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Old 2nd April 2006, 23:43   #1
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Default lean fuel mixture = hotter engine? why?

I have a m800 carb Euro - 1 , i noticed that whenever the fuel mixture is set to a bit too lean, the engine runs hotter and knocks(esp. at high revs). If i make the fuel mixture richer, the engine runs cooler and hence the knocking too dissapears!

Does this mean that a lean fuel mixture mean a hotter running engine? if yes ..Why?
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Old 3rd April 2006, 00:11   #2
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Quote:
Originally Posted by techn0l0gist
Does this mean that a lean fuel mixture mean a hotter running engine? if yes ..Why?
Yes, a temperature increase is one of the signs of a lean fuel mix.

However, why this happens i am not sure and would love to know as well. Good Q.

cya
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Old 3rd April 2006, 00:43   #3
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but rehaan is it bad in any way to run a lean mixture.by installing a K&N filter you automatically run a lean mixture unless you temper wit the carb so what do you do in that case ?

and technologist ?? what do you mean by KNOCKIN at HIGH RPMS ?


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Old 3rd April 2006, 04:07   #4
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In a rich mixture the excess fuel absorbs the heat.
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Old 3rd April 2006, 07:49   #5
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Quote:
Originally Posted by revtech
but rehaan is it bad in any way to run a lean mixture.by installing a K&N filter you automatically run a lean mixture unless you temper wit the carb so what do you do in that case ?

and technologist ?? what do you mean by KNOCKIN at HIGH RPMS ?
Revtech,

When you install a K&N on a carb car you would make adjustments to the carb for the correct mix.
On a MPFI car, the MAF sensor notices the extra air comming through and adjusts the fuel accordingly.

And YES it is bad to run a lean mix. It causes the engine to overheat, causing certain internal parts of the engine to get extremely hot, which causes pre-ignition, which is commonly known as knocking, which is bad for your engine.

Also i would imagine that emmisions are worse when you are running lean? (anyone?)

cya
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Old 3rd April 2006, 07:53   #6
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Check distributor advance setting. If itís more than the recommended (handbook) setting position..knocking happens. Supported by lean mixture supply. Often one keeps the advance setting high in order to get a little better pick up.

Also remember that itís summer and youíre in Goa. Your engine is getting hot much faster.
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Old 3rd April 2006, 09:05   #7
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lean fuel settings tends to provide less 'food' to a 'stomach'.

and the burden of carrying same amount of weight gets exaggerated on a lesser-filled stomach.

the engine needs a decent amount of fuel to provide enough power to carry the weight with a speed. if there is very less supply of fuel to the engine then it has to work upto the same amount of load but without proper amount of fuel. so several parts tends to make engine heatup.

correct fuel mixture is a crucial thing to maintain to get proper performance without wasting the fuel (in rich settings, fuel does gets wasted and power output also declines.)
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Old 3rd April 2006, 12:30   #8
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Hey..guys i have a 1998 carbureted 800 run 61000 km...earlier the temperature needle used to be in between the temperature..but after the maruti guy did some adjustment..it always settles down to less than the half on the temperature..even though the temperature in mumbai has increased a lot from what it was before...how come...is it because of a too rich mixture..?or is it proper..??there is no knockin as such...car runs smoothly..get a mileage of around 16 to 18 in city with A/C
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Old 3rd April 2006, 15:09   #9
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but dude why to use a lean fuel mixture in your car . there are other ways to kill engine life
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Old 3rd April 2006, 15:17   #10
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so rehaan if you readjust the carb to compensate for the extra air entering because of the K&N wont that reduce FE ( not That im interested) but arent K&N supposed to Give better FE.so how does that happen.


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Old 3rd April 2006, 16:15   #11
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Quote:
Originally Posted by revtech
and technologist ?? what do you mean by KNOCKIN at HIGH RPMS ?
revtech,

usually when the engine knocks, it does so because of driving at too low rpm in a higher gear that what one should be in. What i meant was, in my case when i downshift from say 4th to 3rd at abt 50kmph to overtake some one, as the revs rise, the engine starts knocking.

Quote:
Originally Posted by Rehaan
Revtech,
Also i would imagine that emmisions are worse when you are running lean? (anyone?)
Rehaan,

The fuel mixture setting was done at a MASS with one of those exhaust gas analyser thingy's, the lesser the "CO" reading, the leaner the mixture is (or so the guys there told me).


Quote:
Originally Posted by Rudra Sen
Check distributor advance setting. If itís more than the recommended (handbook) setting position..knocking happens. Supported by lean mixture supply. Often one keeps the advance setting high in order to get a little better pick up.

Also remember that itís summer and youíre in Goa. Your engine is getting hot much faster.
Rudra,

Distributor advance setting is the "dwell" setting right? The ignition timing and dwell angle was set to the recommended value, i was just experimenting with the fuel mixture to see what happens with a leaner mixture(had tried earlier making the mixture a bit too rich - didnt do much, apart from using too much fuel by m800 standards, and leaving a black soot like deposit on the spark plugs).Now that i found out that the leaner mixture makes the engine run hotter...was just wondering why.

Quote:
Originally Posted by Mpower
In a rich mixture the excess fuel absorbs the heat.
Mpower,

Someone else had also told me the same thing...but how does this happen? I thought the Euro-2+ spec engines run with less fuel (so i assume a leaner fuel mixture than the carb. versions) so do the Euro-2+ spec engines run hotter than the carb versions? Or does the ECM do something differently that prevents the engine from running hotter.

Quote:
Originally Posted by garvit2002
but dude why to use a lean fuel mixture in your car . there are other ways to kill engine life
garvit2002,
I am not using a lean fuel mixture anymore - i dont want to kill the engine, but , i was just trying to learn by experimenting i did set the mixture back to what it should be, when i found that that the engine heats up!

Regards,

Amit.
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Old 3rd April 2006, 16:39   #12
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Leaner Mixture leads to higher exhaust temps as the flash point is lower and it burns faster, this leads to excessive heating at the cylinder head leading to a hotter engine, this coupled with the increased advance will defn caus knocking.
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Old 4th April 2006, 10:29   #13
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Hey techno , i just thought i mite beable to help u out with this one. from my experience as a flight instructor we usually have our students run a little rich of lean in the plane , this helps cause it keeps the fuel burnt to a min and the (free) air to the max and this gives your engine the best operating temperature. However in your car u cant set the mixture on demand youll need to find where its running the leanest and then increase it a tad.
The reason for this being The fuel/air distribution to the cylinders is not always even, so one cylinder might be running much richer than another; by the time you lean far enough to get the richest cylinder , the leanest cylinder might no longer have enough fuel to ignite at all, and the engine will start vibrating violently. Carbureted engines mix the fuel and air together in a single place (the carburetor) then send the mixture to all of the cylinders, where it arrives in various states (sometimes more air will get through, and sometimes more fuel). Fuel-injected engines actually mix the fuel and air separately for each cylinder, so it should be possible to adjust them so that all cylinders get exactly the same mixture. Thats the reason your engine knocks at wide open throttle as well , theres way too much air going thru and not enough fuel to burn it evenly. thus the knock. For your carb i would suggest u set it just a little past the full lean , maybe turn the screw a full turn and a half more. That would definately take care of your high rev knocking, save some fuel and get your engine running free of em knocks.
Hope that helps.

Last edited by NikN : 4th April 2006 at 10:44.
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