Buick 455 idle questions

Very interesting read!

I got a new disk with magnets for the Pertronix system yesterday. Trued out the centrifugal advance plate where it is mounted, set all clearences and boy oh boy does it run better! Which is rather expected when going from 7 to 8 cylinders, but still Ignition is way more consistent, timing between cylinder 1 and cylinder 6 is at least within 2 degrees from one another, on a quick checkup. Manifold vacuum is up from 15 in-hg to 18-19 in-hg, measured using the same tool as before. Still idle is a bit rough and too high though. So more questions:

1. How should the mixture screws act over their whole range on this engine? In general with single point adjustments a good sign is if the extremes of the adjustments yield some undesirable results and an optimum would be somewhere in between the extremes. Should I expect that on this system? We expect the engine to stall when a mixture screw is fully closed, but do we also expect the engine to clearly run worse when it is fully opened? Worse when compared to some point in between fully closed and fully opened then. This is what I expect from smaller engines I have experience with, but car engines is a rather new experience for me. I am thinking I have an air leak otherwise, if the engine still "wants" more fuel.
2. I do get occasional firing through the carb when revving a bit and dropping the throttle to closed What would you guess would be the reason for that, that it leans out a bit during the transient phase where the throttle is dropped? Is is something to be expected from the 455?

 

The idle circuit is pretty simple and as you stated, and you have the "logic" correct. (And, no, you should not expect or accept your engine to run like it does, it's fixable.)

Mixture screws all in, no air so no fuel is drawn in through the idle circuit, as you open them equally about 2 turns out, then adjust to highest vacuum reading, (a little back and forth on both screws) then about a 1/4 turn in on both. (engine warm, timing correct, and vacuum gage on manifold, not ported on carb).

Idle fuel is "pulled in by vacuum" when the idle mixture screws are opened and allow the fuel to be drawn in the slots where the primary throttle plates seal against the bores and proper mix for idle. As soon as the front throttle plates open, the idle circuit is no longer "in play" and the primary metering rods begin moving and signal (air flow/negative pressure) pulls fuel.

If you turn them out too much (with no vacuum leaks, and proper working carb) it will be too rich (high negative pressure air flow) and can cause hard starting, rough idle and inconstant vacuum at idle.

If you have vacuum leak, then it can be very hard to adjust the idle air/fuel. You often get rich/lean (surging/hunting idle) and just a very unhappy car at idle and off idle tip in.

If any work has been done, intake, cam, heads, it can lead to a carb that is not in tune with those changes over the range of operation and be very difficult to "tune out" without changing jets/metering rods to deal with the effects of the changes.

Those can cause symptoms that mimic vacuum leaks, timing/vacuum advance issues, or several issues with a maladjusted carb.
Bent external rods that control choke pull off, secondary air valves etc., loose bell cranks, shafts, throttle shafts, warped air horn, and such can cause all manner of symptoms of lean/rich conditions that often seem contradictory.

But you are on the right track, dealing with one thing at a time and eliminating possibilities one at a time.
Change one thing at a time and test everything and make notes.

Older cars have often had so many things done incorrectly before you ever got your hands on it, and it takes some time to untangle the mess.

 

Thank you for sharing those experiences! Continuing to dig, lets see if I can get where I want

 

Flushing the cooling system today and found that the thermostat is stamped at 160 fahrenehit, while stock it should be 190 fahrenheit. I understand racers experiment with lower temperature thermostats for several reasons. But I guess that for my street car purposes there is no reason at all to not go with the stock opening temperature of 190 fahrenheit on this 1970 Buick 455 engine?

 

It seems indeed that 190F thermostats are off the market. I guess that a lower temperature thermostat can allow for a lower radiator pressure as well which might be beneficial on some aged cooling system. I guess I'll buy a 180 and 195F thermostat and see how it goes.

I started to work on the car again and I cannot get it to run well, especially not at idle. Among other things I think I have a vacuum leak as I have not been able to stall the engine by unscrewing the low idle screw. I have disconnected and plugged all vacuum hoses but the one for the carburetor vacuum pots. I did leakage test the vacuum pots before assembling the carburetor. The high idle is fully deactivated. I cannot find any binding in the throttle linkage. I went over the intake and carb with some propane but did not find anything. I am thinking, maybe the throttle blades are a bit wind or something? I put them in at the exact same place they were before rebushing, having indexed them before disassembly. I aligned them as well as I could but there are some tiny gaps (0.05 mm maybe) at certain points around the circumference of the primary blades. Picture attached below. Seems very unlikely in my world that a 7.5 liter engine would not stall on such a small gap though?

Maybe the PCV valve? I bought a PCV valve looking exactly like the one mounted previously at an American car parts store. But maybe these are easy to get wrong? I cannot find many options for PCV valves online so I am under the impression that there is a one-size-fits-all PCV valve for older GM cars but yeah... Intake gasket blown on the side of the intake facing the crankcase, so it does not react on propane?

 

With the blade screws slightly loose, so the blades will "float", adjust the curb idle screw and choke so that the plates will move the fully closed position and tap the carb body and throttle shafts for the primaries and secondary's to get them to "settle" in place, then tighten the screws, and check the blades again using a light from one side of the plates while looking at them from the other side so see if they appear correct.

 

Thermostat temperature has little to do with pressure in the radiator. Any reasonable engine temperature will be enough to cause the pressure cap to release some pressure, until the system stabilizes. The rad cap sets radiator pressure.

Many Q-Jet carburetors have an idle-air bypass system. Air is routed around the throttle blades so the blades can be closed more than they would be otherwise. This allows the idle and off-idle fuel circuits to work better than if the throttle blades were opened farther.

Does your carb have an idle-air bypass?

How about a hot-idle compensator that isn't sealing properly? Another easily-missed air entry point for some Q-Jets.

Do the REAR throttle blades close all the way like they should? I've seen rear throttles that are held partly-open for a variety of reasons.

PCV valves are kinda hateful. There's no source of info describing the pressures they're designed to react to, or the CFM of their flow. There used to be about a hundred PCV part numbers; and I think a lot of them have been discontinued. Best you can do is to find a PCV valve from a similar-cubic-inch engine that came with a similar-grind camshaft to what you're using...and hope for the best. That, or buy the mega-expensive adjustable PCV valve.

If you have a vacuum leak due to the intake gasket leaking into the crankcase/lifter valley area, you'll have substantial vacuum in the valve covers even with the PCV valve removed from the crankcase. Blow propane or carb cleaner spray into the crankcase, see if the idle changes--AFTER you remove the PCV valve, and PCV fresh-air tube from the air cleaner.

 

I tried to do this, but I was not able to achieve a better seal than what you could see in the picture above.

Yes, what I meant is that e.g. a 195F thermostat makes the coolant operate pretty close to the coolant boiling point at atmospheric pressure. So we have to raise the system pressure a tad to have some operational margin. With e.g. a 160F thermostat there is already quiet some margin, and I guess we could choose a cap with a lower opening pressure if we wanted?

Did not know that! Consulted the chassis service manual and this seems to be the case.

This carburetor has a system that mechanically raises the idle a bit when the choke is active. However this is external to the carburetor and operates directly on the throttle blade shaft. Is this what you mean?

I think so, but it is a bit hard to see with the carburetor mounted. If I manually push the air doors open at idle, the RPM raises slightly. Could that be an indication of leakage? I am thinking that unless there is some bypass here too the system should probably be invariant to the air door position when the secondaries are closed.

Hmm I see :/ I did not even find a part number in the chassis service manual and all PCV valves I find online are for Buick GS, which are hot cammed. Today I will try to temporarily seal the PCV connection. If I still cannot stall it, I know there is some leak elsewhere at least.

That sounds it could to the trick! A bit worried the propane would ignite inside the crankcase though.

 

The blades should have a number stamped on them,(on the top side) and you may be able to find replacements for them.
I had to do that on a nailhead carb years ago. The bores and the area the blades seat against were fine, but the blades were not seating uniformly.
The new blades fixed it for me. If the shaft the blades are on is bent/warped, it can also cause seating issues.

 

There's NO reason to lower the radiator cap pressure in a healthy cooling system. The more pressure, the more efficiently the pump works, and the higher the temp can go without boiling.

Putting in a low-pressure rad cap is only useful for folks who know they have corrosion problems, or faulty hoses. The better solution is to fix the cooling system, and keep the high-pressure cap.

There's a reason for the engine not stalling with the curb idle speed screw turned out.

No. The fast-idle cam system works when the engine is cold. The hot-idle compensator works when the engine is overly-warm. If the hot idle compensator wasn't sealing properly, you'd have a vacuum leak that wouldn't be affected by throttle position.

Usually on the very rear of the carb, under a tall-narrow rectangular cover (if it exists on that carb at all.) Underneath the cover, there's a bi-metallic leaf spring that uncovers/unseals a vacuum port when hot.

I'd look into the air-door idle change. Doesn't seem right to me.

