I was having some idle problems on my Demon 750,so I went to tinkering with it.The fuel level was a little to high and the throttle blades cracked a little too far open and that was causing fuel to be drawn through the boosters.I got that set right ("fixed") and the idle quality got worse.More tinkering.Found a fuel leak on the primary side,fuel seeping from between the squirter and the throttle body.Put in a new gasket and took some tension off of the accelerator pump to close the circuit.Now I have no idle quality at all because there is no idle.So now I've deducted that it was idling through the 2 fuel "leaks".Between the low vacuum of the camshaft and the size of the carb,too big,(Speed Demon 750=Holley 825 hp) there isn't enough vacuum signal to the carb.s baseplate to draw fuel through the idle circuit.The driving range runs excellent,idle is where the problem is at.

So I pull the Demon off,and weigh my options.I can drill a 3/16 hole in each of the primary throttle blades allowing air to be drawn through,"tricking" the idle circuit as if there were more vacuum,or I can further modify the idle circuit-enlarge the idle feed restrictors in the metering block more and make the idle ar bleeds in the throttle body smaller.Either will work,I'll probably just do the throttle blades.

In the meantime I've put the Holley 3310-c back on(750 vacuum 2ndaries),this carb allows the engine to start with ease and idle right off the bat,no need to babysit while it warms up,drivability is awesome as well,excellent street manners.But.There's always a but.This carb,as good as it allows the engine to run,is rich as hell at idle and for some reason the engine pulls even less vacuum(3-5 inches at idle as opposed to 8-10 with the Demon).I'm sure that I need to change the power valve to a different size,probably on both,but for the moment it stays put.Also when you put the pedal to the mat you can tell a very noticable difference,the Holley doesn't have anywhere near the upper mid to top rpm range acceleration,that kind of goes without being said though.Not sure if I want to go through the trouble of tuning the vacuum springs or not on a carb that will only be on temporarily.

This is probably the strangest thing,the street friendly Holley doesn't get near the fuel mileage of the track friendly Demon(brand H getting 8mpg +/- and brand D getting 15+/- on same set up).Obviously the Demon will run leaner at idle,but in the driving ranges it is usually assumed that the vac 2nd carb will get better mpg.Maybe switching to a metering block on the 2ndary side instead of a plate along with spring tuning would help some,but it's a huge difference to make up.And it's only temporary.

Not really any questions here,just my own personal observations.I'm open to any comments or suggestions.

I think that you have 2 messed up carbs. I don't believe that you can't pull enough vacuum with that Demon to activate the idle circuit.
What is the cam that you are running?

I didn't think a carb change would change the idle vacuum so much! Do you have a good vac gauge? Mine was leaking from the vacuum gauge hose and giving me faults readings. 3 to 5 inches sounds very low. At 3 inches of vacuum I don't think you can get a power valve low enough. I use a power valve 2 inches lowwer than my idle vacuum. A power valve that is too big will be dropping fuel into the motor a idle, causing a bad idle. Does the exhaust smell rich when idling? Dave

do you have idle eze on that demon??
shane

Originally posted by ovelle:
do you have idle eze on that demon??
shane No,it's too old,there's supposed to be a retro fit kit for it,haven't found it yet though,I was wanting to do that instead of drilling the throttle plates.It's supposed to be on Demons built after Sept. '01 according to the Demon book.

It pulls the same vacuum on 2 gauges,the one that's in the car and my test gauge.On the small block,with the XE 268 cam the 3310 pulls around 15-17 inches and the Demon,using a 2" 4 hole spacer,pulled 20 in. +/-.

The cam in the 402 is probably too big,solid 248* @ .050,.602 gross lift,110* lsa.It runs good though.Power brakes are a little iffy sometimes.

More info on this from Barry Grant.Has anyone done this and have a part # for the conversion kit?I haven't found a part # at the Barry Grant site,Jegs or Summit.


Idle-Eze Solves Camshaft Problems

By Sam Moore

http://www.barrygrant.com/news/articles/images/IdleEze%204BBL%20cutaway2.jpg
The Demon Idle-Eze, an indispensable tuning aid
for matching camshafts to carburetors
Patent-pending

When upgrading the camshaft of a street vehicle, a difficulty at engine idle may appear where there was none before. Until recently, the most common remedy for a camshaft-induced poor idle has been for engine builders to drill a hole in each butterfly or throttle plate within the carburetor. But why can the quality of idle be so adversely affected by the installation of a performance or racing camshaft? And why would such customized holes be used to improve the idle? What would be the correct diameter of the holes; how would you gauge that; and what are the consequences if the holes are drilled too large in the butterflies? Perhaps the best question would be: is there a better way to retain idle quality and throttle response within the engine while taking advantage of the extra power provided by a modified camshaft?

