I don't know if I should drill out the Idle Discharge Ports and the idle tubes on my Q-Jet. I changed the jets to .77 and the engine is a 462 with 11.1 Compression Ratio pistons and TA's 292-02H cam. If I should drill them out, what size should I go with? Is there a chart I can follow? Any help is appericated. Thanks!
Cliff, Sorry I just saw your reply now....it is a 73 Buick 455 carb...I'll get you the number tomorrow. Thanks!
The 1973 455 carb is probably 7043240. 77 jets are HUGE for that carb, it should be using much smaller jets. I see quite a few early Buick carbs with 76-80 jets installed in them simply because they are lean at part throttle due to being emission calibrated. Your carb will need the idle system recalibrated for the engine combo. I would get one of our books, and look at the 3rd recipe for the idle system. The idle and main airbleeds remain stock in those carbs, just open up the idle tubes, idle down channel restrictions, bypass air, and holes under the mixture screws. For jetting I would use 74 main jets and 43 metering rods if you can free up the APT system in the baseplate. If not, 39 to 41 rods with the 74 jets will usually be enough part throttle fuel.....Cliff
Yep that's the carb...7043240. The engine runs great when the throttle is opened but when it tries to idle it gets rough and dies out. It backfries throguh the carb too, showing a lean condition. I thought I was going to have to modify the idle circuit, but was not sure. I just bought your book and it should be here in a few days. If I can get this engine to idle I'll be real happy! Thanks for your help Cliff, and if I have any other questions I'll let you know.:TU:
Backfires up thru the carb? When does it do this, normal driving, quick hit on the primary side only, going to full throttle, etc?.....Cliff
Does not occur when the throttle is open, it happened when I went to go start the engine a few times(before it kicked over) and also if I let the throttle close and the engine dies, sometimes then right as it's dying out, it will backfire through the carb. I looked up the specs the carb originally came with..originally it had .073 jets and .048 primary rods. I installed .077 jets and .045 rods. I'll will look into getting smaller rods, since the engine seems that it needs more fuel, but before I do that, I recieved the book in the mail today! I see the areas you told me I will have to modify, I'm just a little nervous because I don't want to open the passage up TOO much. But I'll take it little by little, and that should allow the engine to idle at least...I hope. Do you have any "baseline" specs for these carbs I can follow for drilling. I know every carb is different, but just so I know what ballpark to be around. Thanks again for your help Cliff. Greg, your right, the book seems very helpful. Thanks for your time on the phone the other day. Keep an eye out for an order from me soon, I think I'm gonna need alot of gaskets, if I keep having to drill out the idle tubes several times. Testing and tuning...what fun. Gonna need a rebuild kit too for this Stage 1 car anyways.:TU:
Keep in mind when tuning with the Q-jet, that changine jets effects the entire fuel curve, light, heavy and full throttle. .077" main jets are too big for that carburetor in terms of fuel delivered at heavy and full throttle. If it is acting lean at idle, light throttle, low rpm's, "normal" driving, etc, open up the idle system to effectively feed the engine at idle/off idle, then tune with metering rods to get plenty of fuel at light/part throttle. I've never to date had to go beyond a .074" main jet on one of those carbs. They act like they want huge jets because they used a pretty big primary metering rod to lean them up at part throttle to get thru emissions. If you can free up the APT system in the baseplate, they are easy to dial in without taking it apart and changing the metering rods.....Cliff
I have a brand new set of .075 jets I can try in it, .075 came standard in the Stage 1 carb....note this is a different carb though. I know that when I was rebuilding the carburetor, I noticed the APT system, but wasn't sure what it did. I don't recall be able to get it to free up. Is it supossed to move up and down, or back and forth in the casting? Thanks!
