Air_Adam

Hey guys,

Just a question... I've never really done much tuning on 4-corner idle carbs, but have recently got a 750 DP Holley (4779).

I know how to tune the idle on a primary idle Holley (where only the primary metering block has adjustment screws) as I've got a 750 VS 3310 and a 650 DP 4777 like this.

How does tuning the 4-corner idle carb differ from the other two I've had? I do have a vacuum guage, which helps, but I'm just not familiar with how the 4-corner system works... do all 4 idle screws have to be set the same? Do all 4 throttle blades have to be opened the same amount at idle speed? This is the kind of thing I'd like to know.

This is on a 350 4-spd manual car by the way.

five7kid

iTrader: (14)

Well, they all have idle in the secondaries as well. The difference is you can't adjust the secondary idle mixture on the typical universal carb.

I can't give you much more detail than that, but you just add the secondary adjustment to the mix. Especially helps if you have a lumpy cam.

xpndbl3

iTrader: (5)

the transition slots and mixture screws should all be at the same opening, but I've had it before on some intakes were 1/8 turn more for the primary or secondaries smoothed out the idle much more, probably a fuel distribution issue.

Air_Adam

Do all 4 throttle blades have to be open the same? Or can I just use the primary idle speed screw to adjust idle speed? Whats a good starting point to set the mixture screws to, to start out with? I used 1.5 turns on my old carbs, does that work for this one as well?

xpndbl3

iTrader: (5)

depending on how much air your motor needs, all 4 should be open the same. Now if it idles really low in the rpm range the the rears can be closed off IIRC. With all the motors I've used 4 corner systems on, they all needed 1000 out of gear and 800-900 in gear so all 4 slots have been even. I'm sure a milder combo wouldn't need that much slot open. You can start at 1 turn out and tune from there, honestly as long as the motor runs at idle when warmed up you can go in or out with the screws to find the best vacuum in gear for an automatic or out of gear for a stick.

Also I don't have all the answers, only what I've picked up along the way, hopefully someone with more tuning experience can chime in, but this is what has worked for me at least Seems lately whenever I post to help out I'm shot down and told not to post "if I don't know what I'm talking about" so maybe that's the way this site is headed. I'm not really sure.

Sonix

iTrader: (1)

hey, hey! Who let xpndbl3 out of his cage?
Yea xpndbl3, just because you have a really fast, good looking car doesn't mean we should trust you

adam, you have the 10:1 350 with an xe274? Is the weather there good enough to take the car out yet?
I'm looking forward to track results out of your car, I should be able to finally get mine to the strip this summer and compare

Air_Adam

11:1 actually, and what do you mean 'taken out'? Its been out all winter being driven lol.

xpndbl3

iTrader: (5)

shhhh don't tell anyone that i'm out of my cage. Makes me laugh because I had some shots of me behind the rear bars of my cage while putting it in and everyone on the chicago board called it the gorilla locked in his car for a long time now.

PS Sonix, who ever said this car was fast It's a show car and I'm sticking to it.

urmomshot77

Well its been about mid 40s here this week and i have my quad idle screws turned out 1 1/4 turns, and its running good. The secondary adjustment screw is there as another way to tune your idle. However don't rely on it too much or you will actually open the secondary barrels to a point were your manifold vaccuum will start drawing fuel prematurely out of the secondaries.

xpndbl3

iTrader: (5)

both the front and rear slots are the same length, and they both should be opened to the same slot size; so opening the rears up too much would have to be a 2500 idle easily before the boosters would see a signal and draw fuel out if the fronts and rears were open the same amount.

Sonix

iTrader: (1)

Holy cow adam, you're a brave guy driving that thing in the winter. I couldn't even be able to get mine to drive forward in the cul-de-sac, took me 5 minutes to get across it, it's like trying to run on ice with duct tape on the bottom of your shoes- curling style. OUCH!
11:1 eh? Do you notice any knocking? When you lug it? Do you run less than ideal timing?

haha, I don't know Steve, a TH350, moser 9" and a full out cage ? I'd think it'd be a little fast. I mean, if it wasn't that fast i'd use a 700R4 and enjoy the overdrive, but if you're fast enough to break one of those...
Is the snow melted over in Illinois yet?

Yea, I have nothing useful to add to this thread, don't mind me.

xpndbl3

iTrader: (5)

nah...the trans came out of IHI's car off this board, so you know that took off at least .4 in the quarter. Since he's flying wheels up all the time at the track. Well, there's no more snow here, well at least that sticks to the ground. Right now we had flurries today with a high of 32 and a low of 20. Sucks....it was 60s earlier this week, supposed to be 40 or below for the next 10 days off weather.com so I don't like working on my back finishing up the 9" install with the concrete being frozen still in the garage.

Air_Adam

Yea... the motor isn't very happy with pump premium lol. I had to keep the timing down to 8 inital, although its much happier around 16* idle except for the pinging lol. I have to run it a little bit rich at idle too when its really cold out, which doesn't help things much.

The pinging is actually not really a problem if you tune conservatively and don't get too heavy on the throttle if you are in too high a gear. I'm sure its got alot more in it though if I could tune it the way it wants to be run and use good fuel in it lol.

I'll be changing the heads for a set of 64cc heads though (have 55cc now), so it should knock it down to a more pump fuel friendly compression level.