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new carb(rochester quadrajet) even after extensive tuning will barrly run?

2K views 21 replies 7 participants last post by  yellowsnow 
#1 ·
Let me start off by saying im new here and relatively new to chevy world.
Some background: 86 stepside silverado
Sbc350
Mildcam
Bone stock as far as i know.
A couple weeks back i noticed that the car would start backfiring after i turned off it and that when shifting the idle would drop so low and fast that the car would die. Soon the car was undrivable as it would not run in any other gear than park.
Naturally i took to forums and local advice and decided to methodically go through and see what the problem was, i checked for vacum leaks, the plugs, distributor, and tried to readjust the timing because i could vaguely remember that last owner telling me it wasnt correct.however the harmonic balancer is impossible to see and /or get to even after removing evrything possible in its way, ie. Fan and housing. It seems that there is a water pump or something on the manifold that makes it impossible to see as well as get a timing light in there.so after checking and rechecking for any kind of vacum leaks i was told to start assesing the carb as it might need to be replaced. After removing it and seeing its condition as well as how it operates i immediately ordered a remanufactured one ( rochester quad) and stuck it on. Thankfully the backfiring stopped as well as the idle drop when shifting but now the car seems to be at a complete loss for power and can only go about 15mph at full throttle, going to recjeck for vacum leaks today. Not sure where to go from here. Any advice at all on the new carb or if its possible to time without a light is much appreciated.
Thanks. Hope i left enough info..
 
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#2 ·
If the carb is otherwise good (some reman carbs are not very high quality), then my guess is that the throttle cable is misadjusted. This is a fairly easy mistake to make when you replace the carburetor.

You need to verify that the throttle moves in its full travel when you push down the accelerator. The bracket is adjustable, and the throttle plates must be pulled full open when the accelerator pedal is on the floor. You also need to verify that the TV cable is adjusted right, if this is a TH700 overdrive transmission.

You should be able to see the timing marks on this engine, even with all the accessories in the way. The stock tab is on the driver's side down behind and to the passenger side of the power steering reservoir. It should be directly under the two bolts that hold the driver's side of the water pump.

Paint the tab setting/line you want with some white fingernail polish or model paint, and also paint the line on the balancer. Its easier to paint the line of the balancer from underneath, and I have used a paintbrush taped to a dowel to reach the pointer. If you look straight down next to the reservoir on the side near the engine centerline, you should be able to see the tab when you use the timing light. Make sure you have the light connected to #1 cylinder.

Bruce
 
#5 ·
can you add a few pictures of the underhood... so we can see what you are dealing with..

click the manage attachments link below.. it will allow you to select images directly off your computer..

you can also take a video and upload it to youtube and link it here.

be sure to include the numbers off the drivers side of the quadrajet..
 
#6 ·
Her are pictures

here are some pictures of the carb and how its hooked up, I also included the closest picture i could get of the hbalancers location. ive been playing with the idle all morning but it seems to run very rich then die as well as get no power... any ideas?
 

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#8 ·
first off.. your primary choke pull off is disconnected... why is that so. the hose sticking out the front toward the passenger side is supposed to hook up to the primary choke pull off.. it pulls the choke open about 1/8 of an inch just after cold start so it does not load up.

i would bet that your secondary choke pull off is also disconnected.. that is the one on the passenger side rear of the carb... that is usually hooked up to the ported vacuum switch on the air cleaner housing.. so when the hot air comes up the duct from the exhaust manifold. it goes into the air cleaner housing. heats up the thermal vacuum switch and causes the secondary choke pull off to retract.. this pulls the fast idle cam down to all the engine to idle.. its set up this way to heat the cat up..

i did not see it in your pictures.. as you got way too close with your camera.. but i expect to see a vacuum operated device on the passenger side exhaust manifold down where it connects to the downpipe.. when there is no vacuum on that.. the trap door will be closed.. blocking the exhaust flow out that manifold and forcing it thru the passage under the carb thru the intake and out the other side.. this will greatly reduce your power output. will also cause the carb to overheat..

usually on that model.. i like to see a slightly thicker carb mounting gasket. its usually 5/16 thick. but i could be wrong.


as for seeing you timing marks... that involves some acrobatics... the only way to see them is looking straight down behind the water pump..

look at the emission label.. if it still exists.. it will have the timing spec.. you can ... turn the engine to bring the timing mark to the proper spot using a socket on the alternator pulley nut.. then pull the distributor cap and rotor.. align the tips pickup coil with the tips of the reluctor by turning the housing slightly.. base timing on engines with pickup coils are always where the tips of the reluctor on the distributor shaft align perfectly with the tips of the pickup coil.. knowing this allows you to set the timing with the engine NOT running.. usually it will be within 2 degrees..
 
#9 ·
Here are some more pictures i rehooked all the lines and checked for any possible vacuum leaks. the car will barely idle if at all and dies almost immediately and it smells very strong? It seems like I have everything hooked up correctly any ideas? so primary choke pull is reconnected,secondary choke pull off is hooked to bottom of carb as it came, and in terms of the vacum operated device on the passenger side exhaust manifold, im pretty sure i rehooked up everything to how it was with the original carb. DOes it look okay and what else looks like it might be a problem.
 

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#10 ·
oh... i see the problem. unplug the bowl vent cap..


please stick something in to hold the choke open also.. that looks like its wedged tight closed..

if you apply vacuum to the primary choke pull off.. does it open the choke blade about 1/8 of an inch???

i still could not read the carb numbers off the drivers side of the carb..

i even zoomed in on the picture.. i could see them..but there was not enough contrast to tell what they were...

that is an M4ME carb..

here is the delco service manual... so you can go over the various components in depth..

http://www.gmcmi.com/wp-content/uploads/2014/05/Quadrajet_78-Service-Manual.pdf
 
#14 ·
That is not the vent. The bowl vent points straight up off the top and does not look plugged. It looks correct.

Without setting the timing I think you are fighting a loosing battle. Does it need the idle turned way up, like into part throttle?

With mine (Q-jet and big cam) I had to advance the timing, which raised the idle speed, until I could back off the idle screw down out of the part throttle range and back into the idle range.

You could use this to set the timing.
 
#15 ·
Everytime i turn the idle/ throttle way up, i can get it to run but it runs very rough has no power and the idle increases rapidly til it dies, iknow the timing wasnt to bad before i switched carbs so im not sure it would have that much negative affect on how its running at the moment. Just not quite sure where to start now if, everything is indeed hooked up correctly.
 
#22 ·
And any tips to unsticking or checking for a stuck float, will update tommorow.

With an electric fuel pump it will overflow when the key is turned on but not started. With a mechanical pump.....I don't know honestly. That could be tricky to tell.

To unstick you try tapping the carb on the front. If that doesn't work then it requires taking the top plate off the carb off to get to the float.

All your problems can be attributed to running way rich. Did you change fuel pumps when the problem started? A Q-jet can only handle 5.5psi of fuel pressure. If you have a regulator and return line check to see if your return line isn't clogged or regulator bad.
 
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