First carb rebuild....success

[CamaroAdam73]

Yeah, this is the first attempt i've ever made at rebuilding a carb myself, and it was a failure.

It's the stock Q-jet from my 79 trans am, oldsmobile 403. I bought the kit, as well as book from cliffs high performance and read it cover to cover twice. once before and once during rebuild.

I go to fire the car up today, and fuel starts rushing out of the vent tube. what would cause this?
I thought i installed the float right, but obviously not.

 

[SBC84Cutty]

Re: First carb rebuild....fail

I had the same thing happen to me on a spare carb I tried rebuilding for a little bit of learning... id like to know too lol

 

[CamaroAdam73]

Re: First carb rebuild....fail

To the best of my knowledge, it seems like a flaot issue. That's the only reason the fuel bowl could overfill?

 

[Bonnewagon]

Re: First carb rebuild....fail

Sounds like float needle. Did you clip the needle's little spring onto the float arm correctly? Did you remember the tiny flat steel washer under the needle's seat (fuel inlet)? Did you set the float level measuring from the edge of the bowl to the end of the float? And lastly, did you turn it upside down after it was together? The last one is a sometimes thing on a Q-jet but an all-the-time thing on a 2GC. The 2GC's design can allow the needle to unseat. The Q-jet is supposed to be immune to that, yet I've learned the hard way -NEVER turn it upside down for no reason. And a big THUMBS UP on the effort. Time to backtrack and see what went wrong. I've been rebuilding Q-jets forever and I still screw up. When you think it's right, rig a fuel line with a length of hose so you can blow into the carb. A properly seated needle will allow you to blow in with an empty bowl, but not when it's held upside down. I know I said never to turn it upside down but that's the test for a needle and seat.

 

[supkylesup]

Re: First carb rebuild....fail

Damn I really thought this thread was going to be about me when I read the title.
I rebuilt my carb 3 days ago and haven't had it running right since.
Pulled it apart 5 times so far ad tomorrow will be the 6th to adjust my float.
Goodluck with the carb man if I had any input or knowledge I'd help but I'm new to it too

 

[CamaroAdam73]

Re: First carb rebuild....fail

Damn, Didn't bother to adjust/measure the float at all. AND i didn't clip the needles spring. Now i know why it's blowing fuel from the vent tube like a water fountain. I feel like an idiot, hah.

On a good note, i did however let it air out for 3 hours and then fire it up with no fuel source to the pump, and it fired right up with nice response 😀 so i've got a feeling once i properly adjust the float I'll be smooth sailing, thanks for all the help guys, it's good to have this place as a fall back when things don't go as planned.

It's not just the internet that is the 'ultimate resource" but the people behind it.

 

[billyjack]

Re: First carb rebuild....fail

Q=jets are really sensitive to float adjustment, so it's an area in which you really need to have some patience and be precise. If you feel you'll be removing the air horn repeatedly for tuning, spray the gasket with WD-40 to prevent damage. I've never read Ruggle's book but I've followed Doug Roe's book for over 30 years, since I first got interested in tuning Q-jets.
In the words of Mr. Myagi, focus Daniel-san, focus.

Bill

 

[G-Body_Vet]

Re: First carb rebuild....fail

One other little detail to remember when you set that float is to NOT put pressure on the needle when you're doing it. In other words don't use the needle for leverage when bending the float's tang for adjustment. I prefer needle nose pliers myself.

 

[CamaroAdam73]

Re: First carb rebuild....fail

SUCCESS! almost anyways, no longer leaks fuel at least. When i was "adjsuting" the float, my method was to just remove it, bend it only slightly more, place in back in and check, i did this untill the float sat 1/4" from the top of the main body.

On to the next issue, i thought i was getting backfire through the exhaust but i was wrong, because i'm seeing slight smoke coming up through the primarys, meaning it's backfiring at the carb so that rules out timing, which makes sense because the timing was perfect when i test fired it with another carb.

Does anybody know off hand what a 79 403 carb adjustment would be? by that i mean the adjustment for the APT, as well as the fuel/air mixture screws on the forward face of the carb.

 

[Bonnewagon]

Re: First carb rebuild....fail

I'm glad you kept at it. Feels good,right? The 1/4" float setting is a general, one-size-fits-all setting for "most performance applications" to quote Doug Roe (my hero too). You should use the actual float setting that is specified for your particular carb number. Is it 17059 followed by 250,253,258, or 553? The 250 and 253 are common, Federal with AC and automatic and both use 13/32 as a float level. The APT should never have been touched as it is set at the factory but if it is, it should have been screwed in, the turns counted and recorded so it could be returned to what it was. Worst case, a rule of thumb is 3 to 4 turns, and you can fine tune the performance slightly since the APT holds the primary needles in the jets and you can raise/lower them to adjust throttle response. Idle mixture screws need to be adjusted with a vacuum gauge and tach so you can see what the affects are. Screw them out, then in to get the smoothest idle and highest vacuum possible, while maintaining the proper curb idle rpm. Backfiring at the carb DOES indicate timing, firing order, or distributor cap issues. Hang in there, you are doing fine!