craig_007
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| posted on 15/5/10 at 10:26 PM |
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AFR setting/readings
I've just installed a AEM wideband too my car.
Now the car has bike carbs fitted and I'm wondering how to go about setting the fuelling
Is it a case of selecting 4th gear and going WOT from say 2500rpm through to redline and noting the readings and adjusting to suit ?
I've already changed the main jet as I was getting 11.5 reading on the afr gauge, at 7000rpm.
After a jet change I've got a reading between 12.7/13.6 approx which I'm led to believe is alright but when I sink the throttle at 2500 in
4th gear the afr readings are 10.5 until the car gets "on cam" so to speak at 4000rpm then the reading come back to approx 12.7/13.6 !
Any advice would be much appreciated
[Edited on 15/5/10 by craig_007]
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FrankP
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| posted on 16/5/10 at 05:27 AM |
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from the megasquirt manual 14.7 AFR is the most efficient mixture and is used for cruising, overrun can be a weak mixture but you may experience some
poping through the exhaust if you go too lean. Max power is usually produced at around 12.5-13 AFR.
The dip in AFR on hitting the throttle maybe a design as on injection cars you need to set acceleration enrichment which has the same effect.
cheers
Frank
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matt_gsxr
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| posted on 16/5/10 at 08:18 AM |
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Craig,
I think that there are some useful pages on Mikuni set-up. These carbs are a bit complex, the fueling at different points are managed by different
parts of the carb.
I am only repeating what I have read, but the Main jet controlled the WOT high flow state. The idle jets are self explanatory. The interesting
mid-range is controlled by the needle profile. You can either change the needles to ones with other profiles, or lower or raise the needles (there is
a little circlip on the top of the needle, and you can move the needle up and down).
The needle has no effect at idle, or at WOT and high RPM. But during the intermediate states lots of things play a role.
I think you need to lower the needle by a notch or two which would reduce the fuel flow at intermediate air flow. Maybe give it a go and see what the
AFR looks like. With AFR of 10 you are in no danger of damaging the engine (not so true for the environment).
I attach a link to a site that explains this better than I can. http://www.iwt.com.au/mikunicarb.htm
Also worth checking the plugs to be sure that all the cylinders are doing the same thing. I had some problems when setting up which turned to be a
fuel leak on one cylinder. The only solution to this would be an AFR on each header, but that is beyond my nerdiness/budget.
Matt
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whitestu
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| posted on 16/5/10 at 09:23 AM |
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+1 on adjusting the needles, though not all bike carbs have adjustable needles.
The guys at Atspeed in Essex reprofile the needles [with a file] to get the mixture right, thoug not so sure this is easy to do.
I got mine pretty close - 15 ish cruising and at tickover and 12 ish at WOT - by adjusting the needles.
Over run mixture is 17+ but doesn't seem to cause any problems other than a bit of popping.
Stu
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craig_007
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| posted on 16/5/10 at 11:21 AM |
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Thanks for the replies.
Good link there also.
I'm going to try altering the needle height today to see if I can lean out the midrange a little then I'll go a for a spin and check the
plugs after a WOT blast.
Thanks again
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Canada EH!
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| posted on 16/5/10 at 03:14 PM |
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If you can a proper plug check is done by turning off the engine and hitting the clutch at WOT then coast to a stop and pull the plugs.
Take a small magnifying glass with you and check at the base of the plug were the insulator and the metal meet.
The insulator should be light beige in colour.
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