What Causes a High RPM Holley Cough?

[elmariachi]

I have all the kinks worked out of this Holley 3255-1 Le Mans except for one issue. If I really lean on it moving up through the gears, I sometimes get a single hard cough/belch/backfire, generally at the top of 3rd or 4th gear and above 4K RPMS. And the pedal is all the way to the floor when it happens so the butterflies would be wide open. It only happens once during the acceleration curve and it is very brief, not like a stall or electrical loss.

Thoughts on what might be causing this??

Jim

[dcdoug]

Did you have to throw it across the room to get it dialed in properly?

[elmariachi]

Originally Posted by dcdoug

Did you have to throw it across the room to get it dialed in properly?

Hahaha, no, but I did do one of those tequila-induced voodoo exorcisms on it. Here is the deal with this carb, for those who care:

1. Setting the float levels with the cathedral bowels is a pain in the butt because it involves removing the bowels, then the floats themselves and bending the tabs. Real scientific there Mr. Holley. The way the engine sits in the car and the slant of the intake means you might have to deviate from normal float settings. LOTS of trial and error.

2. The horizontal needle and seat design is an invitation for trouble because gravity ain't on your side. Somehow my front one stuck open on the UPS ride back from Keith and it flooded the intake and a couple of cylinders with lots of raw fuel. That was a real mess.

3. My Mighty Demon 750 uses 72P and 77S jets. To get this one right I am at 77P and 85S, a lot more jetting than I would have expected with a Sidewinder intake and stock MR heads. Ironically, I think those are the same size jets that came from Shelby.

4. I had to take all the play out of the pump arm and upgrade to a 31 discharge nozzle to get a healthy pump shot. I am using the pink cam in the #2 position.

5. PV is 4.5, I am using the lightest vacuum spring in the Holley kit, and I am a fat 1.75 turns out on the idle mixture screws.

Timing is set at 18* base, it idles nicely at 900-950 rpms and runs good. Other than the cough I'd say its sorted out and behaves predictably. Looks cool, but doesn't run quite as stong as with the Demon double pumper though. I am still moving on to the 2x4s.

[Hotfingrs]

I'd would guess, because the LeMans bowls are bigger, you're not running out of fuel until 3rd gear. Either your fuel pump can't keep up, or you need to put jet extensions on the secondaries, you'll also need the float that goes with the extensions.

[elmariachi]

Fair comment, but if its running out of fuel, then where does the fuel come from that resumes the engine in the 1/10th of a second this cough lasts?

[Hotfingrs]

Only guessing here, but when it coughs, the car slows down, and when it slows down, the secondary jets are covered again.

[vector1]

sounds like vacuum secondaries? that's where i'd look.

[undy]

Tough one... Could be ignition too. Inside of dizzy cap (rotor button and contacts) look OK?

I've been kicking around the idea of installing an "in dash" wide band. It's not only great for dialing in the carb(s); it's the shiznit for troubleshooting too.

http://www.amazon.com/Innovate-Motor...=pd_sbs_auto_6

http://www.compcams.com/Technical/Ca...pdf/page18.pdf

[elmariachi]

Originally Posted by vector1

sounds like vacuum secondaries? that's where i'd look.

That's just it, there is no noticeable loss of power or momentum, just a hard half second hiccup.

Originally Posted by Hotfingrs

Only guessing here, but when it coughs, the car slows down, and when it slows down, the secondary jets are covered again.

Can you be more specific on what I should be looking for? What would cause the secondaries to COMPLETELY drop out for a split second in a straight line under full throttle? Its not like a cylinder miss, its a complete carb backfire.

Originally Posted by undy

Tough one... Could be ignition too. Inside of dizzy cap (rotor button and contacts) look OK?

I've been kicking around the idea of installing an "in dash" wide band. It's not only great for dialing in the carb(s); it's the shiznit for troubleshooting too.

Just to rule that out Dave I just added a new cap, and rotor and plugs. It only does it with this carb, not the Demon so its gotta be carb-related. I do like the idea of an AF gauge in the cockpit. Been thinking about it in a fold-down panel that I can hide up under the dash. (Dont want this thing looking all new-school and stuff. )

[vettestr]

Don't overlook the ignition as mentioned earlier by Undy maybe? A tight valve(adjustment) or floating a intake valve is more obscure but possible, nope look at ignition.

[Rick Parker]

Does the car nose over entirely, or is it just act like one cylinder? If it noses over and then immediatly takes off again, check for a leak in secondary diaphram, or incorporate jet extensions in the secondaries. Do you have a an on board fuel presure gauge?

[Jac Mac]

To me it sounds like the secondaries are opening too soon/quickly causing a momentary lean condition, try going up one or two grades on the vac secondary spring...

[RICK LAKE]

Elmariachi Jim how long does the shift take?? The other thing is could this be the weights for the advance in the distributor returning to base for a split second. Heavy advance weight will do this or the motor may backfire through the pipes. No power loss just noise and flames. You are at WOT and lift of the throttle pedal for a moment, motor gets high vacuum, carb is partially closed and you get a pop. It's a timing thing. Advance the timing 2-4 degrees than go and road test the car. see if it is doing the same thing. Rick L.

