What causes an engine to 'buck' at low speeds?

[mreid]

It seems most on here assume that it must not be tuned correctly (of course all of ours are perfectly tuned and if we owned the car in question it would never buck). I completely disagree. We put barely streetable, high performance engines in a light-weight race car and then complain that it doesn't drive like mom's minivan (okay, that's a bit of an exaggeration to make my point). Fact is, low vacuum at low rpm equals low torque and bucking if load is applied too fast.

I just don't want Natalie to get the message that any cobra that bucks, must need a tune or have other problems.

[HI Cobra]

Any stick car, truck, whatever, I have driven will buck if you get the rpm's
below a certain point - shift it down a gear and it stops. No problem. If I am in creeping traffic then I just take it in and out of gear as needed to keep it
rolling - I don't ride the clutch.

[Dwight]

we call it "trailer hitch jerk"

most of the time it's the cam, poor tuning makes it worst


Dwight

[RICK LAKE]

Flygirl Natalie It's not a single thing. Starting with tune up, Fouled plugs could cause this at idle with a carb running rich or lean.
Weight of flywheel Depending on size of motor(cubes) and camshaft with LSA number this could all cause lugging of the drive train, or hickps. A larger flywheel stores more energy that a lite one. this helps stop the pulsation of each cylinder being fired at low rpms.
Gearing in the trans and rearend also can cause lugging of the car. The lower the rearend gear the wider the trans gears need to be to not have the motor running at 600 rpms. Unless you have a large camshaft with 110 LSA and choppy idle the lugging should be minimal. I have a 482 with 22 pound flywheel and FI system. Both trannies have a 3.00 or higher first gear. With a 3.31 rearend I can idle drive the car without the lugging like a parade speed. If you step on the throttle it will delay for a second because of the fuel dump and air change into the motor. The best thing to do is not let the car drive at idle speed, 1000 rpms with large camshaft and 800 rpms with a small street camshaft. If you can't run in these rpms, change the rearend ratio or trans mission first gear ratio. I can say the any gear higher than a 3.00 in a cobra is a waste of time unless you are running a gearing under 3.00 in the rearend. Lugging a car can also due damage to bearings on the crank and rods by pounding them, low oil pressure to the upper end of the motor, damage to the drivetrain on the rearend backlash of gears, same for inside a trans, a solid hub clutch has no springs to absorb the pulses from the motor at idle speed. As said it's a combo of things. Rick L. Ps missed the call.

[westcott cobra]

With a vacuum meter and a wideband A/F Controller i tried to find the root couse for the same problem as you describe it now.

With my FE it was definitely a mismatch in carb tuning because everytime I felt the bog I was always rolling in idle rpm and just pushing the throttle slowly a bit to keep cruise speed. Vacuum went down imediately in these situations.
Corresponding to the suddenly low vac the A/F meter showed a very lean condition about 16:1 or higher.

The bog went after finding the correct powervalve but was still there just a bit. I adjusted idle to 800 and with that rpm the problem was solved completely( before idle was at 650) and the A/F meter showed never leaner as 14:5 to 13.8 at low and middle rpms and about 12.3 to 12.5 at WOT.

Engine had aluminum flywheel and the car has very long rear end with 2.88 and 5 speed tremec.

200-300 rpms higher in high gear rolling around and nobody could have noticed the lean condition.

First step in tuning was changing the primary idle air bleeds from .033 to .036 because the car idled too rich and the adjustment the four corner screws allowed was to small.

Westcott

[Bobcat]

Had the same problem on mine and found that it was in the idle circuit just off idle as I moved off from a dead stop . Changed the idle feed restrictor jets ( everyone does have removable hi speed , idle air bleeds and IFR jets don`t they ?? ) from .035" to .037" on the secondary side and eliminated 95% of the problem . The .035" are stock on the Holley I have . I`ll replace the primary IFR jets this weekend and that should eliminate the problem completely . I also have the ability to change the transfer slot restrictors , so I have another avenue to explore .... or screw up on .