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Help please -- Carb Adjustment question (Quick Fuel)
'78 Vette, 3spd auto, 3.55 rear end. 650cfm SS QFT carb, Mechanical secondary, Elec Choke. Mechanical fuel pump (stock). Edelbrock 2101 intake, original heads, True dual exhaust, no x or h pipe, hooker headers, no cats, open mufflers (no baffles). Stock motor as far as I know. No cam or oversize bore. HEI ignition (unknown specifics).
My question surrounds the mechanical secondary and specifically when it opens.
"Super progressive. 60% of primary throttle before secondary opens"
"Medium Progressive. 40% of primary before secondary opens"
"1-1. Even opening ratio for primary and secondary"
Just so I have it in my thick head correctly:
Super progressive would (should?) allow for better fuel mileage since the secondaries would be open less. 1-1 would be the opposite, more fuel going into the carb at all times. Medium progressive would be a "happy medium" between Super progressive and 1-1, correct? Or am i missing something?
Sorry, a bit of a back story, and I'll try and keep it short. I have a better than basic understanding of small engine carbs. Automotive, ehh, not so much, but I'm learning, slowly, but learning. Terminology is my issue.
Figured out my idle mixture screw(s) adjustment. The car surged at cruise speed on the highway, no surge on acceleration, no popping on rapid deceleration. Raised the floats a 1/8" turn, no change. Opened up the mixture screw adjustment 1/2 then 1/4 then 1/8 turn, then back to 1/4" and that seems to have taken care of the surging issue. It idles nice, but rich. Sometimes its hard to start. Exhaust smells of what I think is unburnt fuel. An Email to QFT and they say to "swap out the idle air bleeds for two sizes bigger". In my mind this will allow more air into the idle circuit and lean out (more air:same amount of fuel) and hopefully get rid of the "raw fuel" smell. Correct?
For example: if the current air bleeds are ".025-.035" then two sizes higher (larger) would be ".045-.055", right? (see pic above, sorry its so large), OR do I go from .025 to .027? (im just using those numbers for size reference, I have no idea whats in the carb now, but is listed on the tech page that came with the carb, Ill just double check it before I order the right sized air bleed screws (ha, look at me using the terminology ).
Then, unrelated to the above, I was thinking to improve the fuel mileage (or at least attempt to) perhaps I should adjust the secondary linkage to "super progressive" (if its not already). The last few tanks of fuel were in the 8.5 MPG to 10.5 MPG area. Was hoping to get to 15-18 or so. No idea if that obtainable or not with my cars configuration.
Is my theory flawed somewhere, or am I on the right track?
dodosmike
Last edited by Dodosmike; May 26, 2015 at 04:28 PM.
Are you using a wideband A/F meter to try and dial the carb in? I was running lean and made the adjustments. Now a bit rich but performance is night and day. I can understand why many have swapped over to efi whether it be Megasquirt or the TBI units. I have heard that the new Fitech units are shipping, looking fwd to reading reviews.
Are you using a wideband A/F meter to try and dial the carb in?
Like a vaccuum gauge? yep. Buddy did it for me after install. All he said was "its fine now". That was last fall before storage.
Upon restart this spring its stumbling when cruising on the highway. Seems to be ok "in town" driving, don't notice it anyway. Got that stumle adjusted out, just wondering what to do with the raw fuel smell and gas mileage now.
Some garages have portable units that you stick the O2 sensor in the tail pipe vs having one installed in your headers. It will give a reading of ie. 13.5, 12.7 14.1 or whatever your carb is setup at (lbs of gas to 1 lb of air). There are a number of guys on this forum who can tell you what you should be aiming at during idle and wot.
You really need to drop the float level to the recommend setting. Too high will have fuel seeping everywhere, too low and you'll have surge or bogg. Then mess with the idle screws. If you are more than 3-1/2 turns out, then mess with the air bleeds. Make sure you stuff a rag in the carb when changing air bleeds!!!
I'm assuming you made sure you have no vac leaks from your base gasket, brake booster,vac advance, ect...
[QUOTE= It idles nice, but rich. Sometimes its hard to start. Exhaust smells of what I think is unburnt fuel.
dodosmike[/QUOTE]
I have essentially the same question emailed into Quick Fuel for a HR780VS. I'll follow you on this post for the expert answers. I've got a strip kit for jetting Edelbrocks and I know how to use it, but I know next to nothing about this QF.
Finally succeeded in getting the idle to settle down after closing the secondary throttle plate adjustment screw. Who knew you'd have to do this on a new carb?
FYI, regarding the rough start/rich idle, Quick Fuel told me to go up on the idle bleeds about 3 numbers. I guess I'll be looking to source those jets from somewhere. Good luck with your secondary actuation. I hope it turns out right.
How many turns are your idle mixture screws from fully seated? Are they between 1 and 1.5 turns out? Have you adjusted the secondary throttle blades properly? I wouldn't think a near-stock motor would require much air bleed tuning.
How many turns are your idle mixture screws from fully seated? Are they between 1 and 1.5 turns out? Have you adjusted the secondary throttle blades properly? I wouldn't think a near-stock motor would require much air bleed tuning.
