Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Diagnose a Quadrajet carburetor problem => Topic started by: F250 Restorer on September 07, 2020, 12:58:27 PM
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I bought a 17059247 on ebay. To my surprise, I found some of the v.tubes in the base plate have been filled with what I believe is plaster of paris.
I want to rebuild the entire carb, but first I need to determine if I can use the base plate. If that is rebuildable, I will go further. Does anyone know how to clean out the tubes? Is it possible? Is it worth the trouble?
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Tubes in the baseplate? Can you put up some pics?
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Yeah, the v. tubes in the baseplate. I just took a drill and drilled into the white filling and it all fell out, in all the tubes.
(https://i.imgur.com/u4NsQ0W.jpg)
The mix screws were still factory sealed so I though it good baseplate. Then I found that it was cracked at the throttle shaft, and the shaft had already been bushed.
Are you sure I can't use a baseplate from another carb? another 1705(9)
(https://i.imgur.com/QoEvNfU.jpg)
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That small crack will not hurt anything. You can drill and set a bushing deeper into the hole and it will be fine. It's been dropped pretty hard at some point so the shaft will also need straightened after a new bushing is set deeper......
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So...drill out the existing bushing and install the new one deeper so that it is further recessed with more of the 'meat' around it. OK.
What is the horizontal slit in ONE of the baseplate's primary bores? Below:
(https://i.imgur.com/ICpLhrv.jpg)
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One more question: Why does this carb only have a idle bypass hole in one of the primary bores? yet have smaller bypass holes in both bores?
(https://i.imgur.com/NnqItF7.jpg)
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What is the horizontal slit in ONE of the baseplate's primary bores? Below:
(https://i.imgur.com/ICpLhrv.jpg)
The slit is the vacuum source for the EGR-valve.
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One more question: Why does this carb only have a idle bypass hole in one of the primary bores? yet have smaller bypass holes in both bores?
(https://i.imgur.com/NnqItF7.jpg)
The smaller holes are the vacuum source for the EVAP system (fuel tank ventilation).
I do not see any holes for idle bypass air in your picture.
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Thanks, you're right. The one bypass hole can't be seen in that photo. In this new photo below, the single hole is visible.
(https://i.imgur.com/eDpJcAw.jpg)
Thanks. I don't have the GM tank ventilation system, nor an egr, and having bypass in only one primary bore is puzzling. I'm thinking of using a rebuilt baseplate from the same year that has none of that. That way, if I need bypass air, wouldn't it be a simple matter to drill a hole in either primary butterfly?
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That hole is added by someone to have full manifold vacuum in the port on float bowl usually having a ported vacuum source in the bore your plate lacks.
This hole does not lead to the idle bypass cavity in the float bowl.
You may still add idle bypass air by drilling the plate in the right locations.
Put on a gasket and you will see where to drill.
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Thanks. I think at this point the carb may be trash. When someone has drilled one, chances are they have drilled others, or enlarged some, and I don't know what the specs are for each hole. So, rather than spending hours rebuilding a carb that may never be tuneable, I'm going to throw in the towel. What a waste.
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That hole wasn't added by anyone the later Buick carburetors used it to supply manifold vacuum instead of ported vacuum to the fitting just to the right of the fuel filter housing in the main casting. It just happens to come out in the same passage where idle bypass air could have been used.
Do NOT drill the throttle plates to add bypass air. The bypass air system is excellent and a very precisely controlled vacuum leak to effectively lower the throttle angle if/as needed.......Cliff
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I see the throttle plate as the assembly where axle shafts sit, and the part that in some cases needs to be drilled for idle bypass air..
The throttle valves are often referred to as throttle blades.
1969 RAIV -70 Pontiac Ram Air carbs has .118" holes in each throttle blade and .073" idle bypass air in the throttle plate.
Also, many 1969-74 Oldsmobile Quadrajets have holes drilled in the throttle blades added to the idle bypass air in plate.
And, if your carb has a hot air choke it will have some .092" bypass air thru the air path in it.
I have found 1967 Qjets drill in that manner for full manifold vacuum, later units usually uses a slit under the throttle blades for the same function.
Looks like Buick went the old style vacuum source for 1979 units.
Learn something everyday!
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Thanks Kenth and Cliff. What I think Cliff is saying is: Run the carb and determine whether bypass air is needed. That is what I will do. Thanks.
