Vistabrat72

Hello to all! My 1972 Rocket 350 with the (now correct) 7A heads has a carryover problem that appeared a while ago before these new ones were installed. Heads were replaced due to severe valve seat recession due to what was diagnosed as improper valve seat grinding. Past history. The car starts, idles smooth as silk, not a hint of a shake, runs beautifully. It is very strong and does not miss a beat under heavy acceleration or higher speeds. The problem lies here: Under a very slight steady throttle, say 1/8ths or so, there is an annoying part throttle miss, very erratic, when the car is in forth (overdrive) and the converter locked up. It feels like an irregular jerking feeling, and it goes away at once if I back off or just press a touch harder upon the throttle. Spark plugs are all new AC-Delco R45S units, the old ones looked fine, but at 25,000 miles on them and with the new heads being put on, I figured maybe one or two may be fouled or shorted inside. The wires are a few years old, in good condition, even with heat socks on them, and the ignition system is a Petronix points replacement style. The vacuum advance appears to be in proper order, no leaks, hooked up to a ported source, and the mechanical weights have been services and work freely. Am I correct at feeling a weak part in the ignition would really only manifest itself under heavy throttle when more KV's are needed? Could it be the metering rods/spring not reacting to a transition in throttle or something? The jets in the Q-jet are #72's if I remember correctly. Thanking the forum in advance for any help they can give for this, hopefully for somebody, simple solution. Regards, Howie

Rickman48

Bet it's the plug wires!
At night, spray them with water in a spray bottle, and look for arcing.
Might hear it too!

Vistabrat72

Thank you very much, tomorrow evening the wires will have a date with the spray bottle! I do have a nice set of new wires kicking around that may find their way onto the car very soon if the solution is this simple. Intriguing, the old ones are still supple, boots flexible. I guess anything can be possible.

ah64pilot

I have the same problem and it's only under a light load. It doesn't jerk or anything but I just hear a slight miss or something at cruise speed. I suspect the wires as well but mine are fairly new so throwing another $80 at wires for a cruise miss isn't a priority...the car still runs strong at WOT.

Can't wait to see what you find out.

oldcutlass

What ignition system are you running? If points, what is your dwell? What is your timing set to, initial, total without, and with vacuum advance?

brown7373

I had a similar problem on a newer car, a 3.8 V6 in a Bonneville SSE. That engine had 3 separate coils, and one of them was bad. It also missed, jerked and ran lousy at part throttle when the converter locked up. Now I know you have only one coil, but I would check that, or swap another known good one to see if it corrects it.

Vistabrat72

Hello again to all. To answer some of the questions that may lead to a fix for this thing: The ignition system is the old 'window style' distributor seen on most GM cars until the end of the 1974 model year, retrofitted with a Petronix electronic ignition system. Stock - but 3-year old 'Standard Ignition' coil. The timing is set to specs. I did not observe as to what total advance was, but I did see the timing marks with the timing gun being used did react when the vacuum line to the distributor was removed at part throttle, and yes, the mechanical is working (again no specs) because the timing certainly did advance when the throttle was opened. Because I have a new coil and wires, cap and rotor too, when I have time, I will throw them on. I know this is 'parts replacing' but the fact I do have them means it will only cost a bit of my own time. Thanking all who have answered with their advice to this time, Howie.

rocketraider

Since it occurs only in OD/lockup- can you disable converter lockup to see if it's transmission related?

rjohnson442

I"ll go the other way with this....If electrical componets check out. I'd say a leaking vac hose could cause the vac advance on the distributor to sputter OR its a gas issue and the bowls aren't getting enough gas or fuel pressure pressure.

Vistabrat72

Thanks again to all so far. With the manual converter switch in the off position, I can faintly feel the miss, obviously the converter, when engaged, cancels out the natural dampening action. I guess this is what called my attention in the first place, the direct mechanical connection amplifying the problem. Vacuum lines will be checked, as will all mentioned. Darn, it has to be something so annoyingly simple! Howie

oldcutlass

Disconnect and plug your vacuum advance and see if it goes away. Basically your asking us to diagnose a problem by simply saying it's broke. The trans will not cause a misfire, it's either fuel, ignition, or vacuum related.

Vistabrat72

I appreciate your input. I am not asking for diagnosis of a problem because something is broken. I am fully aware the transmission will not cause a misfire, I am quite fully versed in the operation of these units. My question was posed lest anybody else (or a few perhaps) have encountered this, and it could be a 'common' occurrence on these engines. Thank you, regards, Howie.

