750 Demon VS running VERY lean on wideband - Hot Rod Forum : Hotrodders Bulletin Board
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Old 08-01-2010, 04:47 PM
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750 Demon VS running VERY lean on wideband

My buddy gave me a 750 Demon VS(Speed Demon) when he upgraded to a larger Demon. I had some extra time on my hands this weekend so I decided to try it out on the Regal. The Regal already had a basic 750 Holley VS that I had tuned extensively and for all intensive purposes works perfectly. I wanted to try the Demon because the main body looked like it would flow much better than the Holley, considering its downleg boosters and machined main body(on the Demon).

First thing I did was look up the specs on the Demon and put everything back to stock. Jets are 76/83, 6.5 power valve, plain secondary spring. I put the carb on and it fired right up. It idled pretty good with the AFR at 14.0. Took it out for a drive and it ran very lean. At a light cruise or even at WOT it was around 16.0:1. I ended up going up in jets twice and now I'm sitting at 83/90 and went with a 3.5 power valve. It is running much better at a cruise, perhaps even a little on the rich side now. It runs between 12.5-13.5 at a cruise now. At WOT i am now between 13.5-14.0 on the wideband. It also runs extremely lean when the secondaries are opening, even with the brown spring in (second stiffest). When the secondaries are opening it is around 16.0:1. I have never had to put jets over 90's in a 750 and I'm getting worried that something is wrong here.

I know Demons have a reputation for metal shavings in the passages and I was sure to hose it out very well when I took it apart. I did not see any metal in it at all.

My question is this.... Do I keep jetting up or look for a problem elsewhere in the Demon carb? To reiterate.... The car runs perfectly with the old Holley!

For reference here is the car in question:

84 Regal
383 sbc
Forged flat top pistons w/2 valve reliefs
Eagle rods, stock crank (all balanced of course)
Engine Quest Heads. 200cc intake runners, 64 cc chambers
Crower Solid Roller 00426S grind
ADV Duration 288/290 Lift .570 .584
Duration @ .050 250/252
108 Lobe Center
.026/.028 Valve clearance
Cam was degreed
Comp 1.5 Stainless Rockers
Crower springs (matched to cam)
Cloyes aluminum timing cover with thrust bearing
Victor JR intake
Holley 750 Vacuum Secondary w/ choke horn removed. Jets 72/81, high flow power valve(4.5")
Holley Red fuel pump
MSD Pro Billet HEI distributor, MSD Wires, Accel shorty plugs
Hooker headers, small primaries, 3" collectors
3" dual exhaust w/x-pipe, no cats, mandrel bent
700R4 Trans, fully built, full manual valve body
JW Custom 4000 Stall 10" w/full size lockup plate.

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Old 08-01-2010, 06:06 PM
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Two more things...

The carb has not been drilled out or modified in any way. Fuel pressure is 6.5 psi.
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Old 08-01-2010, 07:37 PM
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For the heck of it I jetted it up to 83/93 and switched to the black spring in the secondaries. It still goes VERY lean while the secondaries open and is still too lean at WOT. When I switched the jets again I blew out every passage in the carb. It all looked perfect to begin with but I did it anyways. At this point this carb looks a lot better than it works. Check out the attached log from my wideband.

Black line- RPM
Purple- AFR
Red- Fuel Pressure
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Name:	Demon Carb 83.93 black spring 8.1.2010.JPG
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Size:	165.3 KB
ID:	47122  
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Old 08-01-2010, 08:11 PM
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Fuel pressure seems low between 6.25.5 and 6.26, then again at 6.27.5 to 6.29.

I don't see it at 6.5 any where. Try raising your fuel pressure a bit.

The pressure does seem to recover as you get into the run. Do you suppose the acceleration is slowing the flow in the line from the pump, or from the tank to the pump.

Do you have a rear facing sump outlet to the fuel pump?
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Old 08-01-2010, 09:18 PM
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I think that dip is associated with the initial opening of the secondary needle and seat. It doesn't drop below 3.5 psi and I believe that is adequate. With the other carb the fuel pressure is exactly the same and I can make it run as rich as I want. If it was a fuel delivery problem I would expect to see the fuel pressure progressively drop at high rpm. I can be cruising at 3000 rpm in 3rd, semi-slowly floor the pedal and it bogs badly as the secondaries open. In other words- I don't have to be accelerating all-out for it to occur. The pickup tube is factory style(but 3/8 instead of 5/16). Lets keep in mind this car runs perfectly with my good ol' ugly Holley 750 VS. I'm only trying the Demon because it was on the shelf looking sexy and I was hoping it might make a tad bit more power once tuned. I'm not trying to solve some other problem with the car. Now just for the sake of principal I want to make it work properly. I'm fine putting my old carb back on, but I think there shouldn't be any reason this Demon can't be made to run correctly.

