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.

"Typical" 850 cfm calibrations, be it Holley, Barry Grant, Quick Fuel, etc HISTORICALLY have had the IDLE FEED RESTRICTION in the lower position and with a .037 or .038 drilled opening, NOT .032.

Where did that IFR number come from?

Tom V.

Tom,
I made my own IFR’s along with HSAB, IAB, etc. from brass set screws.

What area of the annular should the squirter be contacting?

If I am reading your question correctly, the fuel spray should contact the lower 1/2 of the booster on the side. People have tried using extended tip shooters but sometimes they can drip or syphon fuel from the Pump Circuit.
Some metering blocks have a small hole drilled where the fuel in the metering block makes a right hand turn in the metering block before entering the main body.
This feature kills the pull-over issue so then you can use the extended tip shooters.

Tom V.

Yup I've got my IFR at .031 and it runs great, no pig rich cruise anymore. I want to try 029 and see if it likes it

Do both of you have a 4 corner idle carb?

Tom V.

Yes my 850 MD has 4 corner idle.

That would explain part of the reduction in the idle orifice size as now the idle mixture flow is divided up by 4 IFRs, vs 2 for a typical 4781-2 carb.

In the old days the idle screws were turned out 1-1/2 to 2 turns out on the Primary Metering Block. With the 4 corner idle stuff, the normal setting is 3/4 turn to one turn out. So how far out are your idle mixture screws?

I believe I have a theory on why you are using .031 IFRs.

Tom V.

Tom,
All 4 idle screws are approx. 7/8 of a full turn out.

johnmachota78,
I was reviewing the countless past data log graphs I have from tuning this carb and found I went as low as .022 IFR with no real ill affect on the fuel curve, ended up at .032 IFR for 2 reasons:

a. Smoothest AFR
b. Gave me the widest IAB dial in range.

4781 Holley Carbs came with (as I posted). TWO .038" IFRs in the low position

With 19 cfm of normal airflow going thru the engine at idle they were able to control the proper Idle air/fuel mixture to the engine.

Enter 4 corner Idle Systems. It would make sense that the engine still wants the same amount of idle air/fuel mixture but now you have 4 sources for it and you don't need 4 .038" IFRs in the metering blocks. You need a reduced area amount of fuel or smaller IFR orifices.

Tom V.

The 850 MD Holley version specs show 36 IFR.

Ok so explain something to me.

The 850 Mighty Demon/Holley specs are 36 IFRs, not that too different from the .038" Holley specs. On post number 28 you said, you went down to .022 IFRs without any ill effects. Yet on the same post you said that .032 gave you them smoothest idle.

So you do not consider a difference in the idle behavior an effect?

I am getting a lot of different stories on this thread.

Tom V.

Most times carb calibrators compromise on the posted "as built Carb Specs" vs the specs that the individual engine likes for best drive.

Tom,
As i mentioned in an earlier post I’m not an engineer so I’m not qualified to explain how or why. Also I said ‘no real ill affect’ I did not say it was the same as the 32 IFR. Just meant the 22 IFR wasnt radically different than the 32 IFR. Sorry for any confusion.

No Worries. Most Idle Quality is subjective anyway, nasty idle to one person is mild to another.

Tom V.

Update.:
Secondary Opening Stumble

Replace Sec. MJ from 76+6.5PV to 86.
Checked vent whistle length OK at 1 5/16".
Remove Pri. MJ extensions.

No change to secondary opening stumble when stabbing throttle from cruise, AFR goes of the chart lean spike (engine will cut out if I don't let up on throttle). Also, I now have a bad stutter/stumble when staying on the primaries and lightly stabbing throttle (not enough to open secondaries) from a light cruise.

Is it possible that a spark related issue would cause this type of problem?
It's HEI cap and rotor are good (checked them today).
Can someone tell me how to test the coil and module, I have a multimeter but would need step by step instructions.

Update:
Hard Braking Stall

Removed and plug brake booster vacuum source.

Engine continues to stall with hard braking. Does this rule out the booster as a possible contributing factor?
When I reviewed the data logger chart for the brake stall and compared it to last week I noticed as vacuum and RPM drop the AFR doesn't really go crazy rich. For example last week setup with sec. PV in the hard brake AFR was 11 and this week with sec. PV plug and 86 jets braking AFR was down to only 12.

I'm starting to think something else is going on here to.

Might need to check the two small wires going to the pickup coil.

Vacuum drops when you mash on the loud pedal and when the vac advance lets off it wiggles the wires to the pickup. May just be insulation holding broke wires butted together.

Clay

Carb always gets blamed first, That is a fact.

Tom V.

It all looks and feels intact.

Using an aftermarket module?

Tom V.

