Quadrajet stumble... Any suggestions??

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I just got an '86 Monte Carlo LS with a 305 and Quadrajet. Problem is when giving it too much throttle (maybe last 1/3 of pedal travel) the engine falls flat on it's face. Acts like it would if it were not getting enough
accelerator pump shot if you floor from a stop or low speed. Even out on the road doing 40-50 MPH it will still stumble and fall flat on it's face. Holding WOT it will kinda pick up a little and then die a bit and slowly pick up some more. It will still stumble even if you gradually roll into the throttle, slowly giving it more until it stumbles.
I was wondering if those huge secondaries are flopping open too quickly and at too low of an RPM and giving an awful lean condition. I've never worked with a Quadrajet before so I don't know much about them. I've only used and rebuild Autolites and Holleys. I've downloaded some very detailed Quadrajet service manuals and am thinking I'll pick up a kit and rebuild it. I'm a little nervous about it as it seems like such a complex carb.
In any case, if anyone has any ideas about that stumble/bog I'd appreciate it. Thanks.
Cory
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Check the vacuum advance and centrifugal advance. Sometimes the weights get dirty or rusty and won't move. H
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Yes Hairy is right, but in 86 that distributor probably has that crap EST system with no weights or springs. The timing is advanced by the ECM and the module in the base of the distributor. Also, does it have a mixture control solenoid on the top front of the carb? If so, it could be stuck lean which would cause a problem very similar to the one you describe. Also check fuel pressure and volume, along with float level. Biggest problem ever with Q-jets is when people rebuild them they over-tighten the bolts and screws in the top plate and warp the plate and main body.

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I didn't see any vacuum advance unit and there were a few wires coming out of the distributor so it may very well be one of those POS distributors. Not good for performance or mileage from teh sounds of it.
On the top front passenger side of the carb there is an electrical plug going into the carb. IIRC there is also one other wire plugged into it on the other side, and then of course the electric choke wire. I have no idea why there would be wires going into a carburetor, so I'm kinda confused about that. Is that the mixture solenoid you mention? Can I get rid of that and bypass it to work like a normal carb? Shouldn't the mixture in a Quadrajet be controlled by the idle screws, the jets, and the stages of the metering rod??
Cory

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accelerator pump is not working properly

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If you had read my original post you would know it's not an accelerator pump issue. The problem occurs when at high RPM in first gear and mashing the throttle, and even at high RPM in 1st gear and gently giving it more throttle. Accelerator pump plays no meaningful role under those conditions.
Unless the secondaries on a Quadrajet are designed to flop right open instantly and there is an accelerator pump on the secondary side there should be no problem with the accelerator pump, as the one on the primary side is doing it's job nicely. The car will actually take off reasonably quick from a dead stop if you mash it, just don't mash it too far.
I've also driven 300 miles straight with no accelerator pump, 75 of those miles were stop and go traffic (frustrating to no end!). I know what an accelerator pump issue feels like and how the engine will act without one. I also know how to drive a car with no accelerator pump. This is not an accelerator pump issue I'm having, it's definitely something else. I think it is in the carb but others have mentioned it may be a timing issue, so I may check that today and see if the distributor is advancing the timing as it should. I have a feeling the secondaries are flopping open too qucikly though, so I may try tying them closed and going for a test drive. I welcome any input, but please, read my post and think a little before posting something that makes no sense and you haven't even used simple grammer such as capitalization and periods. Going by a name like "I'm Right" I figgure you may just be a troll, so maybe I've wasted a minute or two of my time here. Oh well.
Cory

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There is a spring that sets the tension on the flapper door above the secondary throttle plates. It's loose. Just look at it and stick your finger in the carb and move the plate and you'll see what I'm talking about. It's a very common problem. There is a "wind up" screw and a "lock screw." Turn the wind up screw until the plate just touches the body, then the specified amount more. Usually between 1/2 and one full turn, look in a service manual. Then tighten the lock screw.
Al
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Thanks, that's goes along the lines of waht it felt like was happening. The darned thing acts like it wants to stall, then will kinda jerkily get the revs back up and smooth out a little. I'll check that spring and see how the flapper and mechanicsm looks to be set up. Hopefully that's the problem.
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On Wed, 11 May 2005 00:29:46 -0400, Cory Dunkle wrote:

1. Accelerator pump bad or linkage out of adjust. 2. Ignition Timing (most likely retarded too much) 3. Vaccum leak. Usually at base of carb. 4. Secondary jets too big. 5. Mixture screws adjusted wrong.
Get a rebuild kit and CAREFULLY rebuild the unit. Yes it IS a complex unit and you have to watch out for little ball bearings falling out of passages (check valves) when you disassemble. They will fit in many places, but there is only one correct place :) Not sure if this unit has them, but be careful.
Get an egg carton to save parts in. Take pictures with a digital camera so you know how to put it back together. Understand many rebuild kits are generic, so take the instructions with that in mind. If you can get a specific kit, buy that one and make sure you get the advanced kit as the tune up kits usually only contain float, needle seat and valve with minimul gaskets. You want a complete teardown kit.
In my day I have rebuilt a number of carbs and while it is not rocket science you MUST document how you took it apart, get the float level correct and use correct torque on all bolts to prevent cracking or warpage.
Take your time, follow the instructions and it will work like a charm. These beast are actually pretty forgiving and you have to screw up bad for it to not work at all. Of course if you want the best performance you must adjust it to spec.
good luck
Spaldeen
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Thanks for the tips. I'll probably rebuild it one of these days. It looks like fuel may be seeping through one of the side gaskets or something, as looked a bit damp. I can do Autolites and Holleys in my sleep, by the Quadrajet does make me a little nervous. Looks like a lot of parts packed in tightly. I will do as you say and take pictures along the way, and get a complete rebuild kit.
Cory
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86' has the "lovely" electronic Q-Jet. It has a primary mixture control solenoid and TPS sensor inside as well as an idle control motor connected to the computer. If the mixture control solenoid is sticking it can act this way. Also it sounds like the secondary metering rods/jets are dirty, causing it to lean out. I would run a can of cleaner through it, then spray the interior down good. Then see how it acts. These carbs are not real hard to work on BUT there are a few parts you have to watch out for. Mainly the TPS plunger under the air horn and the MCS connector.
-
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What role does the computer play on this model of engine and car? I believe it controls the timing, is that it? What would it take to get rid of the computer, a new carb and distributor? Just thinking what to do should this computer stuff become a big annoying problem.
In any case, what role does the mixture control solenoid play? Is this in place of a normal metering rod and jet? Or does it control the position of the metering rod instead of manifold vacuum? How do I test it to see if it is worknig correctly?
Also, waht is an MCS connector??
Thanks for the help!
Cory
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Mixture Control Solenoid = MCS connection.
The computer controls that carb almost the same way it does a throttle body injection system. It uses the TPS (Throttle Position Sensor) to see where you have the pedal and then looks at the engine RPMs and Timing to see how many pulses it should hold the solenoid open to give the correct fuel mix for that throttle position. As the TPS moves it tells the computer to either richen the mix (pulses are held on longer) or lean it out (pulses are shut off sooner) Then it looks at the O2 sensor in the exhaust and sees if the mix is correct based on the voltage it is seeing there. The Idle control does almost the same thing but it controls airflow not fuel. The MCS controls the primary metering rods in place of the power piston that operates off vacuum.
As for converting it isn't hard. You need an HEI distributor off an older small block, the ignition module for a 305, A Q-Jet off of an earlier non-computer car with a 305/307/350 small block (big blocks use higher flow metering and sometimes the large bore Q-Jet, you don't want it on a stock small block) . Remove the light bulb from the check engine light in the dash. Remove the wiring to the carb and tape it so it cannot short out (in case you ever wanted to convert it back) and remove the computer fuse (or remove the computer itself and store it) Bolt the carb on, bolt the dizzy in, set the timing. Start the engine and check the timing again.
All bets are off if you live in any area that does smog testing since most of what you did during the conversion isn't allowed..
-
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Thanks for the detailed description. So the comptuer on this car really doesn't do much, huh? I don't see why a well tuned standard carb shouldn't be able to pass emissions. Heck, I'm sure I could get better mileage out of a normal carb and distributor, and better power too. Just takes a bit of tuning. Of course it's a learning experience with Chevy's, as personally the vast majority of my experience is with classic Fords (first car was a '67 Galaxie, now my daily drifver is a '68 Galaxie). They do emissions checks here, but they don't check under the hood, just what comes out the exhaust.
This is just a daily driver type car so I'd rather not get into that stuff just yet. Is there any way to adjust the timing, or am I stuck with what the stock curve is? I can adjust initial timing, right? From that will the computers curve always be the same? Will it increase total timnig when I increase initial timing?
Sorry for so many questions, but I'm not used to working on anything newer than the early '70s.
Cory
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Nope, not much, just fuel delivery control, ignition timing, air pump switching and torque convertor lock up. Pretty much the same as model year 2005 vehicles.

Because they couldn't control the fuel mixture precisely enough to satisfy the feed gasses to the catalytic convertor.

A correctly operating mid 80s 305 with feedback controls could easily achieve mid 20s MPG in a "B" body (full size), and probably slightly higher in a "G" body like the Monte Carlo. The HO 305 in a Monte Carlo SS performed quite well for what it was.

You'd be best advised to learn how this system works before you go tearing into it. Shit canning the electronics because it's too technical for you is amateurish at best.

Things can always change in that regard, and either way, you'd still be violating federal law, creating increased pollution and likely contributing to even more draconian emissions laws, rules and regulations and added repair costs to your fellow citizens.