Vistabrat72

Update to the erratic miss - solved. Hello once again, I am sorry for the delay in posting outcomes, a few things non car related took precedence for a few weeks in my life, all is in order now thankfully. I'd like to thank those who helped with suggestions of where to look. It turned out to be an ignition wire. At night we saw, when increasing the idle to the RPM's where the miss seemed to be taking place, it could be seen where there was a little sparking from the last wire on the passenger's side going from the wire itself to the valve cover. Of course it had to be the most buried of all the wires. Off with the old, on with new wires, over and done with. Odd thing is, the old wires looked like new (put on in 2008 when the original plugs were put in), were anything but perished looking. Probably poor quality control. Live and learn.

oldcutlass

Glad you got it fixed, and reported what your issue was for others to read.

Yellowstatue

A friend of mine had a brand new set of expensive wires and they would not even fire the engine!! Every wire was defective.

Vistabrat72

Oh believe me, it was much to my surprise to see the arcing in my own wires. I will examine the bad wire more closely, it was marked with a white ink pen in the area of the arcing, perhaps a bubble in the insulation, who knows. As for your friend, it just goes to show quality and workmanship is what counts, not price. Must have been frustrating to him.

Vistabrat72

Here we go again, the 'missing' beast came to life again. 1972 Oldsmobile 350 in a Vista Cruiser: Not another set of wires I thought, with just a few early spring drives on the car, how could this be. Even with a known good set of wires from a friend's C***y, the part throttle miss and surging was back, but feeling a little different this time. So we knew it could not be wires, and it could not be ignition, for under more throttle pressure the car, all would smooth out, higher cylinder pressures and demands make the ignition system 'work' harder. So I read, and read and read up on things and experimented with stuff. On one test, with a glove on, I partially blocked the air horn by the choke as the miss off throttle (in park) was happening. Wow, it smoothed out at once, even the RPM's built up. OK I thought, maybe some richer jets were needed in the Q-jet, perhaps as the engine ages, things were changing in requirements. I have a nice cache of jets, and ended up with #73's. This helped a little, but the fuel mileage went over like a lead balloon falling from a table. Reading on some Quadrajet forums I was seeing about diagnosis such as hooking dist. vacuum to a ported source (as suggested by oldcutlass initially). The source ended up being ported, I had taken pains to be sure vacuum lines were right on the engine years ago. Stepping into uncharted waters, I read, and read more on the subject, and it seems with reformulated gasolines as of late, with their different burn characteristics, etc are mean to older cars. Researching further, I tried different springs on the metering rod pistons in the primaries on the Q-jet. Results a bit better/worse with the trials, but still the miss/surge. It was suggested to play with the Adjustable Part Throttle (APT) but the factory manual says "DO NOT TOUCH" the apt or else. Well I did take apart the carburetor and knocked out the cap for the apt, and turned the screw 2/3rds of a turn in after soaking it in carb solvent. I put #67 jets into the carburetor, and voila, all is perfect once again! No more surge, no more miss, and the gas mileage has gone up nicely! So if somebody does have a similar problem, maybe check out this scenario, I just wanted to pass this tip along! Regards, Howie.

Octania

" It was suggested to play with the Adjustable Part Throttle (APT) but the factory manual says "DO NOT TOUCH" the apt or else. Well I did take apart the carburetor and knocked out the cap for the apt, and turned the screw 2/3rds of a turn in after soaking it in carb solvent. I put #67 jets into the carburetor, and voila, all is perfect once again! No more surge, no more miss, and the gas mileage has gone up nicely! "
==============================
VERY Helpful sir
I always found it annoying that the factory CSM does not explain the ramifications of fiddling with the APT setting. "Just don't mess with this...."

I mean, if you are into that level of service, you should be told what to expect from the TWO possible things you can do to that thing- turn it inwards or turn it outwards. And, how to tell if you have gone too far. Somebody, somewhere, at some time has to set that adjustment. Let's share the secret.

Is this gone over in the aftermarket QJet books? I cannot recall...

allyolds68

Cliff Ruggles book talks about it on page 76 but it says to count the number of turns in to seat it, clean, and then re-install the same way. It doesn't really help you if someone has f-ed with it before. There's more talk about the APT system in the Performance Modifications chapter on page 97

Grace Stinespring-Welch

i had the same problem with my 74. it had points ignition,they were set and reset time after time. i took it out and put electronic ignition in it, set it once, runs like a top,and never touched it since, only for tune ups.

