Miles71

Here is my Carburetor issue:


Idle is good. The car will start good, Hot or Cold. Chokes seem ok.
When I step on the gas and go from idle to peddle, the car hesitates and bogs for a second and then accelerates. Cruising speeds are ok, but even on the highway, if I take my foot off the peddle and then apply gas, I can still feel a slight hesitation.


1971, 350, 4bl Rochester, Automatic 350, 20k miles, all stock.
Timing: 1100 RPM, 12 BTDC, Dwell 30, Plugged Air breather tube. Plugged Distributor tube.
Installed vacuum gauge. Plugged Air breather tube. Plugged Distributor tube.
Set idle to 650, Removed Screw caps, Turned both screws all in then back out 6 full turns. Peaked screws with Vacuum gauge to 17.
Final screw settings are 9.5 turns out. That seems too much.
Vacuum gauge bounces between 16-18 (quick oscillations). Vacuum peak and RPM peaks were similar.


Check Accelerator pump externally. Yanking on the throttle I can see and hear a solid jet of fuel going into both of the primaries. This car still has the original accelerator pump and ethanol dissolving parts. This carburetor has never been touched (7041250 MC 0491, February 18, 1971?)

brown7373

If it still has all the original parts since 1971, I would DEFINITELY buy a rebuild kit from Cliff and rebuild the carb. Even if the accelerator pump shoots gas, I'd bet it is still leaking and giving less than a full shot, which would result in exactly what you describe. I know www.cliffshighperformance.com from the Pontiac site, but he has q-jet parts for all different makes. I did my 72 Cutlass, 72 Chevy Cheyenne and 70 Bonneville, and the difference in smoothness and driveability is amazing! He can do a kit for just a basic rebuild including a float, thicker gaskets (for warped air horn) and rubber parts that are absolutely not deteriorated by the ethanol in the gas. He also can do any jets, rods, pick up tubes etc. (if you are experienced enough to attempt modifying for strip use), and the cam that opens the secondary air valves. This cam usually wears and results in lousy secondary opening and closing. His kits cost more than the parts store rebuild kits but they are much, much better.

oldcutlass

Accelarator pump most likely candidate. However I would not rule out timing and/or advance issues either.

Lady72nRob71

Mine does this after she has sat for a long time (over a month or more). After being run over 20 miles on a weekly-bimonthly interval, she is fine.
I am thinking the rubber accellerator pump plunger dries out and shrinks after the fuel in the carb evaporates. This can be fixed with a new plunger (incl with carb rebuild kit).
Also make sure you do not have any vacuum leaks from rotted hoses or vacuum caps or loose carb mount bolts.

Miles71

My long term goal is to have a qualified shop do a full rebuild on the Carb and optimize for street usage. Since the car is mostly original, I want to keep the original carburetor.


Currently I am just trying to fix the easiest things first.


Unfortunately I do not have the tools and time to do more than superficial maintenance. Does anyone know of a shop in Massachusetts or southern NH that I can drive in and get good carb/engine results?


Thanks, Miles

dingusboy

You can try Sparky in Milwaukee. www.sparkyscarbs.com I had the same problem and did all the local stuff. Sent it to Sparky and after some piddlin' around, got running like a champ. Sparky stood by his work because he deals with quite a few folks on this board. You should call him.

brown7373

Many local shops aren't real experienced with Q-jets, because they haven't been made in a while and they don't do many of them anymore, so be careful. You don't need much more than a couple screw drivers and maybe a pair of needle nose pliers to rebuild a carburetor. Also usually a 5/8 wrench and a 1 inch wrench to remove the fuel line, and a 1/2 inch wrench (preferably a socket) to remove the carb to intake bolts. A Service Manual is a must if you plan on maintaining the car yourself. If you don't have one, you should invest in one.

oldcutlass

I am going to correct my original assesment, I guess you edited in some additional info. 9.5 turns out is definately not right!! Should be no more than 2-2.5 max! So you possibly have a vacuum leak or the carb is not functioning correctly!

Your timing and dwell are fine!

Tony72Cutlass'S'

Are you sure we're not just dealing with a clogged fuel filter here?

When was the last time you changed it.

MDchanic

From what you describe, your bog is right off idle under all load and speed conditions,
and it runs well under all other conditions.
This does not point to a clogged fuel filter or to a bad accelerator pump.
A bad accel. pump would bog at any throttle setting, not just off-idle.
A clogged fuel filter would behave worst under high load, not under off-idle.