Matched to the right components, performance and racing camshafts are impressive in their gains of power. But, due to their increased inherent overlap (the period when the inlet and exhaust valves are open simultaneously), idle quality can suffer because the difference in pressures above and below the carburetor butterflies has been altered. The upper side of the butterflies must attain greater than atmospheric pressure, and the underside must be closer to a vacuum, which creates the signal that activates the carburetor and provides the subsequent flush of fuel-and-air mixture. With a large-profile camshaft installed, the carburetor is usually deprived of engine vacuum at idle: the insufficient signal is unable to entice the idle circuits to fully cooperate. Often, in an anxious attempt to improve a poor idle, the idle-speed screws are adjusted and as a consequence butterflies opened – opened further than they were originally intended to be in the idle position. This causes the idle-mixture circuits to cease, or partially cease their fuel delivery within the carburetor. Now, idle fuel is being drawn through the line of least resistance, namely the increasingly exposed transfer slots, which are small slots located in the bores of the baseplate that adjoin the butterflies. The chief function of the transfer slots is not to act as idling circuits, but instead to add fuel during the carburetor’s transition from the idle-mixture circuits to the main circuits. Therefore the tuning (the positioning) of the butterflies in relation to the transfer slots must be correctly made, or symptoms of disproportion will occur: excessively rich idle, bellowing black smoke, teary eyes, and idle-mixture screws that have been rendered useless.

Years ago, some enterprising engineer dreamed up the idea of supplying the plenum and carburetor with a source of air by drilling holes in the carburetor’s butterflies – allowing the atmospheric pressure above them to resolve the problem of poor signal (vacuum). This cleverly adaptive solution allowed engines with modified camshafts to operate at idle as butterflies were returned to their original position and idle-mixture circuits functioned as they were designed. However, given the almost infinite number of engine combinations, how could one decide with any degree of accuracy about the correct drill size? Unfortunately, there was no exact formula, only the educated guess of proportionate size. Further, this sort of change cannot easily be reversed. Once drilled in error the holes can only be plugged – usually less than gracefully. Often the holes would be so detrimental to the carburetor’s condition it would be discarded.

The Idle-Eze™, a new patent-pending concept from Barry Grant’s carburetor manufacturing division, Demon Carburetion of Dahlonega, Georgia, provides the simple solution. It’s a spring-loaded, needle valve and brass seat assembly located in a threaded hole in the center of the baseplate. The Idle-Eze is designed to precisely meter an additional source of air to the intake plenum via four milled channels on the top of the baseplate. After setting the butterflies to their correct idle position, any necessary adjustments to the Idle-Eze can be made by screwdriver through the air cleaner stud hole. The new device not only overcomes idling difficulties with performance camshafts but also allows for a smoother transition from off-idle, corrects the potential for hesitation, and comes as standard fitment on all Speed and Mighty Demon carburetors.

Since all Demon carbs (including their predecessor the Claw) feature four-corner idle-metering circuits, the new Idle-Eze baseplate can be retrospectively fitted to all. The conversion of these early carburetors is fairly straightforward. It involves the removal of the carb from the engine, and removal of the float bowls, the metering blocks and baseplate from the carburetor. To facilitate the adjustment of the new Idle-Eze, a hole needs to be drilled in the bottom of the casting of the carburetor main body via the air cleaner stud hole. Once equipped with the access hole, the new baseplate together with the metering blocks and float bowls are installed on the carburetor. Before placing the carburetor on the engine, adjust the butterflies. With the carb upside down, the butterflies are set such that they expose approximately .020” of the transfer slots (the slot will give the appearance of a small square shape). Next, a screwdriver is inserted through the air cleaner stud hole and the Idle-Eze valve closed by rotating it clockwise until it stops. To return it to a baseline setting, open the Idle-Eze valve by one-and-a-half turns and install the carburetor on the engine.

Now, the carburetor can be primed and the engine started. Use a screwdriver through the air cleaner stud hole to control the engine’s RPM. Once the RPM is set approximately and the engine is running at normal operating temperature, begin to adjust the idle-mixture screws. Adjust the screws either in (leaner) or out (richer) until the engine reaches its optimum idle. Make the adjustments in small increments of approximately Ľ-of-a-turn at a time. Once the idle mixture has been set, adjust the Idle-Eze™ by using a screwdriver through the air cleaner stud hole to reach desired RPM. Once satisfied, install the air cleaner stud and air cleaner and get ready to enjoy your carburetor.