 

Yes, it sounds sane to run at rated pressure. I was just thinking, as they put such a low temperature thermostat in on my engine, maybe they also reduced the pressure because they could not make it leak free otherwise.

Now I see what you mean by hot idle compensation. On Buick 1970 there is a system that enables ignition vacuum advance at a certain threshold temperature. This sits on the front of the 1970 engine. I have kept that system disconnected. On a side-note, I wonder why Buick choose to enable vacuum advance only when overheated. They state that it is for emission regulations compliance but generally it seems the ignition curve enabled by a vacuum advance system yields higher efficiency which should be positive for emissions. Some people online speculate is is because it could reduce specifically NOx emissions.

Hmm yes, it cannot be ruled out! This is what the blades looked liked before I assembled the carburetor and put it back on the car again. Can you see anything out of order in the picture?

 

I can't see anything, but it is hard to capture with a camera sometimes.

As I said, loosen them, then with all adjustments so the blades are in the fully closed position and tap on the shafts/body will help the plates "settle" and then carefully tighten the screws. If you have done that and there is still "gaps" or areas of light, and a carb cannot be properly adjusted for idle /air circuit and curb idle with a vacuum gage, then the next step is to replace the blades. (the only other problem could be bent shaft or wear/damage in the bores themselves where the blades seat.

They are like a properly sharpened cutting blade, angle cut/ground to the edge of the bore and proper diameter mating all around the circumference.

Carb removed and shining a light through may help.

I think that the blades can erode a bit over time or maybe damaged by people messing with/rebuilding the carbs and not taking care of the blade edges, or damaging the bores with tools/scraping or wire brushes.

There are a lot of things that can become maladjusted and make a good carb run/operate very badly, so having good understanding of proper operation and proper adjustment is paramount.

 

The vacuum advance should work in high gear, AND if the engine is excessively warm.

 

The extra advance kicks in when hot to increase the idle RPM and therefore the cooling efficiency of the pump/radiator system.

 

What about the idle mixture screws?? Tune them to get Max vacuum readings in gear

 

Yes indeed! I was more referring to why they made the design choice that they did. If ignition advance at low intake manifold overall seems to improve performance it *probably* is beneficial for efficiency. And better efficiency should imply lower emissions. But they choose not to utilize the advance curve influenced by intake vacuum anyway for most parts, which puzzles me a bit.

Some updates. I couldn't get the engine to crank well. After excluding battery issues, I took off the starter. It had a damage on the housing, probably from some previous tampering, a fleece of metal chopped off at the main part of the housing. Starter had a lot of oil and sand in it that probably intruded from the damage to the housing. Don't think that was the main issue though, the wires to the rotor had their insulation damaged. So I do suspect part of the rotor supply current was shorted. Bought a factory rebuilt starter and it spins like a fury again.

Today I ran a few tests. I have some issues with the engine wire harness I think. It know insulation on some parts is weak and today the overheating indicators on the dash triggered without the engine being hot. The "Stop Engine" light is flashing slowly when triggered which I think was kind of rad. But I am going to redo parts of the engine wire harness, ordered parts for it.

Otherwise, I did not see any change in idle when opening the air doors today. Verified with my ears, vacuum gauge and an RPM meter. Also, wish I had watched this video before rebuilding the quadrajet: According to the video, tolerances are not at all as tight for the secondary throttle as for the primary. I rebushed the secondaries because they felt slack, but it seems they were slack already from factory.

I have had an issue were the engine stalls when warm. Then after a cooldown, it runs again. It really beats me what it is! Guess it could be a vapor lock. But that implies to me that that fuel is supply is disrupted, but ignition should be fine. However, it wouldn't even start on starter fluid (ether based) when warm. In my experience with smaller one cylinder engines, if ignition is fine the engine will almost always react to starter fluid. Also, when warm, I tested for spark and it seemed fine. Then today after cooldown, suddenly it starts up and this time runs alright even after it is warm. Can't really say I changed something significant. The machine just acts inconsistently and I cannot wrap my head around what the issue(s) might be... As it ignores starting fluid, sometimes I am thinking that is the ignition. I have the Pertronix I unit on. I know the distributor has a shaft with too much play both axially and radially. Which we don't want, and this will be fixed. But I cannot clearly see how this would create issues when the engine is warm, specifically. Oh well, lets fix the engine wire harness and the distributor and see were we end up.

 

I'd start looking at the coil. Test the primary and secondary windings for resistance, but even then I'd try a known good coil and see if the problem still exists. Sometimes I've found a coil can test fine but temporarily fail when hot.