Hei
 

Why are numbers up. Shouldn’t it be numbers down on center cam.

ICM is a GM 990 and coil is an Accel at least 20 yrs old.

As for the vac. adv. center cam numbers up I have to dig up my notes when I dialed in the timing. Went through a lot of different weight/cam/spring combinations and the engine seemed to like this setup.

In case anyone would to continue to follow journey I started another thread in the electrical section to help determine potential distributor issues that may be contributing to the stutter and stumble problems.

Here us a direct link:

http://forums.maxperformanceinc.com/...d.php?t=834056

Update.
Resolved the stumble/bog issue with a distributor cap replacement (a couple of the coil screws were stripped in the cap, go figure). After that I decided to replace the 30yr coil and pickup coil.

Now back to the carb, although there is no noticeable stumble when stabbing the throttle there still is a huge lean spike showing up on the data logger. I tested larger squirters and added 50cc accel. pump but no change. Also plugged the secondary PV and up the jets to 86, no impact to the major lean spike with this either.

Went through the carb several times and all is good and clean, I don't know what else to test or check, any ideas are welcome.

Center cam is upside down.
You'll need to turn it over.
Also, do you have the original springs?

Kenth, ok I will flip the center cam and as for the springs they are not the originals. I do have several different sets of springs I can sort through.

Timing when riad tested yesterday was as follows:
Initial 13-14
Mech 23
Total 36-37 all in @ 2000rpm
Manifold Vac Adv 7* starting at 7”hg

I rest timing but not road tested yet as follows:
Initial 10-11
Mech 23
Total 33-34 all in @ 2000rpm
Manifold Vac Adv 7* starting at 7”hg

As for the carb can someone confirm the dry float setting is 0.4” for a BG Mighty Demon?

Does your Demon have sight glasses for the float level? I always used them for final determination, middle to slightly above. As for your spike on the data logger, you could conceivably get a spike from going way rich momentarily. If it runs good, and the plugs look good, I wouldn't let a momentary spike ruin my day. Lots of things are in play, like the location of the sensor, the fact that its reading all 4 cylinders on a bank together(likely, you didn't specifically say how you're monitoring afr) etc. When you're happy with the ignition, try backing down on the squirter, in small increments, or cam position, or go back to the 30cc pump, it's possible your seeing the fuel "dump" at wide open "put out" the sensor momentarily.

Scott65, it’s a bad lean spike not a rich spike and there is a stumble that occurs at that moment. I’m close to resolving the timing issues and once that’s good will reset float then move on to HSAB testing.

Kenth, I flipped the center cam and had to reset timing several times. The detonation is gone :) and these are the current settings:

Initial 17
Mech 17
Switched to ported Vac Adv. 7* starting @ 6”
Total 34 all in @ 3300rpm

I have several springs sets and will try to get it all in somewhere between 2600-3000.

Also found this in my tool box drawer that holds all my distributor spare parts but not sure what it does. Looks like it’s suppose to lay on top of the weights and under the springs or am I way off?

You said the stumble/bog was resolved. We all try to help with the info you provide... Trends are to be given more weight than "spikes". And a slug of fuel will confuse an O2 sensor.

The stumble has reared it’s ugly head again, it was gone for a day or two, what can I say I started celebrating too soon.

Update
I'll start with the bad news, after replacing everything spark related from spark plugs all the way back to pickup coil this issue is still present. Now it wasn't all bad because there were somethings that actually needed replacing and except for the terrible studder/stumble when stabbing the throttle it actually runs a little stronger now.

Good news is after swaping out the 850MD for a 750SD with vac secondaries the studder is gone which means the problem is in the 850 and I can refocus my attention back on to that carb.

I have confirmed the following:
Fuel pressure is good,
Needle & seat is clean,
Power valves functioning properly and removing sec. PV did not help,
No debris in the IAB or HSAB,
Both accelerator pumps react instantly and larger squirters did not help,
Float levels within spec,
There is probably a few other things I checked or tested but ultimately the problem is still there.

I'm open to any and all recommendations.

I know this thread is getting old however I would like to update it in hopes of getting some more advise.

Quick recap the problem occurs when I stab the throttle while cruising and no fuel gets to the engine causing a huge stumble, so bad that if I keep the throttle pinned it will kill the engine.

Distributor is ruled out as the problem.

Fuel pump is ruled out as the problem.

All individual carb componets are clean and functioning properly.

What I found was the stumble only occurs when cruising under 2200rpm. so I'm thinking the issue is related to the transition circuit but I'm not sure where to go from here. The following pics show the pri. and sec. transfer slot relationship to the closed plates. Pic with closed plate is primary side and pic with the black pencil line represents the bottom side of the sec. plate when its closed.

Are these set correctly?