It could point to a bad accelerator pump check valve, allowing the fuel in the column before the nozzles to leak down between shots - a test would be to give it a good pump or two while driving along, then go back down to idle and open up slowly and see if it still bogs. If it doesn't, it's because the fuel hasn't had time to run down through the bad check valve.

It could point to a vacuum leak - follow standard procedures, outlined in this forum, and everywhere else in the world, to check it.

It could point to a clogged idle circuit - rebuild the carb, making sure to remove and clean the areas of the idle air mixture tubes in the carb. body.

I'd say you're going to have to rebuild the carb. regardless of anything else.
Bite the bullet and just do it.
Auto parts store rebuild kits ($20) and floats ($10) are fine for a normal engine, but
Cliff Ruggles's kits and Sparky's are nicer (whether you will see any benefit from the extra niceness is a matter for violent discussion in some other thread).
I would strongly recommend reading both Cliff's book and the Rochester Quadrajet manual. I linked to both of those in another post within the past week or two, and don't have to time to find the links again - use the search function.

- Eric

brown7373

I am 61 years old, and have been playing with Q-jets since 1972. I am far from an expert, especially for modifications to idle circuits or the like, but a basic rebuild is pretty easy, especially if you have Doug Roe's or Cliff Ruggle's book, and/or a factory service manual.

Maybe parts store kits have improved. I used to have fairly good results buying NAPA kits and then buying a separate accelerator pump that was impervious to the various additive and ethanol. The carb would be okay for a while, then the accelerator pump cup would deteriorate. I had some swell and get stuck in the bore, pulling right off the shaft. Maybe some of the parts strore kits have improved, but you can buy a basic kit from Cliff that includes a float for $34.95. I noticed a HUGE improvement, and the rubber parts are not effected at all.

Results from various kits may vary from state to state, because each state has it's own regulations for additives and blends of gasoline, and even they change according to the seasons of the year. I live in south Florida. The gas they blend and sell here is different that the gas in Pennsylvania or Any other state, and may effect the rubber parts in the carb. And what about if they start putting 15% ethanol in our gas, instaed of 10%?

Miles71

Hi all thanks for the good tips.

The long-term plan is to do a full rebuild on the Carb. Since Xmas is coming I need to think about toys for the Wife and Kids. I would like to delay the Carb rebuild until next year.

I think I will retune the screws again this weekend. Ill crank them back in and go out 3 full turns. Then start the adjustment process over again and monitor the Tac and Vacuum gauge and see if I hit the same numbers. Since my idle Vacuum is 17 at 650 RPM, I think that’s good and does not seem like any obvious leaks.
I have a roll of Vacuum hose and I am replacing all of the hoses.

As for the Idle Screw. When you are Idling-only, are the Primary jets fully closed or open a hair?

The fuel pump was replaced by the previous owner this past summer and he stated he did replace the fuel filter when he changed the fuel pump.


Manuals: I do not have any dedicated manuals yet. They are on the list.
Tools: I saved most of my tools from May old cutlass, but will get a 5/8 for the Carb.

I sold my 71 red Convertible in 1991. Just getting back into Cutlasses again.
Thanks everyone.

brown7373

Get a 1 inch too, or a large crescent. You have to hold the large inlet nut while loosenimg the 5/8 fuel line. If you don't, you will almost certainly strip out the threads on the carb which will ruin the carb unless you can heli coil repair it. Same with reassembly. You install and tighten the inlet nut, then hold it while you tighten the 5/8 fuel line fitting to the inlet nut.

Miles71

Fuel Mixture Screw initial and idle settings.


I have read that on initial mixture settings some people use 2.5 -3 turns for the first setting (I assume a turn is a full 360 degrees).
After setting both screws to 3.0 turns.

What should I set the Idle for as this point?

While at 3.0 turns, should I set the running (hot) idle to 600 (automatic). And then tune the mixture screws to maximum RPM and Vacuum.
Then when finished with Maximum Vacuum and RPM, reset the idle stop back to 600 RPM?
Due to the freak storm and 50+ hour power outage I have not had time to play. Thanks Miles

oldcutlass

Well Miles, first set your idle speed, 650 in drive. Then adjust your 2 air/ fuel mixture scews for max vacuum or speed, from where you are you need to turn them cw. Readjust your idle speed again, and repeat to fine tune it!

Miles71

Interesting. I always thought you set the Idle while in park.