You can adjust the base timing to your hearts content, but...

Yes it will, but the system has a knock sensor, so as soon as any knock is heard by the sensor, the timing will be retarded. The GM engineers weren't stupid, there is little to be gained by adjusting to anything other than what's on the VECI label

It all can be learned if you put forth the effort. That and a few special service tools can yield an engine that runs and performs as it should.
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wrote:

Newsflash, catalytic converters have been used on non-electronically controlled carburetors. Perhaps "they" don't know how to tune a carb properly.

I've seen comparable mileage without electronic crap from various '60s cars. No electronics or feedback controls there.

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If you had read my post without jumping to conclusions you would know that the reason for not wanting the electronics was the apparent inability to tune the timing and mixture.

exhaust.
Violating the federal law does not mean increased pollution. I know a guy who swapped the stock 302 in his fox body Mustang for a 408W and no electronic controls and he passes emissions, the thing puts out less than the stock 302 with emissions carb and distributor did, and as you may guess it's a hell of a lot faster too. So long as it's "clean" why should it matter how that is achieved??

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Right... Timing is a compromise between emissions and mileage. I've found on many vehicles that adjusting the timing curve, amount of advance and type and amoutn of vacuum advance several more MPG can be had along with better performance. I don't like computers controlling my engine. I want to control it as I want to know exactly waht it's doing and when it's doing it. If I can tell the comptuer exactly what to do then maybe this computer controlled stuff isn't so bad. Unfortunately that probably involves expensive tools.

newer
"As it should" is nice and dandy, but I want my engines to run better than they "should", as you use "as they should" in terms of exact factory specifications and tune. I want my cars to be running optimally for mileage and performance, and if necessary compromise a little performance for more mileage on the daily driver.
Cory
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Yes, catalytic convertors have been used on non electronically controlled carburetors, but you'd have to search long and hard to find such a set up on a 5 liter engine that was able to meet the federal emissions regulations in 1986, and do so for the required 50,000 miles. Ford certainly didn't have anything, Chrysler either.

Too bad the 60s aren't the 80s, no? Here's an idea; shit can the whole PCV system and replace it with a road draft tube.

No conclusion jumped to. Being as I've been repairing these exact systems on a daily basis since before you were ejected from your daddys nut sack, and training people to repair these systems from about the time you wore diapers, I can state precisely that there is no way in hell that -you- can make any adjustments that would be better or more optimal than what the on-board computer is capable of.

That may be, but the law doesn't look at it that way. Tampering is tampering, there is no "but officer."

Pardon me while I have trouble believing that this "guy" that you know has access to a full tilt federally certified IM test facility and that he went to the trouble to run the eleven day long test sequence to achieve an OEM type emissions certification. IOWs, no it doesn't.

And that is exactly what the on-board computer is capable of doing, hundreds of times per minute. You can't even open and close the hood that fast, much less make an adjustment.

So why did you buy one that does?

I want to fly the space shuttle.

So go to e-bay and buy a scanner. An OTC Monitor 2000 is perfectly capable of outputting every but of serial data in this system and they can probably be had for $20.00

You're befuddled by the carburetor already, how in hell are you going to come up with the smarts to manipulate the programming?

There is a Monitor 2000 on e-bay right now, 29.99 with zero bids, 4 days left to bid. As for re-programming, well, it's already set for optimum mixture and timing at wide open throttle, and it's already set for optimum mixture and timing to achieve maximum fuel economy during cruise, strangely, I find it hard to believe that someone who couldn't afford a set of tires a few months ago is going to improve on what is already there.

Ah, I suppose those idiot engineers at GM hadn't thought of that. Thousands of lines of code and odds are it has more to do with Chicken McNuggets than it does "mileage and performance."
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On Fri, 13 May 2005 01:47:53 GMT, aarcuda69062
...

hey! me too. my 1986 chevy 307 used to stumble and stall and everything. after a lot of money at the garage and a lot of parts changed, it runs ok. nothing special. just ok. i'd have to rebuild the whole engine to make it run like new. what would be the point? the only reason i'm driving this barge is because i can't afford a newer car. like a '96 impala. ...thehick
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<snip>
You're not even worth my time. All I have to say is that last post of yours shows just how truly pathetic you are. I pity you, and your immediate family. God bless them for putting up with you, assuming you have an immediate family that is.
Cory
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If you don't want accurate information with which to solve your problem, what the fuck are you doing here?
You want to play a mechanic and fix your car? Great, do it right instead of shit canning everything and anything which you don't understand or are too lazy to learn to repair correctly.
Or be scoffed at.
Everything on that car of yours can be analyzed with a dwell meter and a digital multi-meter (so much for your expensive equipment beliefs) , you're just too stupid and lazy to do so, and as a result, the rest of us are supposed to gag on what comes out of the tailpipe and suffer tighter and stricter emissions testing requirements.
Typical self centered adolescent.
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