I have a 1966 Toronado with a brake booster rebuilt locally. I seemed to have this same issue before the rebuild, which is why I took it in to be rebuilt in the first place.

When I am sitting at an idle, pumping the brake pedal causes an audible change to the idle. When I drop the transmission into drive and pump the brakes a little bit, I can just about get the engine to die. It takes a couple seconds after releasing the brake pedal for the idle to pick up and smooth out. Did I receive a faulty rebuild? I can't imagine this is normal behavior. I know that a brake booster will take on vacuum during each pedal application, but it couldn't cause the idle to slow down and rough out so much, right?

I don't know if this issue is related, but the car dies out when I get to stop signs, and I can save it by shifting into neutral, sometimes with a little gas. The engine was rebuilt, and the carb was rebuilt from a new to me quadrajet, which is in much better shape than my old one, so I don't know whether this is a carburetor issue or related to my brake booster issue. It didn't die like this with the last carb, and the same booster was on, so I guess it could be carb related. I double checked everything during my carb rebuild and had to lower my float a little bit after install, which didn't fix anything. Raising the idle speed seems to help a little bit, but it's really just a band-aid to raise the idle speed. I can't just raise the idle to 1,000+ rpm in Park.

Carbruetor has the fuel bowl insert in, power piston spring present, accelerator pump checkball in place, choke is holding wide open when car is warmed up. New rebuild w/ needle and seat, float, gaskets, accelerator pump, the whole nine yards. Cleaned carb body and made sure passages were clear. Fuel pump is new as of a couple years ago, never had this issue before this rebuilt carb/rebuilt engine, so I can't imagine that's my problem.

Current setup is stock jets, stock rods, float level a little below correct setting, idle mixture screws tuned to highest rpm, initial timing at 12 degrees (a little higher than stock, but I'm running a different dizzy and giving a little more total advance.) Vacuum advance hooked up to manifold vacuum (factory setting on my '66 Toronado). Idle speed 600 rpm in Drive, 800 rpm in Park. Air valve spring wrap is at correct setting of 3/4 turn past spring contact.

I do get a little bit of fuel seepage out of the air horn gasket. I know that fuel will wick through that gasket, so it doesn't concern me too much, but I will sometimes see the tiniest bit of fuel on the top of the accelerator pump well on the air horn. But not all the time, and mostly after a drive. Maybe I have a tiny bit of a leak in my needle/seat. But wouldn't it also flood out at an idle if that was the issue? It will sit and idle forever, but it's just when I come to a stop that my idle will stumble, and the harder I stop the more it will stumble/stall out.


Sorry for the rant, I'm partially thinking out loud here, and thanks for any and all help and advice!

 

Maybe the boosters check valve ?

Mine went after a couple of years, the car was a bear to drive. In my case seemed to be a wonky aftermarket valve, that was installed with the conversion kit just prior to my ownership. Replaced with a factory unit..., and problem solved.

Venting is a good thing, it keeps us from blowing our top...

 

The check valve seems to be functioning. I went out to the car, and as I tried to wiggle the valve out of the grommet, I heard hissing coming from the booster. It seems to be holding vacuum properly.

 

Sounds like your booster has issues. Contact the rebuilder.

 

I guess I should have written two different posts. To clarify, these brake booster issues were present before my stalling issue came about with the new carb, so I would be inclined to consider these two issues independent from each other.

 

I might just do that, I wonder if there's a valve or diaphragm inside that didn't go in right.

 

Ok. I can't recall how obvious the culprit was, it's been like 9 years. But if I recall correctly it was a bit of a mystery at first. And when it was replaced the problem was solved. In other words the check valve was replaced as a maybe, that turned out to be correct.

My booster finally went bad last year but did not share your symptoms. It was not really altering the engines idle, and our idle park/drive rpms are similar. In my case the booster was audible leaking, you could hear the prominent hissing. That got worse and worse over time. And it fought me pedal wise, it would pull so hard it would depress the pedal on its own. To the point where the brake lights came on by themselves. I would have to put my foot under it to fight it back up. One time my tail lights stayed on after parking down and drained my battery. Just before I had booster replaced I had to use an oversized car wash sponge to hold up brake pedal after parking.

