I have a 600cfm manual choke carb on a 290hp 350 crate engine. I am trying to tune it. Setting fuel mixture screws I turn in until engine runs rough and then 1/2 turn out correct? I am also experiencing the car falling on its face just off idle and then it oulls hard. Is this a lean condition? I have adjusted accelerator pump rod and it does not help. I have fuel pressure reg set at 5.5., i have the choke unhooked and wired open.

 

the screws on the front of the carb are for idle mix only, they don't affect anything else. Turn them out to increase fuel at idle. I usually turn them all the way closed, then a 1/2 a turn out, start the car and make 1/4 turns until i get a nice smooth idle. (fully warm of course). As far as the stumble goes, It's a 600 so it shouldn't be too big and you said you've messed with the accellerator pump so i'm not sure. The floats might need adj or maybe the springs could be changed as well. I have an owners manual at home i can check for you later if someone more familiar dosn't jump in with an answer.

 

I did adjust the floats as they were off a little. I just put the accelerator pump arm back in the middle hole as it came. I was wondering about the fact I might need to change springs. It olny has the hesitation off of idle. If I am goning down the road and stomp the pedal it will kick open fine.

 

Is the hesitation under hard acceleration off idle or easy?

 

hard acceleration, it falls on its face

 

If I pull in the street and from a stop mash the pedal it bogs or whatever its doing.

 

This is going to be a jet/rod issue. Springs don't affect hard acceleration much at all. With the 600 CFM, Edelbrock tunes for efficiency out of the box. Try stepping up the rods to the next size which will actually be smaller. If that doesn't work but helps, change out the jets to the next biggest size. Keep going till you get it right. I'm assuming it's a lean condition. If not, try going the other way with it. Hope this helps.

 

The idle carburation is the most important part of a carburetor tune!!!
It will affect the transition zone (from off-idle to 2500 RPM) and the road driveability very strongly!

You need a vacuum-Meter connected to the manifold !!!!

1) Set both idle screw at 1 or 1-1/2 turn off seat.

2) Warm-up the engine and be shure of the correct setting of the advance.

3) Then fix the RPM you like at idle and adjust-it with the throttle-screw.

4) Start to turn one idle mixture screw.... and look for the direction where the RPM will rise a bit. Move the other screw in the same direction of the same quantity.

5) Unscrew the throttle screw in order to restore the correct RPM wich was increased from you action on the mix screws.

6) Repeat the procedure from point #4 looking for the condition where the Vacuum-Meter will show the HIGHER vacuum level.

With this procedure you will found the condition with the correct RPM..... and with the lower aperture of the throttle

We need this condition because is the only way to have the most transition slot avalaible in off-idle.

The next thing to check is the needle-springs...... a too stiff springs will richen the carburation too early..... a too soft spring will give you a bog in the transition to power-enrichment
A standard rule is a spring with a nominal seat level in the range of 50% of the idle vacuum level (ex. if at idle you have 18 Hg of vacuum..... you should need a spring between 8 and 10 Hg of seat vacuum)

The needle can be changed...... but in 90% of cases will be OK!

Finally you can check if your main-jet are Ok or not!
The only way to do that is a Wide-band O2 sensor..... if you have it, do it..... othewise leave the jet you have..... because in 95% of case will be in the safe zone (rich).

All the previous checks should be done keeping closed the secondaires!

At full throttle, with the secondaires working, you have 2 thing to check:

1) The carburation at full throttle under load at high RPM (all 4 barrels opened and the vacuum butterfly fully opened)

2) The transition from 2 to 4 barrels

The first thing will need the mentioned Wide Band sensor....

The second thing can be tuned even visually with a friend looking at you tail pipes during a very full throttle acceleration of your car.

You should see a very small cloud of black smoke coming out..... but if you notice a bog..... you have too light couterweight on the butterfly.

My opinion?

Go with the correct tuning of the first section..... and you are in the ballpark of wath you need!!!!

I hope my english if enough clear...... sorry!

 

Good post.

 

very good post. I only have around 10 hg of vaccum at idle. Is there springs that would close?

 

Very good post Stroker....you can buy the spring set from Edelbrock or others ( I got mine at Checker), and also different rods and jets. The Edelbrock owners manual has very good descriptions of the procedures you should use to tune their carbs. I believe you can get a PDF from their website.

