When selecting a carburetor, is it better to consider the cfm or the Venturi and throttle bore diameters? It appears that between manufacturers some of these diameters are different with the same advertised sized carb. I'm guessing that the difference is between the street/strip type carbs compared to the more race oriented ones. I have no real baseline to start with since my current Q850 carb has proven very difficult to tune to have a true baseline to reference. But I can say that I'm currently jetted the same as a stock Q750. I am not sending this back for the money that they want, so I'm gonna just buy another one. So in essence I'm starting over and could use some help / recommendations. If it is general consensus that a 4150 style 850 cfm is not to big of a carburetor for my set up, I was looking at the Holley 850 XP series purchased direct from them. Mainly because of the idle bypass feature and it has all of the same features as my quickfuel. or would I be better off trying one of the street/strip type carbs?
cfm . i kinda look at the other sizes but just mainly consider the cfm as far as engine needs . the other diameters may effect the way the air enters plenum or velocity etc . and for your listed motor i would say sure , 850 should be a good cfm arena for ya . especially for street , maybe a little larger even if its mostly a track car ? expensive proposition trying dif carbs out . you said your's wasn't responding to tune-up steps ? correct metering blocks on there ? no "race voodoo" done to carb ? sometimes those are steps backward . i would strip it down , inspect , make sure all original oem spec parts are in place etc and reassemble . start from the oem spec carb as a baseline and make small single changes from there - if you want to go through all that . or like you said , just start out with a new carb .
I'll take your advise and give this carb one last try. I called Holley today and asked them for the metering block part number. So the tech says it's 34-4qft for the Q850 so I say, isn't that a 4 emulation block? And he says "yup" standard calibration. I begin to tell him that I have 34-10qft 5 emulation blocks installed from the factory, so which ones are correct? Then I asked, could this be the cause of the problems I've been having. He says even though they fit that it may be the reason for dumping extra fuel causing tuning issues. He said something along the lines of lack of air velocity pulling fuel or something along those lines. By this time I was already getting frustrated because I felt that any one of the other techs from one of the past 5 phone calls should have thought about this as an option for all the tuning trouble I was having. After all I was doing everything that they were suggesting to do with the issues I was having. Suggestions from the phone techs and board members here made small improvements but at the end I still had the same issues. Bottom line here is, I purchased the so called "correct" metering blocks for this carburetor and will set it back to factory specs and give it one more go around. My fingers are crossed, but I'm skeptical.
Yeah , this is why I try and stick to the basic Holley carb series . If someone adjusted or switched something it could send you down a path with no direct way out when working with an HP series etc . Plus dif techs = dif answers sometimes .
I enjoy my prosystems carb. Took very minor adjustment from new. Any issue in had patrick quickly got me answers. Would recommend them.for any build
Tony, could you give a little more info as to where you carb is having problems such as: idle/transfer, main/cruise, or full throttle. When you isolate the trouble area, you can concentrate on tuning that circuit for best performance. Good luck.
Here's the thread, http://www.v8buick.com/index.php?threads/q850-idle-tuning-help-needed.300178/ Been at this for a while. I think the bottom line is that I'm getting extra fuel at idle somehow which is causing the mixture screws to be unresponsive in the normal sense.
Thanks for the confidence boost Del. I have a solid base line now instead of just throwing money at a new carb.