Keep in mind my limited automotive carb terminology and how the different functions affect and relate to other items.
I have no idea how many turns out they are. They're out not quite a half turn from the "out of the box" position. If I run them in anymore it "hesitates" at speed on the highway, which is 90% of my driving.
I have no idea how (or ever heard of) adjusting the secondary blades. Is this what I was referring to above, the "60% - 40% - 1-1" settings?
It was suggested by QFT to "go up on the idle air bleeds two numbers".
Papers from QFT show "71"'s in the carb now. Is that .071? no idea. So I'd move to 73's, or .073? Waiting on QFT to confirm to me.
Keep in mind my limited automotive carb terminology and how the different functions affect and relate to other items.
I have no idea how many turns out they are. They're out not quite a half turn from the "out of the box" position. If I run them in anymore it "hesitates" at speed on the highway, which is 90% of my driving.
I have no idea how (or ever heard of) adjusting the secondary blades. Is this what I was referring to above, the "60% - 40% - 1-1" settings?
It was suggested by QFT to "go up on the idle air bleeds two numbers".
Papers from QFT show "71"'s in the carb now. Is that .071? no idea. So I'd move to 73's, or .073? Waiting on QFT to confirm to me.
dodosmike
If you are within 1/2 turn of the factory setting you are probably in the ballpark. But you can always just turn the mixture screw clockwise until it seats and then back it out the same amount to determine it's position.
You want to adjust the secondary blades so that your primary throttle blades expose the proper amount of the transfer slot. If too much of the transfer slot is exposed, it will run rich. See below.
You want to adjust the secondary blades so that your primary throttle blades expose the proper amount of the transfer slot. If too much of the transfer slot is exposed, it will run rich. See below.
I'll check it out, thanks.
Any thoughts on my secondary link question? Super vs medium vs 1:1?
Super progressive should (in my mind) provide the most opportunity for better fuel mileage then the medium or 1:1 option.
Any thoughts on my secondary link question? Super vs medium vs 1:1?
Super progressive should (in my mind) provide the most opportunity for better fuel mileage then the medium or 1:1 option.
If it's not bogging or hesitating under acceleration I probably wouldn't mess with it. But it wouldn't hurt to try different settings. Your mileage is really primarily determined by the idle circuit and the primary metering circuit to a lesser degree.
Actually the selection should be based off the engine needs.
Super Progressive, 60% of primary throttle is used before secondaries open.
Medium Progressive, 40% of primary throttle is used before secondaries open.
One to one, Both open at the same rate.
If you use Super Progressive you need to know if you are running out of fuel before the secondary's start to open.
Using a wide band would show you the results.
I sent my SS to Lars to have it setup.
Here were some of his findings,
The secondary accel pump adjustment was slack and did not produce instant pump shot
-Mixtures were set very rich (12.2:1)
-Float bowl fuel levels were high - the primary side was very high
-Idle speed was high
-primary/secondary idle speed split was right on the money
-Mixtures set at a perfect 14.2:1
-Idle mixture screws are set at 3/4 turn out
Actually the selection should be based off the engine needs.
Super Progressive, 60% of primary throttle is used before secondaries open.
Medium Progressive, 40% of primary throttle is used before secondaries open.
One to one, Both open at the same rate.
If you use Super Progressive you need to know if you are running out of fuel before the secondary's start to open.
Using a wide band would show you the results.
Yes, but this would be very difficult to tune without a wideband unless he gets a lean stumble through trial and error.
What were the symptoms prior to the adjustment? What exactly was adjusted?
dodosmike
Hi Mike, sorry didn't see your reply. I have my story going on a similar thread right now, see "Crate 383 won't idle down and diesels". Basically, had trouble getting idle to come down, starts very rough and rich, and diesels after shut off.
So far I've solved the run-on after shutdown, and the 1400rpm idle by slightly closing the secondary throttle blades. Quick Fuel told me to leave them open "a crack", probably because of the four corner idle set up needs air coming into the secondaries even at idle. The solution to the rough/rich startup is said to be the idle air bleeds are too small. I have .70's now, I am picking up some .73's tomorrow and we'll see if that works. That number is visible on top of the bleeder using a magnifying glass. At this time the idle mixture screws barely have any effect and it runs best with those all the way in. Letting more air in with the bigger bleeds will lean the mixture and I should be able to open up the idle mixture screws a bit more and gain actual control over the mixture.
Don't adjust the secondaries unless you're having high idle and ruled out vacuum leaks, sticky linkage and timing. But the way you do it using a 3/32 allen wrench (hex key) to close or open the secondary plate. It's in a well hidden spot down low on the passenger side. Just look for the stop that limits travel of the secondary linkage there. I did the adjustment without pulling the carb but had to slowly turn the hex key with pliers, and I have smallish fingers.
A vacuum gauge is much different than an A/F gauge setup
Look at AEM setups on Summit they are about 200
Never had luck with the ones sticking up the pipe...any leak in the system too much overlap it couldnt keep up
Help please -- Carb Adjustment question (Quick Fuel)
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