I would still consider Oldcutlasses advice. As the rebuilder may be able to point you in the right direction.

Also maybe a Chilton Guide if not a CSM. I remember they would have an issues symptoms checklist, go down the list till problem is solved.

 

The easiest way to troubleshoot this is to disconnect and plug the line to the booster and see if the engine runs better.

 

Plug the vacuum line to the booster and drive around without power brakes. If the problem is the same, its not the booster. Do this before going back to your booster guy.

 

To clarify, unplug on the engine side and put a vacuum cap over the barb. Vacuum lines themselves can also go bad. Don't stick things in vacuum lines to plug them unless you like them not fitting the barb tightly any more.

If you have a weak idle, you may be lean and cranked up on idle screw to get right idle. I would check idle fuel mixture.

 

There are many here more knowledgeable than me, but if it were my car (I have a couple of ‘66 big cars), I’d try to isolate whether the booster is the issue.

First - it’s the thing that’s changed most recently. Next, if it’s leaking vacuum, aka causing an engine vacuum leak, it’ll likely be a _big_ one. So cap it off to separate it from the engine and see if the engine works or sounds different.

Another way to go is to cap off all vacuum accessories to the engine to confirm the engine is sound. Then add ‘em back one by one to see if things change. If they change in a negative way, well, that’s something to chase down. ~60 years later, all kinds of rubber hoses could be old & leaking vacuum…

Yet another way to go is to simply replace _all_ the 60 year old vacuum lines to be sure what you can see is sound. This is the opposite of diagnosis, but may solve today’s problem and help avoid future problems.

Personally, I’d suspect the local booster rebuild. It was a tricky high surface area (therefore high leak potential) job 60 years ago. And it’s not like auto techs today are being taught how to rebuilt 60 year old brake boosters - they’re just too rare to be profitable. So you may have found a bold person who wanted to take a shot at it and didn’t quite get there. Not for lack of effort, but more due to how hard/rare the job of redoing the booster really is.

If I were in your shoes, I’d pull the booster and have it confirmed to be vacuum sound on the bench. Then at least that’s eliminated as a problem

Hope that helps
Chris

 

Good advice already provided. Use a vacuum gauge to measure engine vacuum to establish a baseline of engine vacuum @ idle. You should be able to achieve a STEADY vacuum between 15"Hg - 22"Hg.

Perform a good tune-up based on instructions in the 1966 CSM. After establishing proper dwell & timing, dial-in your carburetor idle adjustment screws individually (one at a time) using the vacuum gauge. You're striving to achieve the highest steady vacuum possible.

 

I've already gone through that process, a couple times actually. I'm at 4400 ft elevation and pull about 16 inches of vacuum in park, very steady reading. I used both a vacuum gauge and a tach. Do I need to make the adjustments in drive?

 

No, you don't need to make adjustments to A/F idle mixture screws while in drive, only in stopped/Park position. Only adjustment, if one is necessary, would be to place vehicle in drive to adjust low idle adjustment stop to correct RPM as per the CSM which sounds as though you have done correctly as you state in your 1st post "Idle speed 600 rpm in Drive, 800 rpm in Park". 16"Hg vacuum @ idle is a good vacuum. As has already been suggested, re-evaluate the brake booster beginning w/ most likely a replacement of the check valve (least expensive & easiest) then move on to brake booster itself, which Eric and yourself have discussed. Diaphragm inside brake booster may be bad, not seated, whatever but you have a good vacuum (16"Hg) @ idle.

It should go w/o saying ensure the vacuum line to the brake booster is solid, no cracks, etc. replace if not sure.

 

Tony (video) is a good old-fashioned mechanic. You might have a listen.

Since these cars had an ignition timing curve established by the factory for normal street usage @ MSL (Mean Sea Level), consider advancing your timing a couple degrees to see if that might make a difference. Timing chains wear out & get sloppy w/ years + mileage, add to it an increase in elevation for an older engine. You might find a bump (advance) in initial timing could help.

 

"Generally" bump/advance the initial timing ~1° for every 1,000 ft in elevation.

