New Motor High idle

[RD_Racing]

Sorry for the long winded post but I was hoping much of it will keep anyone from asking about something I already checked and verified.
My new motor wont idle below 1200 without it sounding like its to low and wants to stall and will actually stall at 1000rpm when i put it into gear. Or is it possible thats just the nature of this build with a tunnel ram and it wants a higher idle? I had the same intake and carbs on a 355 with fairly large cam and had no problem idleing at 1000rpm.. I also know someone with a 406 and the same cam but he has less compression and runs a single plane intake and has no problem ideling at 1000rpm and it sounds wicked.. It seems my motor prefers a idle at around 1400rpm.. I have checked everything I could think of and have even replaced both carbs and even put a different set of heads on it and I still get the same results.

Here is the build
SBC 400 block (511 block)
3.75 Scat 4340 crank
6" Carrlio rods
Wiseco pistons (domed 12-1 compression)
Comp Cam (#12-908-9) solid roller .630 lift, dur 264/270 @ 050 and 106 lSA/IC
Cam was degreed in
lash is set to cam card .026/.028
Promaxx 225 shocker aluminum heads cnc ported (225cc intake runners)
Edelbrock street tunnel ram that was welded and ported to match the larger port heads
pair of custom 600cfm quickfuel carbs (vacuum secondaries)
timing locked out at 36 degrees
verified TDC with an indicator
I have sprayed everything with carb cleaner cant find any vacuum leaks
I have upgraded the heads (had 215cc heads on it)
I have installed a different cam (had a similar cam profile but it was a Lunati)
I have replaced both carbs
I verified the transition slot on the secondaries are barely cracked open
installed 3.5 PV's
Vacuum gauge is steady with no bounce at 1400rpm
Idle mixture screw are open 1/4 turn, anymore than that and it will start to fould plug
I have tried reduced the IFR's

None of the above had any effect on being able to lower the idle.. I cant really get the idle down to 1000rpm to get an actual vacuum reading at that RPM becasue it almost wants to stall at 1000rpm.

Also, when idleing I cannot turn the timing down any lower than 30 degrees without it sounding like it isnt running right and wants to stall
Otherwise the motor seems to run great as long as the idle is at 1400rpm minimum and timing is at 34 minimum.. I have seen Small blocks with bigger cams that idle at 1000-1100prm with no problem. I thought maybe i had a vacuum leak under the intake but i have since put new heads on it and had to re-install the intake and there was no change..

The main issue I have when ideling at 1400rpm is the convertor is to tight and wants to drag the RPM down when i put it in gear and its hard on the u-joints (3500 stall)
Any thoughts?

Picture of car

[PRH]

When it’s idling at 1400, is there any fuel dripping from the boosters?

The big red flag for me is the mixture screws at a 1/4 turn.

Are there dedicated idle discharge holes for the secondaries, along with the transfer slots?

What’s the manifold vacuum at 1400?
1000?

[RD_Racing]

When it’s idling at 1400, is there any fuel dripping from the boosters?

The big red flag for me is the mixture screws at a 1/4 turn.

That's something I will have to check. The other carbs I had on there were 450cfm holley mechanical secondaires and the idle mixture screws were out about 1-1/4 but I had the same results with the idle

[PRH]

I added a couple questions above......

And I’ll add another couple here.....

If you have it idling at 1400 in neutral, and put it in gear........ how much does the vacuum change, and what’s the rpm in gear?

Mufflers? If so, what kind and size? Have you tried removing them?

[RD_Racing]

Are there dedicated idle discharge holes for the secondaries, along with the transfer slots?

What’s the manifold vacuum at 1400?
1000?

At 1400 vacuum is at 9hg
Picture of bottom of secodaries

[RD_Racing]

I added a couple questions above......

And I’ll add another couple here.....

If you have it idling at 1400 in neutral, and put it in gear........ how much does the vacuum change, and what’s the rpm in gear?

Mufflers? If so, what kind and size? Have you tried removing them?

That's something I have to check but idle drops about 200rpm when I put it in gear
Right now its running open headers and hasn't had mufflers on it yet. Primary tubes are 1-3/4 with 3" collectors

[PRH]

Do those carbs have 2 or 4 corner idle?

