Demon Carbs

Jack21 · #1 (**permalink**) 05-12-2003, 06:48 PM

Just about everything I read in the rags extol the virtues of Demon over Holley.

Holley 4778 700DP on 351W was driving me nuts getting it to stay idling correctly, and trying to dial out flat spot on acceleration from cruise.

Just bolted on Speed Demon 650 DP. Initial impression, it didn't even sound like the same motor. Sounds much smoother and healthier with rock steady idle. Pulls nearly 17" vacuum at idle with Comp 224/224 .533/.533 hydraulic roller, where the Holley could only pull 15". Throttle response is much crisper, and no flat spot anywhere. I'm impressed. Now for the dyno.

George Snyder · #2 (**permalink**) 05-12-2003, 07:48 PM

They tune great, they run great and they look great.

The four idle adjustments and the separate outlets for manifold and ported vacuum are a plus.

I have a 650 on a SBC and what a difference in performance. I have seen decussions stating that they flow more air than a Holley the same size also.
I was cheap and used a Holley on the Cobra because it was collecting dust on the shelf. It is only a Crate Motor, so I am OK for now.
I am sure you will be pleased with it's performance on the Dyno.

Bud

Roscoe · #3 (**permalink**) 05-13-2003, 06:36 AM

I have a 351W with a 650 Speed Demon, mechanical secondary. Right out of the box it was fine. Easy to tune and adjust. Most, if not all, carbs come a bit rich and mine was no exception. I brought my jets down about 4 numbers, front and back, and the engine runs strong with no hesitation, backfiring and idles well. They also have excellent tech support.

When I dyno'd my car last year I was a bit disappointed with the initial numbers. I faxed the sheets to BG and they recommended about 4 things to do. The next week I picked up 35hp on the dyno!

Let me know if you have any other questions.

Roscoe

Tongue Pirate · #4 (**permalink**) 05-13-2003, 08:06 AM

I had one on a BB Cheby which ran great except the Vac Secondaries wouldn't open correctly. Replaced it after endless fiddling. Would buy another in a second but DP!

Mr.Fixit · #5 (**permalink**) 05-13-2003, 08:26 AM

Properly set up, a Holley brand holley-style carb is just as good, I like them better than than the BG carbs. I know of at least one BG carb that was replaced with a Holley, with very improved results.

Roscoe · #6 (**permalink**) 05-13-2003, 08:47 AM

I hear more and more about the vacuum secondaries having problems. I have the mechanical and may that's why I have no problems.

Roscoe

Mr.Fixit · #7 (**permalink**) 05-13-2003, 09:03 AM

If you have a heavy car, or not very much gear, than the vacuum secondaries will make the car faster. There is a rubber diaphram that actuates them, that can tear, but I haven't seen it happen. If you remove the choke from your vac secondary Holley, there is a hole you need to plug. Don't remove the checkball from the vac secondary circuit either. The biggest problem is when guys try to change the spring that controls when the secondaries open, then reasssemble the vacuum pot incorrectly, then they have no secondaries.

Tom Wells · #8 (**permalink**) 05-13-2003, 10:51 AM

I can add a "nay" to the vacuum secondary Demon rating. It NEVER ran OK, even after a factory visit and several knowledgeable Holley guys tried to set it correctly.

Now it has been replaced with an EFI.

Maybe I shoulda tried a mechanical secondary one

Tom

jwoodard · #9 (**permalink**) 05-13-2003, 11:19 AM

a 750 Speed Demon DP and it was the best thing I ever did. That after trying an Edelbrock and a Holley.

Much smoother out of the box and with a little tuning excellent performance all the way through the RPM scale.

I'd definitely buy another one and don't hesitate recommending them.

Jim

Tongue Pirate · #10 (**permalink**) 05-13-2003, 11:32 AM

I talked a friend into putting a "Road Demon" on his 69 Camaro which he was putting a crate motor into. I set the choke & idle for him & he has looked back since. He loves that carb. & tells everyone about it.

turnpike boy · #11 (**permalink**) 05-13-2003, 11:43 AM

Bought a 650 Speed Demon after conferring with BG tech reps for my setup. On fire up, so rich that plugs were fouled after about 35 minutes of run time. OK, rejet down 4 sizes front and rear, replace plugs and try again. Now able to at least set idle mixture, altho the four screws are NOT more than 1/3 out from closed. Another call to BG, snotty S.O.B. who answers says I must have screwed up settings....No, assh*le, it's out of the box except for rejetting. Well, then, you just gotta drill out the airbleeds, don't you? - and make sure the throttle plates are closed front and rear.

My time is kind of limited these days, and tuning any carburetor is a given on any new application. I will get this carb straightened out (with time), but the off-idle stumble is just about bearable right now.

Got many years' experience with airbleed carbs (all 2-stroke Jap bikes used 'em) and plenty of time spent on Holleys too; much misspent youth drag-racing tunnel-rammed Nova.

That you folks were fortunate enough to get one that worked, out of the box, is great but ought not imply that will be the case every time.