So each Half turn, if i still see an increase in vacuum / RPM, i should re-adjust the Idle back down to 650 rpm (in park)?

oldcutlass

I would do it in 1/8 turn intervals! Try and keep them close to even. After you achieved your highest vacuum or rpm in neutral, then reset you idle speed.

You set your idle speed in drive, should be around 650 rpm. Your curb idle rpm will be somewhere from 700-750 in neutral or park.

Miles71

Ok, will try again this weekend. Thanks.
Just got the AM Radio running. Now need to figure out how to replace a bad Speaker.

Miles71

Carburetor Adjustments

The Engine running much better now.

I set the idle to 650 RPM (in-gear).
Attached the Vacuum gauge to the Air breathers port, disconnected the Distributor and plug the connection.

The initial mixture screw settings were 3.0, Vacuum 10, RPM 650.
For some reason I went CCW, richer, the Vacuum peaked at 11.5 and 720 RPM and then began to drop. I also forgot to put the car back in park after setting the idle. Ok am a beginner.

I back off to my best setting of 4.5 turns on each mixture screw.
Then went for a test drive
90% of the hesitation is gone.

I did experience a backfire under the new settings once. It was during a slow acceleration from 20 mph.
The backfiring concerns me, not sure if it’s an indication of sticking intake values?
The backfiring was more common with my previous wrong 9.5 turns setting.

If I have time this weekend ill go back to 3.0 and go down gradually to 2.0 range and see what happens. I am slowly getting there. Thanks all.

oldcutlass

After you adj you air/ fuel miixture screws and reach you highest vacuum, turn them in 1/4 turn. Then reset your idle speed.

The backfire is aconcern, is your choke pulling off completely? And, what are your dwell and timing set to now?

Miles71

Close enough.

I think it’s now close enough to last me until I get it to a professional and have the carburetor rebuilt this winter (original carburetor, never touched)

I checked the timing, at 1100 rpm, timing was 12 degrees and dwell was 30. No change since I set it 2 weeks ago.

Park idle = 800 rpm, In-gear idle 600.
I tested a whole range of mixture values from 2.5 – 6.0 turns at quarter turn intervals (park idle).

Here is a subset of the values tested:
Turns RPM Vacuum
2.5 525 11.0
3.0 600 13.0
4.0 750 15.0
5.0 800 16.5
5.25 820 17.0
5.50 840 17.0
5.75 860 17.0
6.0 880 17.0

As you can see by the numbers, the mixture peaked at about 5.25 turns and just leveled off from there. The Vacuum stayed about the same and rpm did creep up a little.
I decided to run with 5.0 turns at 800 RPM and 16.5 vacuum.

Question on the Vacuum:
On a typical cutlass 350, should the vacuum needed be steady while at idle (800 rpm aprox). I noticed my needle oscillates quickly plus/minus a quarter psi at all times.
What is normal for a cutlass?


I only had time for a short road test (its Halloween tonight, too many goblins on the road for a real workout). If it stays dry, ill do a few more road tests this week. Its cold in Boston and I will not have a heater core until next weekend.

I still have a bog between Idle and Gas, but it’s only a half-second before the engine picks up. Much better than last month. Did not notice any backfires today.
The choke system seems to be working, I can hear the step changes. The car starts usually first try Cold. And usually first try warm, No gas, just turn the key.
Thanks All. Booo

Miles71

This morning's temp in Boston was 28F. I took the old girl out for a spin to see how my Carburetor adjustments went the night before.

She started first try.



However the First mile of driving was tough. I did not let the car warm up, I just drove away slowly. I think there are three choke idle steps? Fast, medium and off? I tapped the gas and kick off to the medium choke step and cruised down the road. She was slow and backfired (about 4 times in the first mile) and liked to bog down.



Stops signs were difficult to get going without drowning the engine and causing more backfires. I kept the speeds below 30 mph. After about 1 mile of driving she was happy and began to accelerate smoothly and by 2 miles, all was normal. About 5 miles later I made it to the Highway, morning traffic kept my speed to less than 95 mph.


