Ok, rebuilt my carb for the 455 Wagon and it seems to run well, but at idle to about 1200 RPM I have an extremely loud whistle coming form inside the front barrells. Its not an external vacuum line etc. I used a piece of heater hose to determine the whistle is coming from inside the horn on the drivers side. Carb didn't do this before rebuild, so I am wondering what could have changed that would cause this? Anyone ever experience this problem before? Thanks, Jon
On the big block carbs there is the idle air bypass opening in the bottom of the air horn, one on each barrel of the primaries. Looking into the primaries from the front of the vehicle, the small rectangular openings are found at about 5:00 on the drivers side barrel and about 7:00 on the passengers side barrel. This passage allows air to "bypass" the booster venturi and enter the motor without increasing main metering signal - helps avoid nozzle drip. I have found that sometimes just the right "edge" on the air horn gasket or the opening itself can create that whistle. I have eliminated the problem at times by taking a flat bladed screwdriver and lightly "chamfering" the edges of the opening and the gasket. Just take it easy and feel around the opening a bit, rubbing the edges. It has stopped the whistle for me at times. There could be some other issue like the wrong gasket or a warped air horn, but I have had luck on occasion with the above procedure. good luck!
Interesting... I will look for that while I draw a vacuum on the A/C tonight. This is a 350 carb on a big block.
That is a problem. Small block Q-jets don't have the fixed idle air bypass. The 350 doesn't have the same idle air requirements as a big block.
You may be able to add it. Ken would know better than I. The big block needs it. Without it, the idle speed will need to be much higher out of gear, for it to idle in gear the way you want it. 1200 RPM in Park is too high. My engine idles in Park at about 850-900 RPM, and will idle in gear at 750 all day long.
hmm well the 350 carbs dont have it so i guess thats not the cause of the whistle. youre sure its a 350 carb? As for adding the idle air bypass it can be done....If you wanted to attempt it I suggest getting the Cliff Ruggles Qjet book....it documents idle air bypass much better than I could explain it to you here. however I think you would be better off just getting a nice 455 carb because of the overall calibration. 1976+ will be 800cfm and electric choke capable.
Yes, it is a 73 350 carb. I will prob go the route you are suggesting and build a BBB carb. I keep meaning to pick up Cliffs book, I hear nothing but good things about it. Guess it is time to start scavenging ebay for a core. Looking down in the bowl, I can see the butterflies have burs on them and are open more than I want, so that may be it. I wondered why my idle was so inconsistant. Thanks guys! - Jon
Yes, idle bypass air can be added to all models, even early ones where the passages were not present. Q-jets don't respond as well to holes drilled into the throttle plates like larger bore carbs do. In most carbs after about 1968, the system is in place, even if it isn't drilled. Our book shows where the passages are located, and gives some recomendations for hole sizes based on the application. As an example, a few weeks ago I added the entire system to a pair of Marine carburetors that were installed on fresh 454 Marine engines. For some reason the engine builder used 244/254/108 cams in them. Marine engines require a low idle rpm, and stable idle in gear. The WORST thing you want is to be idling thru a Marina full of $200,000 boats and have your engine(s) stall out and slam into one of them! The engine builder did his own carb builds with our parts and recomendations, but after many attempts to get the engines to idle well in and out of gear by only drilling the throttle plates, he hired us to drill the castings and baseplate for bypass air. Despite his best efforts, some "nozzle drip" was present at idle, and the engines would stall out if left in gear for more than a few seconds at idle speed. The effort was successful, and the engines ended up idling fine in and out of gear, even with the big/tight LSA cams in them. After the mods they reported back that idle speed is steady, and barely slows when the marine gears are engaged. It took .140" worth of idle bypass air and pretty "generous" idle fuel to the mixture screws to make this happen! I have another customer with a street/strip GTO that runs 9.90's at 136mph. His engine makes around 730hp, and uses a 274/286/108 solid roller camshaft. He has a 1000cfm Holley "clone" at his disposal, but prefers to use the Q-jet for racing, because is idles better, more stable in gear, and comes up "smoother" on the brake. His carb also uses .140" idle bypass air, and quite a bit of fuel to the mixture screws. For the Holley guys out there, he sees no more than about .02 difference between the custom 1000cfm carb and the large cfm q-jet we built for him, on any track in any weather. His car leaves with the front wheels about 14" off the ground and carries them out most of the way thru first gear!.........Cliff
Cliff, good point on the boats! I wouldn't want to stall in a marina like that either. Like driving blindfolded into a car show.... I will have to buy your book. My buddy has pulled some real performance using q-jets and references your book all the time. Its cheaper than buying a new carb. p I have heard that all Buick q-jets are 800 CFM after 72 is that right? Thanks again! -Jon
Front inlet "mod-quad" units are all the larger castings (non CCC) carbs after 1974. They made some side inlet Chevy style smaller castings from 1975 to about 1980. Don't be afraid of the smaller carbs. I have several customers running into the mid 9's in Super Stock applications with them! The only real advantage of them for most street/strip cars, is more cfm available on the primary side for "normal" driving. This provides less throttle openign to get the same thing done, cruising, moderate speed passing, etc........Cliff