[mag2555]

I guess you had these same Carbs on a duel plane 2 4 bbl set up ?
If so you now have a bunch more Plenum volume which slows down the air passing thru each bbl of the Carb.
I would bet that your plugs are now showing a dry back soot , no?

[RevTheory]

It sounds like this combo is really wanting a pair of 4-corner idle carbs, maybe even 750s. How do you guys feel about a functioning PCV on a tunnel ram? Holes in the throttle plates?

[F-BIRD'88]

Don't know why you chose those vac sec carbs for this.
They are never right on a tunnel ram.
Should have used 2 eddy carbs ..
But.. Try advancing the timing more ,say 42-45 deg..
Now try slowing the idle speed..

Better? Could be TDC timing mark is off .
Could be reversed diz magnetic pickup polarity
Try reversing the pick up lead.
Leak.... TR plenum gasket?
Wrong carb throttle plate base to carb body gasket?

How much primary T slot throttle exposure at idle?
If carbs are too far open at idle the timing is off.
If carbs are too far closed at idle you got a leak somewhere. (Pri T slot exposure)
Do those carbs have a "idle eze"? Below center air cleaner stud hole. Missing screw valve or screw set too far open in the idle eze. ... Have a look.
Its getting extra air from somewhere like that.
Who talked you into those vs carbs for this, anyway?
The biggest error on a tunnel ram is using THE WRONG CARBS...
But look for that air leak (idle eze open wide)

[RD_Racing]

Do those carbs have 2 or 4 corner idle?

2 per carb

[RD_Racing]

It sounds like this combo is really wanting a pair of 4-corner idle carbs, maybe even 750s. How do you guys feel about a functioning PCV on a tunnel ram? Holes in the throttle plates?

I know a guy on my Vega forum that just put the same carbs on a SBC 421 stroker with tunnel ram and fairly stout cam and they work perfect for him and with a 1000rpm idle too.. This is my second set of carbs and Im getting the same results with both.

[F-BIRD'88]

Post the carb model number etc.
What is "custom" about them?
What got changed?

[PRH]

If the carbs are 2 corner idle, here’s my concern......

The “normal”(to me) idle discharge port for a 2 corner idle carb is a tiny little hole under the transfer slot.

The giant hole in the pic of the OP’s carb would be dumping a lot idle mixture without some means to regulate it(like an idle mixture screw).

So, if the carbs are 2 corner idle, then I’d pull it apart a little to trace the idle circuit and see where the flow restriction is(if there is one), and see what size it is.
Not the ifr, but a restriction to regulate the amount of emulsified idle mix that’s being discharged out of the idle port.

Pic is of an old 3310 baseplate rear idle discharge port.
It’s the tiny dot above(but located “below”) the transfer slot.

[RD_Racing]

Don't know why you chose those vac sec carbs for this.
They are never right on a tunnel ram.
Should have used 2 eddy carbs ..
But.. Try advancing the timing more ,say 42-45 deg..
Now try slowing the idle speed..

Better? Could be TDC timing mark is off .
Could be reversed diz magnetic pickup polarity
Try reversing the pick up lead.
Leak.... TR plenum gasket?
Wrong carb throttle plate base to carb body gasket?

How much primary T slot throttle exposure at idle?
If carbs are too far open at idle the timing is off.
If carbs are too far closed at idle you got a leak somewhere. (Pri T slot exposure)
Do those carbs have a "idle eze"? Below center air cleaner stud hole. Missing screw valve or screw set too far open in the idle eze. ... Have a look.
Its getting extra air from somewhere like that.
Who talked you into those vs carbs for this, anyway?
The biggest error on a tunnel ram is using THE WRONG CARBS...
But look for that air leak (idle eze open wide)

I was just thinking about adding more timing too.. I like that idea and will try it later today or tomorrow. I know the timing mark is spot on, I checked it with an indicator right on the piston when I had the heads off..

Carbs do not have an idle eze like the BG carbs. I tried a set of Holley 450cfm mechanical secondary carbs and got the exact same results. I even tried the ole trick with the wire in the ifr's when I had those carbs on it..