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Author Topic:   Need help some 4412 carb questions
novajohnb
Dirt Roller

Total posts: 6
posted May 09, 2005 09:50 AM  
OK guys- I need a little help w. this carb setup. I have experience w. 4bbl carbs on drag engines, but the 2bbl on circle track has me a little stumped.


Engine is a 13:1 sbc 350, cam 259* @0.050" 106* (choppy idle.) Timing locked out at 38* (HEI) timing chain stretch/ module retards timing to ~36* at redline. Timing rock steady until high speeds.

Engine idles at only 2" vacuum at ~1,000 RPM, 5" at 1200RPM, 10+" at 1800RPM, then vaccum drops back down when it revs up. Due to big cam.

Idle screws not responsive at 1000 RPM. I want it to idle at 1000 RPM. Have 4.5 power valve, 74 jets (more on this later.) 6psi fuel pressure, floats set at bottom of sight hole, no fuel dripping out at idle or at speed. I believe 4412 carb to be otherwise stock. Carb is mounted in "normal" passenger car position with float in front of car. Have high rise single plane intake with adjusatable insulating wood 2bbl-4bbl adapter with carb roughly centered over manifold.

Running 134 autolite racing plugs. #2 is fouling out. Spark plug wires new- resistance checks out OK. Right hand side plugs looks richer than left hand.

1. How big can you drill holes in butterflies? Holes started at 3/32" (0.094") I opened up to .110" not much difference noticed. With 3/32" butterfly holes I had about 0.060" of idle transition slot showing. I know this should be about 0.030" so I have butterflies open way too far at 1000 RPM idle. I just dont wanna drill too far and have to start over...

2. I have a K&N flow control air cleaner (looks like sub stack on bottom with a profiled lid.) According to K&N need to jet up with this unit- that's reason for 74 jets. Anyone have any experience with this unit? Is it worth any power? Do jets sound like they're in the ballpark?

3. Can the carb work OK mounted front- back? Should I stagger jet with smaller jet on right side larger on left? Or. should I just mount the carb sideways- with float bowl pointed left?

Engine runs fine except for fouling issue and loading up on idle.
Thanks guys!


ss99
Dirt Maniac

Total posts: 120
posted May 09, 2005 01:01 PM  
Holes in our butterflies are drilled to .125. With those, we can idle as low as 700 once it's warm.

I'd say open 'em up a little more. You should be able to close the butterflies a little more that way to expose less of the idle transfer slot and regain your idle mixture screw control.

leapinlizard
Dirt Freak

Total posts: 402
posted May 09, 2005 01:45 PM  
Why do you want this thing to idle at 1000?


It's race engine, who cares where it idles.

dirtbuster
Dirt Forum Champ
Total posts: 2007
posted May 09, 2005 01:50 PM  
Same question why would you want to idle it at 1000? It would actually be better to idle it at 1500-1700. Keeps more oil on the cam.

But you might also need to drill them as suggested if the screws arent doing anything because you might have too much of hte transfer slot exposed.

[This message has been edited by dirtbuster (edited May 09, 2005).]

john7771
Dirt Roller

Total posts: 14
posted May 09, 2005 05:42 PM  
I have drilled mine out to a shade under 3/16, throttle plate completely closed, 76 jets with same k&n air cleaner, 6.5 power valve, same float level adj. Hints: if your using an adapter, slide the carb back to the rear of the engine an 1/8" at a time and make sure no fuel is dripping out of the boosters. My 355 will idle @ about 800 rpm with a .500 lift 240/250 cam. Hope this helps!

novajohnb
Dirt Roller

Total posts: 6
posted May 09, 2005 07:30 PM  
Thanks guys!
John 7771 now I will try to drill butterflies a little bigger- I didn't have a good feel for how big you can go. I was thinking about sliding carb back maybe take some fuel from #2- will try it.

As for why to idle down so low- actually I figured it's a race motor- let it idle at like 1500 to splash oil around the cam, etc...
But the previous owner mentioned that under yellows or a restart- idling that high you'll be riding the brakes. The car is a (northeast type) DIRT sportsman with a Bert tranny- once you're in high you really can't get out of high on a restart. Only raced twice- but I can see where if the leader is doing "brake checks" on a restart, where idle speed too high will make you run over the car in front. Same thing going off the track into pits. I tried popping it out of gear to avoid cars when goin off hte track and then into low or high- GRRRIND!

Thanks again- at least I know I'm going in the right direction. I need all the free help I can get- runnin against guys with $15,000 engines and new cars!

ss99
Dirt Maniac

Total posts: 120
posted May 10, 2005 07:07 AM  
quote:
Originally posted by leapinlizard:
Why do you want this thing to idle at 1000?


It's race engine, who cares where it idles.



It's not so much about where it idles in terms of the RPMs as it is about the location of the throttle butterflies in relation to the idle transfer slot. Getting this right leads to better throttle response. If you've got the idle set at 1500-1700, you've got too much of the idle transfer slot exposed - not the best scenario for off-corner throttle response.

On the track, the rpms obviously aren't going to be down to idle, but the throttle butterflies will be in the "idle" position when you're off the gas. It is from that point that you begin your acceleration when you step on the gas. This is where the idle setting actually comes into play on the track. Maybe Earl Parker will chime in and explain in better.

leapinlizard
Dirt Freak

Total posts: 402
posted May 10, 2005 09:36 AM  
Or you can idle it up to 1200 and get some of both idle circiut response and cam lube without worry of running over you buddy in front of you. If you need more out of throttle response you can use the accel. pump to cover the lean spot.

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