Need Expert advice \ 1400 Idle

B

Brandonium

Member
Ok, here's my scenario:

engine:345
carb: edelbrock 1400
spacer: squarebore to spreadbore
cam: stock
dist: dui
wires: livewires
plugs: ngk 3332
gap: .054
intake: rpt spreadbore
egr: intact
primary: .098
secondary: .095
metering rods: .073 x .047
step-up spring: orange 5"hg
thermostat: rs 180*

here is my dilemma. I've got the vehicle running fine. The carb is rebuilt, new floats measured and set, new primary and secondary metering jets, accelerator pump, metering rods and step-up springs. My vacuum at idle (650 rpm) is 18hg at 12* btdc / 19 hg 14* btdc. No vacuum leaks and idle mixture screws set to achieve highest vacuum. I'm running manifold vacuum so at 12* btdc it'll advance to 24* btdc when connected, rpm's jump about 100 or so rpm. At 14* btdc my timing advances to 28/29* btdc when manifold vacuum is connected, idle raises the same as mentioned earlier.

The issue I am having is that I am unable to lower my idle past 600/650 without vacuum connected and with it connected the 750 idle is the lowest I can get it. There are no blockages in the linkage, throttle cable disconnected and the fast-idle cam is not interfering as the choke is wide open. Flipping the carb over and when checking the primary throttle blades you can see a very small bit (not a total square in appearance) of transfer slot on each of the primaries. The curb-idle screw will continue to back out even if the cam stops when the primary butterflies are closed. All four of the throttle blades appear to be closed in the barrel properly, no large or small gaps seen.

I'm not really sure why I have no additional idle adjustment. I get to the point where when I get the engine humming and I need to idle it down I'm stuck with a higher idle. I know you're not supposed to idle at 450, and I am not trying to.. But I don't think aiming for 600 rpm with manifold vacuum is asking too much is it?
 
Suggestion, run ported vacuum, that is the way IH did it from the factory. That'll get you right where you want to be and it is just moving a hose. Assuming this carb has ported vacuum.
 
I'm not trying to be an azzhat here but....
Despite you believing that you do not have a vacuum leak somewhere, you have to. With the throttle plates shut the engine will not run. Air is coming from some where or the carb is not really shut tight.
Regardless of where the advance is if you have not air or fuel to burn there will be nothing to run to advance.
 
I can try to check again. I have been over every bolt, gasket, intake nipple, gasket and hose with a propane torch and it has not flinched even once on the idle/rpm. I mean, not 1 rpm. I didn't wave it about either but put the torch directly over each bolt and held it there waiting to see a response.

If I've done this around every area of the intake and carb where something is mounted and nothing fluctuates then where else could it be? Brake booster? That is the only place I have not checked.

Also, if I screw the mixture screws in it will die, it will not continue running.. I am by no means an expert but if that happens doesn't that tell me that there's no unmetered air being taken in?

Also, I have a steady 18/19hg at between 650 and 800 rpm's. Wouldn't this show on a vacuum gauge if there were a leak? The needle has never been steadier or higher for this engine.

I welcome any input which is why I posted. Not offended.
 
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The small round hole below the throttle plate is the port fed by the mixture screw. It only adds a fuel emulsion. It dies because the mixture is leaned by turning the screws in to the point that it is to lean to run. That says nothing about unmetered air.
Engines will not run without air and fuel.

I have no love for the new performer carburetors. The are marginal at best. The older ones were good units but of late the quality has tanked. I will only run holleys now.

There are many places for alternate air to be introduced that will not show up by gassing the outside.
The throttle plates have to shut light tight. Shine a flash light from the top and looking through the bottom, see if any light sneaks past. Look around the throttle shafts where they enter the bores on primary and secondary's. Some times you have to loosen up the plate screws and realign them to shut properly. It will take some doing but it will help.

The power needle pistons can be leaking. Put your thumb or clay over the top cover plate and the side hole for each piston.
 
Robert, here is what I just did. I can say with certainty that when I disassembled the carb I only removed the secondary throttle shafts, not the primary. I didn't think to check that. Would gassing that tell me if air is getting through via the shaft?

photo 1.jpg

photo 2.jpg

photo 3.jpg

photo 4.jpg
 
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Robert, how much of an intake leak can there be, if any, if he's getting a vacuum reading of 18"???
 
And just to reiterate, at 12* btdc my vacuum is at 18hg. When I bump to 14* btdc it steadies at 19hg. Prior to replacing my intake manifold gasket I was running about 14hg with a slow fluctuation of about 1hg.
 
That's about as good as it gets, which is why I suggested to just run ported vacuum.

Just to confirm, have you checked to see that the choke cam is not preventing the throttle plates from closing further? (didn't look at your photo's yet)
 
Yes, because it's bitten me before. It's easier to do when it's off the engine because it's always closed/cold. When it was running the choke was fully open and the fast-idle cam was removed from the fast-idle screw allowing it to fall on the curb-idle cam.

I had my hopes up to run manifold but prior to that when I did run ported (back when it had vacuum leaks and a load of other issues) I was still able to idle it down. I would think that if desired I should be able to stall it out with lowering the idle too low but I don't even have that option. And if by chance I wanted to drop it to let's say 550 or 500 (used to idle great at 500 a decade ago) I have no additional adjustment available.

It makes me feel like something is off and while ported would get me to my only idle option at this point it seems like there's an underlying issue that's lurking. Perhaps it's the carb itself. I was tempted to replace it but I hate tossing parts at it. I'd love to try a Holley but it would require too much change. Fuel connection on the other side, my edelbrock adapter, lokar setup and my almost nil clearance with the air cleaner..just a can of worms waiting for me with that.

