Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
1903 Won't Respond to Tip-In Procedure
bry593:
Thanks Cliff, I'll give that a try.
For sure the direct horn UIAB is much more sensitive to tiny changes in the circuit. Also a lot easier to get plugged up than the larger and down pointed body bleeds.
I have another question which assumes a return to original 1903 calibration. Is it possible to tap the main body UIAB #4-40, insert a brass SS and drill the center to .070? Or will this not work the same as the original .070 drilled hole? I didn't see this technique mentioned in your book, only the Al tig rod and resize. I have both 3/32 and 1/8 rod on hand, but it seems like it would be easier to drill a setscrew.
Cliff Ruggles:
About the biggest you can drill a #4-40 will be .059" if you stay on center.
The stock idle airbleed sizes in the 1903 are fine for what you are doing.
The 1910's showed up with several different calibrations as it seems like Edelbrock was playing around some there but they are the exact same unit. The most common 1910's have .038" idle tubes but I've seen them with .040's.
They used the same smaller MAB's and 74 jets with 50M rods, pretty sure the 1903's got 73 jets.
Looking at your engine combo it has a very small cam in it for the CID and compression ratio. Not sure why they ground it on a 109LSA as that would close the intake pretty early with such short seat timing and may make your engine octane sensitive, or at least you may experience some pinging at light throttle if you add too much vacuum advance and heavy/full throttle as well if the mechanical comes on too quick or too much of it.
Have you taken a vacuum reading at idle speed, 700-750 rpms with around 10-12 initial timing and no additional timing from the advance?
Anytime I've tuned larger engines with tiny cams in them they have not wanted, liked, or responded well to a lot of timing at idle speed and adding timing has made them difficult to tune as the throttle plates were pretty well closed at idle speed.
As mentioned previously the only thing I'm not sure of is the small cam on such a tight LSA. I did work on a 355 last year with a 194/204/108 cam in it. The owner brought it here for idle issues. I did the carb for it about 10 years ago for the stock 305 engine and this new combo wasn't happy about it.
I went in and opened up the idle tubes .002" and DCR's to .055" and that's all it needed. Even so I was not impressed with the engine anyplace. It had a "quirky" idle and even though the engine builder dubbed that cam a "torque monster" in a 350 build, I thought is was "weak" compared to 350's I've done with the 194/204/112 cam in them......FWIW......Cliff
bry593:
Haven't checked the vac since last year, but it seems like it was around 18" Hg.
Yes, you know how GM is with crate motors, they just throw whatever they have lying around at it. The cam is left over from the Ram-Jet 350. This HT383 also has cold piston slap due to the short 880 block bore combined with a stock 5.7 rod. I wouldn't recommend it.
For drilling setscrews, I made a jig to hold the screws. It's just a piece of 20 gauge sheet with a 4-40, 6-32 and 8-32 nut epoxied to the sheet. You simply thread the setscrew into the nut with the cup point up. The cup point acts as a centering pilot for your pin drills. Maybe not the best method, but works okay for me.
Cliff Ruggles:
I usually drill them in place but have a piece of flat aluminum about 3/16" thick tapped part way with 4-40, 6-32, 8-32 and 10-32. I just seat the brass set screws and drill the desired hole size in them, not really fussy and only take a few seconds.
Will it make that much vacuum without a lot of timing and down around 700-750rpms?
I've been building SBC's for over 40 years and have never done or wanted to do a "383" build.
I have several excellent "recipes" for 355's and stick with them. They make great power with improved rod length to stroke and bore to stroke parameters.
I actually prefer a 327 or using the shorter 3.25" stroke cranks, but they didn't make a lot of large journal 307's and 327's so they are difficult to obtain.
Compression is also your friend with SBC engines and they are fine on pump gas WAY beyond the proverbial "brick wall" of 9.5 to 1 we see regurgitated on every Forum you log onto by folks who probably haven't built an engine that made chit for power or even have an SBC powered car or truck that moves!.......LOL....
bry593:
Thanks for all the advice Cliff! The off-idle hesitation problem is fixed.
The power piston spring had tilted and slightly slipped into the vacuum channel and those coils to bind. This had the effect of a shorter spring and would not lift the primary needles out of the jets at light throttle (too much vacuum with this tiny cam). Note that I'm using the stiffest (DrkBlu) piston spring.
To reduce the vacuum channel diameter, I dropped a #8 brass washer into the piston bore. The OD was too big too fit, so I "turned" it down using the following method. The washer was sandwiched between the head and nut on a screw, chucked in my drill and sanded with a die grinder while the drill was activated. Deburred by flat sanding on a piece of 400 grit.
Retained the lean idle calibration (.031 IT, .041 UIAB, .064 LIAB), and added .052 IABP. Also installed a 195F thermostat to improve efficiency per your recommendation. With the spring fixed, throttle is instant and seamless through the primaries with only a slight delay in the 2ndy.
Will run this calibration a few weeks to get a feel for it. I'm hoping the smaller idle tubes will help achieve a target of 18mpg @ 75mph (1971 3/4 Chevy, 383, 4L80E, 4.10 HO52).
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version