I recently replaced my two barrel with a q-jet on a 350, the car does feel like it accelerates faster from a roll, but its a little more sluggish off the line at part throttle. A lot of people here have probably done the same swap and I'm just wondering if you've had the same experiences or is there something I should fix
You are probably going to want to change the timing a little with the 4bbl. Also make sure the choke is working properly. I think alot of the guys here are running around 10* initial with a stock cam. I think the vacuum advance is also different from the 2 or 4 bbl models. You may want to get a curve kit that has different bushings and springs to get the timing in a little sooner. You can pick those up for around 10-15 bucks at about any parts house. Mike
Timing will cause this too, but make sure your accellerator pump is working. Look into the carb with the motor not running, and briskly open the throttle. You should see a steady unbroken stream of gas squirting into the primaries. If not, the rubber plunger needs to be replaced.:Smarty:
Thanks for the help guys; I already had the timing running at about 12 degrees initial, but I've never gotten around to changing the timing curve. IMO the choke and accelerator pump work fine, Ive spent some time adjusting the choke and I don't think there's a problem with the pump, if anything it sprays more fuel then it should. I thought it might have been the timing but I'm also wondering if its simply because the primaries are smaller then the bores on a two barrel and provide less air flow without the secondaries open. Sorry for the long post but I'm just typing while I think here.
Steve, Did you change the accelerator cable? Have someone look down the carb, with the motor off, while you depress the pedal wide open. Make sure the secondaries are opening all the way. You'll have to hold the airvalves open to see the secondary blades. The cable may be too short or long, I've seen this many times before. Check the simple things first.
I did the same thing 18 years ago on the 2NABOT. Big difference! Where did the Q-jet come from? Start by making sure the jets and rods, etc are correct for your year/model. Then, find a copy of Doug Roe's book "Rochester Carburetors", by HP books, available at most any large bookstore. Great bathroom reading!!! There are a LOT of things on a Q-jet that can be adjusted/misadjusted, like secondaries not opening fully, secondary rods too low/high, secondary wind-up spring too tight/loose...that's just a few of the things related to the secondary side, not mentioning the primaries and transition circuits. READ, then take your time, a Q-jet is a great carb! Then, I'd seriously recommend you do two things, which may seem expensive at first, but considering all the time and effort I spent piddling around in the dark, would have been WAY cheaper for me. 1. Send your Q-jet to John Osborne (search this board for contact info), he's a highly rated carb guy (I'm sending him mine once I make up my mind about a 455). He'll want some specific info, but for about $250, you'll get a high quality rebuild and a dead-on tuned carb for YOUR car. 2. Send your distributor to IgnitionMan (also on this board) for his rebuild and HEI conversion. For $175, you get your ignition set up dead-on for YOUR car. Total cost, about $425, plus/minus some. I'll bet I spent that and more, between carb kits, junkyard parts, and time banging my head against the hood latch! ou:
Thanks for the replies. Once I get back to her I'll check the throttle valves and other odds and ends and I should probably get a copy of that book. Too bad my college won't let freshman bring cars, I have to wait till March to see if any of this works:ball: . The carb is off a stock '69 350, and my car is almost stock so I'm assuming the jets and rods are in the ballpark for my application. The carb was sitting for a little bit and there was some corroded aluminum in the float bowl and into the main jets, but my father (who cleaned it) believes he got it all out. The car idles better then ever so I don't think the corrosion is a problem. I've heard a lot about the good work that John Osborne and the Ignitionman do, but, right now, college doesn't leave me with much money to "waste" on my hobby. I'll have to think about it later when mom and pops aren't quite like this :rant:
Make sure your vac advance can is working properly by checking the timing with it connected also. It should read about 15 degrees more than the initial mechanical 10-12 degrees you set it at. If it is not reading 10-15 degrees more, then check to make sure you have it plumbed to a full manifold vacuum source. Also check all your vacuum routing, as you may have a vac leak? 2bbl vac hose routing can be much different than 4bbl routing. So you have a 72 chassis manual?
hmmm....nope, I don't have a 72 Chassis manual. I've mostly been going on what I can use from Chilton's and a 73 Olds manual we already have, but that gives me something to look for at swap meets. About the full manifold vacuum to the distributer, I know a lot of people use it, but it doesn 't make sense to me. When you floor it, the advance would drop almost to zero and when you are decelerating it would be at the max. Wouldn't you want more advance during acceleration ( without knock of course)? is there something I don't understand?
If you have EGR, then I may be feeding you wrong info with the manifold vacuum with your vac advance... I can try to explain this the best I can - judging by mine and others experiences... Well, here you go: this looks like a good read - http://www.v8buick.com/showthread.php?s=&threadid=31412&highlight=vacuum+advance Make sure you read the WHOLE thing.....it has LOTS of info in there - some wrong...some right.
The motor doesn't have EGR, I just figured that if it came from the factory with ported vacuum, then that was the best way. Course, the factory isn't alway looking out for the performance enthuiasists interests.p
Steve, Ported vacuum will drop to zero when you floor it also. You do want more advance upon acceleration, and the mechanical advance should provide that. Get the right amount of advance, and get it in sooner, and you won't believe the difference. Vacuum advance is for gas mileage and throttle response, and is load sensitive. An engine can tolerate 40-45* at light load(loafing along at 60) When you put your foot in it, you want it to go away quickly, otherwise it's PING time. When you modify the existing mechanical advance so that it comes in sooner(2000-2500), you must restrict the vacuum degrees(8-10). Using manifold vacuum gives better throttle response, and a smoother idle, and it goes away a little quicker than ported, just what you want. On decel, your at light load, so full advance is no problem.
thanks to all I appreciate all the help you guys have given me. Now I have all this tuning to look forward to when I get back home and am able to drive her again. :3gears:
Get that 350 ignition and carb tuned up "by the board", and you'll be amazed at how much power you'll have! Mine has 10.25 pistons, but no headers like yours, and it'll romp my 5000-lb LeSabre around pretty well. If I had headers and some more cam... I'm just trying to figure out what to do with a perfectly good 350 once I get the 455 done...I'm thinking maybe a REALLY stripped-down streetrod...