It seems as though all fingers might be pointing to the carb if my vacuum is what it is, my dist being new and timing sticking to where I set it. I'm not sure where else the issue could be lurking.

fdchappie said:
that's about as good as it gets, which is why I suggested to just run ported vacuum.

Just to confirm, have you checked to see that the choke cam is not preventing the throttle plates from closing further? (didn't look at your photo's yet)
Click to expand...
 
fdchappie said:
Robert, how much of an intake leak can there be, if any, if he's getting a vacuum reading of 18"???
Click to expand...

18" is not over the moon if you are using that as ammunition to reinforce your point. I've seen better on old high mileage 304's

so fd what is a carburetor? An air leak by definition!
When the air leak originated in the plenum is coupled with the proper amount of fuel it is no different that the idle being set and the engine will make normal manifold vacuum.
If you have an air leak close to an intake port the engine will miss fire and loose manifold vacuum.

I hope you can grasp this as I have no desire to argue with you while trying to help the op.
 
brandonium said:
Robert, here is what I just did. I can say with certainty that when I disassembled the carb I only removed the secondary throttle shafts, not the primary. I didn't think to check that. Would gassing that tell me if air is getting through via the shaft?
Click to expand...

I take it you have a nice smooth idle? That tells me that you have air being introduced at or near the carb. It it was anywhere else like the vacuum tree, booster or intake runner you would have an miss or lame cylinder and a vacuum gauge fluctuation which I take it you do not.

Have you removed the hose and blocked the pcv port?

See the photos that I added arrows to. That shows a significant air leak path and balanced.
When the throttle plated shut tight you should feel a little sticking. The afb's have some linkage that can hinder complete closing also.

photo%202.jpg

photo%201.jpg
 
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Robert, you are correct. The idle is very smooth and there is no stumble and roughness. The pcv valve is new along with the grommet and hose. I have the hose routed to the front of the carb, it used to be routed to the power brake connection in the rear. Brake booster is now connected to the #8 intake runner and my mani-vacuum is routed from the rear spreadbore/squarebore adapter which is below the throttle plates. Vacuum from the egr is running through a 2 port vacuum temp switch and from there to the ported vacuum on the drivers side.



Robert kenney said:
I take it you have a nice smooth idle? That tells me that you have air being introduced at or near the carb. It it was anywhere else like the vacuum tree, booster or intake runner you would have an miss or lame cylinder and a vacuum gauge fluctuation which I take it you do not.

Have you removed the hose and blocked the pcv port?

See the photos that I added arrows to. That shows a significant air leak path and balanced.
When the throttle plated shut tight you should feel a little sticking. The afb's have some linkage that can hinder complete closing also.

View attachment 26033

View attachment 26034
Click to expand...
 
Have you tried to see if disconnecting and blocking the booster and pcv helps to lower the idle?

I have to tell you that other than those two and the way you have them connected the only other option is to tighten up the leakage around the throttle plates.
 
Just throwing my 2 cents worth. Not saying there isn't a vacuum leak of any type but the vacuum advance should be hooked up to ported vacuum as fdchappie has already mentioned. If you're setting the base timing and idle speed and then hooking up the vacuum advance to manifold vacuum after those adjustments are made you are applying the vacuum advance at idle which will increase the rpm's.
 
Jeff I agree but :d
don't think the increase in idle when the advance is connected to constant vac vs timed is the op's concern but rather the fact that he can not lower is.

The idle speed should be adjustable to 0 or there is a considerable source of air getting to the engine from some where.
 
I would be less concerned it is true if I would be able to idle it down on one or both, but even on ported I'm out of adjustment at about 600/650 rpm. Like Robert mentioned (once again, I'm no mechanic) I was under the assumption that the curb-idle adjustment would let me idle it low enough to stall, or well beyond the point where I would typically set it. I feel that I might be lucky that if I wanted to let it go that I could get by with it this way, but something feels wrong.

I'm going to have another look at the throttle plates. Let me ask this. It appears that the transfer slots are almost obscured in the photos I posted. In reading as to configuring the throttle plates it's mentioned that it should show at least as a square, or .020/.030 of transfer slot. Is this a problem with my configuration? That would only affect off idle transition correct? And to adjust the throttle plates? Is it as simple as loosening the screws a bit and turning the throttle bracket to see how they position themselves in the bore, then retighten? I'm assuming I need to be aware of the transfer slots and that I don't cover them completely correct?
 
You don't have the ability to alter the transition slot location only the throttle angle. You need to loosen the throttle plate screws and see if you can get them to fit better. They need to shut tight on the bores perpendicular to the shafts.

I don't see where you ever answered my question, "have you tried to see if disconnecting and blocking the booster and pcv helps to lower the idle?"

I would appreciate it if you would... :icon_rotate:
 
Apologies Robert, I totally forgot to add that in. I will absolutely test it for you, it will probably be Friday or Saturday. Installed my dui this weekend and the ignition module failed on it so I am putting another one in this weekend, until then it won't run. Once I get her running again I will plug both and report back. :)

how about adding gasket sealant to the carb base and adapter plate? Right now I only have the primary gaskets but nothing else. It was snugged down but just curious.

Robert kenney said:
you don't have the ability to alter the transition slot location only the throttle angle. You need to loosen the throttle plate screws and see if you can get them to fit better. They need to shut tight on the bores perpendicular to the shafts.

I don't see where you ever answered my question, "have you tried to see if disconnecting and blocking the booster and pcv helps to lower the idle?"

I would appreciate it if you would... :icon_rotate:
Click to expand...
 
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