Damn This Demon...I need a new carb - Hot Rod Forum : Hotrodders Bulletin Board
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Old 11-13-2005, 07:11 PM
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Damn This Demon...I need a new carb

After almost a year of tinkering with this demon, i think i need to get me something else. I want to go to a Holley classic gold, mech. secondaries, double pumper. Im just not sure what size to get, 650 or 750. heres my specs

1987 Monte Carlo SS (about 3500 lbs)

350 .040 (357)
MSD 6AL ignition
MSD Blaster 2 Coil
MSD Billet Distributor
Comp Cams Hydraulic Cam - 236 duration, 520 intake lift, 490 exhaust lift
Flat Top Pistons making a 10.5:1 CR
Edelbrock Performer RPM Manifold
Dart Iron Eagle Heads, 200cc, 2.05 intake valves, 1.60 exhaust valves
1 5/8 headers with dual 2 1/2 inch exhaust and Flowmaster 40 series mufflers

Currently have 200-r4 tranny, but will be replacing it with a 2 speed 'glide, and once the glide goes in i will be dropping the rear gear from 3.73 to a 3.fifty something. There will be a 2500 stall on the tranny.

I think that should be all the determining factors for carb selection.



Last time i was at the dyno, i only managed 247hp and 306ft/lb to the wheels...running at a pig rich 9.5:1 AFR! and a possibly slipping converter. I know it is capable of more. Im thinking it should be in the high 300's to the wheels

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Old 11-13-2005, 07:41 PM
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But other than running rich, what other problems are you having?
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Old 11-13-2005, 08:14 PM
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Have you tried speaking with the TECH guys at BG ?
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Old 11-13-2005, 08:32 PM
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what size is the demon and what kind is it. is it a race demon. and is it for sale
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Old 11-13-2005, 09:57 PM
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IMO those heads are way too big. I bet its a real dog on bottom end.
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Old 11-14-2005, 08:01 AM
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- other than running rich...it doesnt respond to tuning at all. i was playing with it yesterday and the AFR was bouncing from 11.5 to 15.5! I also noticed that there is a small fuel drip coming from the primary butterly shaft (where the pump cam is)

-i tried talking to the BG guys and frankly they are useless

-its a 650 speed demon vac secondary

-No its not a dog, actually down low is where it is the most responsive...it also does well up top, like on the highway. The only place it seems to lack power is mid range. I have thought about the size of those heads tho...id change a cam before i got new heads. I would like to get a hydraulic roller.

I want to see how much i can get out of this block...it is only a 2 bolt but everything is held together with studs instead of bolts. i figure maybe 500 at the crank will be pushing it.
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Old 11-14-2005, 08:14 AM
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How are you making 10.5cr on a flat top with 64cc chambers?
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Old 11-14-2005, 08:53 AM
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well thats what i was told by the machine shop that built it and summit when i bought the pistons. I think it may be becuse im using a thin head gasket too.
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Old 11-14-2005, 08:56 AM
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A 650 carb should be good for a 350 engine. A 357 turning 7000 rpm at 90% efficiency (I doubt you'll get much better than that!) needs 578.5 cfm. 8000 rpm needs 661. You do want a little extra "room", but unless you're turning near 8K carb size isn't the problem. Is the AFR meter you're using in good condition? Could be a dirty sensor causing the ratio to jump around like that. Also, is it a new engine? Could be just not broke in good yet, that can cause the mixture to change some, though I doubt that much.

Sounds like you have a bunch of good parts, but they aren't working well together. Could be a problem with the carb, but I doubt it. One brand of carb is generally as good as another as long as it's sized and working right, and tuned to the engine. You will find more people familiar with the Holley 4V because so many are in use, so that would make it easier to get help tuning it.
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Old 11-14-2005, 09:00 AM
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Its an LM1 wideband and that sensor prolly only has a 1000 miles or so on it. The motor isnt new, it was built over a year ago.
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Old 11-14-2005, 09:04 AM
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carb

If it's zero decked and you have 4 valve pockets then you are probably closer to 10:2 compression.... but that and the size of your heads is not the problem........

If that carb is leaking fuel from the throttle blade shaft then the carb is probably getting fuel on the main body side of the meetering block.... I know we talked about this carb before but i really do not remember all the details....

Have you had the carb apart???

What is you initial and total timing???

Is the mounting gasket wet with fuel when you remove the carb????

Have you been changing plugs with the carb chages????

Float level and fuel pressure?????

Keith
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Old 11-14-2005, 01:00 PM
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Have you had the carb apart??? - yes

What is you initial and total timing??? - initial is 14, total is 26-28 i believe

Is the mounting gasket wet with fuel when you remove the carb???? - yes

Have you been changing plugs with the carb chages???? - sometimes

Float level and fuel pressure????? -float level is just below the sight plug and the pressure is 7psi
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Old 11-14-2005, 01:08 PM
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carb

Ok we are getting closer...

Your initial timing sounds close but you need to get your total up to 36 / 38 degrees.....

The problem with the wet mounting gasket + the fuel running out the throttle shafts is the symtom... now we need to find the problem..
Your going to have to look real close at the meetering block and the gasket. See if the gasket is sealing properly. Look at the powervalve and see if it's bottoming out on the main body. look at the powervalve mounting gasket and see if it's sealing properly...

I have had brand new bg carbs that needed the main body side skim milled to make them flat enough to seal....

keith
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Old 11-14-2005, 01:33 PM
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Normally fuel dripping out of the throttle shaft would be from fuel dripping onto the butterflies from the boosters. Than can be caused by dirt, improper butterfly setting, or most commonly fuel level too high for the specific engine. Try lowering the float level down a little first before doing anything else, this may explain why the carburetor is not reacting consistantly.
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Old 11-14-2005, 02:50 PM
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the butterflies are set to .020 and the fuel level is actually lower than recommended.
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