Front inlet "mod-quad" units are all the larger castings (non CCC) carbs after 1974. They made some side inlet Chevy style smaller castings from 1975 to about 1980. Don't be afraid of the smaller carbs. I have several customers running into the mid 9's in Super Stock applications with them! The only real advantage of them for most street/strip cars, is more cfm available on the primary side for "normal" driving. This provides less throttle openign to get the same thing done, cruising, moderate speed passing, etc........Cliff
Ok, so I just reran my carb number and it is a 1978 model from a 350 Buick. So I am starting with the 800 CFM model if I understand your post correctly. So in your thoughts, am I best to buy your book and modify this one for my application instead of buying another core and buying your book and modifying it? My Carb # is 17058241 Thanks, Jon
The 17058241 is an excellent unit. The linkage is a bit "goofy", otherwise a perfect starting point for a really nice high performance carburetor. I use a nearly indentical 17057274 on my own engine. It was provided FLAWLESS performance for 4 different engines that have powered my car, from about 400hp to near 600hp. I've dyno and drag strip tested it scores of times against all sorts of Holley carbs, and even the new Edelbrock AVS. It outpowers them on the dyno and runs quicker on the track every single time. We were even asked to back to back test it for the Popular Hot Rodding Engine Masters and High Performance Pontiac magazine articles we did a few years back. Right on the dyno it made 2 more HP than a well set-up 850 Holley DP dyno carb. We carried both carbs to the track, the Q-jet outran the Holley by .02 seconds and .30mph on back to back runs. You'd will NOT read about that in the articles that were published a few months later...wonder why?......Cliff
Its so funny how people are afraid to back the q-jet. I have a lot to learn about them, but I have seen others pull more HP out of and better driving feel than a holley. Every Holley I have ever had always ended up leaking and making a mess of my engine, and heaven forbid the thing ever backfire. Glad to hear you like the carb I currently have. I get the book delivered sometime this week and I will start reading it. I have a mild cam from T/A, I will have to pull the specs off it for you, but I am running 10.25:1 compression in a Buick 455 .030 over. I have full advance right off idle since it has a 3.90 gear in it, and its a light g-body. If you can point me in the right direction to get me started, it is definately appreciated. What info do you need to provide some recommendations for a starting point? Thanks, Jon
The carb you have chosen isn't going to need a lot of help. Slight recalibration of the idle system, jets, metering rods, high flow needle/seat assembly, complete accl pump assembly, power piston spring, float, gaskets and small parts. Let me know when you are ready to set it up....Cliff
"Its so funny how people are afraid to back the q-jet. I have a lot to learn about them, but I have seen others pull more HP out of and better driving feel than a holley." What's going on here is that a Holley carb is an old design, and was built to delivery a LOT of air and fuel to an engine at heavy/full throttle. The basic design was not intended to be efficient at light load and light throttle openings. They have very generous idle and all idle fuel, and they have to be that way, since the signal to the boosters is poor and it takes significant throttle opening to start pulling fuel from them. What's happened in recent years, is that folks have scienced out improved booster designs for them, and went into the metering blocks to help them out as well. I've tested modern versions with the large annular boosters, and they are much "smoother" than the older versions for "normal" driving. Problem is, the large boosters cut back condideraby on CFM for any given model. The Q-jet was designed right from the start as an efficient carburetor for light throttle openings and "normal" driving. They quickly worked the "bugs" out of them found in early designs, and they pretty much obsoleted everything else out there, and became standard production for just about every 4bbl equipped GM engine that left Detroit. The efficiency is unrivaled by any aftermarket carb. If we apply some modern technology to them, they quickly become EXCELLENT high performance carburetors. They will right with, and often outrun aftermarket designs, even on some pretty high HP set-ups. We don't go around waving the flag and trying to get folks to dump aftermarket carburetors and install Q-jets. What I do tell folks in public Forum, is that you an spend some time with a Q-jet, and run right with, and even outrun high dollar aftermarket "high performance" carburetors, and reap the benefits of a much more efficient design for cars that see a lot of street driving. I've made one track outing this season. Drove over to a little Friday night "test and tune" session at our local 1/8th mile track. Made 6 runs back to back as quickly as I could get them in. Didn't cool the car down at all. Here are the numbers from my last two runs, made 23 minutes apart: R/T: .505 .514 60': 1.6230 1.6289 ET: 7.3169 7.3188 MPH: 94.35 94.36 All other runs except for one where the tires spun just a tad produced the same 60' times and just a tad slower because it was hotter. Those two runs were made right at dusk, DA about 3000'. If you don't think a q-jet makes a good racing carburetor, the R/T's for the other 4 runs were .502, .505, .505, and .530 (spun just a tad).......Cliff
