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72 Chevy Cheyenne
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217 Posts
Discussion Starter #1
I did a search but couldn't find anything related directly.

Here's the problem, I have a Rochester 750 Q Jet on my 350, a couple of weeks ago it started running a little rough and then I noticed that after getting off the freeway it ran really, really rough. It was leaking out of the side so I fugured time to rebuild. Last night I went to a buddies shop and we tore it apart, inspected every part, put on new gaskets and put it back on. Now it stumbles at low RPM, stinks heavily of unburned fuel and surges at idle, it wants to die at idle also. We leaned it out as much as we could but the air/fuel screws don't seem to make any difference all the way or out.

What did we miss? This carb is only about a year old and has worked fine until now.

Any help would be great and let me know if you need more info.

Thanks,

Dion
 

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Dion, you may want to check the float level, the float itself to make sure it still floats, check the needle, make sure the rubber tip is still on it, check the seat for trash, and check the fuel pressure, Quadrajets don`t like over 5 psi, anything over it`ll flood.
 

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72 Chevy Cheyenne
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217 Posts
Discussion Starter #5
Three votes for the float, that must be it then. I actualy used a metal float from one of the parts q jets he had laying around instead of the one that was in there. I also used a new needle from the rebuild kit.

Makes sense now. I'll put the other back in and see what happens.

Thanks again guys,

Dion
 

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Opps jsut noticed your original problem..."after getting off freeway" that tells me you back butterflys are not closing properly could be a linkage problem and there is an adjustment to hold them open of some models for cold start if your choke is not hooked up you may be experiencing that. Look on the passengers side on the base plate and see if you don't have a screw that controls the basic cracking of the back buterflys...its a tricky devel you can easily 'not see what I am talking about' but 'after freeway' is also after open throttle and that brings in the back buterflys. If you want to take it apart you can see what I am talking too or look at some of the old junkers you have been scrounging parts from.
 

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72 Chevy Cheyenne
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217 Posts
Discussion Starter #7
wally said:
Opps jsut noticed your original problem..."after getting off freeway" that tells me you back butterflys are not closing properly could be a linkage problem and there is an adjustment to hold them open of some models for cold start if your choke is not hooked up you may be experiencing that. Look on the passengers side on the base plate and see if you don't have a screw that controls the basic cracking of the back buterflys...its a tricky devel you can easily 'not see what I am talking about' but 'after freeway' is also after open throttle and that brings in the back buterflys. If you want to take it apart you can see what I am talking too or look at some of the old junkers you have been scrounging parts from.
Thanks wally, that part of the problem seems to have gone away but I'll give it a look anyway, my choke isn't hooked up.
 

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Q~jet

Doc here:pimp:

You said you put in a new kit, Did you also boil the Carb out?

I'd do that next if you didn't.

If you put the Float needle retainer Clip in backwards , it will hang and hold the float up, you can just run without it (mine is) and it will be fine.

Your Leak could be the lead soft plugs in the bottom of the float bowl, above the base plate, Leaking down while sitting. This is usually evidenced by hard "Cold" Starts. If this is the case, replace them or Epoxy them up.

It could also be worn bushings at the Throttle Linkage. This usually drips or seeps fuel on the manifold through the base plate. If this is the case, You need to replace the Brass bushings or get a new baseplate.

If your Idle jets are having no effect, They are probably bad. Did you inspect them for Scars or Dull points, or pits? You may need to replace those (not usually provided in a kit ) You should be able to turn them in all the way and the engine should run very badly or die if they are functioning.

Turn your Jets in one at a time while idling until the car stalls, or runs sounds like it will stall, then back them out 6 one half turns (3 turns total) and slowly Adjust for highest idle, no random stumble, no fuel smell out the back.

Wait about 15 seconds for the engine to stabilize after you think it's right, then check again. You may have to do this several times until you find the "Sweet Spot".

If you still have trouble, There is a jet/power valve (for want of a better word) on the top of the Carb, on the primary side of the choke plate. Recessed down inside the Air horn. Use a slotted screw driver, and slowly turn it in/out while listening to the air being sucked in the carb, It will suck less air or change tone when you hit the right spot, you may have to play with it to get the Feel & Sound right.

Try that, Should get you going again.

Doc :pimp:


PHOTO: Q~jet assembly
 

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There are a couple things you can do to just see if it put together right. On the top front is a tube sticks up higher than rest of 'horn' it 's a vent for the power pistons. You can stick a screwdriver down inside and feel if the power pistons are loose there should be some spring action you can activate them manually...they maybe stuck. Maybe not in their seats or the gasket has fouled their action. Also one to one and half turn from full shut is what you need on the throat adjustments. I hope you don't have those special 'D' looking screws that tool is expensive. I suspect your still getting fuel from the back throats.....thats why it seems too run without no idle effects check the ugly dog idler and see if it isn't screwed in too much.
If fuel is coming onto the top of the carb at the acc pump its almost always floats unless you got an electric fuel pump they have a habit of pushing the floats off their seat and causing flooding. Of course there is always the vaccuum leak if you got that it a whole new ballgame. You get that if you changed base gaskets and the new one is different from the old one. And then the obvious filter clogging...but I have never seen one bad enough to shut you down.
 
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