Author Topic: Freshly rebuilt carb running ruff  (Read 671 times)

Offline cj7

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Freshly rebuilt carb running ruff
« on: January 16, 2020, 05:51:07 PM »
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

Offline Cliff Ruggles

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Re: Freshly rebuilt carb running ruff
« Reply #1 on: January 17, 2020, 04:01:14 AM »
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

Offline cj7

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Re: Freshly rebuilt carb running ruff
« Reply #2 on: January 17, 2020, 01:29:33 PM »
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

Offline Cliff Ruggles

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Re: Freshly rebuilt carb running ruff
« Reply #3 on: January 18, 2020, 05:55:53 AM »
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....

Offline Cliff Ruggles

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Re: Freshly rebuilt carb running ruff
« Reply #4 on: January 18, 2020, 05:56:06 AM »

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


Offline cj7

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Re: Freshly rebuilt carb running ruff
« Reply #5 on: January 26, 2020, 01:52:05 PM »
Alright, finally opened up my carb again and got some measurements
 and bumped up fuel PSI to 5.

idle tubes are .042
idle down channels are huge at .054
Upper and lower  idle air bleeds at .063
mixture holes are at .055
there are also idle bypass holes at in the base plate

I'm not exactly sure what to enlarge or by how much, so I could use some help with that. one thing I am confused on is in addition to the 2 pairs of idle air bleeds in the body, I have a pair of larger holes in the air horn right next to the scews located inside the chamber. the holes seem to share the same channel as the upper idle air bleeds, so I'm slightly concerned I have mix-matched parts. I will attach a pic.

I also have been tinkering with the APT, and it cruises a lot better, but still leans out under light throttle and going uphill a bit.

Offline cj7

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Re: Freshly rebuilt carb running ruff
« Reply #6 on: January 26, 2020, 02:00:15 PM »
After reading more of cliffs book, I think the large holes are just very large MAB, correct me if I'm wrong.

Offline cj7

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Re: Freshly rebuilt carb running ruff
« Reply #7 on: January 26, 2020, 02:59:04 PM »
Lower idle air bleed is actually closer to .059

Offline Kenth

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Re: Freshly rebuilt carb running ruff
« Reply #8 on: January 27, 2020, 12:45:15 AM »
Here is the original recipe:

      454 Calif.
Carb #      17058512
Main jets      71   
prim.rods      35L
idle channels       .054
idle tubes      .041
upper idle air      .069
lower idle air      .063
idle air bypass      .049
idle discharge      .054
main air horn      .120
main air body      .066
main air nozzle      .029
sec rods / hanger      DR   /   K
sec acc. / pri acc      .035 / .025
sec air tube / Fill      .025 / .039
idle screw   taper   L 3,5 /
sec air valve/APT      31mm     / 4,0

For a 350 i would use .038" idle tubes, use .050" main air bleed inserts in air horn and float bowl with the original jetting, if you only have #76 jets and about #46-#48 primary rods i would open the float bowl main air bleeds to .120".
Open the idle air bypass to .080" with choke channel closed off.
Also, i would open the sec. acc. POE to .052" and use DA secondary rods.

Offline cj7

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Re: Freshly rebuilt carb running ruff
« Reply #9 on: January 27, 2020, 01:19:34 PM »
Would there be any benefit e.g. fuel economy, from going to the smaller inserts and jets? my MABs are already .120. I do like the idea of opening up the idle bypass, but is that for both pairs or just the upper idle air bleed pair?. Also, I'm not entirely sure what you mean by "with choke channel closed off."

Offline Kenth

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Re: Freshly rebuilt carb running ruff
« Reply #10 on: January 28, 2020, 12:58:08 AM »
With proper main jetting for the current main air bleeds there would be no difference in fuel economy compared to smaller sized main air bleeds and main jetting.

Idle bypass air holes is located in the throttle plate, under the throttle blades, and is not a direct part of the idle mixture circuit air bleeds.

The choke channel in carb pulls hot air thru the choke housing for the bimetal spring to open the choke. This is not needed when converted to an electric choke element and the channel should be closed in bowl or at air inlet on choke housing.
The loss of this added air is compensated by opening the idle air bypass some.

Offline cj7

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Re: Freshly rebuilt carb running ruff
« Reply #11 on: January 28, 2020, 11:10:36 AM »
I already plugged the choke, but i will open up the bypasses. This should lean out the idle? anything else i should open up for performance besides the secondary POE?

Offline Kenth

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Re: Freshly rebuilt carb running ruff
« Reply #12 on: January 29, 2020, 12:57:08 AM »
Opening the bypass air will lean idle mixture some but most of all it will reduce throttle blade opening angle at idle reducing the possibility of mains nozzle drip at idle.

,055" idle mixture needle holes are somewhat small, i like to have them .080"-.085" for better control on these units.

Did you use the "tip-in" method to check cruising mixture at 2200 rpm

Offline Cliff Ruggles

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Re: Freshly rebuilt carb running ruff
« Reply #13 on: January 29, 2020, 04:04:40 AM »
Adding idle bypass air can "lean up the idle" simply because it lowers the angle of the throttle plates and can expose less idle transfer slot.  This puts LESS idle fuel into the engine at idle if you don't move the mixtures screws.  The added air doesn't lean it up as some folks think as it still gets the same amount of air to maintain the same idle speed just pulling it down thru the carb and not re-routing it in from the bypass system......Cliff

Offline cj7

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Re: Freshly rebuilt carb running ruff
« Reply #14 on: January 31, 2020, 03:42:49 PM »
Thanks for all the help. And yes I was using the tip in procedure. I'm going to go ahead and use the 76 jets and make open things up as suggested. Opening up the body main air bleeds all the way to .120 does seem like a large jump, considering they were originally at .066. I will let you all know the results after.