17.8-18.9 AFR @ idle, thoughts?

[whytry]

I have a message to Kevin @ CSU too, but I thought I would see what things you guys come up with so I can try to resolve it.

Carb is currently set with idle mixture screws about 4-4 1/4 turns out! I know its alot and Kevin said the same thing too, but less than that and the car was around 19.7-20.5 AFR.....

I have checked for a vacuum leak, there is none. I went as far as unplugging the Boost Reference for the fuel pump, MAP, etc. No changes to idle or AFR, went back and plugged them back in 1 by one with new hose and still no change..... I pulled the hat off the carb and plugged the 4 air bleeders on the carb itself and the idle sounded rougher (by myself so was not sure officially), and did data log and AFR went down to 15.9-16.5....

Does this mean metering block? Anything else I can try?

[camcojb]

sure wish you had the money to get a carb from Scott at http://www.sd-concepts.com/

I know CSU is supposed to know what they're doing on blowthroughs, but that carb is not right, at least from what I saw playing with it. Scotts carbs have been spot-on for me.

Jody

[Blown353]

Do you have any airleaks in your exhaust system upstream of the O2 sensor? Any slipjoints in your headers, loose/leaking flanges at the head, warped flanges or collectors?

A leaky exhaust which lets outside air get sucked in will skew the A/F readings to the very lean side.

I'm fighting this issue right now on an all-aluminum 427 ford with an F1R that I'm supposed to tune and dyno tonight. The AFR heads have the inner exhaust flange bolts half way into the ports and the dyno guys only have narrow-flange headers-- so the headers are leaking air through the bolt holes and really throwing off the oxygen sensor readings. While the sensor is showing off-scale 16:1 the exhaust is burn-your-eyes rich and the plugs look the part.

If you covered the idle air bleeds though and richened up the mixture a bit, I'm going to say it's probably a carb issue. I'd say the idle air bleeds need to be changed to alter the idle mixture to a more favorable range with the mixture screws at about 1-1.5 turns out, but I'm not the expert at more advanced blowthrough carb tuning so don't go rushing out to do that.

[whytry]

I have the 02 in the collectors before the header flange. I have the 3 bolt flanged collectors, replaced exhaust gaskets in both headers and collectors gaskets recently, checked bolts last night and they were tight.

From the sound of it the idle lowered down when I covered the air bleeds, so that would mean it is running richer right, and could now be too rich? I talked to Kevin and he thinks there is a leak somewhere in the exhaust and causing the AFR number to be high... Because he said covering the air bleeds and the idle dropped is a sign of enough fuel and dumping too much fuel in.... I tried both my FAST Wideband and an Innovate Motorsports wideband (thanks Jody) and got similar readings from both.

I did go for a drive and @ WOT the AFR was around 11.9-12.1, and then when the Methanol/water injection came on (progressive) it went down to 9.6-9.8 range. Could it still be considered an exhaust leak if I am getting readings like that at WOT?

[Blown353]

A small exhaust leak will wreak havoc on the AFR readings at times of low exhaust flow (idle) but will skew the readings less elsewhere.

Rather than tune your idle mixture by the WBO2, tune it the old fashioned way-- tweak the idle mixture screws, while readjusting the idle speed screws as necessary to maintain a steady idle, until you have achieved the most manifold vacuum you can. That is usually a close to ideal idle mixture for the given engine.

[andrewb70]

Double check your ignition. An occasional miss will make the O2 reading artificially lean. If you have one of those nifty laser heat guns check individual cylinder exhaust pipes and look for variances.


Andrew

[whytry]

So had some time to look at it this weekend with some friends, and I got some good news. I went through and checked for leaks header bolts and collector bolts, etc, all were tight, then started checking around the actual 02 sensor bung, and low and behold there were a couple places where it looks like the guy who welded in the bung either thought it was 'good enough' or was just lazy and it was causing me to have an air leak right next to the 02 sensor! I did try to put some duct tape over the piping and cut a hole for the 02 sensor and was able to get it to work for a couple minutes before the tape started leaking again. When it was taped up I was getting 13.5-13.8 @ idle and with the idle screws at 1.5 turns out each...... So shut it down and had the bung welded up properly and it works good now. I will be tinkering with it some more this week, what would I want it to idle @? As close to 14.7 as possible or is there a sweet spot that blow through motors like better?

This was done with my FAST setup, I am going to double check my work with Jody's Innovate setup this week. So I can bring your Innovate setup over maybe this weekend if you are going to be free Jody. I will give you a call.

[Blown353]

That's much better, and an easy fix. That's why I advised to check for exhaust leaks first before diving into anything more expensive. :)

Idle mixture depends completely on your engine and what it likes.

For example, my engine likes idling at 925 rpm, A/F in the 15-15.3:1 range, A/C on, and 32 degrees of timing. Any lower RPM and it isn't happy at any timing/mixture, any leaner and it surges, and with any less timing it has to be much richer so the A/C doesn't kill it when it kicks on an off. What works for some doesn't work for others.

With the carb, you have the compromise that the idle mixture screws also affect immediate tip-in response when taking off from idle. That being said my preference is to run it lean as you can at idle without having it lean-bog on a takeoff from a dead stop. The preferred method for tuning a carb idle is decide on an idle speed and set it there and then adjust the idle mixture screws until you achieve peak vacuum at that RPM. If the RPM changes from tweaking the mixture screws, adjust the idle speed back to your RPM target before taking your vacuum reading. And then when you're done check the transfer slots, if the butterflies are open too far drill holes in the blades so you can close them a bit more and repeat. Fun, isn't it? LOL.

My guess is it will end up somewhere in the 14's.

Remember, don't tune your engine to the A/F meter, give the engine what it likes/performs best and then use the A/F meter as a sanity check.