Tony72Cutlass'S'

I have a 1972 350 Rocket under my bonnet.

Idle: 21 Vacuum

WOT: 0 Vacuum

Part throttle, between 15 and 20 depending on how much i feel like rockin' and rollin' that day.

From idle, When you rev the engine and then snap the throttle plates shut, Your vacuum should increase a little bit. around 23 or 24 then go back to 20 or 21.

Tony72Cutlass'S'

In fact, here is a pictoral representation of what i was trying to say. Print it out.

brown7373

Any adjustements to the carb should be done when it is fully warmed up. That is not to say, the bench settings aren't done on a cold carb, because they are. Things like adjusting the float, the choke settings and pull off etc. are done before the car is started. But the idle mixture screws, or slow or fast idle are done with the car at operating temperature.

Rickman48

I always use the port the vacuum advance plugs into - 2 birds with one stone, so to speak!
Total vacuum readings can vary with the condition of the motor - a bad valve will give you a little 'blip' on the guage.
Just get the highest reading you can, while turning the screw in - takes up any unnecessary fuel flow.

Miles71

Thanks Tony72 for the PDF, I printed it off in Color. It seems like my Idle 16 vacuum is low and should be in the range of 20-22. I will have to look closer for a leak. My vacuum advance plugs into a 3-way solenoid of some kind near the front of the engine. Ill tap into here and see if I get the same range as on the Air breather port.
According to Tony72 chart, my idle vacuum looks closest to the meter labeled "Steady low". Trimming at 1100 rpm is 12 degrees / dwell 30.
My car is still stuck at the shop waiting on a fuel sending. I told the guys to remove the 40 year old differential fluid and transmission fluid and new gasket.

Tony72Cutlass'S'

Are you using your new rebuilt Q-JET?

I had this exact same problem, ended up my quadrajet was warped at the bottom after i tried having it rebuilt a half dozen times.

I was getting 10 vacuum and crappy response.

I then bolted on a 1405 Edelbrock (used an air cleaner spacer to make it look stock. When you look under my hood, you'd never know it was there since i got a 72 stock air cleaner from oldspackrat.

The day i changed my carb, vacuum went up to 21, and throttle response it great.

jpc647

Can you pm me details on this spacer? I would love to be able to use a facotry air cleaner, although I have HEI so I would need to notch backside of the cover to fit. //end hijack.

Also, is there a way to check if the carb bowl is warped? Obviously I'm having similar problems and am following this thread closely.

Tony72Cutlass'S'

Miles,

That's your distributor vacuum control switch. It should have a solenoid on the top of it as well that connects to your transmission. Make sure these hoses are hooked up properly as per the CSM.

What this switch does is it advances the timing when the engine is hot. It also advances the timing when the car is in 3rd gear (the wires on the top).

archeryshooter

Just a reminder the advance port should be above the throttle plates so it wont pull vac until the throttle opens make sure of that and it should be unpluged to check timing

Rickman48

If that 'vacuum control switch' is bad, it could account for all your problems!
Not to mention all the bad lines there could be - or cracks in the switch!
How brittle is 40 yr. old plastic that's been hot/cold a few 1000 times?
Some years/models had these, [another attempt @ pollution control!] and I usually went around them - hose from vacuum advance canister direct to carb, and 'T' off other lines as needed.

Miles71

Here is how I set my timing last month.

I disconnected the vacuum advance to the Distributor at the 3-way output side of the Distributor vacuum control switch. Then Plugged in a 5" tube with a golf tee stuck in the open port on the switch.
The vacuum control switch was technically still getting vacuum from the carb directly from one of the top front vacuum tubes.
Also remove air breather and connected vacuum gauge to this port. Set RPM idle screw to 1100, etc.

If this is wrong, then ill plug my vacuum gauge directly to the carb port where the vacuum control switch was located. Should plug the Input side of the vacuum control switch?

My current carb is the factory original unmodified Rochester 4bl.
The engine is all original with 20k miles.
The following original parts were removed this summer, fuel pump, plugs, wires, distributed cap, rotor and points. The car got its very first tune up last summer. I even had to remove the plastic caps off of the carb mixture screws. The carb is still a virgin.