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Yep, far as I know the later Buick carbs are the only ones to use a drilled round hole in that area to supply manifold vacuum to the fitting on the front of the main casting. All the others will use a source either above (ported) or below (manifold) the throttle plates for vacuum signal.
As far as adding bypass air it is best to add it using the installed idle bypass system if your carburetor has it. Some early units didn't have it so it's just easier to drill the throttle plates instead.
Bypass air is actually a very nice feature and makes getting a Q-jet to idle with a relatively "big" cam much easier. Adding bypass air lowers the throttle angle doing two things, gets the main system off-line and it covers up more transfer slot. It's important to get all the idle fuel coming thru the mixture screw holes and what amount of transfer slot is left. If too much transfer slot is exposed we can still have little if any control with the mixture screws with some set-ups.
So for the most part LESS throttle angle is better than MORE.
I will add that bypass air is NOT a cure for not enough fuel to the mixture screws in the first place, but part of the "recipe" for success with that sort of thing.......Cliff
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I am finished with the recalibration. Following Cliff's advice, I opened the idle tubes to 38, the down angle restriction to 52, the mix holes to 95, float level at 1/4", main bleeds are at 70, as is the idle bleed.
I'm anxious to find whether the small primaries deliver a bit more pep off the line. Today I'll toy with the throttle cable bracket and the adapter to a Holley style intake. I'll post my results.
I was a bit lost about which booster spring to use, but went with one that seemed about mid tension. I'll see if that works for the 14hg I have at idle.
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I got it running. I was surprised to find that the AFR changed quite a bit from when it was warm in the driveway with the hood up, to when I brought it back after a drive. I attribute that a hot engine compartment and the hood being closed. I went from 12.6 AFR at idle, to 15:1 idling after a drive. I had to readjust the mix screws to get it back to 12.6. I opened the screws and it got back to desired ratio.
But, cool air is denser than hot air. So the engine should go rich with hot air supply, right? not lean?
Also noticed that my test drive was w/o vacuum advance hose connected; carb hose connection capped, hose not connected. I look forward to feeling the difference with the v.a. working, ha ha.
Cliff, how can you tell if the booster is not over-coming the engine vacuum and raising the power piston/m.rods?
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Engine vacuum falls off WAY below the spring rate with any very quick throttle movements or going to heavy or full throttle. With the later APT carburetors you can remove the plug over the APT and put a cut piece of a drinking straw down over the APT screw and against the pin on the power piston. Move the throttle quickly to reduce engine vacuum and you can see the straw move up so you know it's working.
You can also reach in with a small flat tip screwdriver with the engine not running and push up and down on the pin to make sure the PP isn't stuck or sticking.......Cliff
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I just want to say 'thanks' to Cliff for all the help. Had you not written your book, I never would have gotten over my hate of QJ's! Now that I understand it, I would rather have a QJ than a Holley, or any of the other name carbs out there.
I tuned today and found that I had to set the AFR mix for when the hood is closed, not for when it is open. It is funny, when I am sitting in the cab, hood shut, and the electric cooling fan comes on, I can see the AFR fluctuate as cooler air is pulled into the engine bay. Love this carb. I can't wait to dial in the APT for max mpg.
Oh, and Cliff, your specs for the engine/cam were dead on. It purrs.
(https://i.imgur.com/uZ5KA6X.jpg)
(https://i.imgur.com/Btfiy1g.jpg)
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8), I love the in-line 6 engines. They were never all that popular but dead solid reliable and last just about forever. Even cooler to see a 4bbl set-up on one!......
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I can't seem to get any flow to my secondaries. I checked and ran a thin copper wire through the four tubes. The innner two are bleeds that vent through the airhorn. The outter two exit through a hole in the air horn. I thought those were the supply to the secondaries discharge tubes, but no. So, how do the secondaries get fuel?
Thanks.
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Fuel flows thru the metering discs in bottom of the float bowl were the secondary rods sits and up the channel were the secondary main well air bleed tubes (the small inner tubes in air horn) sits and out of the main discharge nozzles.
The larger (outer) tubes i air horn are the accelerator tubes that pull fuel out of the holes in front of the air valves when they opens to cover for the incoming air before the main discharge nozzles comes in. This pull over enrichment system acts much like a secondary acc pump.
HTH
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Fuel flows thru the metering discs in bottom of the float bowl were the secondary rods sits and up the channel were the secondary main well air bleed tubes (the small inner tubes in air horn) sits and out of the main discharge nozzles.
