Many of you may remember my WOT lean-out/rich cruise problem I had in the past. I'd made a few changes, so I'd like to ask some more questions.

As I roll onto the throttle, it still progressively leans out. This happens at any speed and any gear, so I don't think it's fuel pressure. The more throttle I give it, the leaner it gets.

I changed the .038" IFRs to .033", and this did help the low-speed AFR. I leaned out the primary jet to lean the cruise from about 12.5:1 to about 13.5:1. I have also tried to up the secondary jet at the same time to keep the WOT AFR the same.

Here's some notes that I've made with the LM-1:
1. Idle vacuum is 11" @ 900RPM
2. Cruise vacuum (30-40MPH) 13-15"
3. AFR at Idle: 14.5:1
4. AFR at cruise: ~13.0-13.5:1
5. AFR at WOT: ~16.0 (idle-6000RPM first gear pull)

Here is how the carb is set up right now:
Mighty Demon 825, downleg boosters
.033" IFRs
78/94 jets, 4.5" PV

Here's my questions:
1. Should I just accept the 13.5:1 cruise AFR, even though it's a bit rich IMO? From the way I see it, the leaner I make the cruise, the leaner WOT gets.

2. Should I keep on increasing the secondary jet to richen WOT? I'm already at a #94 jet, so I can't go up that much higher.

3. Is there something wrong with my carburetor to need such large jets, and it's still 16:1 at WOT? I understand this carb is big for this engine, but it does get good throttle response, and many guys have successfullly used 950CFM Holley HPs on small blocks.

Ideally, I would like 14.0-14.5 on the cruise, and 12.5-13.0 on WOT.

What do you guys think?

Matt, what happens to the WOT ratio when you bump the jets up? Do you get a corresponding drop in ratio or does it stay 16:1?

The carb is NOT too big for your combo.

You know John, now that I think of it, there is a change but it's pretty small. I think two jet sizes (on the secondary only) might change the AFR by about 0.5.

I think I can probably jet up both the primary AND the secondary to get a good WOT AFR, but my cruise would be WAY rich then. I don't know why I'm having such a difficult time with this.

Does it act lean at WOT? Sure your meter is correct?

Here's what I'd do. Jet the carb with the correct spread from front to back (8 sizes) and go up enough to get a WOT reading around 12.7 or so. Don't worry about idle ratio (for now), just see if you can get a proper WOT reading (try measuring in 3rd gear if possible). If you're unable to get down to at least 13:1 without going over 6 jets up on each side from the stock 76/84, look at the fuel delivery or meter first. If the ratio does come around (and the car acts right under WOT) then we can look at the idle circuit.

John,
In the first few tests, yes it did act lean. It kicked and bucked pretty badly when I got on the throttle. Since I've upped the jets, that problem has gone away. Because of this, I believe the meter is correct.

I'm going to try your idea right now.

I'm wondering if the Power Valve circuit is even working?
Could the PV be dead? Or the PV channel orifices somehow blocked?
The Power Valve system is usually eqivelant to 8-10 jet sizes.
I would check to see if you can squirt carb cleaner through the restrictions. Should be 2 holes under the PV.
Also, is the fuel level in the secondary float bowl correct? Something along these lines has to be wrong for it to go lean at WOT.

Ron

I'm with ron948.. I would be checking to see what is happening with your PV. On your carb do you have a PV in both the primary and the secondary? Is either of them plugged? Have you checked the passages to insure that they are not obstructed with anything? What is the opening point of both PV's?

In respect to PV's, they are both timing, and A/F ratio control devices. They can be altered to come in when you need them to, and to richen the mixture up when it is needed most. You should be able to tune your PV restrictors to a point where wide open throttle is exactly where you need it, and still not be too rich during part throttle cruise.

Time to check/inspect your PV circuit both front and rear if so equipped. You indicated what your vacuum readings are at idle, but where is the vacuum at wide open throttle in relation to your PV opening point?

Please reply back and post your results as it will be a good learning tool for those who have not had the experience personally.

Ron and Go69, you bring up a valid point. I just tested the vehicle and here's what I did:

1. To see whether or not the PV circuit is working, I went ahead and threw in a 6.5" valve, to replace the 4.5". The problem with the 4.5" is that when I open the throttle far enough to reach that vacuum, I'm already speeding on public roads. So, the 6.5 should be easier to test.

