tworottiedogs

The quick question is how do I do it? I have never changed jets before.
The rest of the questions:

How can you tell what size is already in there? Any recommendations on sizes? Where do I order them from?

The problem is that I think it is running really rich. I have the screw truned in pretty far. All four are less than 3/4 turns out. I have a little smoke and fuel smell at idle and a lot at WOT.

Thanks


[Modified by tworottiedogs, 6:30 PM 7/24/2002]

Sigforty

First thing I would suggest is getting the book on Demon carbs. I purchased this book and it tells you all about the carbs. Included are sections on theory, principles, and tuning. Here is the review by someone on the forum about the book. One thing you need to know is that jetting the carb will not effect the idling richness. You may need to tune the air bypass on the underside of the carb. https://www.corvetteforum.com/review...483&TopicID=18

Mark B.

QuickVet

if it is like a holley it is simple. first, take of the fuel line that connects to the carb. then just unscrew the four screws on each fuel bowl. then take a big flat head screw driver and unscrew the jets. the size they are is marked on the side of it. go down two sizes. put em back in and put the bowls back on. done. :D

tworottiedogs

Sound easy and I wish I had the book now. Can I get Jets at the the regular auto stores or are they a mail order item? Seems like I'd need a big pile of them and keep swapping it until it works.

tworottiedogs

I found this at summit. Any ideas on what jets come in a Demon 850? BGI-230001-2 Jet Sets (65-82 Jet Range) 2 Of Each Jet Size w/Divided Plastic Tray

tworottiedogs

I should be able to take the bowls off with the way I plumbed the carb.


[Modified by tworottiedogs, 5:29 PM 7/25/2002]

Matt Gruber

if it smokes black floored, it is WAY too rich. mine were 28% too rich(that was a major change, not 2 sizes :lol:

tips in my sig., but it is beyond the scope of most

tworottiedogs

Thanks Matt,

I bookmarked your site a week ago and I know I need an A/F meter to do it right. It's on the list and you described the problem perfectly. "A big black cloud" seems to follow me around.

QuickVet

save yourself some cash, get that kit.

btw, when i say go down to jet sizes, that means untill you have it right. You want to do it slowly till you find you start to loose performance, the go back one step

good luck, it is easy.

Rumpity Rump

Factory Jet Sizes on my Speed Demon 850, dp, mech secs, annular booster model are : 80 primaries and 85 secondaries. After you make a jet size change and run the car for a while....pop out a plug on each Head and see what it looks like. Better to err on the rich side than lean side, so you dont destroy your pistons.

I just had a total engine rebuild . I needed to go with 85 primarys and same for secondaries. I have all 4 mixture screws out 3/4 turn . Idles smooth and eliminated a 1500 rpm bog.

I love the Carb. Very responsive . SHines up nice also.

Theres always the Demon Techline to help you too. Good luck, Dave

Fevre

NICE!

Get the kit that has all the jets in it. You'll spend a little extra now but it is about the price of 4 or 5 jet pairs and will have extra to try in both your primaries and secondaries. You'll be glad you did.

Matt Gruber

i change jets when the engine is cold. less chance of fire.
i remove lower bowl screw and let it drain into a cut open old oil can. then i put an old rag under the carb to catch drips.
and i wear nitrile gloves as gas can't be good absorbed thru the skin.
i smear a little grease on the gasket and i use it over and over.
i have a numbered drill set and can drill many, but not all, sizes.
if the demon has a PVRC study the book to calculate jet sizes.

gkull

http://www.demoncarbs.com/demoncarbs/index2.htm

It's the same as working on any Holley dubble pumper. Remove the four allens holding the end bowls on. Each metering block has 2 jets. I got a big set of two of every holley jet sizes 60 - 100 from Auto Zone or one of those type stores for $44

just like my two Demons you need to drill a 1/8th inch hole in each primary throttle blade and for a very close starting point install 78 primary jets and 82 or 83 rears.

If you gave me a run down on what you motor has in modifications I could guess even closer.

