Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Dialing in your rebuilt Quadrajet carburetor => Topic started by: DieselPower on February 14, 2015, 10:19:30 AM
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Let me start off with specs:
1970 Lemans with auto trans
Pontiac 400 bored .060" over
XE268H cam
224 @ .050" intake
LSA 110
9.5 CR
Edelbrock Performer intake
Stock exhaust manifolds and exhaust
Edelbrock 1910 Qjet
Stock distributor with Pertronix ignitor
B20 vacuum advance can (fully actuated at 12") hooked to manifold vacuum
12* initial, 36* total
Jets - .073"
Primary Metering Rods - 53M
APT Screw - 3 turns
1/4" float level
Idle Tubes - .038"
Upper Idle Air Bleed - .070"
Lower Idle Air Bleed - .060"
Idle Channel Restriction - .059"
Idle Discharge Port - .098"
Idle Air Bypass - .106"
Main Air Bleed - .046"
Secondary Air Bleed - .036”
Light blue Power Piston spring (verified that it is staying closed at idle)
Accelerator Well Hole in main body - .050"
"CE" Secondary Metering Rods
"J" Hanger
Accelerator Pump Discharge - .025"
Carb rebuilt with Cliff's HP kit. Airhorn had maximum .026" warpage. Installed thin gasket and had fuel leaks and very hard starting. Installed thicker airhorn gasket and now starts easily and runs well except for idle. I have two questions:
The thicker airhorn gasket moves the secondary rod hanger up about .025". Is it worth getting a new hanger to bring the secondary rods back to the original location? My current hanger is a "J", and I would need to move to an "O" to bring back to the same height.
My second question is in regards to idle quality. Mixture screws do not do anything. I believe the throttle blades are too far open and I am running off the transition slots. If I pull two vacuum lines, I can close the throttle enough to get some adjustment from the mixture screws. I am trying to get an idle of 850 rpm and be able to put it in gear and turn the wheels without stalling. At 850 rpm, I get about 14" of vacuum, but idle quality is poor and engine dies easily when loaded. Should I increase idle air bypass? I am currently at .106". I measured the size of the vacuum leaks that I created, and to get the same flow area, my IAB diameters would need to be .149"!!! I assume that is too much. Is there something I am missing? From what I have read, this cam should be able to idle at 850 with no problem and make 15" or more of vacuum and not stall when loaded in gear.
Oh, one more question. Is the power piston spring supposed to fit into the bore at the bottom of the power piston bore? It looks like it should, but is slightly too big. It is so close, I just want to make sure my carb was not machined wrong. Cliff says a lot of the Edelbrock carbs had questionable quality control.
Thanks in advance for any help!
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How much advance do you get from vac.? Can you give it more initial, to see if that improves the idle?
Have you pulled the carb to see where the throttle plates are positioned to the transition slots?
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I have a B20 vacuum canister that gives full advance at about 12". This unit gives 16* timing advance giving me 28* timing at idle. Timing is rock steady since I am running about 15" of vacuum which is more than enough to keep the advance unit fully retracted.
I just checked the transition slot. It has way too much exposure at ~.115".
Thanks!
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Idle system is pretty rich for what you are doing, it may be rich at idle instead of lean. Do you see any nozzle drip at idle speed....Cliff
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Cliff,
No nozzle drip that I can see. I used idle recipe 3 from your book which looked like it fit quite well with my engine specs. Is there something I missed? Should I enlarge the IAB?
I was wondering if you could also help me out with my questions about the secondary rod hangers and the power piston bore?
Thanks a ton!
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Take a look at the IABP make sure it's not blocked by the gasket some place, not likely but ya never know.
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Gasket alignment is good.
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dp i had the same problem with a 400 . it was to rich at idle i end up with .037 idle tube and .052 dcr. i know every engine is different and the guy told me he had a bigger cam than what he had. its so hard to build something when they don't know exactly what they have. hth
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How big can I make this bypass? I am tempted just to take it to .130" and see how it runs. If that is not enough, maybe .140". I have seen Cliff say that he never has to go over .120", but I do not see how that is possible unless he added holes in the throttle blades. Is there any reason not to make the IAB .130" or even .140" if it gets the blades where they need to be so I am not running off of the transition slots?
Any thoughts on my questions about the secondary hangers and power piston bore in my original post?
Thanks guys!
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Start at .125 I had one at .14 & it whistled at idle. You can always close it back down by using drilled 1/4 inch set screws to restrict the vertical passage in the baseplate.
