Rebuilt the carb on my 77. Stock jets and rods. Starts great, and seems to run fine, however; it is running rich. Black soot under the tailpipes, and burns my eyes when I stand behind it. Some other issues. I adjusted the carb, and set idle to 750 RPM. That is as low as it goes, without the idle screw even touching. Fast idle screw is not touching the cam. Also, at idle I have vacuum at all the ports at the front of the carb, even above the throttle plates. I am sure this shouldn't be. I have the timing set to the stock specs. What the heck did I do? Thanks.

Tommy

 

Tommy, I had almost the exact same problem with my QJet but after consulting the forum got it narrowed down to a power piston problem. On my third(!) disassembly of the carb I noticed that the bottom of the power piston on my rebuilt-by-Holley carb appeared to have been snipped off with a pair of cutting pliers- the bottom 1/8" or so was gone, allowing too much fuel to get through during idle. With the idle stop screw turned all the way out about 900 rpm was as low as it would go once it was warmed up and it was very gassy coming out of the pipes. I replaced the defective piston with one from a parts carb that I had and no more problem. Also make sure that none of your metering rods have bent or damaged ends- good luck.

 

Thanks, I will check that out. I picked up a couple of carbs out at the junkyard the other day for practice. Any idea on the vacuum issue?

 

How are the idle mixture screws set? Are you sure you got the correct air horn gasket and that it is installed correctly? Most rebuild kits have several gaskets that cover many different numbers. These gaskets are very close around the power piston. They can hang it up and stick it in the full rich position. Check the float level. Make sure you didn't jam the metering rods up. Make sure the air bleeds are clean. Make sure the throttle shafts go to their full closed position.

BigBlockk

Later.....

 

BB is a master on this stuff, and I had the exact same problem as you, at full off throttle, the engine won't go below 950 rpms, so your post inspired me to open up the quadra before i put it back on tommorow, this is what I found:
the power piston is held slightly open with a C spring that pushes up on it from the bottom. I remove the top of the carb to get to the top of the power piston, took out the screw and pushed down on the piston, you can feel the spring force building, then as felt it's travel stop, then with a little extra force the piston move further down about 1/16 of an inch. the off throttle hold up spring 'C' was open too far. Pulled the bottom plate, pulled the spring, felt how the piston seats w/o it, good hard 'click' and solid seat. used a pair of pliers to bend the spring in, put it back in reatched the the bottom plate and felt the piston seat again, problem gone,, good seat. teflon taped the spring front screw (vacume) and put it back in, slightly, then while holding the p/p down, felt it's contact point, two revoloutins after that raised it about 1/32 of an inch from full seat. put it back on tuesday, think this was the problem. C.









Full Seat

 

Great shots...I did match up my old gaskets (assuming they were original), and did make sure I guided the needles into the jets. Now, about this C spring you are talking about. Do they all have this spring? There is a spring under the power piston, but it looks like the spring out of a pen. I am still confused as to the middle port on the baseplate of your carb in the picture. Is there a screw that you remove from that area to seat the piston? Thanks again.

Tommy

 

I had the same problem with my '74. I noticed that the power piston spring on rebuild kit was longer than stock and throttle body leaks air on base gasket. reinstalled the "standard" spring, replaced throttle body with another good used, new gasket and everything works fine.

 

Runningman's carburetor is "704" series unit with the first design APT. Yours is a "170" series carburetor with the second design APT. These are two completely different designs. Your APT is adjusted from above, through the air horn.

BigBlockk

Later.....

 

A standard rebuild kit like an Echlin does not come with a new power piston spring. You must reuse the original one. Power piston springs are not a ware item. They are reused unless they are damaged by rough handling or botched modifications.

BigBlockk

Later.....

 

Didn't I read somewhere that there are different spring tensions for different applications? It could be that a stiffer spring has been installed which would make it run rich at idle wouldn't it??
I think maybe it was in Cliff Ruggles book.

 

I have a 75 that has trouble idling below 1000. I have a cam that pulls about 6 to 7 inHg at idle. I have been working with the idle for a while. I found that below 1000 rpm the vacuum was so low that the sock power piston spring would raise the power piston. I know you carb guy will yell at me; but until I find the right spring, I cut 1.25 coils off the pp spring. It has smoothed out the idle, but it still needs work.

 

Back in the day there were probably 7 or 8 different power piston springs if you take into account all of GM. I personally have seen 5 different springs. I believe there are 4 different springs on the market right now.

The different spring rates were to accommodate different camshaft profiles. A mild camshaft will idle and cruise at a high manifold vacuum (17" or 18"). A wilder camshaft will run at a somewhat lower vacuum (maybe it will cruise at 12"). You need to have the power piston spring about 2 or 3 inches below the cruising vacuum. Whatever your cruise vacuum is, you want the power piston closed so you can get some economy. At high vacuum cruise (part throttle cruise), say 60 MPH, you want the fuel/air mixture as lean as possible without the engine serging. For most this can be in the 15.5/1 to 17/1 range. Some exceptional combo's can do the 18/1 range.

NOTE: You need a very good ignition system if you're going to run this lean so you don't get lean misfires.

Anyway, you want the power piston down (closed) during normal cruise. However, you need it to open whenever you get into the throttle. This will give you a very responsive throttle and the car will be a joy to drive but, (there's always a but) you want it to close quickly after the maneuver is complete. This will maximize economy. It will also keep the engine running clean inside.

Now, if that's not confusing enough consider this. At 50 to 60 MPH the Quadrajet (one of the most sensitive and precise carburetors ever designed) will be providing fuel to the engine through three different systems (idle, transfer slot and primary boosters) simultaneously. However, they all get their fuel from the same place, the main jets. If you stop and think about it it's a heck of a juggling act. Then you press the accelerator a little bit and the power system passes even more fuel through those two main jets. Four fuel systems flowing just so your car can go up that grade on the highway.

This is why making a change in one area can change other things.

Isn't carburetor tuning fun?

BigBlockk

Later.....

 

Edelbrock makes power piston springs. You can get them at Jegs.

BigBlockk

Later.....

 

Boy! Much good info..what do you think about the vacuum readings at idle? Should I get those readings at idle? The ports above the throttle plates read vacuum at idle. That doesn't seem correct to me.

Tommy

 

The vacuum tap down low on the drivers side front of the carburetor is ported (timed) vacuum. If all carburetor and ignition adjustments are correct it should not have vacuum at idle. The vacuum tap up on the body on the passenger side front should have vacuum at all times. This is manifold vacuum.

I went back and looked at your first post. You may want to look at the base gasket. The one between the throttle plate and the body. It may be hanging the throttle plates up. It may be holding them open. I've seen this before. I have also seen the secondary throttle rod hold the primaries open when not adjusted properly.

BigBlockk

Later.....

 

Yeah, these things are an amazing piece of engineering. I've read Ruggles and Roe's stuff on how they work, when you read about the paths the fuel takes inside these things and how one parameter affects another you wonder how the things can possibly work. Lots of good info in your post, I didn't know you could still get springs. Thanks

 

http://www.jegs.com/p/Edelbrock/753848/10002/-1/10285

BigBlockk

Later.....

 

Thanks

 

Thanks BigBlockk..Looks like I have some more work to do. I am going to recheck my vacuum readings, then yank the carb back off and do some inspecting. Learning more and more everyday. Cheers

Tommy