2. To eliminate some variables, I disconnected the secondaries like the circle track guys do when they tune carbs.

3. In the primary, I upped the #77 jets to #86. Yes, it's a big jump, but at least this way it will be obvious if there is a change.

4. With the LM-1 recording data, I went on a lonley road and tested a 25MPH cruise, 35MPH cruise, and WOT. The WOT is in 3rd gear only.

Results...
1. 25MPH cruise is about 12.5:1 to 12.8:1. 35MPH is about the same.

2. When I open the throttle past 6.5" Hg (say, at a 30MPH cruise), the PV opens and the AFR dips to about 11.5:1. Way rich, but at least I know that circuit is functioning.

3. At WOT, I get about 12.7:1 to 13.1:1. Good! As a note, because the secondaries were disconnected, the RPMs hovered around 4500-5000 or so...so I'm not sure if this would be the same if the engine could rev to 6000.

Ok guys, what do you make of it? I don't know about you, but I'm confused on how a 9-size jump on the primary jets only fattened the cruise mixture from 13.5:1 to about 12.5:1. Maybe because of the low air velocity past the boosters? I dunno, maybe it's not important.

I think I'm headed in the right direction. Where should I go now?

Matt, reconnect the secondaries and take note of the A/F ratio and the jetting on a 3rd gear pull. Once its around 12.7 (around, not exactly) then lets look at the idle circuit and also how the carb is on tip-in (transition). Keep in mind here that I'm assuming the float levels are set correctly along with around 7.5psi fuel pressure.

What's the cam specs?

By doing the pulls in 3rd gear with all 4 holes working we're also verifying your fuel system adequacy (somewhat). 2 holes may not be enough to uncover a fuel delivery problem.

when you get done with all of that,get a smaller carb and compare them?
E

The 4.5 was too small so you're going in the right direction there. What does your vacuum show when you tip into the throttle? My experience is every two jet sizes is approx .5 difference in A/F. Maybe Eric68 will chime in but I recall he and others had to significantly increase jetting.

rick did my advice work on the carb?

The 4.5 was too small so you're going in the right direction there. What does your vacuum show when you tip into the throttle? My experience is every two jet sizes is approx .5 difference in A/F. Maybe Eric68 will chime in but I recall he and others had to significantly increase jetting.

When "tipping-in" from a 30-35MPH cruise, it decreases to about 8-10". Remember my idle vacuum is 10-11, so perhaps a 7-8" powervalve?

Also, I'm a bit confused. When the power valve opens when I open the throttle enough, say when climbing a hill, the AFR dips to about 11.5:1. But, at WOT, it says around 12.8:1. Is it leaner at WOT due to the increased load?

Thanks for the advice so far guys, it really helps out! :thumbsup: :thumbsup:

Matt, have you tried running the carb WOT with all 4 holes yet? If so, whats the ratio now.

16:1 at WOT is not right and somethings up. It looks as though your PV circuit is working and frankly, even with the 4.5 pv, you're not going to pull enough vacumn at WOT to close the pv and lean out the mixture.

Matt - as the rpm's climb so will the vacuum and the PV may be closing => going back lean(er).

Hi, Brian - I took out the yellow spring and went with the stock one, upped the PV to 9.5 (based on the vaccum I pull) and dropped the primary jets to 70's. Works great!! My Innovate A/F meter shows good ratios. Nice carb :)

Matt - as the rpm's climb so will the vacuum and the PV may be closing => going back lean(er).



Isn't the vacuum at WOT constantly zero, no matter what the RPM is? I would think if I had, say, 6.5" vacuum at full throttle, the carb would be extremely small...

Good discussion, Guys,, may I ask? If your vacumn is hi at wot,,, doesn't that indicated that you dont have enuff air speed velocity? ie,, carb is too large? smaller the throat , stronger the signal?

On my 355 the vacuum will approach zero when I punch it. As the rpm's climb the vacuum will increase. I don't beileve I'm undercarb'd with a 700DP modified by the Carb Shop. I also read this in a book by David Emmanuel on modifying Holley's. Perhaps others will chime in and explain more fully. Matt, what does your vacuum gauge show when you accelerate?