The early APT system isn't covered in our book. We were limited for space by the publisher, and it's one of the things that we omitted. Mostly because it can be difficult to get the APT screw free'd up anyhow. I have a near 100 percent success rate here. From what I've seen, the success rate by most is about 2 percent! The trick is to make a good fitting screwdriver, and apply some heat to the casting from the underside BEFORE trying to remove it. If it doesn't want to turn, don't force it. Apply heat and penetrating oil until it starts to move, then quickly remove it from the casting. We make an external screw as shown in the pic in the previous post. Once you get the APT screw free'd up, the tuner will have full control of the A/F ratio at part throttle (assuming the correct jets and rods are selected) without taking the carb apart. It's a great feature on the early Buick carburetors. It was also used on some Oldsmobile and Chevrolet units....Cliff
So if I understand you correctly Cliff, if I can not get the APT screw to free up, that is okay, and I can compensate that with different jets and rods? I will do my best to, but don't want to destroy any part of the baseplate. I'm gonna take the carb off tomorrow and tear it back apart....
The carb will need to be tuned by changing metering rods for part throttle fuel, and jets to change part throttle and heavy throttle fuel. If the APT system is operational, it's a LOT easier to dial the carb in exactly and it doesn't have to come back apart scores of times. When tuning the q-jet, determine the correct jet size FIRST. This is done by measuring the primary main airbleeds, and looking at what the factory installed, then selecting a jet size. For most application, the stock factory jet, or at most about .001" larger is best. Drive the car and climb some steep grades at heavy throttle, primary side only. Do some "aggressive" driving, loading the engine hard but stay out of the secondaries. The vehicles performance will quickly tell you if the jet size is correct. The engine should not surge, buck, feel "flat" etc once the correct jet size is selected. Keep in mind here that going too rich will mask any of the symptoms above, and why most folks end up WAY too rich with jet selection. It's better to start out at the stock size and move up if/as needed. If no improvment is noted, go back to the last size tested. Go after metering rods next. Test at light throttle openings, smoothness right off idle, steady cruising at light load, any high vacuum driving situations. If the APT system is in place, do a "tip in" test before driving the car. This is done by putting the engine at apprx 2000rpm's, then adjusting the APT screw so you see a very slight rpm increase when the choke flap is "tipped in". Shoot for about 50-100rpm's. This is you base setting. If you can not get the engine to respond to the tip in test, the metering rod selected is too small. Using the APT system and a fully tapered rod about 25 to 30 numbers smaller than the jet is a good place to be. For example, with the big Buick carbs (71-74), a 73 or 74 jet teamed up with a 43, 44 or 45 tapered rod is a good place to be. I get those carb in here all the time with a 76 thru 80 jets in them using 45B metering rods in them, where the tuner knew they needed more part throttle fuel, but lacked the parts and/or knowledge to tune them effectively for enough part throttle fuel. Once you get the primary side tuned correctly, LEAVE IT ALONE. You would not believe the folks who rip the top off the carb to add fuel at full throttle by changing jet sizes. Dyno shops do this all the time, and every single time this happens, I get the carb back because it is WAY off in the "normal" driving range. All full throttle tuning is done with secondary metering rods, after you are done tuning the primary side for best A/F ratio for all primary side only driving conditions.....Cliff PS: one small note to add. Another reason we see HUGE jets showing up in the primary side of these carbs, is that the builder/tuner doesn't add enough idle fuel for big cams/low vacuum. The idle fuel effects transition onto the main system as it puts some fuel to the transfer slots and lower idle airbleeds. The fix here is to get the idle system up to par, not compensate with a huge jet.
That's really helpful! I started taking the carb back apart today, just took the air horn off. I did notice that the baseplate gasket of the carb was wet with fuel when I took it off the intake....when I first rebuilt the carb, I did epoxy the plugs on the bottom, both front and rear, but I am going to do your test that you mention in the book, to make sure they aren't leaking. I'll see if I can free up the APT system this weekend.
Don't epoxy them.Tap and screw plug in with good sealer.Some Locktite product.Epoxy will leak no matter what You do. Peace Kacper.
Yes I did but this did not help my problem. I literally could not get the engine to idle at all.....like I said it would run okay with the throttle open, but as soon as I would let off it would try to die out, backfire, etc. Anyways, finally got some drillbits and just ordered some new gaskets. Going to try an remove the idle tubes tomorrow. The other 3 mods to the idle system seem fairly simply........Still a little nervous with how much I should increase these holes......guess I'll make small adjustments first but I feel that I'm gonna end up buying alot more drill bits.ou:
