65air_coupe

SBC406, 10.7:1, Demon 750, Edlebrock Performer, XE288 hydraulic roller.

I'm having a problem setting the idle down to a reasonable level without closing off the transfer slots completely, creating a worse problem. If I set the throttle blades for correct transfer slot exposure (0.025"), the idle is 1200 rpm. I suspect a vacuum leak but it will idle reasonably smooth at 850 with about 15" of vacuum if I close the throttle down more so it can't be a significant leak.

If I disconnect and plug the line to the PCV and brake booster, the idle drops about 100-125 rpm. Re-connecting with the PCV only, the idle goes back up the same amount so it doesn't appear to be a leaky booster.

So just how much air does a properly functioning PCV allow into the system at idle? FWIW, it's a brand new valve.

My next step is too remove the carb insulator and just bolt the carb directly to the manifold with a single gasket just to eliminate any possible leaks in that area.

tbarb

A few questions, is the carburetor a vacuum secondary or double pumper. How much initial timing and how is the vacuum advance connected, ported or full manifold vacuum.

From your report it sounds like the PCV is good. The transfer slot on the primary can be set even with the bottom of the throttle blade at the 850rpm position.

If it's a double pumper set the secondary slot at the top of the blade position at the 850 rpm idle, if it's a vacuum secondary use the secondary stop screw and set the rear blades closed plus 1/8 turn on the screw just so they don't stick.

See if any of that helps and report back, If you get no joy here the primary's can be set closed further and the slot filed down even with the bottom of the blade.

You should be able to run approx 10* initial with 15* full manifold vacuum advance with a camshaft that makes 15" vacuum at 850 RPM and maybe slow it to 800RPM. No need to remove the insulator.

One last thing to remember, every time you change the throttle blade position at idle (idle speed screw), you should double check the A/F emulsion screws. These screws trim the IDLE speed A/F mixture and taper off as the blade goes up the slot.

buns

Does your Demon have the "Idle-Eze" ™ feature? If so, there is a screw down below the air cleaner stud hole for adjusting the idle.

65air_coupe

It's a mechanical double pumper, early model without the 'idle-eze' feature. Ported vacuum, B26 can, 12* initial, 36* total. Idle mixture adjusted with tach and vac gauge at whatever rpm.

And you're saying that a 100-125 increase in rpm when connecting the pcv is normal?

And I don't understand your statement "If it's a double pumper set the secondary slot at the top of the blade position at the 850 rpm idle". I can't open the secondaries at all or the rpm is even higher.

I wasn't able to bolt up the carb without the insulator as the primary accelerator pump housing and screws hit the manifold. My insulator looks pretty bad so I've ordered a new one anyway.

MikeM

The only test I've seen published is a 50 rpm drop between removing the valve from the engine and plugging it back in the crankcase.

My guess is you have a valve not calibrated for your application or a defective valve.

65air_coupe

Any idea how to do that considering PCV specs are non-existant?

All I've found is a general consensus that a valve with the number '6' stamped on the end of the plunger will work better for low vacuum engines but that's probably a manufacturer specific coding system and mine is stamped with an 'F'.

MikeM

Read this thread. There's lots of mystery about this little device.

http://www.chevytalk.org/fusionbb/sh...hp?tid/291749/

MikeM

Read this thread. There's lots of mystery about this little device.

http://www.chevytalk.org/fusionbb/sh...hp?tid/291749/

65air_coupe

Read all that before posting here, no help there.

Bought two different valves with no specs and will try to trace back their differences using Google and test them on the car.

MikeM

You might try playing with a fixed restrictor orfice instead of a variable flow valve.

If your idle mixture is way off, that could cause your idle speed to rise too much when you pull the valve out of the engine.

65tripleblack

Although "50-70" RPM drop is what's considered rule-of-thumb, I have my doubts about it and don't know how that range was determined. I always tell people that "about 100" RPM drop is about right so your 125 RPM drop doesn't seem far enough out of range to be causing your condition.

First, as you probably know, a 4BBL carb when properly adjusted has pri and sec throttle blades adjusted equally. Right now, yours are not...........a compensation for your high idle speed, I suspect.

IIWY, I'd start by eliminating the PCV valve temporarily by plugging the vacuum feed and keeping the crankcase open and vented. Then, adjust carb so that all 4 throttle blades are equally open.

Somehow, I don't think that this will solve your issue, but it will, at least, eliminate the PCV valve as a suspect.

I believe that either you have a moderate vacuum leak or your timing measurements are not accurate (too much spark advance). Do you know how to check your timing indexing with a piston stop tool? Did the engine builder use the right harmonic damper and timing cover?

65air_coupe

Although it may help in diagnosis, a fixed restrictor isn't going to do a good job at crankcase ventilation overall so I'm not going to go down that path for now. There's no reason to believe the mixtures are off, they've been set, re-set, and checked again. And the rpm increases with the PCV in place, not decreases.

That's good to know, hadn't heard those numbers before.

Exactly! I've had this discussion with the folks at BG shortly after installing this carb and being unable to properly set it up.

I've been down that road and can't there from here with regards to an optimum idle rpm and correct transfer slot opening.