An engine pulling 2" or more of vacumn at WOT may benefit from a larger carb. There's just no way an 825cfm carb on that motor will pull enough vacumn at WOT to close a 4.5pv. On my own 422 with a 750 BG (smaller at the venturi and throttle bore compared to the 825) it was pulling 1.8" of vacumn at 6200rpm on the engine dyno. A tad small, but not terrible.

Silver69-

I think you are going to want to increase the size of your power valve channel restriction holes to get your WOT A/F back in the appropriate range....since you have basically maxed out the secondary jets. Problem is...is that increasing the primary PVCRs (power valve channel restriction holes) will affect your mid throttle cruise/accel (before secondaries are open) especially if you move up to a 6.5 or higher power valve. With the primary PV open and secondaries closed and the PVCRs opened to get the WOT A/F to 13:1.....that mid throttle cruise (6-8 inches of vacuum) will be way too rich.

Here are some options that involve trade offs but might be tolerable for your driving style:

Keep the lower number power valve (4.5 or 2.5?)....so that it basically only opens when you are at WOT....then increase the primary PVCRs to get the WOT A/F in the good range while using "normal" size secondary jets. But now your mid throttle cruise will be too lean (since the power valve is closed in the 5-8 inch vacuum range). You'll most likely need PVCR enrichment in this range of throttle opening to get 13:1....but maybe its not too lean for your driving requirements.

Here is an option that I would try:

Get a secondary metering block that has a provision for a power valve (I'm not sure if your Demon's secondary block has a provision for one or not). Put a 6.5- 8.5 power valve in your primary metering block and use the primary PVCRs and primary jets to tune the part throttle accels in the 6.5+ vacuum range to get your desired cruise A/F.

Now with the secondary metering block (that can now take a power valve)...put in a 2.5 or 3.5 power valve...one that only opens in your driving vacuum cycle when you are at WOT....and now tune the secondary power valve channel restrictions (drill to make bigger or tap for removable in order to make smaller) and also use the secondary jets to get your WOT air fuel to the 13:1 range. At WOT, Your fuel will be coming from the primary jets, primary PVCRs, secondary jets and now the secondary PVCRs. With a .055 inch diameter secondary PVCR holes your required secondary jetting should come back down to the 80 size range of jets.....and help get the WOT A/F down to the 13:1 range.

The Holley 850 DPs all came with secondary metering blocks that accept power valves....Ive always used a plug to block them off because I have never really needed the extra secondary PVCR fuel to get my WOT A/F in the desired range. But after hearing your experience and Eric68s experience with needing to use large sized jets (or drilling larger the primary PVCRs), its an interesting trend with the Demon carbs.

Many of you may remember my WOT lean-out/rich cruise problem I had in the past. I'd made a few changes, so I'd like to ask some more questions.

As I roll onto the throttle, it still progressively leans out. This happens at any speed and any gear, so I don't think it's fuel pressure. The more throttle I give it, the leaner it gets.

I changed the .038" IFRs to .033", and this did help the low-speed AFR. I leaned out the primary jet to lean the cruise from about 12.5:1 to about 13.5:1. I have also tried to up the secondary jet at the same time to keep the WOT AFR the same.

Here's some notes that I've made with the LM-1:
1. Idle vacuum is 11" @ 900RPM
2. Cruise vacuum (30-40MPH) 13-15"
3. AFR at Idle: 14.5:1
4. AFR at cruise: ~13.0-13.5:1
5. AFR at WOT: ~16.0 (idle-6000RPM first gear pull)

Here is how the carb is set up right now:
Mighty Demon 825, downleg boosters
.033" IFRs
78/94 jets, 4.5" PV

Here's my questions:
1. Should I just accept the 13.5:1 cruise AFR, even though it's a bit rich IMO? From the way I see it, the leaner I make the cruise, the leaner WOT gets.

2. Should I keep on increasing the secondary jet to richen WOT? I'm already at a #94 jet, so I can't go up that much higher.

3. Is there something wrong with my carburetor to need such large jets, and it's still 16:1 at WOT? I understand this carb is big for this engine, but it does get good throttle response, and many guys have successfullly used 950CFM Holley HPs on small blocks.