The larger (outer) tubes i air horn are the accelerator tubes that pull fuel out of the holes in front of the air valves when they opens to cover for the incoming air before the main discharge nozzles comes in. This pull over enrichment system acts much like a secondary acc pump.
HTH
Thanks for the explanation. So, when I look up the discharge tube, I can see that the inner tubes meet the top of the airhorn. Shouldn't there be a gap so that fuel can be pulled out? And if so, how do drop them down a bit? Thanks again.
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The smaller tubes adds air to the mixture and are smaller than the channels for the fuel to pass.
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Thanks again.
I think I got it, ha ha. Fuel is pulled in when secondary rods are lifted. That fuel passes the bottom of the inner tubes and air is then added as it is pulled past. It travels up the outer tubes and across the airhorn to the discharge tubes. Why is there a hole in that passage, in the airhorn, between the top of each outer tube and the discharge tubes? Each hole sits above each air flap. That has to be like an acc. pump squirt as the air flaps open, right?
I can't get air past those holes to the discharge tubes. If I plug each hole and blow through each outer tube, I got nothing coming out the discharge tubes.
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The outer tubes in air horn are parts of the secondary initial POE system using their own fuel wells in bowl and has nothing to do with the secondary main discharge nozzles.
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Thanks for taking the time to lead me through this.
The inner supply tubes on my carb go directly to the top of the airhorn. No fuel can escape them into the discharge tubes. How can I pull/tab them down a bit so that they can supply fuel to the discharge tubes?
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The inner tubes are sitting fine and supply air into the fuel and the emulsified fuel feeds thru the nozzles.
Click on the picture to enlarge and see better.
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We have a disconnect. I understand your explanation, but I don't think you are following what I am trying to say or explain. My secondary supply tubes are solid and extend to the top of the airhorn. There is NO way for the fuel to mix with air, unless it spews out over the top of the carb and is then sucked back down the secondaries. My understanding, as in your photo, is that the tubes have to terminate somewhere within the circumference of the discharge tubes so that the fuel can mix with air before being sucked down. Mine do not.
CLIFF, can you please tell me how to rectify this?
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The small brass tubes extend to the top of the air horn and adds air into the fuel in the fuel supply channels, not fuel to the nozzles.
The fuel to the secondary main nozzles are delivered up thru the fuel supply channels cast into the float bowl.
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Yellow is fuel, blue is air and red is emulsified A/F mixture mixed in channels and out the nozzles.
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The long inner brass tubes are nothing more than airbleeds. The size of the hole in the bottom of them varies with application. Some models will even have an additional set of holes in the airhorn next to the tubes. As the fuel is pulled up thru the large passages around the tubes it turns and makes it's exit via the large round nozzles.
The long tubes simply add some air to the fuel before it reaches the large discharge tubes and eventually into the intake and on down to the combustion space.
One can actually leave the tubes out completely and plug the holes if you want to richen up the A/F ratio a bit and are enable to with small tipped secondary metering rods. A few factory carburetors didn't use the tubes at all and we've had several customers that ended up blocking them off when converting to E-85 as it requires about 30 percent more of it that regular pump gas.
For what you are doing I'd leave them in place. I'd also add here that some "builders" recommend shortening the tubes about 3/16" or so to get them up higher in the passages and out of the "short turn" (potential restriction) in the bottom of the main casting . It is said that they can restrict fuel flow to the nozzles if left full length. I've tested that theory and see no benefit anyplace shortening the tubes but it also doesn't seem to hurt anything either........Cliff
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Thank you for taking the time to lead me through that. It took me a while to get it, ha ha, but I think the penny has dropped. Cheers.
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Either my secondaries are not opening or I just can't feel it. If they are opening I have to say that power-wise I seem lacking. It is as though I'm missing 25% power at the top end. I've checked the secondary cam, the flap spring tension, and disconnected and plugged the vacuum activated lock out.
Any ideas?
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Loosen the flap spring tension 1/8 turn until you notice a bog or hesitation at WOT, then tighten 1/8 turn.
Also, you may need richer secondary rods.
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I had to take apart the choke. I couldn't get the small latch to disengage from the secondary shaft. Now it seems to work correctly. My secondary rods are pretty rich, DP's.
I think I'll have to get one of Cliff's adjustable choke pull off diaphrams. Mine is retracted all the way at 2.5 hg, and never releases until there is zero vacuum.