incomplete combustion diagnosis

**janmike** · August 8th, 2008 01:17 PM

i have signs on my spark plugs that my engine is having an incomplete combustion (black carbon deposits). aside from not knowing the cause i am baffled that most of my spark plugs suffer the same condition except for my no. 4 which is clean and (it seems as) showing a complete combustion! any ideas please?

**ghosthunter** · August 8th, 2008 01:31 PM

Few info first

What is your car. How old and mileage? What previous repairs (if any) has been done?

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You sure you aren't burning oil? Have you checked your oil level lately? Maybe a compression test would solve some light?

**[archie]** · August 8th, 2008 01:46 PM

^^ +1 parang may pumapasok na oil sa chamber. usually if the tip of the spark plug is black then its burning oil.

**janmike** · August 8th, 2008 02:47 PM

no oil..just dry carbon deposit except for 1 plug. its an old 2T engine btw

**ghosthunter** · August 8th, 2008 03:52 PM

Originally Posted by janmike

no oil..just dry carbon deposit except for 1 plug. its an old 2T engine btw

definitely do a compression test.

**janmike** · August 8th, 2008 04:44 PM

been digging the net and i have read a few possible causes as to why. this include over rich a/f mixture(carb), ignition timing, valve leaks and intake manifolds including low compression. additional info is the engine eats up lotsa gas, carb obviously not in good condition, no knocking, pinging or hard starting, smell of unburned gas in exhaust, backfires on decelleration and loss of power on steep climb.

*ghosthunter
if bad compression is found would the engine require overhauling? TIA

**janmike** · August 12th, 2008 10:57 AM

i started fiddling with the engine beginning with the carb, i'm going to test first for a very rich mixture diagnosis that may cause the incomplete combustion. visual inspection and looking down the throat of the carb gave me a picture of how it works. with a bit googling and youtube class i had a good idea where to start. float seems to work fine (i can see fuel in the bowl and looks like it's closing because the flow of gas stopped on my soda bottle).
my question is, before i start tweaking, when we talk of A/F mixture adjustments are we talking about the idle mixture screw? and what does it regulate the air or the fuel? like if i fully closed it does it mean i'm starving the engine of air or fuel? or both?
help please!

**kinyo** · August 12th, 2008 06:08 PM

If you have a rich mixture, wouldn't ALL spark plugs be affected?

**4JGtootsie** · August 13th, 2008 07:32 AM

You have a fine old school engine.
I would simply begin with valve adjustments, good compression will greatly rely on good seating of the valves giving a tight seal, worry about worn piston rings and walls when you have already done all the other test.

The cylinder head gasket will be the next.
The third will be cylinder head check up (dismount head from block), take a close look of the valve condition, seats, guides and seals. Warpage and leaktest of the head should also be checked.

**janmike** · August 13th, 2008 09:26 AM

Originally Posted by kinyo

If you have a rich mixture, wouldn't ALL spark plugs be affected?

Theoretically, yes it should. that is what's puzzling me until now. i have searched for a number of info's regarding this and how to fix it. for now i am trying to DIY and hope to gain some valuable experience regarding this matter. My plan is simple elimination starting with most accessible, most obvious and serviceable part - the A/F mixture. if it does not show any significant change then on to dwell(point gap) and timing because late timing can also cause incomplete combustion.