Ideally, I would like 14.0-14.5 on the cruise, and 12.5-13.0 on WOT.

What do you guys think?

Real interesting thread . . . and as always good information Erik :thumbsup:

Let me clarify a couple things:

1st -- The need for lots of jet on my 750 Speeed Demon carb was unfortunately an aspirin solution . . . turns out the cause of the headache was a weak spark. I verified that I had two, yes 2, bad sets of MSD 8.5mm wires. In addition, I found that the NGK racing # 8 plug is on the cold side for my combo and was not self cleaning very well. I went to a set of Accell 8.8 wires and an Autolight extended tip plug (hotter and self cleans better) and was able to get my consistency back.

2nd -- I drilled the front PVCRs out in my Speed Demon the equivalent of 3 main jet sizes and went to the box stock front jets. In other words, when the math is all said and done to keep total jet area the same in front I would need to go down 3 jets sizes in front to compensate for the larger PVCRs. I am running rear jets 2 sizes over stock. It might like a little more jet in the back for best power, but the car feels close now.

In this particular case I would go the bigger PVCR route. I think you could play with PV timing and get the "mid-throttle" mixture close enough. Personally, I don't ever use "mid-throttle" :D Its either drive it easy, have my foot into the gas a little coming off a stop light or pulling a hill, or pedal to the metal. 1/2 to 3/4 throttle on my car is a rarity . . .

Also, I think smaller IFRs will create a need for more fuel throught the RPM band. I don't think that the fuel fed the motor at low speeds goes away at higher speeds -- it is only added to by other circuits . . .

PS. Took runner up last night at the track -- I might just take my winnings and buy an LM 1

Eric, good feedback and glad all is now well. What specific Autolite's are you now tunning.

An LM1 sure is useful.

Rick, I'm running 3924's. They're actually a little hot . . . I'm going to put in a set of 3923's soon.

Eric & Eric,
I'm glad you guys chimed in. I'll just make comments in a bulleted list.

1. No, I haven't yet made any WOT tests with the secondaries open. I'm already accelerating way too fast for public roads with the primaries only...but I'll hook up the secondaries and try again.

2. My secondary metering block has a provision for a PV.

3. So, I'm thinking, using the math (flow area) from Eric68, I should open the primary PVCRs enough to bring down the primary jet size to what comes from the factory (76 or so?). I assume I should be testing this theory with the secondaries disconnected. Or, I can tune my cruise to about 14.0:1 by jetting down, then open the PVCR until my primary-only WOT reaches 13:1.

4. Then, install a late-opening PV inthe secondary and basically see what happens...and jet down until I get the desired AFR.

Man, that's gonna be alot of fuel running through that carb. How strange.

Matt, I'd just jet down the primary until you get the right cruise AF. Should be close to the stock jetting. Then you could open up the PVCR to fatten it up at WOT.

I'm wondering if it might not be better to hook up the secondaries again and see what WOT mixture is like on all 4 barrels before drilling on the PVCRs -- especially if you have to increase PVCR area a lot to get WOT mixture where you want it.

Wow, I'm confused.......so many variables to play with....I cant keep it all straight.....IFRs, PVCRs primary, power valves, primary jets, air bleeds, secondary PVCRs, sec IFRs, secondary jets, secondary power valves.....oh boy.

If we would all just drive like Eric68......either at idle or mashed it would be much easier....Eric, Maybe cut out the carpet and padding from underneath your gas pedal....might put you into the 10.8 range.............Just kidding Eric :-)

Matt- This is really an interesting experiment you are going through....trying to dial in the carb for multiple throttle positions/vacuum readings. I think what you might find and what we will find by following your results is how versatile the carb is with the multiple power valve option......somebody was thinking when they designed these carbs way back when.