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I've got a 1910 that i bought for a project I've never taken it apart. on the quads the spring goes up inside i can't tell you on the 1910. your second question the thicker gasket id see how the thing performs before id change it.i would not be afraid to drill the bypass a little bigger to .130 and maybe smallen your idle tube and dcr. .ive noticed it only takes a couple .000 to make a large diff.fwiw
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how did you get power piston down all the way if the spring doesn't go up inside? you should have bad coil bind. on the other note i reread your setup. i would drill it to .130 and make the dcr to low side of 50 and leave the idle tube alone .whats sad is you should do them 1 at a time . but that means lots of bending over in the car or lots of removal. fwiw
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Thank you for the replies!
I will drill out the IAB to .130" and let you guys know what happens.
I decided not to worry about the secondary hanger. After doing some full throttle runs and pulling the plugs and seeing that they are a perfect light tan, I am going to call it good enough.
With the power piston concern I had, if you look down into the power piston bore, you see another small bore that is obviously the vacuum source. The thing is, that inner bore looks almost big enough to put the spring into, but not quite. I did not experience any binding, but obviously if the spring is supposed to sit in that inner bore, my spring tension will be way off.
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you are correct . it does not go in the vacuum source, it just sits right on top of the little hole. i was confused ,i thought you were asking about the power piston internal bore . sorry for the confusion. hth
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on a side not ,i like your login . i used to run diesel in my 79 f250 when diesel was 30 cents . i had a 11.5.to 1 429 and i had dual tanks . i would fill the rear with diesel and front with gas . id start it on gas and when i got it warmed up going down the road id switch over . then id switch over before i shut it off. it ran pretty darn good.now i just run quads on my fords. fwiw.
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A Pontiac 400 with 9.5 to 1 compression and that cam doesn't need a lot of idle bypass air.
If the throttle angle is too great, you will see two things. Nozzle drip at idle and no control with the mixture screws.
If you have no nozzle drip, and no control with the mixture screws, it may be getting enough fuel thru the transition slots even with the mixture screws seated.
What is the vacuum at idle, and idle quality?
With that cam it should be relatively "smooth" with a deep/heavy sound and little if any "lope".....Cliff
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Cliff,
At 850 rpm, I am seeing a very steady 14" vacuum with no nozzle drip. However, in gear, it is on the edge of stalling and any input into the power steering and the engine dies.
The mixture screws do almost nothing. It seems to run slightly better when they are full closed. Anything more than 1/2 turn out and there is no difference.
Thanks
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I was just thinking, does the carb gets some "bypass air" from the PCV system? Is it possible that some carbs have different sized PCV ports, thus the difference in required IAB diameter in similar applications? For example, if someone else has an engine with the exact same specs as mine, but he has a factory carb and I have an Edelbrock 1910, is is possible that the 1910 has a smaller PCV port and thus less airflow for a given IAB diameter?
EDIT: I just remembered that Pontiac engines have the PCV port in the intake manifold. I wonder what the flow area of the elbow fitting is compared to the PCV port on my Edelbrock 1910?
Maybe the PCV valve itself is the limiting factor to airflow?
Either way, I still cannot get this thing to idle like it should. I have the IAB at .130" now with little change. I was able to close the throttle about 1/2 a turn. Mixture screws still do basically nothing, but engine runs best with the screws all the way in.
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Sounds like too much idle fuel for the application.
I'd also check the timing, and make sure it is rock solid in and out of gear......Cliff
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Cliff,
Timing is steady as long as vacuum stays above 12" or so. The vacuum advance can I installed is fully actuated above that point.
The idle recipe I used is per emails from you and recipe 3 from your book. If the idle circuit is too rich, how does that keep the throttle blades from being closed more so less transition slot is exposed? Not arguing, just trying to understand.
Thanks!
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None of the symptoms you are seeing are typical for that camshaft if a 400 build at 9.5 to 1 compression.
I have tuned several of them, and they idled fine in and out of gear with less idle rpm, less idle fuel, and less idle bypass air. I also didn't run nearly the timing either, about 12 initial and used ported vacuum to the advance.
So why is your engine wanting all that bypass air with the timing at 24 degrees? It should idle fine in and out of gear with less timing.
I have ran into several engine combinations that hated a lot of timing at idle speed, so you may want to try less timing by using a ported source to the advance, and start the idle tuning process all over.
With the HUGE idle fuel delivery from the Edelbrock 1910, you should be fine without all that timing, and very little idle bypass air, at least from what I've seen here tuning similar set-ups with the same carburetor.....Cliff
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What does the exhaust smell like?
