urban_cowboy

Wow, what a difference this carb is vs. the standard double pumper. I got a chance to do a little low speed circuit and primary tuning on Sunday after the rains stopped. I still need to work on the accelerator pump cam, nozzle, and secondary jetting, but man this is great to work with. I have been able to tune the car so that it stays in the high 13s and low 14s through the entire rpm band and throttle position up to when the secondaries open. It is still usable at WOT, but needs some tweaking.

What I currently have is the primary and seconday LSAB and HSAB set the same. I have all four idle screws set the same. I need to open the secondary throttle a little more and close the primary a little more to totally prevent dieseling. The idle is not as stable as I would like, but I think that is because I do not have the idle set as high as I should because of the dieseling issues. The motor seams to like idling at 950-1000 which is where the vacuum is most stable and the engine does not lop as much. The problem is the primary throttle is open too much at that level which causes dieseling. I have also noticed a drop in idle rpm and observed a rich idle condition when I let out of the throttle at say a stop light. Again, I think this will be better with a little more idle speed. I plan to change the oil out this week, and then do some WOT pulls to set the secondary jetting up. At that point, I will schedule a dyno tune day where I can camp out on a dyno one Saturday.

I do not know if the lack of a choke tower (dominator style main body) or the ability to fine tune with air bleeds is the reason for the better tune, but I never has as stable of a air fuel curve with the other carb as I do with this one.

my77stang

if you go to smaller LSAB's you should be able to idle it down further without it dying, which should eliminate your dieseling

urban_cowboy

Good point. I am running 71s for LSAB front and back. That gives me a high 13s AFR at transition (1500rpm). I could try say 65s or 67s front and back and see what that does to the light cruise AFR. I do not want to sacrifice light cruise AFR for no dieseling because the throttle response off idle and thru transition is very good at it is set right now.

I have also ordered a second set of throttle plates to try drilling them to see if that will help. Am I right in thinking that the more I can close the throttle plates, the less likely it will diesel? My thoughts are drilling the plates will give me the air I need to idle correctly, but will let me close the throttle plates some to prevent dieseling. On the other hand, it may just be the high rpm idle speed is what causes it regardless. In that case, I will just have to live with dieseling or live with a rougher idle. If it is carbon build up from the dead rich stuff I was dealing with with the old carb, I have a fix for that. I plan to run BG 109, BG MOA, and BG 44k to clean this thing out now that the tune is right.

I am assuming at this point that raising the idle speed will help eliminate that rpm drop and temporary rich condition I am observing when the throttle is close as in coming to a stop sign. The idle speed and AFR always comes back after second or two, but it is annoying. I am not totally sure what causes that, but my old carb did the same thing while I was trying to tune it.