I need some advice from you carb experts out there. I have a stock 350 with the Q-jet. I'm looking to add some power. Should I switch to another type of carb or keep the Q-jet? Can you modify the Q-Jet with different jets to make more power? Sorry to be so naive...I'm carb-challenged.
I say keep the q-jet, Its very versatile and is actually quite easy to understand once you spend some time reading up on it. I've actually been planning on playing with my jets, rods and hangers soon, and I've found a lot of information using the search function on this board as well as carb sections from the buick parts directory and buick performance websites. There is also a rochester carb book by Doug Roe thats highly recommanded, I personally don't have it, but I've heard only good things about it. Hope this helps :TU:
I agree with this assessment. The Rochester is a very good carb; it works well in almost any application. The book, which I have a copy of, is very well written, and informative. Using this book, every attempted change in metering was successful. My suggestion, however, is to remember that the carb only supports fuel delivery to an engine. It cannot be used to increase engine performance. It can only optimize what the engine delivers. Ray
I always felt metering rod carbs. were more accurate. The small primaries are better for fuel mileage too in my opinion. Keep the quadrajet!! But what do I know. Mike D.
Actually, the small primaries are intended to maximize air flow signals to the venturis. The smaller the primaries are, the quicker they respond to changes in throttle positioning. Though the result is better fuel economy, the intention is fuel control accuracy. The metering rods are intended for more accuracy in fuel metering; they also last longer than those !@#$%^&* power valves. Ray
The Q-jet is more than adequate. Pick up a copy of Doug Roe's book on the Rochester Carbs and you'll find losts of tricks for getting more out of the Q.
I agree with Leviathan's assessment of the book on the Q. The book is well written and should be on everyone's to get list if they use or plan to use a Rochester carb for anything. Ray
The Q-jet is the best all around carb ever made. I am sorry for all the bad things I said about them in my ignorance. I wish I could put one on the Ford 302 I am currently fighting with. The Holly 4150/60 series are such pieces of junk.
Thanks for the advice, y'all... My Q-jet has been really funny about staying in tune...lots of rough idles, stalls, etc....it gets tuned and before too long, it's back to acting up again. Maybe it isn't the carb. I've heard that adding spacers to the carb will help build horsepower...can this be done on a Q-jet?
Yes, it can. Bear in mind, however, that what you gain in horsepower you lose in torque. Spacers come in one of two iterations; the first combines the barrels, allowing them to feed in tandem. The result is a division of the signal between the two primaries. The other is a four hole spacer, which will strengther the signals by further isolating them, causing the loss of additive feeding, losing you horsepower. By trying both, you can decide what you want the engine to do. It is a subjective call. My question, though is:what problem are you encountering with the carb? Is the mixture inconsistent, or a hesitation? I can help fix the problem, possibly better satisfying you with your carb. Ray
I would say buy some metering rods and/or jets from Edelbrock (or junk carbs if you can find them. I experimented on my 70 350 driver a few years back with base plates, Holley carbs, stock manifolds, aftermarket, and even a Thermoquad. The thermoquad gave great launches, but was way to much carb. I ended up with a TA manifold, no carb spacer, a Kenne-Bell Cam, stock exhaust manifolds and a 1972 Buick Q-jet (800 cfm) with 1973 Cadillac primary and secondary rods and jets. It gave great all around power. Another thing that I found important (more so on my small block than big block) was playing around with the tension on the secondary air valves. The set screw and adjustment screw on on the pass. side (hold a screw driver on the screw before loosening the allen head set screw). You can tighten or loosen that spring tension as need-be which adjusts the speed at which the secondary air valves open (and thus the metering rods). Typically they are loose due to fatigue and tightening them will reduce the infamous Q-jet bog and allows your engine to ramp up a lot quicker. Be careful not to tighten that spring too much as it can break loose or the little stalk it's connected to can break!!
