I suggested that also early on Derek. To Jose- You might want to check to make sure that when you press the gas pedal to the floor, its opening the carburetor fully. Even with the factory carburetor, many times the carburetor is only opening 3/4 of the way when the gas pedal is floored. On the Q-Jet, there is a tang you can bend to adjust the secondaries to open fully. Im sure the Street Demon has a similar set up 18 inches of vacuum is fine as long as its a steady reading. Have you adjusted the mixture screws to the highest vacuum reading?
I need to find a new A/F meter, a while ago just tried to turn on my old wideband but I think is dead. Maybe in the meantime I can go to a smoke test machine that a friend owns and have an A/F report of idle. I'll check it later Already check pedal travel and the carb is opening fine with pedal to the metal. I think I've two problems. The first one is a bog at WOT when the secondaries opens but that could be fixed with the secondaries air door adjustment just like you said, I'll try to check it later today after work. The second issue is the lack of power at part throttle but I have to try first a better jetting or metering rods chane I'm going to try to check the A/F today and post the results If the A/F for the idle isn't right then, could it affect the part throttle performance?
Honestly, I wouldn't worry about the idle A/F ratio, I think it is wrong all over. At 5600', you need to lean it out, not richen it. It needs fatter primary rods.
A properly-functioning TWO-BARREL carb on a properly-tuned engine that size would obliterate the rear tires on heavy acceleration at low RPM. Carb CFM RATING is NOT THE PROBLEM here. Remember, a small-cfm carb on a big engine may need to be leaned-out. A big engine (that runs right) provides lots of vacuum signal to the carb, and it's vacuum/velocity that pulls fuel from the carb. God bless Daniel Bernoulli. Terrible gas mileage, no power? Yes, it could be a FAULTY carb. Could be FAULTY ignition. Could be seized brake calipers. Could be restricted exhaust. Could be improper valve adjustment. Could be improper cam timing. If this car had an automatic trans, it could be a failed (freewheeling) sprag on the stator in the torque converter. You tested valve leakage using...gasoline? Kerosene? That's a fairly crappy test. I pull a vacuum on the ports to see how much thin air gets pulled past the valve and seat. This requires limiting how much air can bleed down the valve stem. And with all of that, you're not testing the ring seal, or how the valves seal once the head is stressed by the head bolt torque--or how pushrod length affects valve closing. In MY driveway, that thing would get a cranking compression test. Any low cylinders would get a cylinder leakdown test using compressed air and a dedicated gauge-and-orifice system. I'd also do a cylinder-balance test. A cylinder can seal just fine, but if the cam timing is wrong, it won't build compression. If you have compression and cylinder seal, the cylinder may still not run due to fuel or ignition problems. Cranking compression, Leakdown, and Cylinder Balance are the "Three Sisters" of engine performance diagnosis. When I had the leakdown tester connected to #1, and pressure regulated to about 10--20 psi, it's easy to find TDC. I'd look at the timing marks to verify that the damper hadn't slipped, or that the wrong timing marks are being used. After that, I'd drive the car with a vacuum gauge attached, to see actual vacuum measurements prior to, and when the car is acting up. And, of course, the vehicle needs a bumper-to-bumper inspection of the brakes, exhaust, etc. As was alluded-to earlier, the OEM exhaust was often made from double-wall pipe. The inner wall can collapse without affecting how the outer wall looks. The exhaust can be terribly choked--and it looks fine from the outside.
A 78 carb is calibrated for an egr valve, If your car is not egr equipped, that carb will always give you bog
I have some A/F numbers now but just for idle Cold engine idle: 13.2:1 Hot engine idle: 14.1:1 Also have some Hydrocarbons going on the exhaust but nothing crazy, I think is normal for a car of this age but just want to comment that in case that somebody things that means something else @Schurkey rear and front brakes were inspected two weeks ago and aren't dragging. The leakdown test was done using vacuum on the machine shop and I repeated it for 3 hours using gasoline. How can I check for a clogged exhaust? I know that in modern cars the oxygen sensor can be removed and use that hole to check for positive pressure on the exhaust but I don't have idea how to do it on these cars Already leaned it a bit more, I'm using 80 primary jets with 6052 rods (60 OD in the lean side and 52 OD on the rich side), didn't notice any difference By the way I think I found a Pontiac 455 Quadrajet on a junk yard shop, I'm going to check it today and see if it actually is what the guy says it is
Excellent. This is wonderful for the machine shop that performed the valve job. It is of no importance to this situation. You need to pump compressed air into the cylinders using a commercially-available leakdown tester to determine if each cylinder is holding appropriate air pressure. SOME leakage past the rings is inevitable. Too much past the rings--or any out the valves or head gasket or engine castings--is a sign of trouble. This is the tester I have. I had to "special order" the 14mm spark plug adapter (no extra charge) as it normally ships with an 18mm adapter suitable for aircraft use. https://www.amazon.com/Aircraft-Too...9H24/ref=sr_1_1?ie=UTF8&qid=1549401474&sr=8-1 Disconnect the exhaust at the manifolds or headers. Tie the exhaust pipes up with wire so they don't drag and get damaged. If the engine runs better with the pipes disconnected, you may need to remove and inspect the various pipes to see where the restriction is.