My plan is to drive her on nice days until the snow falls. Then pull the carburetor off and ship it out somewhere to get overhauled. Now I need to find some other part of the car to play with, maybe a new vintage FM radio or hidden 2-meter transceiver.

thanks all, 73 Miles

69CutlassAlex

When rebuilding my 455 I had the exact same problem with an old quadra-HOG. If it is and electric choke it may not be functioning properly. Im not sure if your using a new carb or not but that is what my problem was. Instead of rebulding I ended up but a nice 750cmf Edelbrock. Good Luck

Miles71

Hi 69CutlassAlex:

I am using the older style Heat tube from the intake manifold to send heat to the choke coil. I believe I only have 2 carb issues left (ethanol rebuild in near future) and Cold first mile drives. The air-filter stove pipe is all intact and working. I will double check my choke linkages next weekend.
When I watch the choke plate on the primary during a cold start, I do see the plate slowly open up all the way as the engine warms up. I have two theories as to why the first cold mile is bad.
#1 Sticking intake value during cold mornings (engine sounds real quite when cold, No tappet sounds at all, until it's hot). Then when hot I will hear 1 tappet very slightly clicking away near the #7 cylinder area. Suggestions welcome.
#2 Clocked jets and or related areas inside the carb causing uneven gas flows. This part will be resolved during the winters rebuild.
Overall, I am in good shape. I will just baby the car for the first mile (and or longer warm-ups) until I get the carburetor overhauled. I have heard rumors there is a local place called" Hobbs Carburetor Shop in Pelham, NH" that I may look into, or ill ship the carb out. I want to keep the original carburetor at least for a year or so.
Does anyone have any first hand information on "Hobbs Carburetor Shop"?
Thanks, Miles

brown7373

If your Q-jet is not structurally damaged, it can be rebuilt to start, idle and run perfectly. The hesitation just off idle sounds to me like an accelerator pump not delivering a proper shot of fuel, so it is momentarily lean and stumbles, and could cause a backfire. As the accelerator is held down, the engine will begin to gain rpms, and then it will accelerste fine. The accelerator pump only pumps gas when you push the pedal down. It doesn't do anything when the pedal is held in a constant position. Many stores still sell kits with inferior accelerator pumps that deteriorate rapidly when exposed to ethanol, and other additives in today's gas.

A few on this thread suggested known quality Q-jet suppliers and rebuilders. Use one of them and fix the problem. Or, you can spend hundreds of dollars for a new aftermarket carb, that in some cases is a "one size fits all" solution. You can be critical of GM for a lot of things, but they didn't spend millions of dollars in engineering and developing specific carbs for different applications if one or two different models would have worked across the product lines.

I don't know what your plans are for your car in the future, but if it is to keep it close to original, having the correct carb makes it worth more. If you plan to modify it, then it doesn't matter. Just don't buy into the Quadra-Junk or other name some people use when talking about the carb. Q-jets still perform excellently 40 to 45 years after they were first introduced, if they are properly rebuilt with the proper rubber parts (accelerator pump and nedle and seat), and properly adjusted.

MDchanic

+1.

Either have it rebuilt by someone experienced or rebuild it yourself (not difficult).
It's probably a bad accel pump skirt and/or check ball, and I suspect your choke isn't adjusted properly and is coming off too soon. The adjustment instructions are in your Chassis Service Manual.

Cliff Ruggles's book is well written and illustrated, and explains the function and rebuilding of Quadrajets very well.

- Eric

Miles71

I do plan on keeping this car a long time.
I also plan on keeping it close to stock and original as possible (no visible modifications)
I would like to do some shows next year as a mostly original car (Millage 19,996.1) I will go over 20k on the way home today.

I do believe the check ball and Accelerator pump maybe issues. However my mechanical stills are a bit rusty, so I will defer the carburetor rebuild to one of the recommended experts. Thank you everyone for very helpful suggestions, Miles

Rickman48

Since it's worked for others on here, I'm going to suggest tightening all the screws on the top of the carb, and then the 4 mounting nuts holding the carb to the manifold. Especially the one's inside the air horn. They can loosen-up over the years!
Seems you're a little light on vacuum - my '69 442 pulled 23 inches after a new carb and valve springs - one reason for tightening things up.
And I'd go back to your 5.25 setting - you want the highest vacuum reading @ the lowest RPM.
Does your idle drop after you unplug the vacuum advance?
If not, the vacuum advance canister could be bad, causing a vacuum leak, and would account for ALL your problems.
I'd also squirt the carb linkage with a little WD-40.
As with any 'low mile' vehicle, there's also a chance your distributer is 'gummed-up' internally, not letting either the vacuum or centrifical advance to operate, at all.
Easy check - pull the cap out of the way, and see if the rotor will twist a inch or two, and return by itself. If not, that's another story!
[had that problem in a L-82 Corvette, that no one could find]

brown7373

Also a good suggestion Rickman48, but if you do decide to tighten, be VERY CAREFUL! One of the most common problems with Q-jets, resulting in people calling them "junk", is over tightened airhorns, that will bend the airhorn and fuel bowl section, resulting in a vacuum leak that is very hard or impossible to overcome. The torque setting for the 4 bolts that attach it to the carb to the manifold are 7 or 8 ft pounds (I forget off the top of my head). That is very light for a 1/2 inch bolt and easy to overtighten and ruin an otherwise good carb.