Vistabrat72

Oh, I just did a top up of the tank to see oif the mileage REALLY improved, or if I was imagining things. Very roughly, because on a Vista Cruiser you can see just how high the gas is in the tank through the filler, I filled to the exact same bench marked height, it would seem I only used 2/3rds (oh that 2/3rd's thing again!) dollar-wise to go almost the exact same route before I did this modification and the jets. Every penny helps these days! Thanks to all for the additional inputs and comments given so far. Howie.

oldcutlass

Thanks for posting your fix, so many don't. It's hard sometimes to diagnose issues from a description. I know the frustration of having a car and just not being able to get it dialed in. I personally don't have a lot of experience with Quadrajets and alway wondered why they did not go into great detail on the APT assy, other than what you found.

pcard

I saw a youtube video of a guy changing his jets and he used the tip over method to get a preliminary adjustment to the position of the power piston. I thought of it immediately when reading your post from today.
here it is:
this is for a later carb with an adjustable APT, where your carb has the earlier setup which is factory sealed (but you already broke the seal).

Vistabrat72

Thanks pcard, this is a pretty useful tip on the adjustment, along the lines of what I was reading on some forums. The newer style (75 and up Q-jets) have a much easier procedure to adjust, and a lot less carbon-jamming prone APT screw. One can tap the carburetor cover, put a screw in, and then be able to adjust the APT by merely removing the air cleaner lid, removing said screw and adjusting the APT. On my older Q-jet, the screw (tiny and fragile) is under the fuel filter inlet, and needs tiny hands and a tiny screwdriver to adjust. I will make a movie on it in due time.

Vistabrat72

You are very welcome! I know the feeling of folks not posting or at least giving the courtesy of a short answer the very least. I just finished (via PM on You Tube, nothing to do with Classicolds) helping a fellow with a 200-4R that burned out twice on him due to rolled seals on reassembly, led him through tricks, pitfalls, modifications, etc. I saw he had the car running on a short clip he posted on Y-tube. I asked him what he found. No reply. Oh well, so it goes. Will I still help folks? Yes. As far as your words on getting a car dialed in, sigh, it is a total frustration, when it seems to be so easy but very elusive the outcome. The Q-jet is a very maligned carburetor, but once it is understood, and finally tuned, in my opinion (subjective) is probably the best metering/performing street carburetors out there, bar none. Comments on the APT and the manual else where in another reply.

Vistabrat72

Closing the barn door after the poor horse is gone (they are back now, feeding well!) I'd like to get a copy of that book! My unit was not touched. Though very clean overall, the screw was still jammed and needed work to get it to turn that 2/3rds turn.

Vistabrat72

Thank you very much, if it keeps others from going through the same fits I did, all the better, it gives one more thing to look at. As for why the book says "DO NOT TOUCH" I think it has to do with emissions requirements of the day. But now gasolines have changed, and part throttle misses and surging mean unburned hydrocarbons and a car very annoying to drive. I can actually have the back tail gate window of the car down a bit now and not smell the sickening smell of unburned hydrocarbons and have stinging eyes when the surging/misfiring was happening. What worked then may not now, much like the Pilot Operated Absolute Pressure Valves (POA) on our Oldsmobiles when converting to R134A: One is told NOT TO ADJUST the screw in the POA in the book. Now one has to turn the screw perhaps 1/2 a turn to lower evaporator inlet pressures to get the desired cooling effect with new refrigerants! It goes on and on. Regards, Howie.

Vistabrat72

At first, I was going down that path, and truly thought it was a wire when I saw the arcing of electricity from one of the wires. It seemed like it was fixed, but much to my dismay, it was not!

pcard

Howie - I would love to see pictures of your APT work. I know the carb is now installed and that makes it hard, but even photos of the fitting, and the tools you used would help. A step by step narrative too!
Luckily my part throttle operation works great so I have no need to touch the main jets or APT system.

Cheers, Peter

Vistabrat72

Hi Peter, actually, I am going to do the same thing on my other Q-jet (a '72 350 unit as well) and will document the whole thing. Usually I do, but this time I just wanted to get the project over and done with. Stay tuned! Literally and figuratively!