Last edited by iambigperm; 08-01-2010 at 09:30 PM.
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Old 08-02-2010, 12:08 AM
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For comparison here is a log from the old Holley. This log was actually done the last time I was at the track....
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Size:	158.1 KB
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Old 08-02-2010, 05:13 AM
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There might be something to be said for tuning the accelerator pump, but it looks like at the shifts, the secondaries are closing and until back fully open, you're lean.

The thing I'm getting at, is do you thing allowing the secondaries to remain open would be preferred?
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Old 08-02-2010, 05:36 AM
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What size are the Idle Air Bleeds and High Speed Air Bleeds, front and rear??

How close is air cleaner lid to the bowl vents??

Have you checked the rear blades/transfer slot setting??

Check to make sure there is a clear path from metering block to transfer slot.
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Old 08-02-2010, 06:54 AM
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Along w/Eric's insightful suggestions, you can also be sure the secondary enrichment holes are unobstructed- they supply extra fuel, somewhat like a secondary accelerator pump shot (Q-jet uses the same scheme). This fuel is supposed to be supplied just at tip in of the secondary air valve. It will come from small holes w/passages down into the bowl, w/air supplied through the usual bleeds Eric mentioned above. IIRC, the fuel for them is actually supplied through the secondary idle restriction (0.031" on a 3310 Holley- although the Holley 3310 is not the same carb as your BG, by any stretch).

Now, my suggestion is based on how a vacuum secondary Holley carb works. Disregard if this isn't how BG does things.
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Old 08-02-2010, 08:16 AM
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The air bleeds are stock and they are non-removable as this is the cheap Demon. What is the best way to measure them? They have not been drilled at all.

I'll measure the distance from the bowl vents to the air cleaner lid when I get home. I do run a strange air cleaner so perhaps this is the problem.

I made sure the rear blades were set as the Demon manual recommended.

The Demon log was taken on the street and I let off a bit when I shifted because the tires were spinning. Hence the secondaries opened more than once.

I will double check those secondary enrichment holes tonight.

Thanks guys!
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Old 08-02-2010, 10:36 PM
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I have at least 1.5" between the bowl vents and the air cleaner lid so that should be fine.

I pulled it apart once again and checked every hole and orifice that I could find. Everything was clear and looked good. Used carb cleaner and compressed air on each passage. I am starting to wish I had an extra carb so I could try swapping center sections or secondary metering blocks with. I don't want swap parts with my good holley and I'm not even sure if the holley metering blocks could be used with a Demon main body.

Just to be clear I have replaced all the gaskets on the carb as well without any change to the drive-ability. On both sets of gaskets all the holes and passages seem to line up fine. The first gaskets were the BG gaskets and the second set were holley.

I called my buddy that gave me the carb and he said that when he ran it on his 440 mopar that it ran lean out of the box. He had to jet up several sizes and was never sure if it was getting enough fuel. His and my experience with this carb definitely goes against what most people say about Demons. 9 out of 10 people seem to say Demons are pig rich from the factory.

Short of swapping hard carb parts.... Whats left? Paperweight?

Here is grabbing at straws... I could swap the metering blocks front to back leaving the smaller jets in the front. Then I could drive it and see if it ran leaner at a cruise with that block in front. Of course the power valve would now be in the rear, but I should be able to tell at a light cruise if it changed the afr at all. If it was significantly leaner it would suggest that the metering block was the culprit. I wouldn't even need to get into the secondaries. Think its worth trying?
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Old 08-03-2010, 12:49 PM
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I called the Demon tech line this morning and he said that there was NO WAY that this carb would work on my engine due to the downleg boosters and my relatively small engine size with cam duration over 250@.50. Said I would never have enough signal to make it work. He also said those 750's flow 925. He recommended a 750 Mighty Demon with annular boosters. I guess I won't be using this carb.
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Old 08-03-2010, 01:37 PM
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Demons are NOT your everyday run-of-the-mill carb. These posts illustrate how important it is to call BG and tell them your setup BEFORE you buy one. I know this was a "gift" but your friend made the basic mistake of thinking a Demon is like a Holley. A 750 that flows 925 cfm shows that.
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