Mr.Fixit · #12 (**permalink**) 05-13-2003, 12:20 PM

I currently have one of my Holley 750's (I have a few) on loan to a buddy because he couldn't get his BG carb to work right at all. I didn't fiddle with the BG, just handed him my Holley and told him to try this carb, make sure it's a carb problem. He called me back, car ran great with my Holley, that was a year ago. I have to pick that carb up one of these days, it's a good one. The BG carb was not a demon, but the predecessor to the demon that doesn't have the swoopy fuel bowls. I bet it is for sale cheap.

bmalone · #13 (**permalink**) 05-13-2003, 12:51 PM

Hi Fixit, have any more 750's for sale. I want two.

George Snyder · #14 (**permalink**) 05-13-2003, 04:11 PM

Turnpikeboy,

The carb should have come with 70 & 78 jets. I am using 64 & 67 in my holley on the 302. Are you some where close to that? If you still are rich you may need to raise the power valve setting. It probally is 6.5 which is standard.

You may want to try and set your idle speed more from the secondary screw than the primary. Sometimes this will help with the stumble. It did for me, but mine was very minor.

The Demon on my SBC is a marriage made in heaven. A few minor adjustments, raised the secondary jet to 8o, perfect!

Is your 650 mechanical secondaries? If so and you want to trade your Demon for my Holley and some cash let me know. Holley is 4777C.
Bud

turnpike boy · #15 (**permalink**) 05-13-2003, 06:11 PM

Howdy, GS. The Demon came with 71's front, 78's rear; now down to 65's front / 72's rear. PV is a 6.5 - squirters are 28's both sides. DemonTech is adamant about keeping the secondary's closed at any point other than where linkage lifts 'em - so, in deference to their knowledge, I do. Will keep picking their brains for help until this is resolved; not afraid of drilling out the idle airbleeds tho since replacement tap-ins are available. Since idle mix screws are so far in (about 1/3 turn off closed), they feel that opening up the airbleeds will help - and I agree, based on past experiences. Hasn't stopped me from driving the car (now 215 miles and counting...), just cuts into the overall enjoyment of throttle response.

And thanks for the offer, but I think I'll stick with this carb and make it work like it's supposed to...after all, this is supposed to be a hobby, right?

George Snyder · #16 (**permalink**) 05-13-2003, 07:18 PM

Turnpike Boy,

I am not sure I completely understand what you mean about keeping the secondaries closed until linkage lifts them?

On that carb there are idle adjustment screws on both sides of the carb. I am saying that if you set your idle speed from the one on the passenger side VS. the one one the drivers side it may help your stumble. All the idle speed set screw is doing is opening the throttle plates. Most cases they are open equally in front and back.

What intake do you have on the engine?
Bud

turnpike boy · #17 (**permalink**) 05-13-2003, 09:00 PM

Sorry for the delay in response, George - can't sit for too long or I get antsy.

Yessir, this is a mechanical secondary 650 Speed Demon; double pumper and all. What I was referring to about linkage was that the opening position of the secondarys is linked to the opening of the primarys. Pretty much all mech secondary carbs work that way and I'm sure you already know that; BUT the "idle" adjustment screw on the passenger side (right side) of the carb is for positioning the secondary butterflys relative to closed and to set proper transfer slot exposure. I suppose it could be used for idle adjustments but I would think it likely to have considerable effect on the idle circuits of the secondarys, i.e. very rich in the part-throttle area and/or possibly lean at wide open, depending on secondary jets used. Primarys do open about 60% before the secondarys start opening, and the squirters take care of that inevitable lean period when air starts to slow down with the opening of the secondarys, making fuel draw difficult.

Whew. How's that for long winded?

The intake is an Edelbrock Performer RPM; ignition is all MSD (Pro-billet distrib, 6AL with rev limiter set at 6200), timing initial 16 degrees, total 36 degrees.

Enlarging the airbleeds just serves to lean out fuel flow at high vacuum periods (idle) but is negated when the butterflys are open and main jet circuits are working (thru the boosters). Depending on where the idle mix screws are set obviously determines how much fuel flows too, but the bleeds basically reduce static pressure in the emulsion tubes and, thereby, fuel flow is reduced. If mix screws are too far in (like mine), then the idle circuit tends to go lean during the transition to main jet circuits and the dreaded stumble results. Too big on the airbleeds, mix screws are backed way out (to get decent idle running)and it usually goes rich in the same transition.

Some fun we have here. To these old eyes, there's damn little difference between Holleys and Demons; supposedly the Demon's milled meter blocks are superior but I think that's crap - I don't ever recall seeing bad meter blocks in any Holley I ever used, and I had my hands in them on Z28's (302's and 350's), big block Chevelles, a couple corvettes, and a LT-1 powered 1975 Vega. They all worked, and pretty well at that. Wanted to try a Demon to see if there would be much difference - but so far they seem pretty much the same. After all, 650 cfm is 650 cfm, is it not?

turnpike boy · #18 (**permalink**) 05-13-2003, 09:21 PM

Hey, Bud, you going to the Cobra Revolution June 7? Will be in Valley Forge, and seeing as how you're in S.E. Pa, might be a nice hop for you.

Tongue Pirate · #19 (**permalink**) 05-14-2003, 08:33 AM

A also remember my Demon being very rich in the Idle circuit. Initially I had turn the idle mixture screws way in to get her set right

Roscoe · #20 (**permalink**) 05-14-2003, 11:14 AM

Turnpike,
If your Demon is stock it has a progressive linkage for the secondaries. Demon told me to change that to 1-1 linkage. This is a small rod available at any performance store. Holley, Moroso, etc. makes them.

Roscoe