Major lean AFR spike when secondaries open after stabbing throttle.
 

I recently noticed a major stumble when I punch the throttle to open the secondaries from a cruise rpm. The AFR spikes off the chart and the engine will actually cut out if I keep the throttle in that stabbed position. When viewed on the data logger chart the AFR shows an initial richening for a split second then reverses to a lean spike (18 +).There have been no recent changes that I can point a finger at so I switched out the sec. accel. pump to a 50cc with #42 squirter (was 30cc with #35 squirter).

Torn down the carb completely to inspect it and everything was to spec. as I built it several years ago. Here are some particulars:

Engine:
Stroked 461, 9.6 CR, 6X heads 250cfm @ 28", solid flat tappet 244/252 @ .05", HEI 10* intial, 6* direct manifold vac adv., 33* total all in @ 2500rpm

Transmission/rear axle:
700R4 with lockup, converter stall 2400, 3.42 rear gear

Carb:
850 Mighty Demon with annular boosters,
Main Jet/Power Valve - pri. 74 + 6.5PV and sec 76 + 6.5Pv
IFR - relocated to low side of metering block
Transfer slot - 0.02" pri. TS and 0.0" sec
Accelerator pump/Squirter - 30cc pri./31 and 50cc sec/42
Float level - mid bowl

AFR's
@ idle (in drive) 12.8-13.1
@ cruise 14.5-15.1
@ WOT 12.5

I have not tested from idle to WOT yet.

Any ideas what could be causing this?

Thanks all.

You are trying to jet a 850 Mighty Demon like a 700 cfm Holley.

Tom V.

X2 Tom V.
Under jetted

If it ran OK before, I'd check some things out?
Change gas suppliers?
Does the gas squirt good (and instantly) when throttle is moved?
Also, does front still squirting when secondary moves and starts squirting?


:confused:

Have you always run that secondary power valve in the carb? Do you run E-15 pump gas?

My joke was, normally TRUCKS with 700 cfm carbs run dual power valves.
Typically a high performance carb, especially a 850 Mighty Demon, runs a Power Valve Block on the secondary metering block.

Tom V.

Here are replies to some of the questions being asked:
johnta1,
I use the same gas supplier all the time except recently I was stuck and had no choice, however all that gas has been burned and it's back to the original gas supplier.

Pri. and sec. squirters are good with instant reaction to throttle movement. Need to check whether the pri. squirter continues to flow once sec. squirter starts up.

Tom,
The sec. PV is there to help prevent engine stall on hard braking. It helped some but did not cure this issue 100%.
Gas is 91 octane with no ethanol.
My next change this weekend is reverting back to the blocked sec. PV and install 86 jets which will equal the same total fuel flow area as it currently has with the 76 jets+PVCR. I may have to dial in the HSAB but we'll see after a road test with the blocked PV.

No sure how jetting goes with annular vs regular downleg booster but my old Street Demon 850 cam jetted way fatter than my Holley 850 DP.

The power valve or plug should have no bearing on hard braking. Your floats too high or you need a vent baffle/whistle for that issue. Unrelated to being too lean.

If you are hard braking, you are off the gas unless you are doing a Brake Shoe Wear-out Test, (One foot on the gas and one on the brake at the same time).

So if your foot is off the gas then the engine vacuum will be typically high enough to close the Secondary Power Valve, so what is the Power Valve doing
if it is closed on braking (like a PV plug would be in the same situation).

Help me out with that one.

Tom V.

This....^^^

If you're off the gas and braking the power valve is inop. Power valve channel will not pass fuel until 2 things happen. The throttle blade has to be passed the idle transfer slot, and vacuum has to drop below the PV rating.

So if you're braking you're off the gas, which should be a high vacuum situation and the throttle blades should also be closed covering the transfer slot if adjusted properly. PV isn't a player.

This past week when doing weekly carb maintenance on the circle car I noticed the accel pumps were not instantly dispensing fat fuel squirt at the slightest movements of either pri or secondary throttles. I religiously keep squirter linkage adjusted for instant delivery as needed. Pulled squirters and check valves below, blew out squirters, pushed fuel thru without squirters, gave the needle weights and seats a quick clean.Nothing obvious present.
Reassembled.
Instant fast fat shot from pumps restored....with no other adjustments.
Noticeable visual improvement on starts and restarts, driver commented on how well it launched.
No change in calibration, just some maintenance.
.HTH..
YMMV
:cool:

I had feeling this post was going to take a turn towards the braking issue, maybe it's related to the secondary opening stumble I don't know however I'm open to the feedback.

