Tugs back and forth when downshifting ?

[D111]

Purchased a new quick fuel carburetor to replace my 770 Holley on my Roush 342.

Engine now idles better accelerates very good .Sparkplugs went from a Wet looking black to a light gray especially the two center ones on both sides.only continuing annoyance is when I let off the accelerator Or downshift the car While in gear starts tugging, more noticeable when I come closer to a stop.
I just tried to make a video but it didn't come out well.
Did not have this At all with the Holley which deftly was running richer.wondering if anyone would know A possible reason for this to Occur? I'm not even sure if it's the carburetor.

Even when cruising When I let my foot totally off the accelerator in gear it slightly starts to occur then gets progressively worse as I slowdown for a traffic light etc.

If you think a video would be necessary I will try again.

Think you
Dennis

[Tomcobra17]

Bucking back and forth is generally a lean condition. Try turning your idle screws ( all 4) counter clockwise about a 1/16 of a turn and see if that helps.

Please post the carb specs from the card that came with your quickfuel. How far out from closed are your idle mixture screws? Did you set your primary transfer slot to square when looking from underneath the trottle plate? Once this is done, you adjust your idle speed using the secondary idle speed screw on the secondary side for quickfuel carbs. Where are your float levels set on both primary and secondary? Should be in the middle of the sight window for quickfuel carbs.

We need a little more info to help.

Thanks,

Tom

[jhv48]

Do the four corner idle air screws first. Actually I would start by turning them out about a half turn past highest idle speed. If problem disappears, then slowly screw them all in till it reappears, then back out about an eighth of a turn.

[FWB]

I don't know, maybe i'm reading into the problem differently, i don't think it's a carb issue at all, a surge or buck upon accelerating i'd be with you on carb isn't set up, but the description is decelerating and downshifting, i would be looking at your brakes hanging up or a driveline issue. did the car sit for a while before the new carb was put on? could the old carb have been so bad you didn't notice the other drivability problem?

if there was a lean condition upon decel, there most certainly would be a popping out the exhaust or a pop when shifting......

[D111]

thank you very much for the replies.

I checked the idie screws both of the passenger side were set at half of turn out approximately and both of the driver side were set at almost a full turn out.
should they all for be roughly the same? I Adjusted Them with a vacuum gauge When I stalled the Carburetor to the highest vacuum .roughly
I am a Carburetor beginner to say the least

My box did not come with a packing slip
It's an HR series quick fuel 650 Carburetor that I purchased from B2 motorsports. Gave them the specifications of my motor and he Recommended and set it up for me. I Asked about the problem he was not sure and wanted me to send him a video , the video did not show the condition properly.

Don't get me wrong the car runs very well it's just that I am picky and I really liked down shifting Smoothly to a stop and not having that pulsation, I wish I could go out and try it now but it's raining Currently here in Jersey.

[Tomcobra17]

All 4 should be the same. Primaries vs secondaries can be different in general, but you definitely do not want one side set different than the other.

Try setting them all to 1 turn out and check for bucking, then do as jhb48 suggested to fine tune. Adjust all 4 equally.

Tom

[D111]

The rain stopped So I went out for a spin almost literally :-), First time I ever had it on a wet road pretty scary.

The pulsation or pulling felt the same even with the low-speed idle turned out 1 1/2 turns I tried different increments no real change set them back to three quarters out all around ,, any further and I would have to turn the idle screw up too much to smooth out Idol.

Something else I noticed different with this Carburetor versus the Holley is that when I come to a stop The idle will be around 950 to a thousand And then workes its way slowly down to about 650 To 700 within 45 seconds to a minute Then stays there.

[Jerry Clayton]

The speeds that you are running probably its the advance curve in the ignition not working smoothly---

[jhv48]

Also sounds like your throttle shaft in the base plate may be sticking or your return spring is too weak. Open the engine compartment and rev the engine by hand if the throttle doesn't snap shut when you release it, help it with your finger. If it won't return to idle immediately, then something else is going on. Throttle should snap shut and return to idle speed immediately when throttle pedal released. Could be a sticking throttle shaft in the baseplate or too weak of a return spring. Solve this one first. If you have to, disconnect the throttle cable/rod and manipulate the throttle by hand. It should open smoothly and snap shut completely when you release it.

[D111]

Throttle snaps back hard.

[D111]

Originally Posted by Jerry Clayton

The speeds that you are running probably its the advance curve in the ignition not working smoothly---

Wouldn't it then have occurred with the old Carburetor?

[D111]

.........

[jhv48]

Originally Posted by D111

Throttle snaps back hard.

When it snaps closed, does it also return immediately to idle? If not, push on the throttle lever with your finger and attempt to push it closed further. Does it now return to normal idle speed?

[D111]

Yes it returns to the idle that is approximately 950-1000 RPMs But then after about 30 seconds of idling it'll work its way down to about 650-700.
I have dual Firm Springs And does not move any further with my finger.

I'm at a loss for both the conditions, maybe a vacuum leak?

[carmine]

Take off the distributor cap and rotor and check the centrifical advance weights
and springs.
They may be all rusty causing them to be stiff in there movement.
If so remove the springs then lift out the weights and sand them clean then lubricate them and re-install.

[D111]

I just spoke with
quickfuel they recommend changing my primary jets to 72 from 70.

He feels it might be a lean condition on the low end causing both
situations.

Just waiting to hear back from Brent from B2 motorsports to find out if those jets were already changed, hope it's easy to do Don't really feel like ripping apart the Carburetor.

[Gaz64]

The slow return to idle is typical of a semi-race style carb AND camshaft.

Your idle circuit is a tad lean for idle/ progression conditions.

The idle feed restrictions need to be a little bit larger.

Have you got access to a wideband O2 sensor?

[D111]

Originally Posted by Jerry Clayton

The speeds that you are running probably its the advance curve in the ignition not working smoothly---

How would I check For that condition?

[carmine]

How would I check For that condition?

This is what i posted earlier.

Take off the distributor cap and rotor and check the centrifical advance weights
and springs.
They may be all rusty causing them to be stiff in there movement.
If so remove the springs then lift out the weights and sand them clean then lub

[D111]

These are two photos of the distributor The two sections seen loose but they only move out as far as the second photo,I can't rotate the assembly more than a bit. don't mind me, Haven't got a clue about the stuff I'm willing to learn.