 

Drive?

The engine is brand new, rebuilt with maybe a mile on it. Everything went fantastic during cam break in. Factory timing spec is 7.5 degrees and I have it set to about 12 degrees. I don't want to go too much higher because I don't want too much total advance. Followed the correct tuneup in the factory service manual.

 

Is there a question here?

 

Sorry, I don't know what happened there, there is no question haha

 

Fwiw, the place that rebuilt my booster is a trucking place that sells parts and accessories, relines brake shoes, rebuilds semi truck brake chambers, etc.

 

In the original post you state that you did not have this issue with the "old" carburetor and prior to the booster rebuild. Since you have had the same issue both before and after having the booster rebuilt, have you tried putting the old carburetor on to eliminate the "new to me" rebuilt carburetor as a possible factor here?

 

I had to use parts from my old carburetor to repair and rebuild by current carburetor, so not an easy option. My old airhorn/bowl/base plate is in a lot rougher shape than the one I am currently running, too.

To all of you guys, one thing I neglected to check during the rebuild was the well plugs. They already had epoxy on them from a previous rebuild, so I thought they would be okay, but after finding an external base plate leak, I took the carb off and removed the base plate and found my well plugs were leaking pretty good. The float bowl was leaking down overnight. I will repair tonight with Marine-Tex, and hope that my issues are resulting from that leak causing a rich stall when coming to stops due to increased vacuum and low air flow. I will keep you guys posted.

 

Well, same issue is happening with well plugs epoxied. Seems like it's flooding out. Float is new, needle and seat are new. Double checked everything when I was epoxying the well plugs. There exists a little bit of fuel on the airhorn where the accelerator pump comes through, which usually indicates a flooding issue. Float height is set at 5/16, which is spec for my carb number, checked against old Delco literature for my particular carburetor number. Seems to be worse when the car is warm.

 

It might be time to evaluate what is meant by a "different dizzy". Can you specifically identify what distributor & ignition system you have installed, please? Electronic (HEI?) ignition or contact point ignition? You're certain the distributor is correctly matched to your ignition system? There are differences. Curious if you have some uneven distribution of power.

 

In the car currently is a two year old Pertronix Ignitor II distributor wired to full 12v switched power. 11 degrees initial advance, 24 degrees mechanical advance for a total of 35 degrees all in @ 3,500 rpm. Hooked up to manifold vacuum (correct location according to oldsmobile factory assembly manual.)

Factory distributor is 7.5 degrees initial, 20 degrees mechanical advance for a total of 27.5 degrees all in @ 4,200 rpm, also on manifold vacuum, of course.

Stock replacement plug wires, resistor plugs (R43S), none of that fancy "low impedance race" crap. Gapped at either 0.030" or 0.035".

The coil is about the same age as the distributor and I chose a part number that has an internal resistor due to me bypassing the resistance wire in the harness.

Literally the only thing that has changed to cause these problems is a new carb and a rebuilt engine. The machine shop was thorough and reputable, familiar with Oldsmobile engines. I called them a few times during the rebuild just touching base to make sure they were knowledgeable about oil gallery plug location, different lifter bank angle and lifter diameter, etc etc and they seemed familiar and confident every time I spoke with them. They took care of things wonderfully.

The plugs read rich presumably because of the loading up/flooding that is happening, and I don't see the point of changing them, as new ones will get the same way after a drive.

I checked for and didn't notice any fuel dripping from the boosters at an idle.

There is a film of gasoline on the passenger side of the base plate near the secondary throttle shaft, a place where fuel never should exist. From my research, the only way it's possible to get fuel there is if the carb is dripping from the secondaries somehow, or the carb is cracked or porous. I feel like I've ruled out the bottom fuel wells because of the marine tex epoxy I applied.

 

I'll admit im grasping at straws here, but I bought a new float and needle/seat from Cliff Ruggles and maybe it'll make a difference. I'll install and test, and I'll see if I can re-tap the accelerator pump check-ball just in case that's causing a weird issue.

 

I believe you're convincing yourself it remains a carburetor issue - and, that may be correct.

