and tolerances, and fuel and oil quality, has increased immeasurably since then.

We've got photos now. My apologies for my low quality borescope, it's the best that HarborFreight (cheap American tool store) has to offer. I've used the handy filters on my iPhone to try and improve some of the pictures (both versions are attached). There is a lot of graininess which overexaggerates the texture. The metal of the piston appears no more textured to me than when they were all clean. Also included pics of the spark plug from the same cylinder for the full picture.

Photo Guide:

A & A Filtered: Cyl #3, the filtered version looks more realistic

B & B Filtered: Another photo of the same thing and filtered version

C & C Filtered: Same thing more zoomed in

D: Extreme close up of the piston top. Very grainy makes it look way more textured than reality. Filtering couldn't not help this one. The shiny-ish spot is where I had wiped away some of the buildup with the screwdriver maybe 50 miles ago.

1, 2, 3 & 4: The spark plug photo taken under a nice bright light

5: The same spark plug picture taken outside with natural light very cloudy day

6: The same spark plug picture taken zoomed in and through a big magnifying light. I've circled in this picture a few specs on the inside wall of the plug which I want to know if should give me cause for concern of detonation?

7: I flicked the plug over a piece of clear plastic until the specs (along with some other carbon junk) fell out of the plug. We're not talking a lot of stuff here. Then I took a neodymium magnet to the specs to see what of it was ferrous which 99% of what came out was not. This photo is what is stuck to the magnet. It wasn't stuck very strongly, I could blow it off with my mouth. I think there was just a ferrous spec or two in there mixed in with what appears to be a hair that got picked up in the process and some carbon flecks. This is zoomed in like 10x.

8: The same mass of stuff that was stuck to the magnet but knocked back off onto the clear plastic on the table. This is zoomed in like 50x.

Hello, great pictures the issue, in my opinion, is in the injector or injectors i suggest flow testing and cleaning and retesting in the car. if it persists I would be looking further into maybe a weak/faulty injector drive or power supply causing the issue. and it is an issue that needs attention

Regards Ross

Thanks for taking a look at that Ross. If you don’t mind so I may be able to judge results without having to post back here, in your opinion what “should” piston tops should have looked like after 1k miles? Should they still have been spotless? And down the line what sort of progression of carbon ba time is to be expected / typical? Looking for some sort of barometer so I can evaluate changes and keep an eye on this etc…