I tend to agree with the vacuum leak suspicion although it can't be very significant or I'd have worse problems. I did replace a leaky vac advance can last month and was hoping that was the source of my problem but no dice so that's why I was casting suspicion on the PCV but if 100-125 isn't unusual, I'm looking in the wrong place.

I built the engine with the correct damper and used an adjustable timing indicator that I zeroed before installing the heads.

With these rpms being slightly over 1,000, I'm starting to see centrifugal advance come into play which further complicates things. I may back off my initial to around 10* and see if I can get a better idle speed without closing off the transfer slots.

And I'll test these various PCV's and see if I can get to less than 100 rpm variation with one in place.

tbarb

How old is the carburetor, did you purchase it new. I suggest removing the throttle body and look into a light with the blades closed to see how well they seal and also check the throttle body both sides for flatness. No chance you overlooked a rubber vacuum hose egg shaped causing a slight vacuum leak.

How low can you idle the engine down to using the speed screw.

MikeM

[QUOTE=65air_coupe;1587616187

And the rpm increases with the PCV in place, not decreases.

[/QUOTE]

That's odd.

65air_coupe

Carb was purchased new, used for a few months on my L79 then transplanted to this 406. It's been apart, cleaned and inspected although it was still relatively spotless. I had checked the blades once before when this whole "can't get the idle set" problem started.

It'll idle smoothly at 850, can go to 800 but gets a bit rough and close to stalling. Pretty much what I'd expect considering the cam and compression. The problem is that the transfer slots are completely closed at 850 and its lower throttle, off-idle performance suffers badly.

Mike, I initially said the rpm rises when the PCV is introduced but you and tripleblack have been talking about a drop in rpm and I didn't catch that. Why does rpm drop when you allow more air into the mix through the PCV? Isn't that analgous to opening the throttle blades slightly?

MikeM

I don't know what kind of PCV system you have. If you have a CLOSED system, meaning you have manifold vacuum on one side of the crankcase and the fresh air to feed the PCV system is coming from carb intake air then you should be running a constant, positive vacuum on your crankcase at all times while the engine is running.

If you are simply running a valve plugged into the engine somewhere and drawing fresh air from a valve cover breather or oil fill breather cap, then you do not have that vacuum on the crankcase, just air flow.

In the event you have a closed system, when you pull the PCV valve out of the system, you now have a small vacuum leak and that will speed the engine up if the carb is correctly adjusted.

I'm confused at this point if you're working on a PCV valve problem or a carburetor problem. Or both.

If it's both, I'd bypass the PCV system for now and get the carb straightened out.

tbarb

Approx how far above the blades are the transfer slots at the 850RPM idle. In your opinion, how long are these slots top to bottom.

With the pcv in play the idle will speed up, I think you are OK there but I do agree there may be valves that restrict air more with the pintle pulled from idle vacuum.

65air_coupe

The slots are about 0.250-0.280" long. At 850 rpm, the blade is just barely past the bottom of the slot. Thanks for confirming the increase in rpm with the valve in play. And yes, I agree, I think the valve is not my problem although the slight increase in air flow through it may contribute to another actual leak.

If you re-read my first post, you'll see I'm dealing with a carb problem and was suspicious of the PCV as either a source or contributor. I still haven't heard how to 'calibrate' it to the system as you suggest.

And I don't follow your logic regarding open or closed systems. It's closed but as far as the vacuum in the intake manifold is concerned, it doesn't care where the fresh air is drawn from. And a PCV is a metered leak. I can't see how removal of it and plugging the line constitutes a "small vacuum leak".

In any case, when I removed the carb this last time, I noticed a threaded brass plug screwed into a tapped hole in the manifold just below the secondary bowl. And while it appearred to be tight, it also felt like it was bottomed out rather than tight in the threads. I pulled it out, added some thread sealant and re-installed it. Tomorrow I'll bolt the carb back up and see if there's any difference. I'll also be cranking back on my initial timing and playing with the mix of valves I collected.

tbarb

While the carburetor is off the engine, with the throttle blades closed completely does the bottom of the transfer slot lone up with the top of the closed throttle blade.

Also, take a look at the light around the closed blades and if you are certain they are adjusted good it may be time to make the slots a little longer or remove the throttle blades one at a time and file a 45* edge where the slot is to expose the end of the slot at the 850RPM position. Changing blades with new ones is no big deal so consider this.



The PCV is a calibrated vacuum leak and does a V/G job of cleaning the blow by from the engine, I don't like the idea of a fixed orifice so try to tune with the valve.

Did you have this problem when the carburetor was installed on the L-79? Can you post the specs on the camshaft.

MikeM

It's not "my logic" explaining how the system works. That's the way they're designed.

I can't tell you how to calibrate a valve for your engine and don't know anyone who can. That's why I included the link and also suggested if you can't find a valve calibrated to suit your needs that you use a fixed orfice and size it to your needs.

In the meantime, if you have a closed system, you should be able to remove the oil fill cap with the engine running and vacuum should hold the card in place.

If you have a valve cover installed PCV and you pull it straight out of the valve cover in a vertical position, engine speed should increase. Plug it back in the valve cover (or stick your finger over the end) and engine speed should decrease.