I'm having some trouble with my Holley 750 double pumper. Yesterday I spent the afternoon playing with my new wideband O2 sensor and trying to get idle where I want it. Everything went great except that I had to turn the idle adjustment screw up higher than where I set it to have .020" of the transition slot showing. So after it cooled down I yanked and drained the carb and found that the primaries were open about 1.5 turns more than the secondaries (also meaning that the transition slot was exposed quite a bit more - like .040 or .050"). After reading Lars' papers, it seemed as if the 4 corners being identical was more important than the .020" on the primaries, so I turned the primaries closed 3/4 turn and turned the secondaries 3/4 open so they were identical. Now there's about .030" of the transition slot exposed on the primaries and about .010" exposed on the secondaries. I didn't get a chance to start it because it was getting late.

This afternoon I dumped some gas back in those vertical tubes and started it up. I left the air filter off to watch the electric choke and fast idle. It started right up and I was happy. When the choke got to about 3/4 open, the engine died. I unsuccessfully tried re-starting several times until I got a small carb fire that lasted about 10 seconds (that was a long 10 seconds). I let things calm down for 20 minutes and started it again. It started right up again. This time I watched what happened. As the choke opened up enough for me to see in, I saw gas dripping from the venturies on the primary side. Why is gas coming out of there? Is it because I poured gas down those 2 tubes to fill the floats back up, or could I have a blown power valve or something? All I did was adjust the primary and secondary idles to be identical, nothing else.

 

You have too much transition slot exposure. If you get more than .020" showing, you will start getting fuel discharge out the main discharge nozzles. You need to close the primary side down to no more than .020 and crack the secondary iopen to compensate. Also, if you have vacuum advance to your distrubutor, be sure you run it off of manifold vacuum and not ported vacuum - this will allow you to close the throttles more.
Lars

 

Lars,
OK, I will do that. I thought it was more important to have all 4 corners equal than it was to have the .020", but I see I was wrong. I will make the primaries .020" which will probably make the secondaries show that transition slot also about .020" (with the secondary throttle blades open slightly wider).

But what bothers me is that yesterday it was running great with .040-.050" of the slot showing. I feel like this gas dripping out is something different, but I could be wrong. Is it possible than when I poured gas down those tubes that I over filled the bowls? Could that cause gas to drip out at the venturies?

 

Oh yeah, I do have vacuum advance and it is running off manifold.

 

since it ran ok yesterday, something went wrong
dirt on needle? will cause drip, also high psi failed fuel regulator?
fire not good! KEEP FILTER ON !!!! will snuff out fire/ crank and start ASAP

 

Thanks for the suggestion. I will put a pressure gage on it tonight to see what I have. Any thoughts on the possible over-filling of the fuel bowl?

 

will use it in ~10 seconds.
1st lower float 1/2 turn for a test.

 

Please help me understand something. From all of the reading I've been doing, we can get a car to cruise on the idle and transition circuits only. If having the transition slot exposed by more than .020" at idle can cause fuel discharge from the main nozzles, how can you possibly cruise without the mains working? As soon as you touch the throttle, more than .020" is exposed. I'm confused.

 

if it idles on the transition U CANT SET THE MIXTURE
if it doesn't flow from the booster, and it is out of transition, it backfires, hesitates, runs like crap on accel.

 

Thanks for the help, guys. As an update, this was the problem: When I first tuned the idle with the O2 sensor, to get the A/F correct the idle mixture screws were almost closed. Lars was right that I needed to close the primary idle throttle so that only .020" of transition slot was exposed. Since I had more exposed, it was idling on the T-slot. When I properly adjusted the primary idle throttle, the T-slot no longer ran and the idle holes didn't get nearly enough fuel because the mixture screws were closed too far. So Matt, you were helpful too thanks.

Now that the idle is working and much leaner (about 14:1 to 14.5:1), my idle has no "strength." When my idle was rich, in neutral it was around 900 and in gear was around 700. Now the engine stalls when I put it in gear. Just for an experiment, I raised the idle and tried again. I'm up to over 1100 RPM in neutral and the darn thing goes to about 350 RPM when in gear. It usually stalls within about 5 seconds after that. Is that a product of a lean idle?

This is a 383 stroker with a mild-moderate Isky cam labeled for street/strip and I have a 2400 stall converter in it. Any ideas?

 

idle needs to be 12-13:1
keep richening until it runs ok

 

Set the idle mixture with the car in "drive." The engine goes lean when you put it in gear. Setting the idle mixture to 14.7:1 in neutral will produce a "drive" mixture in the 16:1 range. Set it to 14.7:1 in drive, and it will be around 13:1 in neutral.

 

Wow, sometimes it's scary.
Set idle A/F ratio in gear to 14.7 and A/F ratio in neutral ended up being 13.2! Idles fine (slightly rough, sounds great) at 900 in neutral, 600 in gear. What's also pretty incredible is how fine an adjustment on the idle mix (1/8 turn or less) shows up on the A/F ratio. Next project is to tune the transition slot for better cruise mileage. Thanks all!