Whats going on with my A/F??

-
straightlinespeed

straightlinespeed

Sometimes I pretend to be normal
Joined
Oct 21, 2012
Messages
10,630
Reaction score
4,399
Location
Uvita, Costa Rica
I've been dealing with carbon fouling plugs so I decided to finally install a Wideband in the car to see what is happening. Specs are as follows

LA360, XE268H Cam, stock pistons, Magnum heads, air gap, tti's, qft slayer 750 carb. Timing is 20 initial and 35 total.

I swapped the idle air bleeds in the front to 74's and the rear to 43's. 4 sizes larger as a recommendation from QFT.

Initially, before the gauge I tuned the car to the highest vacuum reading I could get. Which was only 13.5. Upon first start up, after the gauge was installed it was reading 10-11 on the gauge. So I readjusted the mixture screws and got it reading from 13.2 to 14 at idle and in gear 13.8-14.8. This was not at my highest vacuum reading. It actually dropped my vacuum to around 12. If I tried to adjust it to a higher vacuum reading at idle, then when I would put it in gear it would be in the high 15's to 16's.

I took it out for a drive and the first thing I noticed is coming up to a stop light it sort of stumbled, when I went to take off. I looked at the A/F gauge and it was in the high 16's. I continued to drive and at highway speeds the A/F guage was from 12.6 to 13.9. At 60mph Im turning 3k. I tried to find out when it would switch from the idle circuit to the main and its around 2200-2400rpm. Which is around 45 mph. Faster than that its on the main circuit. Cruising the idle circuit is about 13.2 to 14.0.

So I continued my drive and noticed every time I came to a stop light or stop sign, especially after coming down from highway speeds the car would stumble and the A/F would spike up to high 16's and 17's

I got home and shut it off. I let it sit for a few minutes. When I started it up to pull it in the garage it was in the 12.6 to 13.9 range. I pulled it in the garage and it shot up to 15.4 and going blank because it was out of the range of the gauge. I reved it a few times and it would drop but continue to go back high, so I readjusted the mixture screws to get it back into range which was about a 1/3 of a turn out. So now I think I threw off the cold start settings.

I dont understand what is going on and why the large range on it. Especially after coming down from highway speeds to a low speed.
 
The high reading with the stumble is the air / fuel not getting burned and the oxy sensor reading that. IMO, I would go back to what worked and tune from there.
 
Lelo Dart said:
The high reading with the stumble is the air / fuel not getting burned and the oxy sensor reading that. IMO, I would go back to what worked and tune from there.
Click to expand...


Problem is nothing worked yet. Its either I end up carbon fouling my plugs at idle and low speeds with the highest vacuum reading. Or readjust and have it stumble because its going to lean.
 
Are you using 10%ethenal gas in your car? If so you need to drop your a/f ratio down to 13 to 13.5. When driving at 2000 rpm and drop 12 to 12.5 is fine. When I open up my six pack I drop to 10.5. When hot at idle 1000 rpms I'm at 13 to 13.5. I use two gauges one in each header down at the collector. I run a 340 six pack 3500 stall in the tranny.
 

Attachments

  • image.jpg
    65.2 KB · Views: 116
Dave1967 said:
Are you using 10%ethenal gas in your car? If so you need to drop your a/f ratio down to 13 to 13.5. When driving at 2000 rpm and drop 12 to 12.5 is fine. When I open up my six pack I drop to 10.5. When hot at idle 1000 rpms I'm at 13 to 13.5. I use two gauges one in each header down at the collector. I run a 340 six pack 3500 stall in the tranny.
Click to expand...


That is a very shiny engine!!!

Im only using 1 gauge in the drivers side bank.

Im running 91 non-oxy gas almost every tank. That is the best stuff we have around here. Just depends where Im at and if they have it. With that being said, where should I be at?
 
I would not be overly concerned with the high readings when coming to a stop. Off the throttle, high vacuum situations are going to drive A/F ratios up.

Tune it for how when you are going to drive, not the cold start. You don't want the cold start to be crazy rich, but also you don't want it to be the controlling factor in tuning.

Try the orignial air bleeds again, tune idle mixture and go for a drive.
 
Alright, I decided to go play with this more.

First things first! Always check your timing! For some reason, when I set my timing, I thought I knew my marks on the balancer. I wanted to double check the marks so I found TDC. Yep, my marks are right on. I didnt have #'s on the balancer so I took a sharpie and wrote next to each line.

Started the car, let it warm up and checked my timing.... WTH, I had my car timed to 8* initial. :wack: Well crap no wonder I cant get this thing to run correct. I reset the timing to 18*, man did the engine like that. Reset all my mixture screws and the final readings are now this.

Idle 12.4 to 13.2
In Gear 12.8 to 13.9

Its raining here so I didnt take the car out for a cruise to see how it reacts but seriously. What a stupid mistake on my part! :violent1: I bet all my issues have now gone away.
 
8 initial and 23 total isn't gonna cut it... :)

You checked something easy, which is a good method to use when diagnosing.

That may get rid of your off idle stumble.
 
You must log in or register to reply here.
-

Back
Top