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Quadrajet 4bbl M4MC bogging down

  
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Quadrajet 4bbl M4MC bogging down

 
ChadGresham ChadGresham
New User | Posts: 15 | Joined: 12/10
Posted: 12/24/10
08:27 AM

I've been reading alot about quadrajets, and there are people that like them and people that dont. I am one of the ones that like a quadrajet, and i have one stock on my stock 4.3L 262 vortec V6 from 1986. I have read alot about the infamous quadrajet "bog" and mine does the same thing. People have suggested many things to fix this, including replacing the stock fuel filter with an in line filter in a different location. Done that. the next thing is metering rods? people say go for richer metering rods, which makes sense, except that they are talking about putting them on race motors and big V8s. Well since i have a stock V6, is it possible my stock metering rods could be too rich? They are labeled EH. If not than can anyone suggest anything else i can try to do away with the "bog"? reseal the carb maybe? Thank you  

waynep7122 waynep7122
Enthusiast | Posts: 594 | Joined: 08/09
Posted: 12/24/10
09:08 AM

a few things first....... you mention its an M4MC...   thats a NON FEEDback carb...   it will only probably have a single wire to the choke...

depending upon where the truck was first sold... it might have a feed back carb on it.. with an electrical connector over the right front hold down bolt...  a 3 wire connector on the front below the accelerator pump....

a few things....    installing a carb kit can make a huge difference...   but please.. read the instructions first...  the electronic version will need a few special tools to make the adjustments...  not expensive.. some of them might be included in the kit..

the NON feed back carbs have between the needle and seat and the primary rod holder an adjustment screw down in a hole....  its got a double D shaped head...   DO NOT TURN THIS>>> do not adjust it... there is NO way to put it back where you found it...   if this has been adjusted...  its called the APT...  adjustable part throttle..  it being out of adjustment can cause your bog...

but... on quadra jets... that have never been rebuilt...  i have found more than one carb.. that the brass tubes that are pressed into the bottom of the top cover have fallen out.. and are sitting in the main body...  this prevents the fuel from being picked up by them properly...   think of loosing your straw into your big gulp cup...   how are you going to drink it.. pull hard at the X in the cap...   same effect...   you to will be starving and running lean...

if you have a feed back carb...  its really important .. that you check the rest of the feed back system also... before looking at the carb...   feed back systems will have a green wire with a green connector near the carb.. that does not seem to be hooked to anything...     take your dwell meter.. attach one lead to that wire..  the other to ground..  set the meter to 6 cylinder scale...   start the engine...    if the carb and the system is working properly.. it will be displaying 28 to 35 degrees of dwell..     raise the engine speed slowly.. it should still stay within the same area...   if it sticks below 10 or above 50..   there are charts for that...

you might also..  want to warm the engine... just enough to get the choke open..  shut the engine off.. climb up on top with a bright flash light..  look straight down the throat of the carb... move the throttle lever slightly...   do you get an INSTANT shot of fuel from the primary bores????

so.. post the carb number thats stamped into the drivers side of the main body behind the throttle lever....    1705XXXX  

if you could also post.. year... make .. model...  and info off the emission label..   california emission.. US emissions, canadian emissions..

there are several articles on rebuilding the quadra jet spread out in the magazines.....   not all show how to properly adjust the APT ...   it has to be done by the seat of the pants..  a quarter turn and a few blocks at a time.. or on a smog machine in manual mode...   i can describe it better if you need...

you might also want to look at this article

http://www.highperformancepontiac.com/tech/hppp_0311_rochester_quadrajet_carburetor/index.html

it has a chart of the metering rod sizes.. by id number in order...  so you know which way to go richer or leaner on the secondary...   but i bet your problem is with the primary... and it could just be a soft gasket... a soft vacuum hose... a bad vacuum hose in the EFE system that opens the trap door on the right hand exhaust manifold...   no vacuum.. the trap door is closed.. vacuum pulls it open when the coolant reaches 110F at the ported vacuum switch on the back of the intake... or the front...  

waynep7122 waynep7122
Enthusiast | Posts: 594 | Joined: 08/09
Posted: 12/24/10
09:39 AM

this picture shows the hole you adjust the APT thru.  

http://www.circletrack.com/techarticles/ctrp_0607_quadrajet_tech/photo_05.html

the apt is the lean stop for the primary metering rods....  this is where the metering rods start at...  

if the adjustment is too far down...  the primaries will be leaner when they start flowing...    

if the adjustment is too far UP...       the primaries will be richer than it needs to be when they start flowing...


this will effect the drivability hugely...  and fuel economy....

the quadra jet... with its small primaries.. gets them flowing much earlier than other carbs... so tuning of the primary needles is critical...   turn it no more than 1/4 turn at a time... if you are doing the seat of the pants...


and ONLY adjust this... after you have verified that everything else is working.. and that somebody else has messed with the adjustment..  and that you have rebuilt the carb...... with care and patience...

this apt adjustment is so critical... that if you adjust it by accident... the specs say to replace the carb main body with a new one... as the adjustment cannot be done in the field...  