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someone correct me if my understanding is wrong. the only way the idle screws get fuel if from the size of the idle tubes and and the size of the dcr.another small amount comes from the transition slots .if those 2 are to large you have to ether give it more bypass air or open the throttle plates . now you are not truly running off the idle circuit. my experience has been if the idle is to rich i have to smaller the idle tubes and the dcr so i can get control of the fuel with the idle screws and keep the throttle plates nearly closed so I'm not pulling a bunch of fuel from the transfer slots.this works for me. if my understanding is wrong please correct my way of thinking.fwiw
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The transfer slots are not in the exact same location in every carburetor, so some will provide more idle fuel to the engine with the mixture screws fully seated. This fact is not usually a big player, but once in a while one will run into a carb that's extremely rich at idle with no control of the mixture screws because it's getting plenty of fuel from the transfer slots.
I also see a very similar problem with some engine combinations where folks use manifold vacuum to the advance and put too much timing in at idle speed. Might be a player here, not sure. From what I'm reading the engine isn't very happy with the tune it's getting. Could be simply too much timing at idle combined with a lot of idle fuel available at the mixture screw holes and transfer slots.
It's also no big secret but I am NOT a big fan of Comp XE cam lobes, especially in Pontiac engines. I've tuned a number of them, and not in one single instance have I liked the end result(s). Those cams drop the valves too hard on the seats, and shove them up too fast. LSA is too tight, and intake valve closes too early. This makes for a "quirky" idle quality right to start with.
I actually built a 400 Pontiac WAY back when those cams first came out, with very well prepared #16 heads. I thought at that time I'd buy into the "new" technology and find some additional power. I couldn't get that cam out of that engine fast enough, very disappointed in it at every level......Cliff
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I agree on the comp XE cams. Had one in my 454 chevy. noisey valvetrain, stinky exhaust, poor vacuum... Swapped it out for a summit 1302 cam. Not for the price but for the specs. Did nothing else to the car. Now it has a very quiet valvetrain, Idles like a champ in & out of gear. Picked up 2 inches of vacuum. (The power brakes really improved) & picked up 10lb ft of tourqe on the dyno. Horsepower stayed the same. Came in 200 rpm sooner. Couldn't be happier.
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OK,
Got back to working on this car. Haven't touched it since my last post. Installed a new AC Delco PCV valve and did nothing else. Engine started right up (40* or so in garage) and went to a fast idle speed of 2000 rpm. I had previously had this set to 1400 rpm, so I knew something had changed. Readjusted fast idle and let it warm up. Was able to dial the throttle back quite a bit. Now the mixture screws do something with the best setting about 1.5 turns out. It still stalls if I have idle speed in park set to 800 rpm. Vacuum is about 14" still. Set timing to ported vacuum and changed base timing all the way from 12* to 36* just to see the effect. Vacuum keeps on increasing all the way to 36*. Previously, I had about 28* with manifold vacuum.
My questions now:
1) Is it realistic to have this thing idle in park at 800 rpm and not have it stall when in gear and turning the steering wheel?
2) Should I hook back up to manifold vacuum to get timing at about 28* with base timing of 12* (I have a vacuum cannister that is fully actuated at 12" of vacuum.
Thanks a ton guys!
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My book describes timing engines using manifold and ported vacuum to the advance, and how that deal really works.
In any and all cases you have to give the engine the timing it wants/needs at idle to make things happy, same as fuel delivery.
Using MVA brings in some problems in some applications as the advance can may not fully employ, or stay employed when a load is placed on the engine.
Light distributor springs can also make idle tuning next to impossible. Make sure that the timing is rock steady at idle speed, and not moving around some, especially when a slight load is placed on the engine......Cliff
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Thanks again guys for all of your help. I think I am done tuning for now. Maybe I will go with a slightly smaller cam sometime in the future, possibly a Summit 2801.
For now, engine idle in neutral at about 950 rpm and in gear about 650-700 rpm (stock converter, listed as 1700-1900 rpm stall). I ended up putting a "B28" vacuum canister on the distributor as a "B20" would not fully actuate in gear.
I now have total timing at 36* by 3000 rpm with base timing + manifold vacuum timing at 28*. This engine definitely likes as much timing as it can get at idle and since the vacuum can fully actuates at 8" of vacuum, it is always actuated at idle.
Part throttle, cruise, and WOT are all awesome. This car has only 2.56 gears and 29" tall rear tires but is still able to easily spin the tires easily even with limited slip diff. I am probably going to put shorter tires on it in the future or maybe even a lower ratio to get a little more out of the car, but for now, it is a great highway cruiser.
Thanks again guys!
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Good news! Good results are good results no matter where you end up with these things.
I've never owned an engine that ran worth two squirts of duck poop with 28 degrees initial timing in it, but if that's what it takes to make your engine happy it's a good thing in my book......Cliff