AFR readings

[tubbyG]

Ive just got my AEM fully connected up and went for a spin last night to see what she was doing. so at 12 psi, standard carb, T2/25, piper 285

idle =13.9 - 14.9

part throttle(no boost, -20 vac) = 10 - 10.6?? slight throttle it leaned towards 11.2 - 12 when the boost came in/increased

WOT = bang on 12

do these sound about right? Im concerned part throttle is too rich and wasting fuel, car feels good though and does not hesitate :coffee:

[Gttnutter]

I'm on the same readings, only part throttle it sticks around 10.9-11.2.

I think this is exceptable and is'nt doing any harm.

[Nayls]

That's pretty rich for part throttle! i used to get 14/15's on part/light throttle

[tubbyG]

thats what I thought, but as I am in new territory here I thought best to ask the pro's!

the second I ease off from part throttle, it leans back towards idle readings.

I was assured it was a standard carb when I bought it,looks like I will need to see what jets it has. But it fuels perfect for 12psi so do you think it could have been fettled with?

[Brigsy]

Doesnt sound too bad, check the main jet to see if it has been drilled out if you want to lean it off a bit.

[Woznaldo]

I get the same but even richer (10.1 - 9.9)! perfect during WOT at around 12.0. The back of my car gets little black dots all over the rear end caused by the unburnt fuel being throw out the exhaust.

[tubbyG]

I get the same but even richer (10.1 - 9.9)! perfect during WOT at around 12.0. The back of my car gets little black dots all over the rear end caused by the unburnt fuel being throw out the exhaust.

Im lucky mine aint that bad - untill I put the choke on then you can see a black haze puffing out

[tubbyG]

Doesnt sound too bad, check the main jet to see if it has been drilled out if you want to lean it off a bit.


cheers, I will hopefully have the carb off at the weekend as im wanting to set it to 18psi so will have a look. I have another 2 carbs in the garage I can compare to/swap jets if need be

[Gttnutter]

I was out giving it a run today and came up with these:
22-23 psi
idle 13.5-14.0
part throttle 12.5.13.0
wot 11.9.-12.0

[tubbyG]

been out for a longer run tonight, AFR's at WOT were good untill a long pull in 5th where they leaned to the low 13's and also had slight backfire/poping/hesitation happening.

so ive put the boost down to 11psi, so is now slightly rich at WOT on all gears bar 5th where it is around 12.5

[tubbyG]

whipped the carb off this morning, jets were -

132 main
125 AC

now changed to -

120 main
0.9 AC
1.2 second stage

fuels much better at part throttle but still on the rich side so might go smaller still. Set the boost to 15psi, still slightly rich so once i get home today I will up it a bit more:burnrubber:

[tubbyG]

Have now set boost to 16psi.

Carb is now
120 main
125 AC
130 second stage.

Getting 12.2 @ wot but it is rich and sometimes sluggish untill max boost is reached and I also believe the bowl to empty itself when doing a hard pull through all the gears. I want to get boost to 18PSI so should I put in a smaller AC or bigger main?

[bigdur]

Have you got a 2mm needle valve?

[tubbyG]

Have you got a 2mm needle valve?

thats next on the german ebay shopping list:)

I have been searching the boards and people have been saying that standard should be ok and fill the bowl for up to 20psi? Just wondering if anything is wrong......

I think I may need a smaller main as AFR's of 10-11 on part throttle but I dont know how much bigger I can go with the second stage so may have to put up with it

[tubbyG]

I should add that I have a phase 1 fuel pump fitted although I have not checked fuel pressure yet

[bigdur]

Mine seemed to be emptying way before 20psi. I'd get one on order before you start drilling any jets out.

[tubbyG]

thats good to hear!

I have a number of different sized jets now - due to having a few carbs in pieces :laugh:
so hopefully no more drilling just yet. Whats the largest recommended second stage size?? 140??

the bowl only empties when gunning it through all gears, which is not done very often, and wont be done again untill I get the 2mm needle. On a single pull in 3rd or 4th the bowl stays full but rich and hesitant untill full boost is achieved

[Miller]

Have you read all the info in the articles section? And this one....

http://www.rtoc.org/boards/showthread.php?t=4734

If you are still going lean at top end drop the AC down, the way I normally check part throttle is 5th gear at 70 mph on a flat level road with no up hill and try and maintain that speed, you should be around 13.5 to 15.0. If you do it that way it's in the middle of the rpm range and something that you can repeat easily.

Chris

[Miller]

From what I know their is no point going over 1.3 mm on the second stage as the valve that controls that fixed jet will not allow any more fuel through it.....

[tubbyG]

thanks for that:agree:. i was using 4th as its prob my most used gear when cruising between 30 & 40 so I will see what I get in 5th @70.

I dont recall reading that particular article - some interesting points in it. but I have read plenty of other articles/posts to have a very basic idea of what is required to increase boost.....just the fine tuning I need help with ;)