Bad luck with the Quadrajet I found. It wasn't a Pontiac 455 unit, it was a Cadillac 1978 (maybe 425) in really bad shape, missing parts, a lot of dirt and high price ($150) I'm going to look for a place to do that kind of test driving without the exhausts, If I do it on my neighborhood I'm sure somebody is going to complain with the police . But, running without exhaust manifold wouldn't harm the exhaust valves?
Don't remove the manifolds. Remove the pipes from the outlet end of the manifolds, then tie the pipes up with wire or whatever. The exhaust still goes through the manifolds (or headers) OR, if you have access to a welder, just drill a hole in the pipes somewhere just behind the manifolds, use that hole as an access point to check back-pressure, and weld the hole(s) shut when you're done testing.
Another way to test for exhaust restriction is to note the manifold vacuum at idle in Park/Neutral fully warm. Then open the throttle just enough to get 2000 RPM and hold it steady. The vacuum should remain steady slightly higher than idle. If it slowly drops off, you have an exhaust restriction. Easy to do.
Guys sorry I've been absent due to a work travel but I'm back I used your method and above 2000RPM the vacuum is steady higher than idling, about 21.5" and idling is 19" About carburetor I think I finally found a 455 Quadrajet but it's an Oldsmobile 1974 455 Quadrajet. I'm going to see it tomorrow and if it works maybe I can go ahead planning to buy a real Buick 455 quadrajet In the meantime in the last week I've not used the car more than 80 miles but all this time I've noticed a gas smell in the exhaust, already tried checking the mixture screws and changing the metering rods without any change, a friend told me that maybe some valves are hanging open, but I'm not sure how should I check that
If you had valves holding open, vacuum would not be as high, and you would notice a down tick in the gauge needle every time a valve would stick. If the needle is steady, no need to worry. The mixture needles do not determine air to fuel mixture, they just control the volume of the air fuel mix that is admitted to the venturi. The actual air to fuel mixture is controlled by other fixed passageways in the carburetor. Have you checked the power piston operation? With the engine off, you can take a thin screwdriver and insert it into the vent and depress the power piston. It should spring back up when you take the tension off it. If you then start the engine, vacuum should hold the piston down, and if you wing the throttle, you should see the screw driver jump up. That's one way to test it.
Just bite the bullet and order yourself a Qjet from Ken at Everyday performance! Msg him and give him all your specs and he'll build you a properly tuned carb made for YOUR car specifically. Yes it will be pricey but "buy once cry once" and if you still have issues well then you know it's not the carb anymore. Make sure to tell him where you live so he can tune it according to your altitude. You will not be disappointed with his work trust me.
Yes it does, one for each metering rod. There should be a cover secured by a single screw on each side of the carburetor. Remove the screw and cover and the metering rod will pop up with the power piston and spring. All carburetors have a power enrichment system. Vacuum holds the piston/metering rod down in the jet. With heavy throttle, vacuum drops and the spring pushes the piston metering rod up. That means richer mixtures because more of the jet is exposed. Make sure your metering rods , pistons and spring are intact and move freely in the bore. You may need to tune the carburetor for your altitude.
Pistons are verified and rods too, none of them are vent so I'm quite lost with the issue. Anyway, I had some look and bought the Olds 455 Quadrajet. I'm going to rebuild it and if it works better I'm going to consider the option of buying the Buick QJ. The part number is #7045184
Any carburetor you use at 5600' is going to run rich. There is less oxygen at 5600'. Unless you calibrate the carburetor to run at your altitude, you will still have the same problem. A fuel injection system that tunes itself might be a better bet.
