Lean AFR at launch
 

Hope I am not wearing you guys out with all my questions. New data logging system is really interesting.

I only have two runs logged so far but when I am staged I bring my RPM up to approximately 4,250 rpm on my transbrake and my AFR is reading about 13.75 (I know thats a bit lean and have increased my jet sizes on both sides by +3)

My issue/question is right after I release my transbutton my rpm drops about 470 rpm as the car goes under load and my AFR goes up to about 16.25 by approximately 0.4 seconds into the run. At 0.7 seconds my rpm is back up to about 4,400 rpm and my AFR is back down to about 12.5

Question - Is the spike in AFR due to too small a pump shot?

I have a 750 Holley DP with a 28 Primary Shooter and 31 Secondary Shooter with Pink (330 Cam) on #1 position

Carb also has 30cc pump on both sides. I have seen where Cliff R has suggested in some instances a 50cc pump on the Primary side is better. Any thoughts?

Appreciate any and all of your thoughts especially your patience.
DV

30ccs are for small block chevies. Put a 50cc in there and see what happens.

Agree on the 50cc up front. But how does the car feel? Most AFRs respond really quick and often if it is just a quick lean spike you might not even feel it or the motor react to it. I think most meters are going to show a lean spike when you launch.

Launch seems good, no spin, pretty consistent around 1.65. Would like to reduce the RPM drop but I assume thats to be expected to some degree when car goes under load.

Wasnt sure what is "normal" for AFRs on a launch. Will plan on upsizing the Primary side pump to 50cc.

That loading of the motor and 500 rpm drop is not only using the up the fuel that’s in the air stream, but it’s sucking all the fuel off of the port walls also.

Install the 50 CC pump for starters and see how your AFR responds and your short times.

If the secondary side has a whistle vent shorten it.

Usually a 750 won't respond to a 50cc accelerator pump, front or rear. The 4781 likes them because they have horrendous booster signal. A lot of builders are using 1,000 CFM carbs with 30cc pumps front and rear.

Paul K

Thanks for the reply. The carb has one on both the primary and secondary side. What does shortening the one on the secondary side do?
How much do you recommend shortening it?

Thanks

On the secondary if the whistle is too long when the car accelerates the fuel goes to the back of bowl which covers the whistle. This blocks the vent (from the whistle being covered). So a shorter whistle keeps the vent from being covered.
Usually the secondary has a little piece of metal for the secondary that keeps the fuel from flooding over when stopping hard.

Edit:
I guess they don't make the steel piece anymore? (couldn't find any online)

So a shortened whistle will do.

:confused:

YW, cut off about two thirds. The fuel sloshes to the back of the bowl and blocks of the vent, at that point fuel may not flow into the boosters causing a lean flat spot. IDK if it's been mentioned or done, if not you need to upgrade to jet extensions and a notched float.

Thanks guys for the education. Will look into the jet extensions. I am currently using the floats that came in the renew kit (37-485)

If you' re up on a 2 step and trans break the throttle should already be on the floor before you let go of the button, so pump shot is already completely out of the equation at that point. Chasing pumps and squirters won't fix the lean spike you're seeing when you launch that way.

My guess is Paul k is on the right track, fuel slosh from the hit is uncovering the jets for a split second. Jet extensions may be the solution

That right there has always made me wonder. I run a 2 step, duel 50ccs. So what happens to all that unburnt fuel sitting on the bottom of your plenum ? Engine popping and snorting, cylinders dropping off.
Foot braking it, your engine gets the fuel shot as intended. But with a 2 step, all that is changed.
But you still get great 60 foots with the 2 step. If running a 2 step would it be better to have duel 30cc squirters ?
Its always seemed strange to me.

Did you do away with the power valves? I tried that on an 1150 dominator. It developed a lean stumble when the car left the line. The more I jetted it up the better it got but ran of jet sizes without drilling my own. I ended up putting the power valves back in and it was good. My guess is it’s still lean. Keep adding jets size to it.

Formulajones - At the moment, I am just running a simple transbrake (no 2 step) with an approximate 4,250 rpm launch. Likely going to a 2 step in the future to make things more consistent.

Probird - Only running a PV on the Primary Side (5.5") with the Secondary Side blocked. Planning on checking my vacuum at my starting line launch RPM to see if thats a reasonable size. See Power Valve thread I started earlier which was initiated due to leanness in 2nd & 3rd gear. Going to try larger jets to resolve that issue.

Increased my jet sizes Primary & Secondary by +3 which brought my AFR at launch (4,200 rpm off a transbrake) down from approximately 13.75 to 12.6.

I shortened the whistle vent on the Secondary side but was unable to install the jet extensions and notched float without a leak but the leak ultimately was unrelated and resolved. Try that upgrade again for next time out in late Sept.

I still have a AFR spike just after launch from 12.6 to 14.75. Defintitely threw more fuel at both the Primary and Secondary side via larger nozzles in the mean time but didnt appear to make any difference in AFRs or 60' times.

Sometimes you just have to go with how it feels vs chasing AFR numbers.
Here is one of my better runs Big lean spike at launch -footbraking 3000, then min spikes at gear changes.

Is it possible the pump is not large enough, strong enough to overcome the velocity at the hit?

I agree with that.

What pump and fuel line/regulator(s) etc. do you run?
What fuel pressure does it run?

:confused:

RobbMc 1/2" pickup at stock tank
8AN lines to carb
RobbMc 1100 mechanical pump & RobbMc dead head regulator
6.5 to 7 lbs pressure

Skip Fix - Thanks for the pic and reply. Looks like you have an Innovate system also, what are you using for your MAP sensor?

This is what my run looks like in terms of RPM & AFR
Launch at approx 2:24 on graph