Vacuum Hoses:
So far, I have only found 1 real bad Vacuum hose. That was the air breather hose. The end at the intake had become hard and brittle from the heat. The rest of the hoses are flexible. My plan is to replace as many of the hoses as possible soon. I have identified at least 3 different size (diameter) vacuum hoses on this car.
Does anyone have a list of the different sizes I actually need?

Tony72Cutlass'S'

I would say it probably still *might* need a rebuild, Gas sitting inside a carb for 40 years (especially today's blends) Often causes gaskets to rot away.

You're lucky to have an unmolested carb, mine was in terrible shape so i opted for an edelbrock.

D. Yaros

If the gas has been sitting for 40 years, it certainly was not of the "today's blend" variety!

MDchanic

If the gas has been sitting for forty years, it will be of the "impenetrable varnish" variety.

- Eric

1971Cutlass

95 PERCENT CHANCE

IT WILL BE AIR IN THE FUEL LINE! I HAD THE SAME PROBLEM!
EXPERT SAID IT WAS AIR LEAK IN LINE! check you lines all the way to the tank, even metal!
Hope it helps

jpc647

If there was an "air leak" wouldn't gas be dripping out?

MDchanic

No.

The pump is on the engine, so everything behind it is subject to vacuum, not pressure, and air is easily sucked in.

- Eric

Rickman48

If there's a hole below fuel level, [wherever] and the engine is off, what's holding the fuel in?
Now, if the hole is at the top of the fuel sender, [rare!] I could understand your theory.
95% NO CHANCE!!

MDchanic

I've had it happen - cracked and porous fuel hose from the hard line to the pump - looked okay at a glance, but it was always a little bit "wet," though it never seemed to drip.
When running, it dried out, and the car acted like it had a bad fuel pump.
Sucking air, probably tiny bubbles, creating foam.

Remember, air will pass easily through small holes that gas will only pass through with difficulty.

- Eric

Miles71

Fuel issues seem to be resolved.

I had a new fuel sending installed last week. I also replaced the 3 hoses at the tank end of the car with new hoses and clamps. Fresh rubber to the steel fuel line. On the engine side, the hose from the fuel line to pump was replace. This solved some of the excessive gas odors I have been getting. One of the vent hoses near the tank was cut clean through.

Now it seems I am battling a vacuum leak, most likely in the carburetor. I have examined most of the 8 or so vacuum hoses on the carburetor and manifold. Only one needed replacement. The rest look firmly attached and flexible (still checking).

In the previous episode of “Lost in Vacuum”, our hero had set the carburetor to 5.0 turns, 700 rpm and vacuum to 15.0. The engine runs but does not like switching from Idle to Peddle (bogs and backfires).

In this week’s episode, the carburetor was set to 2.75 turns, 700 rpm and vacuum of 13.5. The initial Hot runs at this setting have been very encouraging. Ill try a cold run tomorrow. I also disconnected the charcoal filter and plugged the carburetor hose with a bolt.

In 2-4 weeks ill pull the Carburetor off and ship it some where. Recommendations are welcome.

Observation:
When Hot and Idling at 700 RPM. If I put my hand over the primaries, the engine speed increases from 700 to 1000 RPM (less air, more RPM).

Happy Thanks Giving everyone

Rickman48

Screws and bolts are not the best plugs - leaks by the threads!!
Golf T's work good in a pinch, and, being tapered, fit many sizes!

Tony72Cutlass'S'

I should think the relationship there would be less air --> More fuel --> Hence perhaps higher RPM.

Are you sure your choke is adjusted properly?

Miles71

The engine is hot and the primary choke is wide open. Choke looks ok.
I am thinking the few supply for the Mixture screws is not enough. Something inside is clogged and may be causing a reduction in Idle mixture fuel? So when i reduce the air quantity, the air fuel mixtures improve and the RPMs increase.

Miles71

I found some data on line by Lars Grimsrud.

To verify a suspected lean condition after this test, simply hold your cupped hand lightly over the choke air horn area with the engine
running at idle, restricting the air flow. If the idle speed and idle quality momentarily increases, you have a verified lean condition.
You need to select a jet/rod combination that will give you a little more Cruise Metering Area.
....

From this test, I can assume that since no mater what mixture screw settings I use, (2.75 or 5.0 turns) I still get a noticable increase in RPM when I cover the choke air horn area. So, my engine is running lean. Since the problem does not appear to be a mixture screw setting, it must be either a big vacuum leak or clogged jets or something inside.
As soon as i find 1 more tool (1" flair wrench), Ill yank off the carb.
Next step is to find a good place to overhaul it.