ChadGresham ChadGresham
New User | Posts: 15 | Joined: 12/10
Posted: 12/24/10
05:36 PM

The choke i believe has a single green wire to it and also there is a connector beside the left front hold down bolt, if you are standing in front of the vehicle looking at the carb, i believe the colors to that are green and pink.  

ChadGresham ChadGresham
New User | Posts: 15 | Joined: 12/10
Posted: 12/24/10
05:39 PM

and carb number is 17086046. I have removed most of the emissions stuff, but plugged unused vacuum hoses properly, the EFE is still in the exhaust, but its vacuum line is plugged up.  

waynep7122 waynep7122
Enthusiast | Posts: 594 | Joined: 08/09
Posted: 12/24/10
09:41 PM

if the EFE vacuum hoses to the valve on the exhaust is disconnected and plugged...

no vacuum... is valve closed blocking exhaust flow...   forcing it around under the carb...

you can unhook the actuator...   wire it open....  i usually... if i don't do one of the others...  hook up a manifold vacuum line to it.. with a check valve to prevent the reduction of manifold pressure under load from closing it..  

waynep7122 waynep7122
Enthusiast | Posts: 594 | Joined: 08/09
Posted: 12/24/10
09:58 PM

http://www.oldcarmanualproject.com/manuals/Carbs/Rochester/QJet/1980/pages/Image056_jpg.htm

http://www.oldcarmanualproject.com/manuals/Carbs/Rochester/QJet/1980/pages/Image057_jpg.htm

M4MED

1985: M4MED electric choke carb with dual capacity pump solenoid used on Federal trucks and various Canadian vehicles



so i am wondering if your bog is caused by the duel capacity pump solenoid is not active... i guess i will have to read up on how this works... ...   i live in california... so all the carbs here that i have gotten my hands and brain into have been feedback...  just looking at the diagrams above are not going to help... but thanks for the carb number sure makes it easier to find the EXACT info...

so check back over the weekend... i will stick my nose in the books.. when i am not stuffing my mouth with turkey and mashed potato's   merry christmas...   oh.,.. leftovers... mmm....

disconnecting stuff .. may also hinder your ability to get rid of the bog....

what sort of electronics are you using for the distributer.. does it have a vacuum advance???  or does it have electronic spark timing.. as that might need some of the stuff you have removed...

last question for the evening..    is this 4.3.. in a factory installation.. or is it swapped into something else..  

waynep7122 waynep7122
Enthusiast | Posts: 594 | Joined: 08/09
Posted: 12/24/10
10:59 PM

M4M Carburetors:

The M4M series (M4MC, M4ME, M4MED and M4MEF) Quadrajet carburetor is a four barrel, two stage unit consisting of three major assemblies: The air horn, float bowl and throttle body.

A single float chamber supplies the fuel to the four carburetor bores. A float, float needle and float seat are used to control the fuel level within the float chamber.

A vacuum operated power piston and metering rods control the air/fuel metering in the primary throttle bores. The tapered metering rods are attached to the power valve piston assembly, moving in fixed metering jets to provide the various fuel flow requirements of the engine.

A factory adjusted part throttle screw is used on all models to properly position the metering rods in the metering jets. The M4MEF model, in addition to the part throttle screw mentioned previously, also contains a rich stop adjusting bushing to position the enrichment portion of the metering rods in the jets.

This bushing enables an adjustable wide open throttle mixture control capability. During wide open throttle operation, the air/fuel mixture in the secondary bores is controlled by air valves and tapered metering rods operating in fixed jets. On M4MEF models, the factory adjusted secondary well air bleed adjusting screw provides additional control of the air/fuel mixture during wide open throttle modes.

The accelerator pump system on all models uses a conventional throttle actuated pump plunger operating in a pump well, to provide additional fuel as needed.

In addition, M4MED models are equipped with float bowl mounted dual capacity pump valve and solenoid assemblies. These assemblies provide a smoother transition from idle to part throttle operation during cold engine driving modes.

All carburetors use either electrically or hot air heated choke coils. The heated coils provide choke valve closing force for cold starts, as well as correct opening timing during warm-up. Vacuum break assemblies control initial choke valve opening during starting and warm-up periods.

Curb idle speeds are controlled, depending on engine and application, by either an idle speed adjusting screw or an idle speed solenoid. On vehicles without air conditioning, the idle speed solenoid provides desired idle speed, while preventing dieseling when the ignition switch is turned off.

On vehicles with air conditioning, the solenoid maintains smooth idle during compressor clutch engagement. Vehicles without an idle speed solenoid are equipped with a throttle kicker assembly. The vacuum operated throttle kicker holds the throttle open during deceleration, thereby reducing exhaust emissions. On some engines, it is also used to increase idle speed depending on accessory load.



the upper left corner is the actual diagram for the secondary accelerator pump solenoid..      