An easy way to tell if your way rich on the idle cuircut is: pull the hose or cap off of the full vacuum port on the passenger side of the carb. If your idle jumps up in rpm and smooths out with an induced vacuum leak you need to drill the primary throttle blades. Which also causes an induced vacuum leak.

Fevre

Norval says he puts a l bent .015 inch (I believe that was the dia) wire in between the metering plate and gasket, if I remember right, into the idle circuit to reduce flow. He says that the Holley dp are to rich on the idle circuit so by restricting flow it leans them out and makes the idle screws more sensitive. I had almost no responce with my idle screws last time I used my 750 dp and am thinking of trying Norvals idea out just to see if it helps. I have also drilled the holes like gkull suggested but it still did not lean it out enough.

norvalwilhelm

Jet do not effect idle richness. Setting the 4 corner idle screws at 3/4 turn is also not right. I thought the demon had replaceable air jets. These effect idle mixture and you want 1 1/2 turns out. Also drilling small holes in each throttle plate allows you to close the throttles tighter and cover the transfer slots.
Idle is effected only by idle mixture and transfer slots. The transfer slots should not be part of the idle circuit but it happens.
Once again jetting will not lean out a rich idle mixture.

gkull

Norval - I'm up here at nearly 5000 feet and the removable air jets weren't big enough. So my hole drilling is a comprimize. They need to be bigger or even additional .125 holes in the secondary plates. I'm really setup for @ 2500 feet and it's lean but OK at sea level. I just don't like to stop and make jetting changes.

My other setup difference is. I have the idle speed adjustment screw on the secondary open more than the pri. throttle blades. This helped in the transition from two barrels to all four when going to WOT.

norvalwilhelm

Fevre. You have a good memory. Remind me not to offend you :) . Even the holley book talks about putting a wire around .013 in the idle circuit. Remove the 4 screws holding the float bowl on, remove the metering plate and the side closest to the carb body has a large gasket. Remove this gasket and look at the two Vees machined in it. At the bottom of the "V" is a hole going straight into the metering block. This is your idle circuit fuel jet. If you look close there is also a drilled passage going off at an angle, this is your powervalve fuel metering jet. Take a 3/4 inch length of wire around .015 and bend it with needle nose pliers into a tight 90 in the middle. Insert one end in this idle circuit fuel metering hole and let the other half lie on the metering block. Do this to the other hole then put the large gasket back on the metering block and the wires are trapped. Reinstall the float bowls , 4 screws and make sure the accelerator pump arm is in the proper place.
This leans out the idle and hopefully makes the idle screws more sensitive. The next step is drilling the throttle plates but that is destructive. The wires do not harm the carb and if you want them out nothing is damages.
I personally drill most carbs to cover the transfer ports.

Fevre

I tried to look that info into memory so when the time comes to drop my 383 back in I could do up the carb so it wasn't so rich. When i bought the car it had a big o solid cam in it, the 750 dp, 1.94 heads and rams horn manifolds. I think it was a terrible matching of parts since my bumbers would get coated with what appeared to gas everytime I drove it. :mad I tried to lean out the carb but I just think the parts mismatch was too great to allow it. My current 350 leaves no residue what so ever. I hope to be able to get the 383 running just as well. When I get ready to start putting the 383 together I am going to post some questions so please keep a look out and chime in when you can.

Thanks Chris

norvalwilhelm

I run 2 850 double pumpers, four corner idles , of coarse heavily reworked but all towards leanness. I have 8 idle circuits to deal with, 8 throttle plates all leaking air etc etc. Even that much carberation can be made to work. I get 12 mpg around town, it does not load up ever, my plugs are 3 years old and look new. On the highway with the oxygen sensor gage it runs around 15-1 ratio. It has crisp throttle response, doesn't stumble.
Big carbs can be made to work, regardless of cam. Also the power valves are very important. Don't discard them and watch the idle vacuum and power valve rating.

tworottiedogs

gkull,

This is the engine I have: http://www.sdpc2000.com/cart.asp?act...id=120&pid=119

Thanks