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View Full Version : afr and cam help please


KeefaGT
07-03-2011, 17:48
Hi people have done a good few miles in the GTT now and now have some afr readings

Idle around 12.6 (Not adjusted mixture screw yet)
Normal moves around between 12.5 and 13.4 never really settles
Wot is around 14.4 since winding boost up (too lean??)

Ive not measured the carb jets yet i know it is a group a (:() and has been recently rebuilt and set up for a similar turbo just wondering where to start with jets as i know little about carbs :ashamed:

Also my other question is about valve clearances as i bought a used bb tuning engine i dont know which cam it has fitted apparently there is a choice of 2.Am i right in thinking the clearances depend on the cam? and does anyone know the clearances for these cams or where i may get them as i dont have a clue what to set them to?

Thanks :)
newbstar*
08-03-2011, 11:02
Whats the cruising reading like not on boost? You might only have to mess with the second stage part of the carb. Group a carbs are hard to fuel correctly, go back to 25mm:agree:
KeefaGT
08-03-2011, 15:48
Whats the cruising reading like not on boost? You might only have to mess with the second stage part of the carb. Group a carbs are hard to fuel correctly, go back to 25mm:agree:

Normal cruising between 12.5 and 13.4 hard to get exact reading. I was hoping if i enlarge the second stage i may be able to reduce the main jet and hopefully lean it out a bit for normal conditions? Does this sound right? I do plan on fitting a 25mm carb eventually:)
GTphil
08-03-2011, 15:59
14.4 on WOT is pretty lean If I were you I would be knocking the boost down untill you get 12.5 at least, I usually aim for 12 to 12.2 on WOT.
5teve L
08-03-2011, 16:05
Either you have a problem with your fuel pump/FPR or AFR gauge or the carb isn't a grp A (or a bored out standard carb with massive main jet as they should be called) as 14.4 is well lean, You need to back that boost off loads.

EDIT * Actually, how much boost are you running as it could be emptying the bowl/need a larger needle jet.
KeefaGT
08-03-2011, 16:17
Am only running 14psi at the moment on my way to about 20psi. The afr gauge is new and the fuel pump has been replaced for a used one that seems ok.
I will whip the carb off and check it is a group a. I think i assumed it was as it has the choke flap etc removed. (hopfully its not:))
James5
08-03-2011, 16:21
Keith, matey I think before you fiddle any further strip the carb down and find out what size jets you have in your carb, also see if your carb actually has a larger venturi aswell.

You need to find out before you do any damage:agree: I would definatly suggest turning the boost down to standard for the time being
KeefaGT
08-03-2011, 16:24
Keith, matey I think before you fiddle any further strip the carb down and find out what size jets you have in your carb, also see if your carb actually has a larger venturi aswell.

You need to find out before you do any damage:agree: I would definatly suggest turning the boost down to standard for the time being


On it now :D
KeefaGT
08-03-2011, 18:32
Carb now stripped the venturi has been bored so looks like a group a :(
Think i may have found the problem the ac jet has 125 stamped on it. Should it not be around 1mm?
Just ordered some micro drill bits and will check all sizes when they turn up.