Well guys, here's what my carb is doing...it idles rough, even when the engine is warmed up...it also stalls sometimes if I am tap the accelerator slightly to move the car, such as while waiting/creeping through a drive-thru lane at a fast food joint. It will also sometimes stall after I start the car and put it into drive or reverse, even when the engine is warmed up. Maybe the throttle is bad? I know next to nothing about carbs...they intimidate me a lot! They seem very complex to me. Leviathan mentioned the book by Doug Roe...does anyone know the title and where I could get it? Would it be suitable for an intimidated carb-rookie, such as myself? Or maybe there's another "carb primer" book you could recommend? Also...I guess I'm more into low-end torque, than top end HP. So, spacers sound like something i wouldn't be interested in. Is there anything to do to the Q-jet to build torque, or is that more of a rear-end thing? By the way, I appreciate all of your replies and time. They really do help me out. Thank God for this forum! Mike
Here's a link to Doug Roe's books. http://www.carbs.net/books.asp Rochester Carburetors (BK-HP14) Without looking at your carb, I will throw these out as POSSIBLE problems, with the first being most likely in my mind. Step 4 and beyond will require you to remove the carb to make sure parts are not lost or dropped into the engine. The attached diagram shows your carb, and it's really not as complex as it may first look. 1. Choke sticking- Start your car and let it warm up or drive it, then remove the air cleaner lid and make sure the choke plate is opening (going straight up and down). 2. Check Idle air screws (number 74 in dia) to make sure they aren't bottomed. There are two in the front on the base. Screw them in all the way, then unscrew them both 3 turns for your starting point. See if that helps and adjust back and forth if it does. 3. Possible vacuum leak - The gaskets could be eroded and/or loose seals around throttle shafts. With cool motor idling spray wd-40 around the base where the throttle shafts are as well as around gaskets. If they are leaking the engine will usually speed up or smooth out. 4. Your primary metering rods are sticking and/or accelerator pump is bad- Best to read the book on how the take the carb apart. 5. Bad vacuum break - not very likely, but possible.
Considering the possibilities with your carb, my suggestion is the same as Tim's: vacuum leaks. One other possibility comes to mind: the hot idle compensator. If so equipped, the back of the carb will have a cover on it. Inside, you will find a bi-metallic spring with a rubber seal on it. Its intent is to allow more air into the carb under hot idle conditions, compensating for the fuel's tendency to vaporize more when hot. If it is open all the time, lean mixtures will result. The other suggestion, as Tim suggested, is the choke tuning. Tim glossed it over; I can go into more detail if necessary. It is also possible that the years have created a debris field in your carb. I strongly recommend the book available at many book stores. It is called Rochester Carburetors, written by Doug Roe. It is valuable regardless of your expertise; I still refer to it with 30+ years tuning, adjusting, and repairing them. I did a lot more when I was younger, but alas, everyone thinks the Holey is better. I intentionally misspelled the word; I hate these pieces of crap. When building torque, remember that cylinder filling at the desired rpm will build torque at the engine speed desired. This is more of a compression, cam, intake and exhaust issue. The carb only supports these efforts by supplying the correct amount of fuel. The same goes for the distributor,. It ignites the mixture at the correct time, based on advance curves, but again, it only supports the engine. Quickening the advances will likely yield better throttle response unless it is already optimized. If it is stock, you will likely benefit from slight modifications. The carb can affect torque if it is incorrectly sized; too big especially. The q is optimized for big and small block engines by changing the primary size slightly. The Q is very forgiving when tuned; the tuning will likely last forever. A good rebuild might benefit you; the book will come in handy here. I strongly suggest shopping for a tech if you want to pay someone. Very few people understood this carb when it was popular, never mind now when half of them have been replaced by Holeys. Keep us informed, we can help. From what I read here, these individuals are all on the right track; troubleshooting is difficult unless one of us can fondle the carb ourselves. Ray
Fondle my carb and I'm yours for the night, Ray.:eek2: Seriously, though, thanks for the advice & diagram, y'all. Getting more technically proficient on the Q-Jet is definitely one of my goals this year...I'm tired of being intimidated by the little bugger...sitting atop my engine, mocking me. Sounds like the book will be a big help.
Once you get comfortable with working on them, they will be your friend. And when you get them cleaned up, working properly and dialed in just right, they make you so happy. Back in the 80's all my buddies thought they were junk and removed their Q-Jets and threw them away opting for AFB's or Holleys. The Q-Jets weren't junk, they were 15+ years old and had never been rebuilt~! What do you expect? Now a couple of those same guys are restoring their cars and kicking themselves as they go through the piles at swap meets looking for that correct code carb.
