Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor

Freshly rebuilt carb running ruff

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cj7:
Recently rebuilt my 17058512 carb for my relatively stock 350 Vortec. was running ruff due to primary shaft play. Used one of Cliffs rebuild kits and bushing kits. while I was at it, I put in new idle tubes and plugged the thermal choke vacuum port as stated in the book since I am using the electric choke. it has 76 jets and matching rods as prescribed by cliff, so the only changed variables are the idle tubes. The main problem is that it runs pretty rich. according to my wideband, it idles at about 12 A/F and slowly climbs as it goes up in RPMs, especially uphill where it will sometimes read at 17 A/F and struggles. Im still fine-tuning the ATP, so hopefully, it will do better at higher RPMs and cruising. but I cannot get the idle any leaner, if I adjust the idle screws, the exhaust starts popping and the vacuum goes down as if it were too lean. I have the strongest PP spring (yes the vacuum does pull it down) and float level is at 1/4", 4 PSI fuel pressure and very high vacuum, almost 25. initial timing is 12 with about 10 degrees of vacuum advance on the manifold vac. I'm kinda leading towards doing something with the air bleeds, but not sure yet. any help would be appreciated

Cliff Ruggles:
Did you measure the original idle tubes and drill the supplied tubes to the same size?

When you block off the supply source to the hot-air choke you lost some idle bypass air.  This puts the throttle plates a little higher at idle and exposes more transfer slot.  In extreme cases it may also cause "nozzle-drip".

Do you have some control with the idle mixture screws?

4 PSI is really low for fuel pressure, do you have a regulator?

The float setting is also pretty high for one of those carburetors if you are trying to build it back to stock specs.

Manifold vacuum to the advance may also throw some problems into the mix with "stock" engines with small cams in them as many don't want, need, like or respond well to a lot of timing at idle speed.......Cliff

cj7:
I did not drill the idle tubes out, I assumed they came in the same size. However, before the rebuild, it was also rich at idle so not sure what size to drill to. would you recommend opening up the choke vac port? I have control over the mixture screws. it actually runs smoothly at idle, it is just very rich. Also, what do you recommend for PSI and float level? The book said if you can't find the float level for your specific carb, to use 1/4"

And as for the carb leaning out under load or uphill, is that an ATP adjustment?

Also, does your recipe 1 from your book apply to my specific Carb? if so ill probably fallow that and drill my tubes to that

Cliff Ruggles:
I don't drill idle tubes unless requested by the customer. 

4 psi is really "low" for fuel pressure and effects the fuel level in the bowl and how well it stays full on hard demand.

IF you have control with the mixture screws then how can it be "rich" at idle?

More times than not super-stinky exhaust is lean, not rich, or caused by using camshafts with tight LSA and a lot of overlap.

If you upgrade a "hot-air" choke carburetor to electric you must block off the vacuum supply BEFORE the choke housing.  The electric choke does not use a gasket under it.  I'd say 4 out of 5 carbs sent here for work that have been previously converted to E-choke will have the vacuum supply open to the choke housing.

If you block off the vacuum supply to the choke housing you loose some idle bypass air.  Doing nothing else this will increase the angle of the throttle plates at idle and expose more transfer slot.  Not a big deal in most cases but if/when you loose control with the mixture screws, or have nozzle drip or other issues at idle speed it's something to look at BEFORE going in and opening up idle tubes, DCR's, etc.

Later carbs are fine with a 1/4" float setting but if/when you are using an original set-up in all respects it is best to use the factory float setting as many later model engines had high pressure fuel pumps on them and the carburetor float settings were pretty "low".

If you are using the carb in a non-stock set-up or moved it to something else 1/4" will be fine provided you don't have a lot of fuel pressure.  Some of the fuel pumps being sold today are up there pretty high for pressure, so if/when one has issues with fuel level in the carb it's something to check before digging into the carb and making changes.

It appears that a company called Airtex is making a LOT of replacement fuel pumps these days.  Hardly a day goes by I don't get a call from someone having carb issues that we end up tracing back to the pump putting out 10psi (or higher) fuel pressure.

The "recipes" in my book are general guidelines that will work for most Quadrajets regardless of year, model, etc.  In all cases one should look at the sizes of both pairs of IAB's BEFORE opening any of them up or closing any of them down.  They have a direct impact on fuel delivery at idle to the engine for all applications based on idle tube and the size of the DCR's.  They also effect fuel delivery off idle via the transfer slots......continued....

Cliff Ruggles:

A little common sense applies here.  If you measure both pairs and they average close to .140", for example, no need to do anything to them.  If they are a little smaller, such as the lowers at .060" and the uppers at .070", they can be left as-is and be a tad more conservative with idle tube/DCR sizes.  If they are a little larger (common with many later carbs) such as .078" and .070" you can go a little larger with idle tubes/DCR's to compensate.  I would add here that large IAB's are more difficult to work with in some applications because even with bigger idle tubes and DCR's they dilute the incoming idle fuel more and with some set-ups it's just a better move all the way around to make them smaller vs going overly large with everything else to compensate.

A very good example of this (general info not necessary related to this thread) is some early to mid-1970's Chevy divorced choke carbs will have LIAB's around .063" and uppers clear up at .090".  Most already have HUGE idle tubes in them, and even going larger will more often than not work poorly for idle fuel delivery, if/when you have a pretty "hefty" cam in the engine and low vacuum at idle speed.  It becomes necessary with some carbs to make changes there vs just drilling everything else to huge dimensions to compensate........Cliff

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