Thanks all.

PS:
Converting from 5.0 turns to 2.75 turns did almost eliminate the bog. Once the car is warm, it is very hard to notice any bog at all. I will still get a few random backfires at low speed acceleration. Highway acceleration is good. Thanks to the tips from this group, the Carb is running 95% better.
As for the Carb Rebuild, it seems that some of the top companys may have long lead times for a full overhaul. While i am still looking into the top companies i will also check some local places in MA or NH.

Tony72Cutlass'S'

Best of luck, get your carburetor rebuilt by a Pro. I tried doing it myself twice and gave up. The amount i spent on two 'name brand' carb kits i probably could have gotten it cleaned, and fixed by a professional.

That's the only reason why i went 1405 edelbrock. It has a manual choke, and after carefully adjusting the float height, it ran like a champ right out of the box. 270 bucks on amazon.com.

Miles71

I found a local shop called " Hobbs Carburetor" Shop in Pelham, NH.

They gave me a good price and claim to have a short turnaround time 2-3 days for a rebuild.
Does anyone have any first hand information on "Hobbs Carburetor Shop"?

Thanks, Miles

Miles71

I was not able to remove the carburetor because the fuel line was attached too tight. The outer nut began to deform, even with the Flair wrench.



Plan-B, I will take the car and carburetor to a local shop with a known good reputation (early in January). The staff at the shop are eager to work on the car. They have rebuilt carburetors in the past. When I asked about Rochester carburetor bushings, the shop owner did not think my carburetor had bushings (7041250 MC 0491).


When the engine is hot, the car performs well. Most issues are when the car is cold, hesitation and backfires. The adjustment tips I received did significantly help me improve the overall performance of the engine (thank you everyone).

Since this is a stock family version of the 71 supreme with Highway gears (R2 Axle), I do not expect it to be a quick car. Acceleration is good, however my 4cyl Camry is faster than my cutlass in an unofficial quarter mile.
Thanks everyone, happy holidays.

Miles71

My existing 6 month old fuel line is so poorly bent, that is hits the thermostat housing and is partially crimped at the bend. The line is twisted near the nut fitting at the Rochester fuel filter and is not moveable. Rather than try to unscrew the fuel line at the carburetor, ill remove the line at the pump first, then disposed of the existing twisted line.

Does anyone know if there are any vendors that have sell pre formed fuel lines for a stock cutlass supreme 350 4bl?
Other suggestions welcome, thanks.

Miles71

A few weeks ago I tried to remove the carburetor to send to a shop for an overhaul. I could not remove the carburetor because the one of the two fuel nuts would not come off. So I decided to just take the whole car to a shop next month. Unfortunately I accidentally loosened the fuel filter in the process. A quick engine pressure check was insufficient for me to detect the loose filter.

I took the car to visit my Dad about 25 miles away. Before I left I filled the trunk with all of my tools just in case. I also noted how much gas I had in the tank (half full). After I arrived 25 miles later, I noticed I used 1/4 tank or about 4-5 gallons to travel 25 miles. That 5 gallons should have covered 60+ miles, not 25 miles.
When I started the car after getting Dad, I noticed a strong gas smell that never seemed to go away.
About 2 miles later I dropped Dad off after a friend’s place for a short visit. I decieded I needed to check out the engine.
I quickly discovered the loose leaking fuel filter. Fortunately I had all the tools and in a few seconds I had the fuel filter nice and tight.
The top of the hot intake manifold had puddles of gasoline, still evaporating away.
My best guess is that I leaked about 2-3 gallons of gas on top of the engine during that 40-minute ride. Most of the gas blew back around the carburetor and across the top of the intake manifold and down the back of the engine and transmission area. Fortunately the 30F temperatures helped keep the engine compartment cool and increased the evaporation rates. I got very lucky.

The ride home was uneventful. I used less than 1/8 tank for the return trip.

Rickman48

I've used lines from 'the parts place' without problems.
They won't be open 'til Tuesday!

Miles71

Ready for Rochester Install.

My carburetor should be back in a few days. I ordered a new fuel line to replace the old bent and twisted line.

