I will give it a shot. Easy since the main body already had a replaceable tsr. And if it cleans up 1 full point at 60 mph, that's an awfully big improvement for a little bit of work.
I would like to play with the IFR's again, but with winter coming this gives me a chance to make a nice difference without too much tinkering before I can no longer drive the car.
Leaning cruise on Quick Fuel 850
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Re: Leaning cruise on Quick Fuel 850
At 60 mph on a level road It should be able to run with A/F leaner than stoichiometric.i82much wrote: ↑Wed Oct 11, 2023 8:22 pm I will give it a shot. Easy since the main body already had a replaceable tsr. And if it cleans up 1 full point at 60 mph, that's an awfully big improvement for a little bit of work.
I would like to play with the IFR's again, but with winter coming this gives me a chance to make a nice difference without too much tinkering before I can no longer drive the car.
At that speed it is in a load range at the beginning of main flow and main is overlapping T-slot, so both T-slot and main jet are influencing factors, but you adjust the main jet to suit the load range of moderate acceleration in the vacuum range 12-10-8 inches Hg.
At that speed (60) on level road, crowd the throttle so the vacuum drops to 10" and observe the A/F. It should be leaner than it is at the steady speed throttle position. If not, reduce the main jet and see how it effects the A/F at both steady speed and moderate acceleration.
You may find your engine gets more responsive in the mid-pedal as the A/F moves to the lean side of stoichiometric. Some engines get a remarkably more 'peppy' feel to the throttle when the mid range is leaned out and the reduced fuel consumption is worth the effort.
Re: Leaning cruise on Quick Fuel 850
Finally got back on this. Thanks for all the help so far. Is this a pressed-in restrictor? I guess i need to pull the carb and drill/tap for replaceable restrictors?
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Re: Leaning cruise on Quick Fuel 850
You said in an earlier post the restrictor is .093" and that seems unusually large because it would have very little or no effect. The picture looks like there may be a smaller orifice at the far end of the pressed in bleed, so check it and be sure what size it is.
You can fiddle with this using the old "put a piece of wire in it" trick. If it is .093" you could try a piece of 1/16" welding rod or ? and that will restrict it to an equivalent area about .067" or .068" which will get its attention and should swing it from too rich to too lean. If that is the case or not, it will give you an indication of the range in the throttle opening where the slot is the main factor.
To know if there is a reasonable possibility of success with tuning the slot you really need to measure the slot width and length. Some of the billet bases have slots too wide and too long and those are a waste of time to try to tune.
For the 1-11/16" throttle bore the ideal slot is .025" (maximum .026") width and 3/16" long (less than .200"). If you have a fubar part the best solution is an old school Holley base from a swap meet, or new base from BLP. Whatever base you buy, measure it yourself or confirm with the supplier before you buy.
The original production 3310 and succeeding 780-800 CFM Chevy carbs from '65 to '72 have the slot as described above ...... and thousands of them ran just fine and the survivors still do ....... imagine that. The original double pumpers during the first few years of manufacture (about a decade) were correct. In the last 20 or 30 years they jump around and some are good and some are funky, you never know until you check. I guess you could say Holley et al FA and we FO. When the billet bases (made in chynah) started to be the New Big Thing nearly all of them were/are funky.
BLP is an exception, they understood the slot length/width importance and 15-20 years ago they were curing the long slot in Dominators with a machined billet plug inserted in the bore to correct the too-long slot.
You can fiddle with this using the old "put a piece of wire in it" trick. If it is .093" you could try a piece of 1/16" welding rod or ? and that will restrict it to an equivalent area about .067" or .068" which will get its attention and should swing it from too rich to too lean. If that is the case or not, it will give you an indication of the range in the throttle opening where the slot is the main factor.
To know if there is a reasonable possibility of success with tuning the slot you really need to measure the slot width and length. Some of the billet bases have slots too wide and too long and those are a waste of time to try to tune.
For the 1-11/16" throttle bore the ideal slot is .025" (maximum .026") width and 3/16" long (less than .200"). If you have a fubar part the best solution is an old school Holley base from a swap meet, or new base from BLP. Whatever base you buy, measure it yourself or confirm with the supplier before you buy.
The original production 3310 and succeeding 780-800 CFM Chevy carbs from '65 to '72 have the slot as described above ...... and thousands of them ran just fine and the survivors still do ....... imagine that. The original double pumpers during the first few years of manufacture (about a decade) were correct. In the last 20 or 30 years they jump around and some are good and some are funky, you never know until you check. I guess you could say Holley et al FA and we FO. When the billet bases (made in chynah) started to be the New Big Thing nearly all of them were/are funky.
