Hello, I just got my 800 cfm carb bolted on my 350 and I was wondering what would be the best way to tune the carb to the engine for the best performance. The gentleman I got it from said it was rebuilt and setup for a stock '73 455. The car runs alot better with it on but, I can't seem to get the secondaries to open up. Can anyone help me here? Also, would it be possible to advance the timing in order to improve performance with this carb? I have a points distributor with a MSD blaster 2 coil. Any help would be appreciated, thanks. Rob
Problem with secondaries also I'm having problems with my carb to. (sorry don't know what it is, it's stock on a buick 350 Skylark I know that much) I've only had my secondaries open up once and that was when I was really mad was running 110 don't the highway and then they kicked in. Felt real good, but is there anyway to get them to kick in any sooner? Like a little less than 5000 rpm would be nice.
Hmm, I'm apt to guess the butterlies aren't opening up... On the left side (vac breaks side) on the airhorn assembly, the end of secondaries you have a square thing sticking out. On the underside is a hex screw and on the end is a flathead screw inside a shaft. Unscrew the hex screw until you can turn the flathead screw. Then turn the flathead screw until you feel resistance from the spring inside, go a-bit past that but don't let the butterflies open up, then tighen the hex screw. Make sure you hold the flat screw inplace while tighening the hex screw or the spring will unwind what you just did. That's my guess and I'm stickin to it.. I sure you'll get a few other ideas from ppl here.. For your Q-Jet Bible Needs, grab Doug Roe's book on Rochester Carbs.. Scott
Okay, I looked over the carb pretty good and I saw a rod running from the primary's butterflies to the secondary's butterflies (which operates the secondary's at WOT) and concluded the rod was too short. When working with the throttle I would floor it for alittle and only seen the secondary's flaps open slightly. Is this normal? Or do I need to get a longer rod so there is less travel in order to open the secondaries more sooner. Thanks, Rob
Hmm, your not confussing the top butterflies with the throttleplates are you? The throttleplates are linked together with rods which opperate on a 40% (or 60 i forget) open primary then starts opening the secondaries. The butterflies on top tho are vacuum operated over spring tension. You should note that the secondaries operate if the primary side is opened up beyond that 40 or 60 percent point, not just a wot.. So, if the throttle plates (sec side) ain't openin up, I'd think you should be able to bend the linkage to fix it. Also make sure if your carb has a choke lockout that it's not binding and holding the secondaries closed. Thats bout all I can come up with.. I'm sure someone will be able to give more ideas and such..
Thanks Scott, I took the choke off when I installed the carb. Will the lockout still affect the carb even though its disconnected? Btw, I'll try bending the rod. Rob
They used two different types (if any) of secondary lockouts during warm-up running.. One is a little lever that runs from the choke side of the primary butterly housing and just slides over the secondary butterfly keeping it from moving up. If the primary side is always fully open on top, then the lockout shouldn't move over to lockout the secondaries. The other is done with a cam, if the choke side of the secondary throttle shaft has a pin driven through it, it has that form of lockout. Poke around the linkage to figure out what cam it hits and when. If you completely removed the choke stuff and don't use nor want it, I'd see if the cam that locks the pin out can be safely removed. I'd tell ya as the one I have downstairs is done that way, but it sits in pieces at the moment. If it can, try removing it.. Otherwise, try to clean the linkages and such with carb cleaner and maybe a small toothbrush. That's bout all there is at the moment..
Isn't 800 CFM too much carb for a 350? Bigger isn't always better and with this much carb, you'll wind up with WOT flat spots and mixture problems when it's tuned to run well at lower RPM An ideal for the 350 is 600-650 CFM, depending on tune... 650 on a real firebreather. Just my shilling's worth.... JMC
I have a reworked 800 Q-jet on my buick 350, its now an 850 CFM! Small block Buicks LOVE bigger carbs, I've had mine on for 5 years now, and I have no complaints. Ya punch the dam thing and all you hear from the engine is the HUGE secondary open up and the air going in. If I was to do it again I'd put the same carb on it now, my Firebird has a Speed Demon 750 on it because it needs it on the small block, 12:1 comp, its 1 healthy Chevy moter (i know a chevy in a pontiac, but its all that i had to put in it, i drive it every day and need a quick fix. Besides a 301 is 1 big piece of YOU KNOW WHAT!!) P.S. The car is know a custom Trans-Am paint, body, roll bar (YEA OFFICER ITS STREET LEGAL) :laugh: Larry
Larry, I don't doubt that a carb that is larger than ideal will work... and sometimes well... at the expense of economy and/or drivability. If the engine can only use 650 CFM at max (reuseable engine) RPM, then the capacity of a larger carb is not utilized. You're right that Buick engines like to breathe, but a larger carb can't make 'em breathe more than the design limit, only more cubes and/or a super/turbo charger will get more volume through the engine. You'll get far better performance and tunability by matching the carb to the engine capacity, then doing the detail work of porting, polishing and matching, tweaking the ignition and installing dual exhaust. JMC
My 350 picked up .4 sec. and 4 mph in the quarter going from the 750 to 800 cfm Q-Jet. The 750 carb. ran good the 800 runs better. My `72 Skylark has no head work only a little better cam, see sig. :grin:
Gotta wonder how it'd run with a properly tuned/jetted matched flow carb on it, then... probably haven't tried that. Then, clean up the heads and see what happens. JMC
John, Believe me, my 350 runs so much better with the 800 cfm carb on. I believe the original carb is a 700 or 750 cfm from factory ('70 350). I think I'm going to check the amount of vacuum my engine is producing, thinking that might be the problem. I hear a small hiss at idle so, I might have a leak. Can anyone tell me the amount of vacuum needed for a 800 cfm carb from a '73 455? Thanks, Rob
John: The 800 cfm will run fine on the 350. I've got a stock 72 conv (loaded) that runs primo with the 800. (15.20) No bog, pulls great at any rpm. It's really only a tad bigger than the original anyway. Sure if it was a 800 double pumper Holley, a 650 would be better. But the air valve makes all the difference allows the motor to take the air/fuel it wants. Bruce BQUICK
