QJet on top of a Weiand 142...
11-23-2006, 09:37 PM
#1
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Car: Bee-Bowdy
Engine: blowd tree-fity
Transmission: sebin hunnerd
Axle/Gears: fo-tins
QJet on top of a Weiand 142...
I've been tuning my QJet for last couple of weeks but it seems like I'm chasing my tail. Originally the carb had 73 jets and 40K rods with an unknown power piston spring. I set out to safely setup the carb for use with a Weiand 142 pushing 4.5 lbs of boost. Idle, off-idle and WOT were ok. The area I felt needed improvement was just before my secondaries opened. The power kinda went flat. My boost gauge showed ZERO vacuum at the time of the flat spot. The gauge is gets signal from below the blower so I realize vacuum there and below the carb may be different.
I tried going with a 75/45K combo and Edelbrocks "black" spring. It ran too rich. WOT performance suffered as well as driveability. Tip-in just after decel on the hiway produced and intolerable bog. The only thing that improved was the ZERO vacuum spot. I seemed to pull harder.
I put the original 73/40K back in with the unknown spring to start over. I learned that a bunch from PROM tuning. Sometimes just have to scrap the whole rather than applying band-aid after band-aid.
Now with the 73/40K combo I installed the black spring once more. My engine pulls 19" of vacuum in gear. The unknown spring was very light so I thought that maybe the hienous vacuum was keeping my rods down. Performance picked up considerably in the ZERO vacuum area.
All that leads me to this...I was giving my bro a ride on the splendidly empty holiday roads. When I hammered it from 2nd it seemed to bog, then once I hit ~3000 rpm it came to life. I'm thinking it was a rich bog, it just seemed lazy then POW.
where to go?...
I tried going with a 75/45K combo and Edelbrocks "black" spring. It ran too rich. WOT performance suffered as well as driveability. Tip-in just after decel on the hiway produced and intolerable bog. The only thing that improved was the ZERO vacuum spot. I seemed to pull harder.
I put the original 73/40K back in with the unknown spring to start over. I learned that a bunch from PROM tuning. Sometimes just have to scrap the whole rather than applying band-aid after band-aid.
Now with the 73/40K combo I installed the black spring once more. My engine pulls 19" of vacuum in gear. The unknown spring was very light so I thought that maybe the hienous vacuum was keeping my rods down. Performance picked up considerably in the ZERO vacuum area.
All that leads me to this...I was giving my bro a ride on the splendidly empty holiday roads. When I hammered it from 2nd it seemed to bog, then once I hit ~3000 rpm it came to life. I'm thinking it was a rich bog, it just seemed lazy then POW.
where to go?...
11-23-2006, 11:33 PM
#2
11-24-2006, 12:36 AM
#4
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Car: 1982 Trans-Am
Engine: 355 w/ ported 416s
Transmission: T10, hurst shifter
Axle/Gears: 10 bolt, true-trac, 3.73
Is that possible?
Wouldn't that expose the power piston to boost, which would blow the piston upwards?
I figured as is, it's referenced to the vacuum below the carb (the blower plenum), wouldn't that just make the carb think it's sitting on a larger cube motor?
.
.
.
ok, brain fart - I get it. The blower will provide suction, which would close the power piston, as if low throttle on a NA vehicle right?
ok, five 7 is onto something
So you're probably not enjoying the full jet size, but once you open the secondaries you get all the power you enjoy. If you can find a way bring the power piston vac hole to below the blower, you might get the full usage of the power system.
This analysis leads to a *lean bog*, since it's sucking the power piston spring down once you spool up the blower.
just a thought.
.
.
I can think of a way to reference the PP spring to manifold, but it's kinda difficult, so i'll keep it to myself in the hopes someone else has a good way to do it, if not, i'll tell ya how i'd do it...
Wouldn't that expose the power piston to boost, which would blow the piston upwards?
I figured as is, it's referenced to the vacuum below the carb (the blower plenum), wouldn't that just make the carb think it's sitting on a larger cube motor?
.
.
.
ok, brain fart - I get it. The blower will provide suction, which would close the power piston, as if low throttle on a NA vehicle right?
ok, five 7 is onto something
So you're probably not enjoying the full jet size, but once you open the secondaries you get all the power you enjoy. If you can find a way bring the power piston vac hole to below the blower, you might get the full usage of the power system.
This analysis leads to a *lean bog*, since it's sucking the power piston spring down once you spool up the blower.
just a thought.
.
.
I can think of a way to reference the PP spring to manifold, but it's kinda difficult, so i'll keep it to myself in the hopes someone else has a good way to do it, if not, i'll tell ya how i'd do it...
11-24-2006, 07:41 PM
#5
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The qjet has limited accelerator pump shot volume.
Just enough for a stock intake manifold's internal volume.
The combined internal volume of the blower and blower manifold has much more internal volume, requireing a much bigger accelerator pump shot when ya slam the throttle open.
Shortening the accelerator pump piston's shaft length will increase the pump volume. Moving the linkage to the inside hole will also help. Make sure the vacuum sec delay diaphram is functioning to delay sec opening a bit. The amount of secondary air door pull over fuel can be increased by modifying the air doors initial open position relitive to the discharge ports. make it so its just open a crack at rest.
Increasing the secondary pull over shot volume requires internal drilling in the carb body and down tubes. get a QJET carb book. By Doug Roe.
Recurve the distributor to allow much more initial advance at idle, yet the same 32-34deg mechanical advance at high speed. Limit the travel of the mechanical advance.
The more initial advance, the more responsive the throttle will be. blower motors like lots of initial advance (18+deg) yet need reduced total mechanical advance to avoid pinging under boost (26-32deg)
Just enough for a stock intake manifold's internal volume.
The combined internal volume of the blower and blower manifold has much more internal volume, requireing a much bigger accelerator pump shot when ya slam the throttle open.
Shortening the accelerator pump piston's shaft length will increase the pump volume. Moving the linkage to the inside hole will also help. Make sure the vacuum sec delay diaphram is functioning to delay sec opening a bit. The amount of secondary air door pull over fuel can be increased by modifying the air doors initial open position relitive to the discharge ports. make it so its just open a crack at rest.
Increasing the secondary pull over shot volume requires internal drilling in the carb body and down tubes. get a QJET carb book. By Doug Roe.
Recurve the distributor to allow much more initial advance at idle, yet the same 32-34deg mechanical advance at high speed. Limit the travel of the mechanical advance.
The more initial advance, the more responsive the throttle will be. blower motors like lots of initial advance (18+deg) yet need reduced total mechanical advance to avoid pinging under boost (26-32deg)
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