M4medwiring  

waynep7122 waynep7122
Enthusiast | Posts: 594 | Joined: 08/09
Posted: 12/24/10
11:09 PM

there is more to the image than is visible... open the image in a new window...   the wires shown in the upper left corner ... hook to the right side of the picture...  the rest is not used on your system ... it just got put in there...


are you catching all this luke...   you have the same style carb..  or.. actually had...  

so.. you need to probably rebuild the carb first.. and figure a tool to adjust the APT screw after you tap and thread the top cover... you can also knock the plug out from the inside when you have the carb apart... as was shown a few links back... i will see if i can find the specs for a APT tool... i own one..

its made by borroughs tool there is not a part number on it....   the top of the APT screw is double d

the tall dimension is .160"
the narrow dimension is .103"

the OD of the tool shaft is .220"  

the shaft length is 5.5"  but thats way to long...

find a drill bit of .160.. grind 2 flats on the sides about a quarter inch down.. so you end up just over .100"  blank end of the drill,   then you have something you can force into a length of copper tubing... to make your own tool...

the threads on the bottom of the APT adjuster are so fine.. if you hammer on it.. you will ruin your carb.. or the adjuster..


i also found this...

the power piston stop or APT screw is set at the factory to give the engine the correct cruise a/f mixture. a good place to start is 3 1/2 turns from the bottom, then use it to adjust the cruise mixture. the way we set the cruise mixture is with a exhaust gas analyzer, the CO reading taken in front of any catalitic convertor and with any smog pump disabled is to target a CO reading of 0.75 -1.25% CO. (any smog shop should be able to give you the readings for just a few$),you could also use a air/fuel meter such as the Innovate Motorsports LM-1 and then target a air/fuel mixture reading in the 14.2-14.5 to 1 range. i know of no easy way to do the APT screw other than those methods or trial and error spark plug readings.  

ChadGresham ChadGresham
New User | Posts: 15 | Joined: 12/10
Posted: 12/25/10
08:18 AM

The vacuum advance on distributor is hooked up and functional, the EFE is fixed in such a way that it does not block exhaust flow, the wires i meantioned earlier are hooked up, the one next to the hold down bolt goes directly to a sensor where the thermostat is housed, and the pink i have no idea where it goes. The 4.3 is stock from the factory, and i have been reading i dont think mine came with a dual capacity pump, doesnt have the extra stuff  

waynep7122 waynep7122
Enthusiast | Posts: 594 | Joined: 08/09
Posted: 12/25/10
09:23 AM

the pink and black wire is switched ignition...   and just those 2 wires shown looped but not connected to the rest of the system seem to be what's on your truck.

what i have posted .. its the most info i have ever seen in one place on this carb....  

ChadGresham ChadGresham
New User | Posts: 15 | Joined: 12/10
Posted: 12/26/10
06:30 AM

Hope ya'll had a good Christmas, i was doing what i do on a normal day, stuck under the hood of my chevy. Well i tried adjusting the tension on the spring that flaps the secondaries closed because someone said try that, to no gain. actually seemed to make my bog worse, so i set it back to how it was... So you suggested rebuilding the carburetor somewhere up there? they sell new seal kits for these things for right around 20 dollars and also i would put a brass float in it, the stock ones are known to sink.. Because i am at a loss here i've tried so many things, i put some DH metering rods in place of my EH metering rods (secondaries) seemed to bog worse also. Any other thoughts you might have, ill try most anything thats reasonable. Thank you  

ChadGresham ChadGresham
New User | Posts: 15 | Joined: 12/10
Posted: 12/26/10
10:10 AM

Update on that, you guys wouldnt believe how much time i spend under the hood workin on this truck, even tho there is usually nothing major wrong with it! haha any how, i took apart the distributor because i read where someone was talkin about distributors on another thread, turns out my ballast resistor was broken off, so i replaced that, seemed to have some positive effect on my problem, so that makes me happy, checked the weights for the vacuum advance everything is functional. SO, i am almost positive my problem is in the secondaries... i am experimenting with rods and hangers now, i bent my hanger down so the rods fit further into the jets i guess is what they are, seemed to lean out at WOT, so im gonna start experimenting with different combinations, i have two quadrajets within walking distance of me, both in 70s model chevys, but any more suggestions again will help me! thanks guys  

tuffnuff tuffnuff
Moderator | Posts: 1375 | Joined: 12/09
Posted: 12/26/10
10:37 AM

IF, you bent the secondary hanger , which by the way is not recommended, you can richen them up again (if you bent them too much) by taking a pair of side cutters to a paper clip and use the small "U" under your hanger, clips come in numerous sizes and thicknesses,,, this will also help those guys that want to "fatten up" their secondaries, for a bit more power.

Smile  
When The Flag Drops.,.

tuffnuff

The Bull ***t Stops.,.
tuffnuff

P. Engineer, Engine Builder

waynep7122 waynep7122
Enthusiast | Posts: 594 | Joined: 08/09
Posted: 12/26/10
01:14 PM

ok... a few things...  since you are messing with the distributer advance weights...    grab the rotor... twist it against the spring tension.. let go.. grab it again... did it return back freely and stop against the limit...

or did it hang up so its not free to return freely...  

what is the vacuum advance hooked to .. intake manifold vacuum or ported vacuum...



now... a little more on the quadrabog...  at what point does your truck BOG...   as soon as you open the throttle... or above 2500 RPMS when the secondaries are starting to open...


if the carb is not working right.. you could play with the timing for months and not cure anything..  

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