The screws also need to be tight, but not overly so. Start with the 2 down in the primaries, then the 2 in the very middle of the carb, then the 1 all the way in the front by the fuel inlet. This effectively seals the primaries. Then tighten the 2 all the way on the back edge of the secondaries, then finally the 2 on either side of the secondary air valves. The correct sequence is in any service manual and usually on rebuild kits.

Rickman48

Sorry I didn't mention it - kinda like driving - doing it for so long, don't even think about it!

Miles71

Looks like I have a few excuses to buy a few more tools.

I will try to verify the torque sequence and values for the Rochester.

And get a new torque wrench. I did get a 5/8 flair wrench, just need a 1" next for later carb removal.


As for the Vacuum. I am planing on replacing most of the vacuum hoses and maybe putting clamps on some of the hoses that are more prone to fall off.

Rickman48

I'd check the distributor first - carbs don't like to play much!
Then the vacuum lines.

MDchanic

If the vacuum hoses weren't worn out and hard, they wouldn't fall off .

And, yes, between the loss of vacuum (especially if connected to a non-ported source), and the lack of advance, a bad advance pot can definitely cause symptoms like these.

- Eric

brown7373

If you buy a torque wrench for the carb, you'll need a small one measured in inch pounds (84 inch pounds = 7 ft pounds). Most bolts you use on a torque wrench start at 20 ft pounds and up, although there are some medium ones that go from 10 to maybe 75. Since the carb only gets 7-8 ft pounds, a bigger torque wrench will be useless.

Miles71

I adjusted the 4 main carburetor bolts to about 5 pounds for a quick test. The two front bolts easily moved a 1/8 turn. The back 2 bolts were solid.

I also checked the 9 screws on top of the carb. 2 screws accepted a 1/8 turn snug. The rest of the screws seemed solid. Hopefully I'll have more time this weekend to do a more detailed check.

Future, ill check the distributor weights and replace a few more vacuum lines.


Where is the best place to attach the vacuum gauge?


Tomorrow I am having a friend replace the heater core, fuel sender, all fuel hoses and check for firewall holes.
This morning's road test (temp 40F) after the slight tightening of the carb, had similar results. First mile rough, bogs and backfires. After she was hot, all ran good.

jpc647

Do you let the car "warm up" at all before just going out and driving? When I start mine, (a 72 Cutlass S, 350 motor 4bbl carb) it's automatically on high idle for a few minutes, and I have to hit the gas to kick it down. If I immediately drove it right away, mine would probably bog down too. I don't think it would backfire, through.

PS, where in Mass are you? I'm in the Worcester area.

Miles71

Hi jpc647:
I am in the Lowell area of Mass. I visit WPI about once a month.
The car was ice cold. I never have this issue as bad with my previous Cutlasses.
I have a little more debugging to do before I pull the carb off in December.
Did not get to play this sunny warm weekend because the car is still in the shop for other maintenance.
Hopefully ill have a working Heater and fuel gauge by Tuesday.
When I do pull the carb, ill make sure to take plenty of pictures of the process and may even open her up to see what the bowls look like after 40 years.

archeryshooter

yes it could be a bad vac advance diaphram if you can suck air through it it is no good and will cause it to fall on its face when you accel.

Miles71

Vacuum questions:
1. Where is the best place to put a temporary vacuum gauge while adjusting the idle? Is one spot better than the other? I am currently using the manifold connection used by the Air breather for my vacuum gauge.


2. Besides disconnecting the vacuum advance from the Distributor (and plugging the open port), what other devices 'Must' be disconnected to get a stable reading.


3. For a stock 1971 Supreme, 350, 4-bl Rochester, Single exhaust, What is a typical Vacuum range while idling (in park, hot, at 650-700 Rpm)?

jpc647

Try letting the car warm up for a little while. I could be wrong here, but when the car is cold, isn't it going to bog out a little. Mine will do it. My dad has a 1966 continental and his will too, before it's completely warmed up. I'm a youngster around here, but I thought that was like SOP with carbs.