I just switched from a 4 barrel quadrajet to a street demon 625 cfm carb. its on a newly rebuilt stock 283. the car ran fine before the carb change, after the change its idling at about 980 RPMs..my timing is set to 32 degrees total advance. everything is running great its just idling to high I read on here that it should be in the neighborhood of about 500-600 RPMs. the demon carb manual says that its already fine tuned out of the box and they STRONGLY recommend no adjustment to the idle screw or needles, so with that what now? Is adjusting the carb the only way to lower the idle after the timing is set? Should I just go ahead and follow the manuals procedures to adjust the idle even though they say I shouldn't..? id sure hate to untune a perfectly good carb. Also is the 500-600 RPM range correct, im just using it as a cruiser and possibly daily driver. all guidance is appreciated.
Idle speed adjustment is different than idle mixture screws. I think they are referring to the idle mixture. What idle speed is determined by several factors, but if your engine is near stock then 600 rpm is good place to start.
Check for a vacuum leak first, then adjust as needed once your sure its not sucking air somewhere else. You should call them and ask if they will send over a factory rep to do the adjusting for you since they feel that you aren't qualified to do it yourself. Just how in the hell are they supposed to "fine tune" a carb at the factory when they don't have a clue what that particular engine that carb is going on?
what they are trying to prevent is idiots cranking the idle speed screw in until the throttle plates are way up into the transition circuit. Have you checked to make sure the throttle linkage isn't holding the primary throttle plates open a bit? How much vacuum is it pulling at idle? the carb is probably set out of the box for an engine that pulls less idle vacuum than yours, that's why it's idling a bit high. Assuming it's not a 4 corner idle circuit, I would start by looking at the secondary throttle plates, they are probably open a hair out of the box, try closing them a bit. If that doesn't do it, close the primary side a little. A LITTLE is the key word here, and make a note of how much you turn it. I like to see no more than .030 of the bottom of the transfer slot exposed at idle, but this is on holleys. Oh, and second what timman says, BEFORE you start messing with th carb, make ABSOLUTELY CERTAIN you aren't pulling air somewhere else.
You had a Quadrajet that ran fine, and now you changed it and it doesnt run right.....click the undo button. Good Luck
I don't do Demons, but my understanding is that the Demon is a square-bore carburetor. You replaced a Q-Jet which definately is a spread-bore carburetor. VERY possible that you now have a vacuum leak where the carb bolts to the intake. And a vacuum leak will artificially significantly raise the idle. So before doing any adjustment to the carb, check for the vacuum leak around the carburetor mount. Not sure why you would want to replace a Q-Jet with a Demon, but guess you had your reasons. Good luck, and please report back what you find. Jon.
Anyone that questions the capabilities of a Quadrajet never witnessed 60's/70's stock and superstock drag racing!
^^^Your fast on the draw Eastwood! "Unless I am wrong, and I am never wrong", the OP is referring to the new "3 barrel" street demon. It is a replacement for spread bore carbs Long gone are the Barry Grant Demons that are bemoaned on this site. These are sup[posed to be good carbs. I own 2 of the BG Demons. One is perfect, one is crap. I too would go with the QJ.
BB - still believe he should check the mounting. A LOT of O.E. Q-Jet manifolds would not seal to that carb without Mickey Mousing gaskets. Jon.
Crap, never even THOUGHT of one of those things! It is probably sucking wind at the baseplate. and I agree, Holley guy myself, but to be fair, the quality control issues with the Barry Grant carbs ten fifteen years ago have long since been rectified. Why the hell anyone would shitcan a q-jet on a mild daily driver small block is beyond comprehension, but hey, nothing new about that, LOTS of choices guys make on here are beyond my comprehension...
Go grief take note of the position of the idle and or mixture screws and tune away. If not happy turn them back to stock specs. Been my experience in the auto parts for many years that many that complain about "junk" products know the very least. My bothers little 66 Mustang with a 332 stoker with speed demon, radical solid roller cam, afr cylinder heads, etc etc. Did it run perfect out of the box? No.how does it run now, high elevens on street rubber and killer on the street. Every motor will require some fine tuning some far more then others as the example I listed. Turn them screws or find someone that will that has carb tuning experience. But rule number one as others have posted make sure everything is correct first such as a vacuum leak etc Cheers. Sent from my SM-G920R4 using Tapatalk
George, you know the power of marketing and advertising, Holley had little competition for years and they threw big money at all the car rags in the form of supplying new products every time the magazine had a project build. When I was a kid, I had my head buried in all the performance magazines, early on, an older friend of mine told me "don't believe everything you read in the magazines". Hell, a new Holley 1850 with an aluminum adapter was one of the first mods on the 283 in my 57 Chevy, replacing the 4GC.
100% on board with that. I have never used this new carb and that is sound advise carbking. Penguins and gearheads are always looking for a tighter seal.
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