Questions:
For the Carburetor Manifold gasket, should I use any specific type of sealer for the gasket?

I also pulled off the DS valve cover to check the condition of the sludge under the cover. To my complete surprise, there was no sludge at all. After 4 hours of scraping, I was able to remove 99% original 41 year old gasket.
I am going to install cheap set cork/rubber gaskets for now, I will install a better set later when I repaint the engine area (its too cold to do any good cleaning or painting now).

Do I need any type of gasket sealer for the cork/rubber value covers?

Thanks all.

Miles71

Rebuilt Carburetor is installed and running.

The initial performance of the rebuilt original Carburetor is very promising. I have been driving the car for 2 days in temperatures well below freezing.
When it warms up, ill hook up my test equipment and check all of the settings (rpm, vacuum, etc.)

One of the questions I had, is what should I expect for vacuum on a stock 71 supreme 350 4bbl. I received a few suggestions from non-stock engine users.
I did find a reference in the Chassis Service Manual, page 6C-11.
It seems there is no simple answer, it depends on distributor, vacuum model and dwell.

I assume that if I see 1,000 RPM 16 – 17.5 in.hg, I should be ok.

When I received the carburetor the idle mixture screws for set for 4 full turns. Ill leave it this way until I have more time to evaluate the system.

Currently the only issues I have are a very slight bog from idle to peddle. The car pauses for a second and then “jumps” away. After the slight jump, the car accelerates smoothly to highway speeds and above. Hot idle is good/steady at full stops, with 15 inches of vacuums.

The only issues I had with the removal and installation was the previously mangled fuel line. I bought a new fuel line from The Parts place, its has all of the right line bends, however it seems to be few inches too long. I reinstalled the old fuel line temporarily by cutting out 4 inches and using fuel hose and many clamps to fill the gap (the bends in the old fuel lines were poorly done).

Carburetor was rebuilt by Hobbs Carburetors in Pelhan NH ($110 with 3-day turnaround).

Rickman48

The quicker you get that carb adjusted, and the timing set, the faster your 'bog' will depart!
Set the timimg first, as that'll affect the vacuum!
And then enjoy a completely 'different' car!

Miles71

Here is my planed order of adjustments. Weather permitting (it’s 10F today).

Measure Vacuum with the following lines plugged (distributor, Vapor canister, and Air filter connection.)

Check Timing and Dwell to see if they have not changed, and adjust as needed (12 btdc, 30 deg at 1100 RPM).

Try the “Lean Test” with hand over primary. Should only see a slight increase in RPM (aprox 50 rpm is OK). Adjust carburetor for peak vacuum and RPM.

Miles71

Found another vacuum leak.

In the Air cleaner assembly, there is a device called the “Air Bleed Valve Sensor” or “THERMAL AIR INTAKE VACUUM SENSOR”.
It’s the temperate switch that controls the Vacuum motor control damper assembly on the snorkel tube.
In theory, when the sensor is Cold (less than 100F) the engine vacuum with go through the sensor to the vacuum motor and cause the Snorkel to close and the hot air side to open.
When the sensor gets hot the sensor closes a valve and blocks the vacuum to the vacuum motor and the hot air side closes and the cooler Snorkel port opens.

On my car, the snorkel is always open, because the Air bleed valve sensor is leaking.

I tested the air clearer assembly with a hand vacuum pump. If I pug the vacuum pump directly into the vacuum motor, the “control damper assembly” opens and closes properly.

If I use the hoses with the “Air Bleed Valve Sensor” in line, I never get a vacuum. I also noticed that with a Temporary vacuum gauge installed in the car, I see a 5 inch drop in vacuum while driving at all speeds when the air cleaner is plugged into the vacuum system.

Vacuum gauge only connected to the air cleaner port (15 inches at idle)
Vacuum gauge and Air cleaner (T-connection) (10 inches at idle).

Attached is a part number and picture of the device. Looks like I need to order one soon.
Its snowing now and the roads are covered with sand, salt and other nasty chemicals. Looks like not much driving for a while.

