68coupe

Hello,
Question for all of you carburetor gurus out there. I recently rebuilt my 1968 corvette and did some modifications to the engine. One on the modifications that I did was with the carburetor. I installed a new Speed Demon 575 with electric choke. The carb has mechanical secondaries.

Here is the story:
Before buying the carb, I called Barry Grant several times. (Actually I bought a demon Jr. 625 and never even used it on the car because Barry Grant stated that it was the wrong carburetor.) All of the technicians that I talked to on the phone recommended this carb for my specific application. (for more details on specs look at the specs at the end of this post) SO I buy the Speed Demon and watch the video on tuning. In addition to that, I also bought and read the book “How to win with Demon Carburetors.” Working with a friend, we install the carb. I fits okay, but the accelerator linkage does not open fully. It only partially opens the secondaries. (that is problem number 1). We just leave it and figure that we can fix that after it starts up and idles. We crank the car over and it starts right up. We make some fine adjustments and then decide to take it out for a test drive. When we left, all of the preset settings from the factory were still the same. (idle screws, float level, etc..)

We get 3 miles down the road and the car stalls and dies. I check the carb and the float bowls are empty. In addtition, whenever I pressed down on the linkage, no gas would come out of the squirter, just air would enter the intake. we get a ride back to my house and get tools and return to the car. I started by adjusting the front bowl to try to increase the float level and nothing happens. After manipulating the carb for an hour I went to the gas tank and looked in, it was bone dry. SO STUPID. Anyways, the car made it home, but the float level has never been the same. (Problem number 2)

Now, The front bowl is empty, the secondary bowl is half filled and the car runs rich as hell. If I let it idle in my garage, the exhaust is so gassy that it makes my eyes burn and yesterday, made my nose bleed. (problem number 3). When I was driving it, the car seem to surge a little and when I accelerated lightly and cruised, it make an awkward sound from the exhaust. (hard to explain a like a backfire through the exhaust.) (problem number 4)

My thought is that the float level needs to be increased, and the idle screw needs to be turned in. I am hoping that this will make the car run leaner at idle and fix the backfire-like sound when cruising. Other people have mentioned a stuck needle, or changing the jets and that is why I am so confused. In addition to that, because of clearance issues, I run a 9 inch air cleaner. I tried the drop base, but it gets in the way of the choke. Is the 9 air cleaner restricting the flow of the carb. In the book “Winning with Demon carb” it says that carbs that are too SMALL run RICH because the CFM rating on the carb (575CFM in my case) stands for air flow and if the carb can only flow 575 CFM of Air to an engine that needs more than that will result in a fuel-rich mixture. Is that true??

I know a lot of people are going to say that a 575CFM is too small for my engine, but I have already bought the carb, based on Demon’s recommendations and I do not have the money to buy another one at 500 bucks a pop. What are you thoughts on the following

Problems Recap:

Problem recap:
#1 The cable only allows the secondaries to partially open. Suggestions? (It is an aftermarket cable kit) (I heard that there is an adjustment on the carb to open the secondaries with less pull on the cable. True or false? If true, how?

#2 Float level is empty in the front. I can’t seem to get the level to go up?Stuck needle when I stupidly tried to fix it on the side of the road? What is the correct setting? Half full?

#3 and #4 Runs rich at idle, but backfires through the exhaust when cruising and light acceleration. Too rich at idle, turn in screws? What about the backfiring at cruising speeds? Raise float level to normal setting?

#5 Does a 9 inch air filter by Kand N provide enough air flow. Is that the reason for hte richness? maybe adding to it?

#6 What is a good idle, currently I am running at about 850. Demon reccommeds higher, closer to 1000? My timing is set to 18 degrees initial, 18 centrifugal, 36 total.(no vacuum) leave this?

Specs:

1968 327 Coupe
Manual transmission

New engine Specs:

Engine Modifications: Engine was bored .030 over and new hypereutectic pistons were installed.

Carburetor: Demon Carburetion Speed Demon 575 CFM vacuum secondaries

Heads: Edelbrock Performer RPM Aluminum Cylinder heads

Notes: comb. chamber: 64cc Int./Exh. Runner: 170cc/60cc Int/Exh valve: 2.02"/1.60" Max Valve lift: .575"

Camshaft: Crane MaxPower Camshaft

Notes: Small Chevy V8 (283-400) RPM Range: 2000-5000 Duration @ .050" Int./Exh.: 216/228 Lift Int./Exh. .454"/480"

Intake Manifold: Performer RPM Aluminum Intake Manifold (single plane)

Notes: for Chevy 262-400 (non-ERG)

Rocker Arms: Proform Rocker Arms ratio 1.5:1

Exhaust System: Hedman Headers. 2.5" pipe. H pipe installed. DynoMax performance super Turbo Mufflers.

Ignition System Upgrades: MSD Pro Billet Distributor (Single pickup). MSD 6AL ignition control unit with rev limiter.

Coolant system upgrades: Be Cool Aluminum Radiator. Perma-cool High Performance Electric Fan (16" Diameter 2950 CFM)

Thanks,
Joe

bobs77vet

Problems Recap:

Problem recap:
#1 The cable only allows the secondaries to partially open. Suggestions? (It is an aftermarket cable kit) (I heard that there is an adjustment on the carb to open the secondaries with less pull on the cable. True or false? If true, how?

#2 Float level is empty in the front. I can’t seem to get the level to go up?Stuck needle when I stupidly tried to fix it on the side of the road? What is the correct setting? Half full?

#3 and #4 Runs rich at idle, but backfires through the exhaust when cruising and light acceleration. Too rich at idle, turn in screws? What about the backfiring at cruising speeds? Raise float level to normal setting?

#5 Does a 9 inch air filter by Kand N provide enough air flow. Is that the reason for hte richness? maybe adding to it?

#6 What is a good idle, currently I am running at about 850. Demon reccommeds higher, closer to 1000? My timing is set to 18 degrees initial, 18 centrifugal, 36 total.(no vacuum) leave this?

Thanks,
Joe[/QUOTE]
Wow you have alot going there...
1. when the carb was in your hand did the secondaries open properly??
2. did you have a filter on the gas line?? is there debris in the valve seat?

3,4,5 &6 fix number 1 and 2 first and then eat the elephant one bite at a time

JoeRags

Your best bet to fix #2 is to just pull the bowls off and reset the floats. I did this to my Demon last week, I had adjusted one of the floats just enough where the adjustment nut wasnt grabbing the float adjuster anymore. I hardly turned that baby and it filled my bowl right up. Pull the 4 allen head bolts out of each end and go from there, not too big of a job. On the Barry Grant site, print the manual for your carb, they will explain in detail how to space the float right.
Also, after each slight adjustment you need to let it run, and rev it up a few good times. The levels dont just change as you turn.

- Joe

Cali,68,L-79

I feel your anguish. I start Sunday to diagnose my problems one of which is an intake leak. The car goes like all hell but I need to fine tune the A/F. I'll be watching this thread closely.

David