Images courtesy of Barry Grant, Inc.

http://www.barrygrant.com/news/articles/images/IdleEze%204BBL%20cutaway2.jpg
The Demon Idle-Eze, an indispensable tuning aid for matching camshafts to carburetors.

http://www.barrygrant.com/news/articles/images/Idle-Eze%20baseplate-tops.jpg
The Idle-Eze assembly as installed in the Demon billet baseplate with accompanying milled slots to provide the necessary source of air to activatethe idle-discharge circuits. Air is drawn along the upper milled slots, through the Idle-Eze valve and exits beneath the butterflies.



http://www.barrygrant.com/news/articles/images/Idle-Eze%20baseplate-bottom2.jpg
Bottom view of baseplate. When converting an existing Demon or Claw carburetor to an Idle-Eze baseplate, remember that the butterfly sizes must not change.

http://www.barrygrant.com/news/articles/images/T%27slotPositionAs.jpg
The perfect positioning for good idle

http://www.barrygrant.com/news/articles/images/T%27SlotPositionBs.jpg
The idle-circuit fuel is discharged through the idle-discharge ports in the baseplate, and the main-circuit fuel via the booster venturii in the main carburetor body. The transfer slots (shown here and exposed by an opening butterfly) are designed to provide a supply of fuel during the transition from idle circuit to main circuit.

The hole in the throttle blade deal is not to trick the carb into thinking it has more vacume. The hole just supplies more air to the mixture to help the engine run on a contaminated fuel charge from the reversion of a large cam's overlap. I think your Demon's idle circuit must have a restriction in it somewhere if there's no fuel getting to the engine. Dirty? The air fuel mixture screws don't work? Or is it getting too much fuel? Black smoke? Low rpms and washed out rings will make for the low 3-4 inches of vacume you're getting. Bob

Nah,it works fine on the smallblock.It's all clean on the inside,took it all apart and checked it out.It's only getting that low of vacuum with the Holley.I just took a 70 mile drive in it,to Plano and back,BIG mileage difference.Demon's going back on asap.

have you tried playing with the idle screws? Where are they set as of now?

Are you using the idle adjustment screw on the primary side only? Or using both? When setting the idle you should adjust both screws an equal amount.

Demon-set all 4 while using vacuum gauge,double checked to make sure all were equal.
Holley-only has 2(2ndary metering plate)-set with vacuum gauge also(sorta),both equal.Leaned it out more,they're about 1 turn out.It's pulling fuel through the enrichment circuit-power valve/low vacuum-is what I'm thinking.Putting the Demon back on tomorrow so I'm not worried about getting the Holley right,I'll put it on one of the smallblocks in the garage.

70- when you say you "set all four" I assume you're talking about the idle mixture screws. I'm speaking of the idle adjustment screws. There's one on the primary throttle shaft and secondary throttle shaft. Make sure you're adjusting both an equal amount.

I tried both a 650 dpeed demon vs and 3310 holley 750 vs on my 350 last year. With the demon I had 2 more " of vacuum also

oops-gotcha-wasn't paying enough attention to what I was reading.
It's been set both ways,using the primary and secondary side was causing a slight stumble at tip in,I called the Barry Grant techline line and they said to just use the primary side for idle on that size of engine.The guy on the phones name was Barry,btw,dunno if it was Grant or not.

How did it idle with both set the same? Better or worse? If better, you might try going back to that setting and uping the primary squirter to a 35. Also, did your carb come with the idle air bleed restrictors that you tap in?

Also, what tranny are you running? Automatic? How about converter stall? If its an auto, is there a big rpm drop when you put it in gear?

Same idle with some adjusting,had a bad tip in flatspot with both open.Playing with the accelerator pump cams helped some.The restrictors are in and have modified the metering block-enlarged the idle feed restrictors.Had to do all that to get it to idle in the first place on the smallblock,along with alot more initial timing.It ran great on the sbc,and runs great on the bbc,it's just when I fixed the 2 leaks on the throttle body(squirter & booster) it killed the idle characteristics,that's where it was getting it's idle fuel from.The biggest difference in the 2 engines is the camshaft.

TH400,just switched from a 2400 to a 3000 just for this purpose.

edit:I just got an e-mail from the Barry Grant tech dept. and the retro fit is P/N 121243 ,but it's the whole baseplate is what I get from message.Not what I was wanting to hear.