My set up. SFT cam. 259/266. Timing all in by 1500. Stick car. Here’s the issue that I have been fighting for several years. My car is a street/strip car. When cruising just off idle, between 1300-2000rpm, I’m getting a “buck”, not a miss, a buck. If I give it a bit more gas, It’s as smooth as you can ask for. Full throttle is also perfect. It’ s Just the frickin’ buck that has me stumped. IFR’s. .035”. HSAB’s. .033”. LSAB’s .073”. PV is a 4.5. Idle is 950 rpm. Oh, Victor intake with RPM heads. Suggestions??

On my street/strip carbs when I encounter that, I usually have to drill out the front and rear IFR's....

I was leaning that way, but wanted to get some more input from others. Thanks for the reply.

Are you saying IFR are the idle fuel feed restrictors or the intermediate (AKA transition circuits) fuel feed restrictors: confused:

. I have always associated the IFR’s to the idle and transition circuit. They feed both the idle discharge hole and the transition slot. I believe that my problem is an off idle lean mixture. I’m looking for confirmation on way or another. This carb is a 1050AN. The only thing I haven’t touched is the IFR’s. Your thoughts???

On my 1050 AN carbs we ended up using the (i think) blue accelerator pump cam in the 2nd hole with a 35/37 squirter setup. 99% sure I decreased the IFR from .035 to .031ish? I would have to look at ,y notes to be 100%

The other place to help the dead spot i.e. where it bucks is transition slot restrictors. The QF t slots are huge and measure all over the place. I have ran my QF 1050ANs on everything from a 451 with cast manifolds to a 511" hot street motor that made 750hp at 7k. Once the fuel curve is somewhat in the ball park it worked okay on most combos with a jet and/or air bleed change.

I also have a QF 1050 down leg that runs good woth similar modifications.

If you can try putting a piece of really thin wire in the upper outboard idle air bleeds on top of the carb to see if it responds to a little richer mixture at light part throttle or not
I agree with Jeremiah on the bucking being a tad to lean at light part throttle driving
God luck, let us know what you do please, that will help others on here

What first and axle gear ratios do you have? Why is the timing “all in” by 1500??? That’s not a good way to do it IMO.

Two other questions. What ignition are you using and how did you come up with a 4.5 power valve?

My opinion is that the 'slot' is lean. That is the transition fuel feed between the idle and main circuits. Issues usually show up more with light throttle changes in that area. With more aggressive throttle motion it is quickly moved through and covered up.

And the 4.5 PV is probably too late, too.

My car actually cruises on the idle circuit and slot much of the time. That make tuning a challenge because the idle, cruise, and light acceleration are all happening on the same circuit that is not set up for all that. Most carbs are designed to cruise on the main circuit with the PV circuit being the enrichment for higher loads. On mine, if I set the mix for a clean cruise it is lean enough to have a buck when a little more throttle is applied. If I set the slot mix richer to eliminate the buck, it has a richer cruise ratio than is best.

My car cruises on the mains after about 2,000rpm.

And if there aren't any detonation issues, our engines typically respond well to your rapid ignition advance.

. Timing is all out for throttle response. Gear ratio is 4:56. Power valve was determined by idle best vacuum (8hg) then divide by two. 4.5 got me close so I used it. Ignition is Digital MSD.

Base your PV off your cruise vacuum. Example 11in at cruise use 5.5 PV.

. Roger that. What I’ve done so far,,,,,,, IFR’s up to .039”. Power valve to 6.5. IAB’s down to .070”. The car runs allot better but with a very, very slight buck. Hardly noticeable for a passenger, but for me as the main driver, I can still feel it. I’m going to live with it for a bit. Thanks everyone for your input. Really appreciate it.

Timing? Have you readjusted the mix screws now that it's close? Are the IFRs in the up or down position? Maybe an .028 MAB will help get the mains started a little sooner.

I. Yep, the idle mixture screws are out 1/2 turn. The IFR’s are up.

You should move them down. Could you open the idle mix some more or does it get all funky? Timing?

. I can’t change the location of the IFR’s without changing metering blocks. Timing is all out at 35*. If I open the mixture screws even 1/8th turn, the idle drops and yes, gets funky.

Less air or more fuel in the idle circuits, idle screws are nothing more than little throttle blades. The less cam vacuum the more the fuel bleed needs opened up. Whatever the cruise vacuum is PV should be 1 step under that (vac 7 put a 6 in it)

I understand all of this. Thanks

A “buck” at 1300–2000 rpm in a stick car? Sounds to me like you are simply not on the power and of the cam at that low of an rpm???

Never set power valve opening by idle vacuum. It’s DEAD WRONG. It’s been wrong since day one and it gets repeated over and over. Check your idle vacuum at CRUISE and then start at two numbers lower than your cruise vacuum. That’s where you start tuning power valve opening.

Also, you need to fix your timing curve. In fact, that needs to be corrected first. With those light springs you loose control of the weights and the timing will be bouncing all over ever time you touch the throttle.

You might think you have better throttle response with basically no timing curve, but you don’t. What happens under a load is far different than what happens under a load.

That bucking is telling you that you have issues. With a 4.56 gear you should be able to creep along at walking speed and not buck.

Between your timing curve and that late opening power valve you are confusing the engine. BTW, when you correct PV timing it will make other changes necessary on your carb tune up. You will probably end up rich on transition. You’ll have to deal with that.