workaholic

I have a 12A Street Ported and freshly rebuilt with a Nikki carb Primary venturies hogged to 24mm, bigger accel pump, 120 primary jets, stock air bleeds. Car is REALLY hard to start without tonnes of starter fluid, stumbles hard between idle and slow, and runs lean whilst cruising (14.5-15:1). A/F at idle is ~13.5:1.
I guess my questions are: Is my jet/ air bleed tuning close for my setup? Is there a recommended starting point for the jet & air bleed sizes?

sommmatt

iTrader: (2)

Are you using the choke still? Does it help or hinder starting?

How is it full throttle?

Cruising AFR is perfect. Idle can afford to be leaner too if it will take it.

Street ports are so mild, I'm concerned your larger accelerator pump is giving you too much fuel and causing your stumble. Might be worth swapping it back to stock and giving it a go.

Carl

iTrader: (2)

I have a similar setup (only difference is 140 primary and 170 secondary) on my race car. When starting cold, I have to pump the heck out of the accelerator. This was actually even the case before the 12A was rebuilt and ported and the carb primary venturis were opened up (to 24mm like yours). I suspect my accelerator pump is a little weak (which doesn't affect driving on the race track, but seems to make it somewhat less reliable during cold start) Once the engine is warmed up it starts right up.

You didn't indicate whether or not you are still using the choke. I think it would probably start up better with the choke, because with it closed there is a higher vacuum pulling fuel into the carb. I left the choke on an SA carb I modified, but haven't had a chance to try it yet to see how much that helps starting.

Did you convert to mechanical secondaries? In my case both the SA and FB carbs I modified did not make enough vacuum with the opened primary venturis to fully open the secondaries using the vacuum actuator. As a result max HP was reduced by about 30%. I switched to mechanical secondaries on the FB (haven't done it yet to the SA which is why I couldn't check the effect of the choke on starting) and works great on track.

If this is a street driven car, I think you will have to do some more modifications. Jeff20B and Benjamin4456 have posted a lot of information on how to modifiy the air bleeds and stuff to make it more street friendly. Search for their posts and you should get pointed in the right direction.

Good Luck,

Carl

workaholic

The choke has been removed, figured I wouldn't need it in the summer (the only time I drive the car). The secondaries are still vac operated for easier driving, I'm only taking it up to 3k for now as there's only ~30 miles on the rebuild. We'll see how the secondaries respond after she's broken in a bit more. I have a pair of 124 jets imma try out too. Thanks for the info tho.

Carl

iTrader: (2)

For a street car, I agree with you that vacuum secondaries are the way to go. Based on my experience with 2 modified carbs, the secondaries will not open (much or at all) with the enlarged primaries. In both cases, I had to manually open the secondaries with the car on the dyno.

Your jetting probably is fine. For racing I am targeting A/F = 12.5 at "full beans". That would seem way too rich for normal street driving. For reference, my dyno experience with the car shows (all with RB collected header): stock port 12A with unmodified Nikki - 135ish HP, street port 12A same unmodified Nikki - 137ish HP, street port 12A modified Nikki (24mm primary) vacuum secondary - 100ish HP, street port 12A same modified Nikki mechanical secondary 150ish HP.

Good luck and have fun.

Carl