(Your English is fine and a hell of a lot better than my Italian!)

 

You have something wrong. 10" of vacuum at idle with that mild an engine is way too low. Check for vacuum leaks or ignition timing. The carb may be way out of adjustment also. you should have between 15 to 20" at idle.
You said you adjusted the accelerater pump, but are you getting a good shot of fuel when you pump the gas?

Paul

 

Too less vacuum....... may your problem is a bad idle tuning !

may too less advance...
may too rich mixture...

try to check it!

 

Had similar problem --- same carb.

Solved this problem by increasing idle timing to 20 deg BTDC and increasing idle spreed to 900 RPMs. Used "ported" vacuum for vacuum advance. You need to make sure your 'ALL IN TIMING" at 2800 RPM is around 50 deg or less and make sure you're not getting any pinging. I did this by using an adjustable Vacuum Advance Can to reduce vacuum advance--- otherwise I got too much advance at "ALL IN" . This was a trial and error solution, but it worked and my engine runs like a CHAMP now --- but I'm not sure of the theory.

Your low vacuum leads me to believe you have a vacuum leak or a timing problem. These carbs usually run perfectly right out of the box .

Good Luck and Let us know your solution

 

my timing is 12* initial, 32* total and 44* with vac hooked up. Its all in by 3300 rpm. I sprayed carb cleaner around and could find no leaks on engine. It has a TH350 trans and I did hear a slight whistling sound down by the trans. Could the module or whatever its called be my problem? I don't even know if that would make any difference. If I plug the vac line that goes there the engine still runs the same so My guess is no, it does not affect it.??

 

You could easily handle more advance and that shold improve overall engine performance. Push your initial timing up to 16 BTDC and increase idle RPM's to 800-900 and see what happens. If that doesn't solve the problem, then increase to 18 BTDC and then 20 BTDC if that doesn't work. If that solves your problem, then just make sure you're not getting any ping at any time. I'm still betting your problem is timing/tuning. I personally found the my engine ran a lot smoother at idle with the higher advance and a richer idle mixture than the "books" would suggest. Don't know the theory --- but it worked for me.

Let us know your ultimate solutiion --- Good luck

 

I will bump my timing tonight and see what it does.

 

I bumped my timing to 17* initial 36* total and 48* w vac/hooked up. Tried running on ported vac. and It wanted to die when I floored it. I went back to manifold vac. I did get 11 hg of vac. on the gauge at idle.

 

Your vacuum advance seems to be giving you 12 deg advance when it kicks "IN". If you set your initial timing at 12 deg BTDC and then you have your vacuum advance hooked up to manifold vacuum, then you are getting 12 + 12 = 24 deg advance when the car is running at idle.

When you move the vacuum advance to PORTED, the vacuum advance is NOT OPERATING at idle, so your advance drops down to what your setting is by turning the distributor --- which is your case was 12 deg. So when you changed (upped) your initial advance setting to 17 deg BTDC but also went to ported vacuum, then your timing at idle is just 17 deg where before you were at 24 deg at idle, so you actually reduced the timing at idle compared to what you had before.

Leave your vacuum advance at manifold, and then up your initial timing setting in 2 deg incriments (from 12 to 14, then to 16, etc) to see if that helps at all.

In my case, I am using ported vacuum and my initial advance setting is 24 deg. When I step on the gas from idle, the ported vacuum advance does NOT kick in because of the large quantity of air being sucked in by the engine --- so I am getting 24 deg at takeoff. In your case, You also have 24 deg at idle (12 + 12), but when you HIT the pedal, the vacuum advance kicks off and drops you immediately down to just 12 deg advance and that what I believe could be causing your problem.

The point to remember is that when you HIT the gas pedal, the vacuum advance kicks OFF in both the ported or direct manifold case.

Let us know how things are working out

 

11" of vacuum with that much initial timing? Something is still wrong here. You're sure there's no vacuum leaks? Could you have some valves adjusted too tight? Cam timing off? Is the vacuum at idle steady or bouncing? Did the engine ever run right? The key is going to be finding what's causing the low vacuum.

Paul