I just bought a new Speed Demon 750 carb with vacuum secondaries, annular boosters, and an electric choke. It is replacing a tired 650 Holley double pumper that blew a needle and seat (and with it my patience ). I simply unbolted the Holley from the manifold and installed the Demon, and to my surprise it did not function very well with the factory adjustment. I've bought Demons before and they basically bolted on and worked great - this one is an exception. Here's my engine setup for reference:
Chevy 355, decked to 5/1000" about 11:1 compression with flat tops (hi-octane fuel)
Ported 305 heads, 58cc chambers, comparable to stock Vortec head flow numbers
XE268 Comp Cam, 224/230 @ 0.050, .477/.480 lift, 110* LSA
1-5/8" headers into 3" single exhaust
Edelbrock RPM Air Gap manifold
Stock stall 700R4 tranny with 3.27 gears.
Basically this is a common 350 buildup, and I don't understand why the factory settings on the Demon shouldn't at least be close. There was nothing wrong with the motor (no vacuum leaks) before the Holley blew and I haven't changed anything since besides the carb itself.
After I let the choke disengage and the engine warm up at idle - a pretty choppy idle - I gave the engine a few revs and discovered that if I lightly tipped the throttle or let it rev just above idle (about 1000-1300), the engine goes very lean and it struggles to keep running. I even had it die a few times. If I punch it hard it doesn't seem to have a problem as the accelerator pump compensates enough for the lean condition. Also, as the main circuit starts to take over after about 1300, the lean condition improves and by 2000 everything seems fine, there's no more white smoke or sooty smell from the exhaust, and the engine is very throttle responsive. It could still be a little lean at this RPM, but it's hard to tell. Also, it doesn't seem to have any problems at idle, with the screws set about 2 turns out. The problem is just after idle speed, i.e. transfer slot territory. If I turn the idle screws out all the way to try to compensate for the lean condition just after idle, it helps a bit, but it is still lean, and the idle is stinky rich. I tried adjusting the throttle blades so the secondaries are closed all the way so I could open the primaries a bit more but it did not help. I tried raising the floats but it did not help either. Both floats are at the top of the sight window instead of the middle now. I took off the carb and checked to see if anything was plugging the transfer slots or idle mixture discharge orifices, but everything checked out. I figure the only other thing I could adjust is the jet sizes, but I thought I better ask here first before I spend the $$$ on a jet set (also Barry Grant Tech Support is off for the weekend). I want to make sure it's not something else, because I can't imagine a brand new Demon carb being this far out of tune from the factory.
If there something else I can try that I may have missed, I would appreciate someone pointing it out to me. I'm going to try farting around with it a bit more tomorrow morning, but if I can't get it right I'll go a buy a jet set and try changing those.
Chevy 355, decked to 5/1000" about 11:1 compression with flat tops (hi-octane fuel)
Ported 305 heads, 58cc chambers, comparable to stock Vortec head flow numbers
XE268 Comp Cam, 224/230 @ 0.050, .477/.480 lift, 110* LSA
1-5/8" headers into 3" single exhaust
Edelbrock RPM Air Gap manifold
Stock stall 700R4 tranny with 3.27 gears.
Basically this is a common 350 buildup, and I don't understand why the factory settings on the Demon shouldn't at least be close. There was nothing wrong with the motor (no vacuum leaks) before the Holley blew and I haven't changed anything since besides the carb itself.
After I let the choke disengage and the engine warm up at idle - a pretty choppy idle - I gave the engine a few revs and discovered that if I lightly tipped the throttle or let it rev just above idle (about 1000-1300), the engine goes very lean and it struggles to keep running. I even had it die a few times. If I punch it hard it doesn't seem to have a problem as the accelerator pump compensates enough for the lean condition. Also, as the main circuit starts to take over after about 1300, the lean condition improves and by 2000 everything seems fine, there's no more white smoke or sooty smell from the exhaust, and the engine is very throttle responsive. It could still be a little lean at this RPM, but it's hard to tell. Also, it doesn't seem to have any problems at idle, with the screws set about 2 turns out. The problem is just after idle speed, i.e. transfer slot territory. If I turn the idle screws out all the way to try to compensate for the lean condition just after idle, it helps a bit, but it is still lean, and the idle is stinky rich. I tried adjusting the throttle blades so the secondaries are closed all the way so I could open the primaries a bit more but it did not help. I tried raising the floats but it did not help either. Both floats are at the top of the sight window instead of the middle now. I took off the carb and checked to see if anything was plugging the transfer slots or idle mixture discharge orifices, but everything checked out. I figure the only other thing I could adjust is the jet sizes, but I thought I better ask here first before I spend the $$$ on a jet set (also Barry Grant Tech Support is off for the weekend). I want to make sure it's not something else, because I can't imagine a brand new Demon carb being this far out of tune from the factory.
If there something else I can try that I may have missed, I would appreciate someone pointing it out to me. I'm going to try farting around with it a bit more tomorrow morning, but if I can't get it right I'll go a buy a jet set and try changing those.