4150 Carb Tuning

I have a two issues with the fuel curve produced by my Quick Fuel Q-1050 4150 carb.  I'm attaching a data logging graph showing AFR and RPM for my best run at the track.  Some info:  Torino Cobra, 472ci (525HP@5.8k\550TQ@4.2k), Top Loader 2.78 1st gear, 4.11 rear, 27" tire, timing - 17 initial, 32 total all in by 2500, no-box...

It bogs on launch.  Either because it's lean or the tires dead hook, or both. The AFR spikes to nearly 16:1.  I'm pedaling the throttle prior to launch to achieve my desired RPM. Might the pump shot be done because of this?  The second part is the AFR gets richer as the RPMs climb. It goes from 14:1 to about 12.3:1 in each gear.  I've tried increasing the HSAB, from 28 to 36, but it's had minimal impacts.  The carb is pretty much stock, minus some HSAB changes.  It runs great on the street, no real complaints there.  Give me some thoughts on how to tune out the issues I'm seeing.  I'm in Minnesota, so I won't be able to test anytime soon, but I'm trying to get some ideas on which way to head.

http://www.torinocobra.com/forum/download/file.php?id=9991&mode=view

The carb will probably need further changes but, start by getting a much larger secondary accelerator pump shooter nozzle ... something around 49 or 52; and be sure to use a hollow nozzle hold-down screw. (it may already have one).

Agree with Randy. The stick shift plays a bigger role in this than you think.

I forgot, I moved the squirters up to .40 and .45. That seemed to help an off idle stumble I had. I haven't gotten back to the strip since though. It sounds like you guys are thinking much larger than that.

How you are launching the car also comes into play. Are you launching relatively low RPM by feathering the throttle and working the clutch??...or are you revving it up to a certain RPM and dropping the clutch??

BBFTorino wrote:How you are launching the car also comes into play. Are you launching relatively low RPM by feathering the throttle and working the clutch??...or are you revving it up to a certain RPM and dropping the clutch??

Thus far, I've revved it to my chosen RPM, dumped the clutch and stabbed the throttle. No feathering or slipping going on. Probably not the best plan, but I've gotten progressively faster as a result.

Are you using jet extensions in the rear fuel bowl??...it could be uncovering the jets, causing a stumble or hesitation for a split second....then when the fuel sloshes back and the jets get some fuel flowing thru them, it gets a sudden rush of fuel and a split second rich condition.
Im just throwing out a possibility here!!

BBFTorino wrote:Are you using jet extensions in the rear fuel bowl??...it could be uncovering the jets, causing a stumble or hesitation for a split second....then when the fuel sloshes back and the jets get some fuel flowing thru them, it gets a sudden rush of fuel and a split second rich condition.
Im just throwing out a possibility here!!  

Thanks, yup, it has jet extensions in the rear.

tall69 wrote:I forgot, I moved the squirters up to .40 and .45.  That seemed to help an off idle stumble I had.  I haven't gotten back to the strip since though.  It sounds like you guys are thinking much larger than that.

Larger will help on the secondary side.
The primary side needs a longer squirt, not simply a big one right away so, a larger 50cc pump and brown cam on the primary side should help a great deal.

rmcomprandy wrote:

tall69 wrote:I forgot, I moved the squirters up to .40 and .45.  That seemed to help an off idle stumble I had.  I haven't gotten back to the strip since though.  It sounds like you guys are thinking much larger than that.

Larger will help on the secondary side.
The primary side needs a longer squirt, not simply a big one right away so, a larger 50cc pump and brown cam on the primary side should help a great deal.

Yes!! Was going to say, the accelerator pump needs to be extended out over a slightly longer duration of time. Right now, it probably gets a quick shot, but depletes the pump volume and then the rush of air causes stumble. A longer squirting pump will help cover it in the RPMs until the transition to the mains is pretty solid.

rmcomprandy wrote:

tall69 wrote:I forgot, I moved the squirters up to .40 and .45.  That seemed to help an off idle stumble I had.  I haven't gotten back to the strip since though.  It sounds like you guys are thinking much larger than that.

Larger will help on the secondary side.
The primary side needs a longer squirt, not simply a big one right away so, a larger 50cc pump and brown cam on the primary side should help a great deal.

I will agree that randy is probably dead on with his suggestion. But depending on which cam is installed now I wouldn't go straight to the biggest one they make, lets say hypothetically if it has a pink which is really small. If a lean condition is causing a bog then if you straight to a rich condition when what you really need is something in the middle you won't really know if you're on the right track because a rich set up produces very similar results. Go up 2-3 sizes at a time then you'll know if you're on the right path. Blue is my go to acc. cam. It hits quicker than any and ramps up faster but just doesn't have as much overall lift as the brown. Accelerator pump selection has as much to do with driving style as it does the engines needs.

Also if the power valve is too small it can cause a flat spot. If the vacuum doesn't fall fast enough to open the valve.

another trick is swapping out the secondary linkage to a progressive arm. This allows the primaries to open farther before the secondary's start to open. This allows the rpms to come up a little more before you get that hit of the back barrels. This is'nt so much to cure a bog but just helps with traction, especially on a street car. HOLLEY PART# 43-4QFT

This is all good info guys. I have not played with accelerator pump cams yet. I have adjusted the squirter size, which has helped an off idle stumble.

Currently, my carb has downleg boosters. What are your thoughts on annulars for my setup? How would this affect things?

tall69 wrote:This is all good info guys.  I have not played with accelerator pump cams yet.  I have adjusted the squirter size, which has helped an off idle stumble.

Currently, my carb has downleg boosters.  What are your thoughts on annulars for my setup?  How would this affect things?

I would say that you are overthinking what is needed here.

rmcomprandy wrote:

tall69 wrote:This is all good info guys.  I have not played with accelerator pump cams yet.  I have adjusted the squirter size, which has helped an off idle stumble.

Currently, my carb has downleg boosters.  What are your thoughts on annulars for my setup?  How would this affect things?

I would say that you are overthinking what is needed here.

That's how I roll.  

I think it has a too much carburetor for starters. A mild engine like that would run much better with at the most an 850 cfm carb.

rmk57 wrote:I think it has a too much carburetor for starters. A mild engine like that would run much better with at the most an 850 cfm carb.

I was thinking the same thing. But I also agree with the pump and cam comments.

Here are 2 videos of my 468ci BBF. The first video is the trucks very first outing with an 850DP with the small 30cc pump. Notice how it stumbled when I mashed the go pedal. The second video is the exact same engine/carb but with a 50cc pump. If I remember correctly, I didn't change anything other then the pump.

https://youtu.be/zHL7NkbW1No

https://youtu.be/rgeLdSPYFtk