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

[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 .

[Marc #561]

Here is my very simple nsolution, as every high performance US Po

[mreid]

Say what...?

[Cobrajames]

Originally Posted by Marc #561

Here is my very simple nsolution, as every high performance US Po

I think that I will try that first thing tomorrow.

[Dwight]

Marc
you need to explain what you mean by "Here is my very simple nsolution, as every high performance US Po"

I don't understand your comment.

Dwight

[SNClocks]

GOOD DAY TO Club Cobra!

Kelly and I bought our Superformance Daytona Coupe (0065) nearly 2 months ago and have put 1,500 miles on her so far. Our Focus, for now, is to wipe out niggling little issues – we have gotten both doors to open and close smoothly, added Freon, quieted her down a bit, started tuning on the carb so she will start and even idle when cold or hot, hooked up the reverse light switch, adjusted the emergency brake (suspect I will need to modify a bit to get more leverage/less travel), and generally are getting used to driving her. We’ve had her in a couple of shows and ‘cars and coffees’ and gotten very kind comments from all and sundry. Heck, a few even knew of these cars.

I am in hopes that, since you folks have had your cars for a while, that you can provide some well-needed technical guidance on the remaining techie issues.

First question that I could use help on: When the engine (427R with 770 Holley) is running at below 2,000 rpm, at minimal to slight throttle opening, the car bucks. From reading on this forum, it sounds like the transition from the idle circuit to the high-speed fuel circuits can be tricky and result in a lean mixture – specifically at slight throttle openings. In our car, opening the throttle more than just a little eliminates the bucking, but with a slight opening, as in cruising down the road at less than 2,000 rpm, it bucks pretty hard.

This forum indicated that changing the “idle feed restrictor jets” on the secondary side from 0.035 to 0.037” eliminated 95% of the problem. So, finally the question: Have any of you with a 427R and a 770 Holley addressed this issue? Somehow I suspect the 750 Holley's are pretty similar.

Looks like it is easy to buy jets on-line. My gut feeling is I need to get an air/fuel ratio meter and use it to troubleshoot – likely we need it anyway to set the triple carbs on our XKE (of course, we have put 130,000 miles on her just setting the carbs by ear – but surely a meter would make her run better). I will admit it is tempting to put fuel injection on the Daytona – it really does make a big difference, and we love the MSD system we put on our Pantera. But, heck, if just some jet work would resolve this issue – it would be a lot easier.

Your comments please!

[Joe's Garage]

Originally Posted by Flygirl

Thanks, guys! It makes sense now. Never considered the cam profile as a contributor, or driveline lash, or PIO either.

And y'all are right, keeping the rpms up just a little helps a lot. The worse case I experienced was probably closer to idle, definitely below 10mph. And in both cases, only in first gear at low speeds with very little throttle opening.

It doesn't really bother me. But experiencing it again had me thinking there was something more seriously wrong that was causing the problem.

The Porsche could be pretty bad, actually, but in that case (come to think of it) it was induced by PIO via the throttle.

Many thanks!
Natalie
:-)

I take it your are convinced you are not just lugging the motor, any manual will lug if your RPM is too low to handle the gearing.

[lippy]

With any carb, make sure your 4 corner idle screws are adjusted first. If those are adjusted properly and the idle screws are, say, 0.75 to 1.5 turns out from all the way in (lightly seated), then your IFRs are ok. At that point make small main jet adjustments. You probably don't need to mess with the IFRs or any of the bleeds. You can also change your squirters if you get a bog when launching but everything else appears to be ok.

[SNClocks]

Thank you for the comments. Joe's Garage - I would think if I was lugging it, it would get worse when I added throttle - nope, it smooths out and pulls. From 1200 rpm, no problem. But, light throttle at 1800 - bucks.

Lippy - Thank you for the guidance on how to tell if the IFR's are sized reasonably. I plan to first verify timing is to build spec, then check out the 4 idle screw settings. Build sheet shows 16 deg BTDC at 900 RPM, 32 at 3000.

Stephen

[blykins]

Originally Posted by SNClocks

GOOD DAY TO Club Cobra!

Kelly and I bought our Superformance Daytona Coupe (0065) nearly 2 months ago and have put 1,500 miles on her so far. Our Focus, for now, is to wipe out niggling little issues – we have gotten both doors to open and close smoothly, added Freon, quieted her down a bit, started tuning on the carb so she will start and even idle when cold or hot, hooked up the reverse light switch, adjusted the emergency brake (suspect I will need to modify a bit to get more leverage/less travel), and generally are getting used to driving her. We’ve had her in a couple of shows and ‘cars and coffees’ and gotten very kind comments from all and sundry. Heck, a few even knew of these cars.

I am in hopes that, since you folks have had your cars for a while, that you can provide some well-needed technical guidance on the remaining techie issues.

First question that I could use help on: When the engine (427R with 770 Holley) is running at below 2,000 rpm, at minimal to slight throttle opening, the car bucks. From reading on this forum, it sounds like the transition from the idle circuit to the high-speed fuel circuits can be tricky and result in a lean mixture – specifically at slight throttle openings. In our car, opening the throttle more than just a little eliminates the bucking, but with a slight opening, as in cruising down the road at less than 2,000 rpm, it bucks pretty hard.

This forum indicated that changing the “idle feed restrictor jets” on the secondary side from 0.035 to 0.037” eliminated 95% of the problem. So, finally the question: Have any of you with a 427R and a 770 Holley addressed this issue? Somehow I suspect the 750 Holley's are pretty similar.

Looks like it is easy to buy jets on-line. My gut feeling is I need to get an air/fuel ratio meter and use it to troubleshoot – likely we need it anyway to set the triple carbs on our XKE (of course, we have put 130,000 miles on her just setting the carbs by ear – but surely a meter would make her run better). I will admit it is tempting to put fuel injection on the Daytona – it really does make a big difference, and we love the MSD system we put on our Pantera. But, heck, if just some jet work would resolve this issue – it would be a lot easier.

Your comments please!

Could just be the camshaft. It's sometimes difficult to balance a "lopey cam" with pleasing low-rpm characteristics.

That Holley Street Avenger is not doing you any favors though....

[Gaz64]

Originally Posted by blykins

Could just be the camshaft. It's sometimes difficult to balance a "lopey cam" with pleasing low-rpm characteristics.

That Holley Street Avenger is not doing you any favors though....

Agree.

Some part throttle roughness just might be the way it is.

There are many carbs available now that would be better than an "out of the box" avenger.

Some of mine have been rough at low speed, I just live with it as a trade off for the explosive mid to top end.