[vector1]

Originally Posted by Jac Mac

To me it sounds like the secondaries are opening too soon/quickly causing a momentary lean condition, try going up one or two grades on the vac secondary spring...

this would be my initial vote. unhook the secondaries and see if you get the hicup.

[ERA Chas]

Originally Posted by Jac Mac

To me it sounds like the secondaries are opening too soon/quickly causing a momentary lean condition, try going up one or two grades on the vac secondary spring...

X2 + vector...

[Tom Kirkham]

Originally Posted by elmariachi

...I sometimes get a single hard cough/belch/backfire...Jim

Just to be clear, is this through the carb or out the exhaust?

[Jerry Clayton]

Couple of things you need to clarify----top of 3rd or 4th---which????same gear all the time, most of the time or completely random???? Each and every time on the run up thru the range?????
Pedal on the floor----do you mean that you don't lift to shift????
Is it a vacume seconary carb????

Do you have a rev limiter in your ignition and at what rpm???

Now---other than those items---have you tried this carb with center pivot float bowls???


IFIFIF ( if )

You have a vacume sec carb, rev limiter, don't lift to shift----you will occasionally get a backfire from any or all---over rev will cause the seconaries to close,changing mixture, and interupting the ignition randomly( so this could be random misfiring cylinder lighting off unburned fuel )

With the rev limiter cutting ignition, ubburnt fuel mixture is passed to the exhaust,where the heat ignites it, causing a backfire back thru the intake while valves are on overlap

When an ignition rev limiter drops cylinders, and then refires--it can cause a crossfire thru parallel running wires to an adjacient cylinder---This is common on Fords left side bank--how are your wires run????

Does your carb use seconary metering blocks or metering plates???

Jerry

[elmariachi]

This is an entirely random event that I have seen happen in both 3rd and 4th gear when at WOT around 4500-5000. It is SO BRIEF, that the car does not nose over, slow down, hesitate, nada. Its not a crack (like an explosion) its truly a cough. Its not a cylinder miss, its an entire cough/cutout/backfire of the engine through the carb/intake. It lasts about as long as the snap of your fingers. And it does not repeat itself during the acceleration. It does not always happen and its not at one repeatable rpm. Totally random but always WOT and generally above 4k rpms.

So for example yesterday morning we had been out running for a couple hours. At one point I was running 55 mph in 4th as we entered a 2 mile straightaway. I downshifted to 3rd, floored it and took off. A few seconds later as it neared 5000 rpms, it coughed once but it remained floored because it happens so fast I didn't even have time to react and lift. I continued to acclerate up through 4th and into 5th and then I finally lifted at about 130 mph. Following that one cough, it never missed a beat until I lifted. And yes, I do lift to shift.

My MSD box is limited at 6200 and my engine stops making power at 5800 so its not an issue of the rev limiter. Wires are properly separated to avoid crossfire. Holley 3255-1 has metering blocks front and rear.

Valve lash was checked and set hot per Comp cam card 300 miles ago, no issues.

Rick, MSD 8594, blue springs, no issues there. I am already at 18 base timing and 36 total all in my about 3k. Remember guys, it DOES NOT do this with the Mighty Demon 750 installed, all other settings being equal.

I will stiffen up the vacuum secondary spring again and see what happens. While dialing this thing in I found the stiffer springs to induce a hesitation when the secondaries opened. Per Holley's instructions, I kept lightening springs until it went away, ending up with the lightest spring. I will unhook the secondaries altogether and see what happens.

Jerry, I was planning on chasing down a different set of bowls if nothing else produced a solution. I have an Innovate LM-1 O2 sensor that has been all but worthless but I plan on hooking it up and going out with the laptop and see what it sees when it coughs.

Thanks guys.

[Jac Mac]

Remember with the Vac secondaries there is no accelerator pump to the rear barrels, you might find that you need a longer duration pump shot ( slightly smaller shooter or larger 50cc pump & cam) to cover the point where it backfires. We used to pull the secondary idle tubes out of 3310's fitted to Big Port 351c Falcon GTHO etc & enlarge them slightly to richen up at the point where the secondaries started to open to get rid of a similar problem on those cars where a spring change didnt work... simply enlarging the rear main jet size might help, but I suspect its happening at the point where the secondaries open & or transfer from their idle to main circuits, therefore would probably be too rich in the overall picture..
One other trick that might help pin down the problem area... Cut a piece of 0.040 SS wire@ wrap it round the aircleaner attach stud, then fed each end into the idle air bleeds inside the air horn on the secondary side, then test drive, if that makes no change then try the main air bleeds & test drive....This would increase the fuel flow/richen either circuit and possibly help point you in the right direction without making any 'non-reversable changes..

[ERA Chas]

Originally Posted by elmariachi

I will stiffen up the vacuum secondary spring again and see what happens. While dialing this thing in I found the stiffer springs to induce a hesitation when the secondaries opened. Per Holley's instructions, I kept lightening springs until it went away, ending up with the lightest spring. I will unhook the secondaries altogether and see what happens.

I agree.

Years ago I had a similar situation when I had gone too light with springs and they were almost snapping the secondaries open. Ideal is when there's a seamless transition into the secondaries.