Pontiac 400 - Idle bypass and Secondary rod hanger questions

[DieselPower]

Let me start off with specs:

1970 Lemans with auto trans
Pontiac 400 bored .060" over
XE268H cam
    224 @ .050" intake
    LSA 110
9.5 CR
Edelbrock Performer intake
Stock exhaust manifolds and exhaust
Edelbrock 1910 Qjet
Stock distributor with Pertronix ignitor
     B20 vacuum advance can (fully actuated at 12") hooked to manifold vacuum
     12* initial, 36* total
Jets -  .073"
Primary Metering Rods -  53M
APT Screw -  3 turns 
1/4" float level
Idle Tubes -  .038"
Upper Idle Air Bleed -  .070"
Lower Idle Air Bleed -  .060"
Idle Channel Restriction -  .059"
Idle Discharge Port -  .098"
Idle Air Bypass -  .106"
Main Air Bleed - .046"
Secondary Air Bleed -   .036”
Light blue Power Piston spring (verified that it is staying closed at idle)
Accelerator Well Hole in main body - .050" 
"CE" Secondary Metering Rods 
"J" Hanger 
Accelerator Pump Discharge - .025"

Carb rebuilt with Cliff's HP kit.  Airhorn had maximum .026" warpage.  Installed thin gasket and had fuel leaks and very hard starting.  Installed thicker airhorn gasket and now starts easily and runs well except for idle.  I have two questions:

The thicker airhorn gasket moves the secondary rod hanger up about .025".  Is it worth getting a new hanger to bring the secondary rods back to the original location?  My current hanger is a "J", and I would need to move to an "O" to bring back to the same height.

My second question is in regards to idle quality.  Mixture screws do not do anything.  I believe the throttle blades are too far open and I am running off the transition slots.  If I pull two vacuum lines, I can close the throttle enough to get some adjustment from the mixture screws.  I am trying to get an idle of 850 rpm and be able to put it in gear and turn the wheels without stalling.  At 850 rpm, I get about 14" of vacuum, but idle quality is poor and engine dies easily when loaded.  Should I increase idle air bypass?  I am currently at .106".  I measured the size of the vacuum leaks that I created, and to get the same flow area, my IAB diameters would need to be .149"!!!  I assume that is too much.  Is there something I am missing?  From what I have read, this cam should be able to idle at 850 with no problem and make 15" or more of vacuum and not stall when loaded in gear. 

Oh, one more question.  Is the power piston spring supposed to fit into the bore at the bottom of the power piston bore?  It looks like it should, but is slightly too big.  It is so close, I just want to make sure my carb was not machined wrong.  Cliff says a lot of the Edelbrock carbs had questionable quality control.

Thanks in advance for any help!

[77cruiser]

How much advance do you get from vac.? Can you give it more initial, to see if that improves the idle?
Have you pulled the carb to see where the throttle plates are positioned to the transition slots?

[DieselPower]

I have a B20 vacuum canister that gives full advance at about 12".  This unit gives 16* timing advance giving me 28* timing at idle.  Timing is rock steady since I am running about 15" of vacuum which is more than enough to keep the advance unit fully retracted.

I just checked the transition slot.  It has way too much exposure at ~.115".

Thanks!

[Cliff Ruggles]

Idle system is pretty rich for what you are doing, it may be rich at idle instead of lean. Do you see any nozzle drip at idle speed....Cliff

[DieselPower]

Cliff,

No nozzle drip that I can see.  I used idle recipe 3 from your book which looked like it fit quite well with my engine specs.  Is there something I missed?  Should I enlarge the IAB?

I was wondering if you could also help me out with my questions about the secondary rod hangers and the power piston bore?

Thanks a ton!

[77cruiser]

Take a look at the IABP make sure it's not blocked by the gasket some place, not likely but ya never know.

[DieselPower]

Gasket alignment is good.

[429bbf]

dp i had the same problem with a 400 . it was to rich at idle i end up with .037 idle tube and .052 dcr. i know every engine is different and the guy told me he had a bigger cam than what he had. its so hard to build something when they don't know exactly what they have. hth

[DieselPower]

How big can I make this bypass?  I am tempted just to take it to .130" and see how it runs.  If that is not enough, maybe .140".  I have seen Cliff say that he never has to go over .120", but I do not see how that is possible unless he added holes in the throttle blades.  Is there any reason not to make the IAB .130" or even .140" if it gets the blades where they need to be so I am not running off of the transition slots?

Any thoughts on my questions about the secondary hangers and power piston bore in my original post?

Thanks guys!

[77cruiser]

Start at .125 I had one at .14 & it whistled at idle. You can always close it back down by using drilled 1/4 inch  set screws to restrict the vertical passage in the baseplate.

[429bbf]

I've got a 1910 that i bought for a project I've never taken it apart. on the quads the spring goes up inside i can't tell you on the 1910. your second question the thicker gasket id see how the thing performs before id change it.i would not be afraid to drill the bypass a little bigger to .130 and maybe smallen your idle tube and dcr. .ive noticed it only takes a couple .000  to make a large diff.fwiw

[429bbf]

how did you get power piston down all the way if the spring doesn't go up inside? you should have bad coil bind. on the other note i reread your setup. i would drill it to .130 and make the dcr to low side of 50 and leave the idle tube alone .whats sad is you should do them 1 at a time . but that means lots of bending over in the car or lots of removal. fwiw

[DieselPower]

Thank you for the replies! 

I will drill out the IAB to .130" and let you guys know what happens.

I decided not to worry about the secondary hanger.  After doing some full throttle runs and pulling the plugs and seeing that they are a perfect light tan, I am going to call it good enough.

With the power piston concern I had, if you look down into the power piston bore, you see another small bore that is obviously the vacuum source.  The thing is, that inner bore looks almost big enough to put the spring into, but not quite.  I did not experience any binding, but obviously if the spring is supposed to sit in that inner bore, my spring tension will be way off.

[429bbf]

you are correct . it does not go in the vacuum source, it just sits right on top of the little hole. i was confused ,i thought you were asking about the power piston internal bore . sorry for the confusion. hth

[429bbf]

on a side not ,i like your login . i used to run diesel in my 79 f250 when diesel was 30 cents . i had a 11.5.to 1 429 and i had dual tanks . i would fill the rear with diesel and front with gas . id start it on gas and when i got it warmed up going down the road id switch over . then id switch over before i shut it off. it ran pretty darn good.now i just run quads on my fords. fwiw.