Like Eric68 said.....I guess its all going to depend on how you are going to drive your car and theres really not just one way to do it since the tuning desired depends on how you will drive the car......What I have always done is just use the primary side for tuning everything from closed throttle to a little more than 1/2 throttle or just until (the vacuum reading at just more than 1/2 throttle) the secondries throttle plates open. You will most likely have to play with a 6.5 or 8.5 power valve in the primarys to see how all this works for your driving style. Based on some of the vacuum readings you listed above....I personally would go with an 8.5 power valve....just my preference.....yours might be different. So then I would tune just the primary side circuits to give me my cruise desired A/F ratio at all the throttle positions from about 6.5 in vacuum on up....I would not do any testing with just the primaies at WOT....or trying to dial in the primary circuits to get a primary WOT A/F ratio in the desired range......all this with the secondarys closed. I would just use the primary jets, and primary PVCRs to dial in the A/F from a bit more than 1/2 throttle to the closed throttle range. I guess you will have to check to see about when the secondaries start opening in relation to how far the primarys are open....I think it might be like 40 degrees of primary opening just before the secondaries start.....when I say "a bit more than 1/2 throttle" I really mean as much primary throttle opening to just before the secondaries open.

For WOT with the secondaries and primarys now open.....I would dial in the secondary jets and the secondary PVCRs (now with a 2.5 or 3.5 power valve installed) to get your full WOT desired A/F.

I know the Demon carbs have excellent resale value so I guess use that as a consideration before you do any drilling in your virgin Demon metering blocks.

I had to use going up a hill to make it easier to test the lower vacuum primary only throttle positions.....dont know if that will help you while trying to tame all that HP.

Keep us posted on what testing results you come up with.

Another thought here . . . if you are testing just the primary, how are the secondaries disconnected? If the secondary side is disconnected by removing the linkage that doesn't eliminate fuel supplied by the secondary idle circuit. I wonder if that could be goofing with you . . .

Now I understand that the roundy-round guys sometimes test WOT with the secondary disconnected, get the fuel mixture perfect on the primary, then reconnect and get it perfect again on all 4 . . . but I don't really know what carbs these guys are running exactly and if they have 4 corner idle circuits or not. I've disconnected the secondaries on my Demon for testing and it seems to work fine to isolate a problem, but I haven't tried tuning with an LM 1 either . . .

An engine pulling 2" or more of vacumn at WOT may benefit from a larger carb. There's just no way an 825cfm carb on that motor will pull enough vacumn at WOT to close a 4.5pv. On my own 422 with a 750 BG (smaller at the venturi and throttle bore compared to the 825) it was pulling 1.8" of vacumn at 6200rpm on the engine dyno. A tad small, but not terrible.

I am trying to understand the above statement concerning the 2" vacumn at WOT and the need for a larger carb. It seems that I read this somewhere in one of the tuning books or maybe Demon manual.. Can anyone tell me where this theory orginated?

I had a chance to do some tuning today with the secondaries reconnected.

I went ahead and installed a larger jet in the secondary (#94, it's all I had). So, with a 84/94 setup, I had 11.7:1 AFR at WOT. So it looks like I'm on the right track. The cruise was consistantly around 13.0:1 to 13.5:1, so maybe if I jet down two sizes on the primary, I can get the cruise and the WOT pretty close.

Update:
Just for kicks, I installed a #79 jet on the primary to see how it effected the cruise. Leaned out to about 13.8-14.0 at a 35 MPH cruise, at WOT is about 12.0-12.1. I think I'll drop the secondary down to a 91 or so and see what happens.

I had a chance to do some tuning today with the secondaries reconnected.

I went ahead and installed a larger jet in the secondary (#94, it's all I had). So, with a 84/94 setup, I had 11.7:1 AFR at WOT. So it looks like I'm on the right track. The cruise was consistantly around 13.0:1 to 13.5:1, so maybe if I jet down two sizes on the primary, I can get the cruise and the WOT pretty close.

Update:
Just for kicks, I installed a #79 jet on the primary to see how it effected the cruise. Leaned out to about 13.8-14.0 at a 35 MPH cruise, at WOT is about 12.0-12.1. I think I'll drop the secondary down to a 91 or so and see what happens.


Matt, I'm curious, did you enlarge the PVCR to help get the AFR back down???
The way I read this thread, you state in the first post that you get a WOT AFR of 16:1 with 78/94 jetting and now your at 79/94 and the WOT is 12:1???? Where's the extra fuel coming from? Thanks.

good question john.did i miss something here or is all that has changed a higher rated power valve,and 1 jet size up on the primary?
maybe he inadvertently fixed a vaccum leak somewhere?
Eric B