Let me first say there has been countless hours spent trying to tune out or figure out what is creating the hard brake stall and some of the changes or testing are as follows:

a. Reset pri. and sec. float level to many different levels
b. Vent whistles cut to remove the 'springboard' section so the end is just the
rectangular opening.
c. Vent tubes are connected to each other via a metal tube with multiple
holes to prevent spillage into the venturis.
d. Inspect all vacuum lines for leaks.
e. Added pri. and sec. jet extensions. Pri. extensions are cut on the bottom
side closest to bowl floor.

I don't have my log book so can't recall if there were any other related changes.

I'm not an engineer and can't explain why this is happening but I can confirm with 100% certainty that when the throttle is released vacuum level will jump to a high reading (18-21) and will remain high until the brakes are applied. At this point vacuum begins to drop the harder I depress the brake pedal. As the nose of the car drops during hard braking, vacuum also drops below the PV opening rate.

Maybe someone can help explain whats going on here.

a) There should only be one Float Level Setting.
Any setting but the correct one screws up the Main Well Calibrations.
Fuel Bowl upside down and set according to the many pictures posted by holley on the float vs mounting screw portion visible.

b) Vent Whistles need to be a certain length: 1-5/16" long from the edge that goes into the metering block to the edge facing the back of the fuel bowl.
Normally I reduplicate the "Springboard (as you call it) once that length has been set.

3) Vent tube holes need to be at least the same combined area as the factory vent tubes. You have two vent tubes. Measure the inside diameter of the brass tubes, then drill the holes so that the area matches the factory area. Assume a total of 8 holes minimum across the center of the carb.

4) Install a different PCV Valve or clean the one you have, If the shuttle is "hanging" the idle airflow will be wrong and the engine will be lean and try to stall.

5) Post up what your idle feed restrictions are and where on the metering blocks they are located at.

Tom V.

Demon carb might be issue. I’ve known several that “ just wouldn’t work” no matter what was done. I know that defies logic that “ anything can be fixed” but......��

Power brakes?
I'd check the power unit's diaphragm?
Try blocking the vacuum hose outlet at the intake/carb.
(bypassing the brake unit basically)

There will be no 'power' brakes so harder to stop, but will isolate the vacuum issue pertaining to the power brakes.

This could also indicate the fuel bowl spilling fuel into carb throat?

The power valve still won't pass fuel in that situation because you're off the gas and the idle transfer slot is covered....IF....the throttle blades are adjusted properly. The power valves have absolutely nothing to do with the idle circuit.

From the way you're explaining it, it sounds like a power brake issue.

Does your Demon have the "idle ease" feature? If so, a problem with that might show up as a vacuum issue. Not sure how likely that is, but it is basically a controlled vacuum leak.

Replies to latest questions.
Tom,
a. Current float setting is as you described in your post #13.
b. I will check vent whistle dimensions this weekend.
c. Vent tube extension holes have a greater area than total combined original vent tube(s) area.
d. PCV had been checked and functioning as it should.
e. 32 IFR relocated to low side of metering block.

johnta1,
I replaced complete brake system a few years ago with Kore3 Z51 4-wheel disc and DSE dual 9"/master cyclinder setup designed for this type of brake conversion. But as most of us know just because its a new part doesn't mean there isn't a defect. I like your suggestion to isolate a potential vacuum issue at the booster, will test it this weekend.

Formulajones,
So there should be no vacuum drop during braking other than the initial high vacuum reading that drops down to the idle vacuum reading as the car comes to a complete stop, correct?

Scott65,
Yes there is an Idle Ease feature however I don't need to enable it with the way things are setup it idles great.

I have to mention that not withstanding the hard brake stall issue and the recent secondary opening stumble the carb just feels perfect in all circuits and this is continuously backed up by the data log numbers and spk plug color, all looks really good.

We have focused a lot on the brake stall problem in the above posts which I'm happy to continue but have to ask the following child like question. Is there a potential that both the brake issue and stumble issue might be related or caused by the same problem?

I only meant to make sure it isn't leaking by.

I wonder if your small front acc pump and squirter up front is causing some issue when punching it from a cruise rpm.
That being said a few years back I switched to an annular main body I had similar issues I could not get the hesitation out of the carb. If I remember correctly the shot from the squirter was actually hitting the annular booster ring. I fought with it for a while whether it was me or just the mismatch of carb parts? I eventually got so fed up with it at the dragstrip I swapped mainbodies on a picnic table and went back to the original downleg mainbody.