However, I'd appreciate it if you could answer a question for me which I don't understand and one which I'd like to understand better.

You have a Pertronix Ignitor II wired to full 12V switched. Normally, I thought you'd use a Pertronix Flame Thrower IGN coil? You stated you disconnected the OEM resistance wire, but you bought (manufacturer unknown) an IGN coil w/ a built-in resistor? Why do that? I'm probably out-of-line w/ my understanding but I thought the Pertronix Ignitor II required a full 12V? Why install an IGN coil with a built-in resistor if it requires a full 12V? Seems that defeats the notion that the Pertronix requires a full 12V doesn't it? I'm lost & confused (as usual). If Pertronix Ignitor II requires full 12V, why would you impede this voltage by using a resistor? I'm learning.....

 

Getting late (for me). I'll make an attempt to validate why I'm asking questions regarding ignition system.

(1) In Park (no load) I don't believe you stated this (maybe you did), but I think the vehicle idles fine;
(2) I've read engine rebuild, carb. rebuild, plugs, wires, vacuum all good;
(3) Recently you noted plugs appear to be fouled or at least "...plugs read rich presumably because of the loading up/flooding that is happening, and I don't see the point of changing them, as new ones will get the same way after a drive.";

Low ignition coil voltage can cause spark plugs to foul if the ignition coil isn't delivering enough voltage to adequately fire the plugs w/ complete combustion. Un-burned fuel pushed into the engine will result in fouled spark plugs from incomplete combustion. I'll note again, I think this might be particular to your case where it appears evident under load - vehicle moving (engine under load) application of brake(s) while under load could be insufficient fuel combustion (fouled spark plugs). I noted I believe you said you had to feather the brake pedal &/or accelerator pedal? I mean it's fine to measure vacuum at idle and be convinced it's a good vacuum reading (16"Hg) but what is the true vacuum reading under load while coming to a complete stop?

I think some electronic ignition manufacturers specifically state you should match the ignition coil to the electronic ignition distributor module(s) - yes? Just a thought.

 

I have read all the responses, and the one thing that no one picked up on is that you don't know much about the carburetor, other than you took parts from the original and installed them on the replacement carburetor. Mixing parts isn't a good idea unless you are extremely familiar with the components and their compatibility from one carburetor to another. Here is a website that might help you ID your carburetors. https://cliffshighperformance.com/ar...ber-chart-1938 . I grew up learning carburetors, and when it came to the quadrajet carburetor, I remember best that they all look alike, but on the inside, they are almost all different. That is the one carburetor that the mechanics of the past almost always disliked because they were very complex to rebuild correctly. Suppose you are at the end of your rope and need help with the carburetor. In that case, I can recommend Bruce at Carburetors Unlimited in Peoria, Arizona: Carburetors Unlimited, 7942 W Gold Dust Ave #1, Peoria, AZ 85345. 623-388-0990 You could send yours to him, and it will return better functionally than a NOS unit in the box!
Another thing I learned about the Quadrajet carburetor is that when you tighten the wing nut on the air cleaner, it should only be tightened 1/4=1/2 turn after it contacts the top of the air cleaner. This is because the threaded shaft only goes down the center of the carburetor a short distance, and if the wing nut is turned too tightly, it will distort the upper casting of the carburetor.
Also, check the intake manifold for possible leaks using starting fluid in short bursts aimed at the intake where it contacts the engine block. If the engine picks up speed, the intake manifold gasket leaks, or the intake manifold is cracked. Be careful when using starting fluid near anything that can spark, since it is pretty volatile. You can also check the carburetor base gasket using the starting fluid, but only when the air cleaner is installed, since you don't want the starting fluid to influence the engine running by being injected at the top of the carburetor.
If the car has vacuum lines to open heating and air conditioning ducts, pinch off the line at the intake manifold to eliminate the possibility that one of the rubber hoses under the dash might be leaking. Vacuum lines in a 60+ year old car can cause you a lot of grief if they develop cracks from age.