Part number from thepartsplaceinc.com
CB10780Z
(1968 - 1972 Cutlass/442) THERMAL AIR INTAKE VACUUM SENSOR (COLD START SENSOR THAT MOUNTS IN AIR CLEANER BASE) - EA $28.00


Distributor:
It was suggested I check the vacuum and free movement of the weights.
The distributor vacuum advance seems to hold its vacuum ok.
The Distributor weights were covered with some sticky rusty crud and did not move freely. I cleaned the weights and they move more freely. I may have found some remains of a dead mouse in the distributor. I will try to remove the distributor for a detailed cleaning and inspection later in the spring.

Thank you everyone for your suggestions.

Miles71

I solved my hesitation / stumbling problem when I accelerate from a stop.

A correct idle requires Fuel and Air from 2 locations. The two Idle-Mixture-screws control the fuel into each of the two primaries separately. The Idle Air supply is fixed and can't be changed. The "Idle-speed-screw", opens the "Primary Throttle Valve plates" to provide more air, and fuel is provided via the Main metering Rods.

If your Idle Mixture screws are set to Idle at a desired speed and vacuum and your "Idle-speed-screw" is closed too much, your engine will hesitate or stumble when you step on the gas.

Every time I adjusted my Idle-Mixture-screws for maximum vacuum and RPM, my final RPM would usually be over 800 RPM. So I would mistakenly turn down the "Idle speed crew" and reset my idle to 800 RPM. This was wrong. I ended up turning off most of the air and fuel from the Throttle Valve.

This was why my Idle-Mixture-screw settings were in the wrong range of 4.0 - 9.0 values, because I had turned off too much Idle-speed-screw. When I stepped on the gas, the Accelerator pump would squirt raw gas onto a set of closed throttle plates. When the plates opened, they dumped raw gas with very little air into the manifold. I guess I was caused a Rich Flooding of the engine momentarily. This caused the engine to hesitate /stumble and sometimes backfire.

Fortunately I found an obscure web references to a similar problem corrected by turning UP the "Idle-speed-screw" and turning down "the Idle-Mixture-screws".

With my hot engine idling at 5.0 mixture screw settings at 800 RPM, I turned up the idle screw to 1000 rpm. Then reset my Idle-Mixture screws to 1.5 turns and tested all ranges up to 5.0 turns. I finally settled on 3.0 turns at 800 RPM and 16.5 vacuums as my bests spot. I used my Idle-Mixture screws to choose my park idle speed.

The two road tests, one hot one cold were great. I do not notice any hesitation from a dead stop. It's gone.

Miles71

A friend loaned me a few tools. The best tool so far, is this carburetor idle mixture adjustment tool, it's great. I no longer have to reach over a Running engine to adjust the Idle-Mixture-screws.

It’s a tube approximately 20 inches long with a 45 degree bend at one end. The fitting will take any Ľ socket. The **** at the other end allows you to turn the socket. It works great with a flat tip screw socket. Much better than so-called flexible screw drivers.
This one is made by blue point and sold by snap on tools, Model TM64 rod, TM83A for the flat tip screw driver socket. I'll buy my own soon.

Rickman48

I'm glad it worked for you, but please get back to us on what your fuel mileage is.
Those adjustments contradict everything I"ve done with carbs for the last 45 years!!
Counting turns on the mixture screws has so many vairables, just doesn't make sense to me, when, at idle, the highest vacuum on a guage is what you're trying to achieve!

Miles71

The settings that worked for me are not my final "Etched in Granit" settings. These test settings identified a problem, my Idle-Speed-screw value was too low and my throttle plates were mostly closed.

Opening up the throttle plates "hair" solved two problems. It now explains why my Idle-Mixture-screw values were in the 5-9 range when most people run 1.5 to 2.5 ranges. With the Throttle plates closed, the Idle Mixture had to do most of the work for idling. The Hesitation problem which was happening 100% of the time has been eliminated.

As weather and time permits, I am going to continue to adjust my ratios (Idle-Mixture-Screw / Idle-Speed-screw) until I get my Mixture down to the 2.x range with a good peak Vacuum and RPM.
Now I have a better idea how these three setting screws work and a better understanding of the Ratios that need to be adjusted.
Thanks for all of the suggestions.

69 Ragtop

x2.

1) With engine idling near RPM spec, use the mixture-at-idle screws to achieve highest RPM/vacuum (usually around 2.5 turns).

2) Adjust idle speed screw to RPM spec.

3) Repeat until adjusting the mixture-at-idle screws to peak causes the engine to idle at the RPM spec. (Usually 2 iterations does it.)