BLP is an exception, they understood the slot length/width importance and 15-20 years ago they were curing the long slot in Dominators with a machined billet plug inserted in the bore to correct the too-long slot.
Re: Leaning cruise on Quick Fuel 850
Finally got around to this. A 47 drill bit was too lean under light throttle cruise, just test drove it after a 46 drill bit and was a lot better. Still seeing some spots where it is around 16:1 on the wideband and gets a little balky so will try a 45 next.
Any way you slice it I think this was a good mod, but I was surprised to see that it only leaned out the carb under a limited range of conditions. I am thinking I will need to drop the front jets and enlarge the PVCR if I want to reliably cruise around at 13:5 to 1 or higher AFR's, still seeing a lot of numbers in the 12's.
Currently jetted at 78/84 with a .069 PVCR. Any thoughts on how much bigger I can go with the PVCR and how far to drop the fronts? WOT is good right now, in Spokane about 2000 ft elevation.
Any way you slice it I think this was a good mod, but I was surprised to see that it only leaned out the carb under a limited range of conditions. I am thinking I will need to drop the front jets and enlarge the PVCR if I want to reliably cruise around at 13:5 to 1 or higher AFR's, still seeing a lot of numbers in the 12's.
Currently jetted at 78/84 with a .069 PVCR. Any thoughts on how much bigger I can go with the PVCR and how far to drop the fronts? WOT is good right now, in Spokane about 2000 ft elevation.
Re: Leaning cruise on Quick Fuel 850
What size is the main air bleed? A smaller main air bleed will delay the start of the main some and it will have less overlap on the T-slot.
Are you using a vacuum gauge? What is the RPM and manifold vacuum in the range you want to be leaner?
Are you saying it is acceptably lean in the light throttle T-slot range, the more or less level road range, and then gets richer as you add a bit more throttle for moderate acceleration?
Are you using a vacuum gauge? What is the RPM and manifold vacuum in the range you want to be leaner?
Are you saying it is acceptably lean in the light throttle T-slot range, the more or less level road range, and then gets richer as you add a bit more throttle for moderate acceleration?
Re: Leaning cruise on Quick Fuel 850
High speed air bleed is 28. About 15" of vacuum, cruising from 2400 rpm on up it is too rich. The transfer slot restrictor helped at light throttle, lower rpm but doesn't do much as you cruise a little faster at wider throttle openings.
Re: Leaning cruise on Quick Fuel 850
This is in a throttle range where float level has significant effect, so maybe down a flat or two on the adjusting nut ?? have you tried this?
However, it sounds like a smaller main jet is the logical thing to do.
This area of load and throttle position is where the main nozzle starts and overlaps the fuel from the T-slot. A smaller main air bleed will reduce the flow from the main circuit in this throttle range where the nozzle in the booster is just starting to discharge.
Because you don't have to open the carb and the MAB is easy to change, I suggest .025", or you could use .022" to see more of the effect of the bleed in this range.
However, it sounds like a smaller main jet is the logical thing to do.
This area of load and throttle position is where the main nozzle starts and overlaps the fuel from the T-slot. A smaller main air bleed will reduce the flow from the main circuit in this throttle range where the nozzle in the booster is just starting to discharge.
Because you don't have to open the carb and the MAB is easy to change, I suggest .025", or you could use .022" to see more of the effect of the bleed in this range.
Re: Leaning cruise on Quick Fuel 850
thanks tuner. i am gonna bump the transfer slot restrictor up one drill size. if i drop the front jet to a 76, would you suggest increasing the PVCR?
Re: Leaning cruise on Quick Fuel 850
Because it is the easiest thing to do and un-do, I would change to a smaller main air bleed. Then, if that does not have the desired effect, go inside the carb and make changes.
Drop the primary jet makes sense, but don't change the PVCR until you drive it and see how much the jet change affects the WOT.
A smaller main air bleed should make it leaner in the early part-throttle range by delaying and reducing flow in the beginning of main flow, and also make it richer at WOT, tilt the A/F leaner down low and richer up top.
Whatever you do, only do one thing at a time and keep notes.
Drop the primary jet makes sense, but don't change the PVCR until you drive it and see how much the jet change affects the WOT.
A smaller main air bleed should make it leaner in the early part-throttle range by delaying and reducing flow in the beginning of main flow, and also make it richer at WOT, tilt the A/F leaner down low and richer up top.
Whatever you do, only do one thing at a time and keep notes.