 

I did quite a bit of extensive research into only sourcing cores by the same application, and sourcing parts by referencing factory Delco/Rochester literature and parts lists. I found a nice AC Delco document from 1971 that covers Oldsmobile models between 1963 and 1969 and gives exploded views as well as application listing, carb casting number, and parts lists for each casting number with GM/Delco part numbers. Everythng down to clips, roll pins, and baffles. That has been a great help to make sure that what I am putting in is compatible with the casting I am using. Having two editions of the Oldsmobile factory parts catalog and online resources for parts interchanges and supersessions helps out so so much as well. That is why I was so confused on it not performing correctly. I appreciate the information for Bruce, it's always so helpful to have info for people like that. Part of the reason I love forums, they being the best people together.

The happy ending is that I ended up fixing the problem earlier tonight. I keep a box of Quadrajet parts (float bowls, air horns, throttle bodies, jets, rods, accelerator pumps, power pistons, screws, hot idle compensators, etc etc etc.) and put together another carburetor. I noticed while I had the suspect one off the car, that the base plate is warped. I must have missed that during the rebuild, oh well. I got the "new" new one assembled in a few hours, slapped it on the car, and my drivability issues went away. I don't know whether it was the warped base plate alone or porosity in the casting or something else I missed, but I guess it doesn't matter at this point. I'm beyond happy I can drive the car without worrying about coming to stops. Now I can focus on putting miles on it to break the new motor in.

Sorry to all of you that this hasn't really come to a definite conclusion on what was wrong, I always hope it does in hopes that it helps someone else out with similar issues. I appreciate all of your responses.

 

Happy you're happy. Nice to be back on the road.

 

Sorry for not responding, I missed your post. My understanding with ignition coils is that when you take a coil made for use with a ballast resistor, and you put full 12v to it, it will cause the coil to heat up more and possibly cause premature failure. My thinking/research honestly could be flawed on that, but it at least made sense in my head, so because I am feeding it with full 12v, I wanted to get a coil that was rated for 12v. Manufacturers basically install an internal ballast resistor when they know there will not be an external one present. This whole theory is pretty irrelevant after the mid-70s, as almost everything by then had moved to electronic ignition. When you look at stock coil specs for the older cars with points, you will see that they are like 6 or 9 volt rated or something like that.

If I kept the resistance wire, I could still connect that to the stock type coil, but I would still have to grab full 12v to send to the distributor from another wire or through a switched relay.

It looks like you are thinking that the distributor receives its power through the coil's internal resistor when that is not the case. The resistor is internal to the coil and is meant for a non-resistor positive feed.

Pertronix only gives a minimum resistance rating for ignition coils to use, but they do not specify that you have to use their brand.

Hope that makes sense, I am terrible at explaining stuff sometimes lol

 

Hey all, I think I may have found the source of the problem. I am inclined to believe that I was missing some sort of seal between my brake booster and master cylinder, causing a vacuum leak which became more pronounced when the brake pedal was pressed due to brake booster pushrod movement. After poring through the factory parts catalog, I found what I believed to be the correct seal, but with that part being discontinued as well as not having any sort of visual/picture online, I could not confirm 100% whether or not there needed to be a seal. I noticed the parts catalog more clearly define a seal on the Delco-Moraine boosters between the master cylinder and brake booster, but there was more vague information on the Bendix setup like I have. I bought a generic o-ring from the auto parts store that fit in the circular indentation on the back of the master cylinder where it bolts to the brake booster, and when I got a vacuum gauge on it later in the day I noticed that I gained an inch and a half of idle vacuum and was able to turn my idle mixture screws leaner than what they were. It seems to stumble and rough up quite a bit less than before. There is still an idle change/what seems to be a little stumble when I pump the brakes at a standstill in drive, more noticeable with the air conditioning on, but I feel like some sort of idle quality change is to be expected because of the booster using up and then purging vacuum as it is being repeatedly pumped. Oddly enough, and I attribute some of this to placebo effect, it seemed like it drove better (significantly improved off idle hesitation, more or less gone), and the power brakes were a little more responsive than before. Only time will tell, but just wanted to update you guys. Thanks for all the help and advice!

 

Hopefully, you've addressed the issue & you have taken the appropriate action(s) to remediate the issue. Nice work on perseverance. I hope it plays out. Good Luck.