incomplete combustion diagnosis

[janmike]

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]

Few info first

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

-----------------------------

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

[[archie]]

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

[janmike]

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

[ghosthunter]

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

definitely do a compression test.

[janmike]

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]

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]

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

[4JGtootsie]

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]

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.

[janmike]

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.

tnx! i really like this engine for it's simplicity and raw power (if i can manage to tame it ). your advise is highly appreciated sir, in fact i had it already on my long list of to do's. as of now i'm on top of my checklist and having it DIY. when it comes to your advise it's best i leave it to a professional - though i may still try to do it myself - but will see . Good thing for now is that i'm not experiencing white/black smoke (signs of internal engine problems), pinging, knocking or overheating (signs of wear).

Correct me if i am wrong please:

Idle Mixture screw - regulates the amount of gas flow? air(vacuum) flow? or both instantenously?

Which part of the carb countinously pours gas on idle - main jet or pilot jet?
where is it located in the carb?

TIA

[4JGtootsie]

tagat ko na dispatcha corolla liftback ko but i think the lowest screw is for air by pass and above it is Air fuel. You can do a lot of things to that engine pwede mo gawin 1800 by replacing the crankshaft from a 3t. Pwede rin gawin twin cam by replacing the head from a 2tg.

[janmike]

later nalang siguro un mods . more on efficiency at FC and objective ko muna . but tnx for the fyi parin.

Note:
i only have 2 adjustable screws in my stock 2bbl aisin carb. one underneath the throttle linkage (idle speed screw) parallel to the screw in the middle of the carb (idle mixture screw). so idle mixture = A/F mixture?

[red888]

about the a/f screw , yes that is the a/f screw ( pang idle mixture ) may i make a suggestion ? With the engine turn off, remove the screw all the way out of the orifice, then screw it back in slowly, taking note of how many times you turn the screw to fully seat the idle mixture screw.Divide this by 2, this gives you the ( rough ) number estimate of how many back turns of the screw you need to get to the mid way ( half open or close ng idle mixture mo). eto kasi yun theoretical na mixture nung carburator ( based on the screw ), from here you may need a vacuum gauge to fine tune the carburator.HTH ( btw, i got 13T engine, a bigger cousin nung 2T mo,and i do my own carburator overhaul, tuning)

[janmike]

it's been a while since i had a reply on this post. was able to learn a bit how carbs work in general within that period. however, though i managed to lower my fc i still have no response from my idle mixture screw. vacuum is good but i may have leaks internally since the engine don't stall when i close the top of the carb. will try to do your suggestion tonight then i'll go ahead with my rebuild on weekend.

[red888]

another test that u can undertake, is that while the engine is running, remove the idle mixture screw all the way, this should stall the engine. if not ,may problema un carburator.saka do u notice kung magalaw yun needle nung vacuum gauge mo ( normal should be between 15 - 17mm Hg ), this is one sign na may leak ka, pero not always.Kung intake manifold leak, look for signs of gasoline ( sa mga joints ) o un basa sya .

[janmike]

another test that u can undertake, is that while the engine is running, remove the idle mixture screw all the way, this should stall the engine. if not ,may problema un carburator.saka do u notice kung magalaw yun needle nung vacuum gauge mo ( normal should be between 15 - 17mm Hg ), this is one sign na may leak ka, pero not always.Kung intake manifold leak, look for signs of gasoline ( sa mga joints ) o un basa sya .

i did the test you mentioned and some other tests too. i had a hunch the idle mixture screw is'nt of the right size along with the jets. which is why i'm having an overly rich mixture and an idle that won't go down. and so as planned i went ahead and rebuilt my carb with a repair kit and carb cleaner. looking at my repair kit and just as i suspected i had the wrong size of mixture screw! alast that explained everything!!!

with high hopes i started to rebuild my carb starting off by thoroughly cleaning it. replaced all gaskets and serviceable parts. could'nt determine the size of the jets coz the marks are unreadable. but based on comparison of the holes i went to the stock jets from the kit for peace of mind

some gaskets aren't provided in the kit so i had to be resourceful and do it pinoy style. like the guy in the auto shop said "eto bossing pedeng pantapal kortehan nyo nalang dun sa base plate. di baleng sumobra ng konte wag lang kulang tapos konting rugby ayos na yan!!".

after putting everything back i did a drip test by filling the gas bowl and moving the carb in slant position to see if it drips. after re-checking for the nth time i finally reseated the carb on the intake manifold and reconnected all lines.

i had few clicks before it cranked, ran then dies. after a couple of attempts it steadied and the first i did was to test the mixture screw and voila! it worked!!! however i was'nt able to properly tune it since it was past midnight when i finished so i had to tune it by giving it a quick turn in till the screw is half way and the engine does'nt stall. happy and contented i went to bed.

afterwork today i'll do the testing and tuning and see if everything worked as expected :D

thanks for all those who posted and thanks tsikot!

[daimaoh]

cleaning your carb is the best way to start this. it's good that you have.

also have your idling, timing, distributor & high tension wires checked.

you can proceed to more labor intensive (and costly) actions if that doesn't work.

[janmike]

finally finished my tuning (i hope). did it twice since the first attempt was under powered (tuning errors due to cold engine). so last night i warmed up the engine first. then with my diy timing light and vacuum gauge i checked the timing. reading from the first timing attempt showed it was over by 10deg (spec is 10' btdc, i had 20'). so i adjusted it to hit 10btdc then checking vacuum gauge to see how accurate my diy timing light is. not dead accurate but so far it was good. after setting the right timing and confirming it with a couple of revs and restarting the engine a few times i moved on to the idle mixture screw.

i started by slowly turning the mixture screw in (clockwise) while closely watching vacuum readings. when the engine started to run sluggish i can see the vacuum reading started to rise and drop. with that i backed 1/4 turn till it started to steady a bit. moving 1/4 turn out till i reached the highest vacuum reading i can get then back 1/8 turn. revved it a couple of times then left it for a while to idle then re-read the vacuum gauge and re-adjust and compensate. when all is good i went for a test drive. with a 500 ml clear bottle with gas i drove the car to a open service road of approximately 1.2 kms from where i am and back. consumption was good it was around 200ml *50-60kph *4th gear (5th gear would open secondaries so i did not engage there to get a reading on city drive). so approximately it was 9-11km/l consumption which is fair considering it's an old engine that used to be 5-6km/l and terrible on idle gas consumption.

my tuning is based on lean best approach (including jetting and secondaries) which keeps the engine a bit under-powered (sakal). my goal right now is to maximimize FC on idle and low end but normal on high end speeds. to do this i still have to understand carbs relationship to engine.

post nalang po dito kung me tips po how to do this. thanks!

[red888]

btw, san nakakabit yun 2T? saka correct ba ang tire size mo ( baka under size yan ). Pero mukhang o.k naman yun adjustments mo.