Hey Have you guys ever heard of this problem,My quadrajet is behaving funny. Around town the engine runs normally on the primaries (except for an unusually low idle when cold). However when I put my foot down to the floor, when the carb seondaries should be opening the carb actually backfires through the secondaries pushing the buterflies past the stops (that's pretty violent). It doesn't happen all the time and happens more frequently when the car is cold (below 180 degrees, usually between 160 and 180 as I won't put the car in drive till it has warmed up to 160). I have checked the Timing and it is fine and replaced the inline fuel filter I have on the car. One time I notced when making a pass at the local strip the car was bucking as the secondaries were opening and closing at wide open throttle (the car launched without secondaries, and then began to buck as they opened and closed repeatedly the whole way down the 1/4 mile). If it were a holley I would say the power valve is shot, but Q-jets don't have power valves. Incidentally (not that this should make a difference) but the carb is actually on my GTO's 400 right now. It was a 1969 Pontiac H.O quadrajet that I had been using to try and get my buick 455 running but then swapped onto my GTO and rejetted. The carb used to run fine on the GTO (it's been on there for 2 years now) but has started giving me trouble since the car has been sitting lately. I'm sorry to post this here but the guys at the pontiac message board were so stumped they didn't answer my post. I figure someone here has to be a quadrajet expert.
How do you know what your secondaries are doing while you drive down the track? Are you running without an air cleaner and Hood? The Q-jet uses a power piston to richen primary metering. The primary rods are attached to this power piston. When the engine is off, a spring raises the piston, which raises the rods out of the jets. When the engine is running, engine vacuum overcomes the spring and holds the rods in the jets for a lean mixture. When manifold vacuum drops, the spring raises the piston, and the mixture is richer. You can insert a small screwdriver down the main vent in the airhorn, and it will rest on the power piston(see picture). With the engine off, you should be able to push the piston down, and let it spring back up. With the motor running, and the screwdriver in place, crack the throttle quickly. You should see the screwdriver jump up, as the piston rises, and falls. If it doesn't, the piston is stuck, and it's rebuild time. The Q-jet is an airvalve carburetor. The airvalves ar on top, on the secondary side. They size carburetor capacity to engine demand. They are supposed to slowly tip in, not flop in suddenly. The airvalve shaft has a spring. With the engine off, push the airvalves in, and let them go. They should return to closed position smoothly and completely. Also there is a vacuum break on the pass side front of the carb, that is linked to the airvalve shaft. The vacuum break slows down airvalve actuation. With the engine running, try to push in the airvalves. If you can, the vacuum break is defective, or the link is missing. Also the airvalve shaft has a small plastic cam in the center of the shaft. It raises the secondary metering rods out of the fixed diameter secondary jets. Sometimes the cam breaks. Push the airvalves in, and make sure the rods rise. Hope that helps.
thanks for the help Larry I'll try it out. How do I know what the carb's doing when the car is running down the track? Well I run an open hood scoop with a ram air pan and a k and N 3" filter and filter top. Outside the car you can hear the car sucking in air through the scoop. Inside the car the engine noise has two different pitches for when the carb is running on two barrels or four. It is a very noticble difference in sound and quite distinctive. I began to recognize the sound of my car at full throttle on two barrels because I wouldn't always hear the backfire and I wanted to know when the secondary butterflies were stuck. I guess in the end I don't really know what is happening under the hood while I go down the track, but I can make an educated guess based upon my expirence with the car. I guess it is something that you have to expirence in person to fully understand.
In answer to both of the above entries, using the screwdriver on the power piston will not work well. This is due to the primary metering rod spring being carefully rated and sized to the manifold vacuum without the additional weight of ANYTHING. You can use the screwdriver to test the power piston with the engine off, but it will not work otherwise. With regard to the secondary backfiring and surge, I suspect a lean out. A dirty fuel filter could cause this situation. Q-jets are also known for eating the phenolic cam operating the secondary metering rod hanger, causing the air valve to feed only air, no fuel. You can check for this by operating the secondary air valves and checking whether the metering rod hanger also moves. A weak fuel pump may do this, though in most passes, the fuel bowl feeds most of what the engine needs unless the WOT is excessive, say in excess of 3/8 mile. You might also check for dirt in the bowl, or even charcoal. I have frequently seen charcoal from the vapor cannister being dumped into the float bowl as the cannister died. These fine pieces can get into fuel feed tubes. There is also a passgae in the secondary system that acts as a secondary accelerator pump, (bet you didn't know that). As the secondaries open, a depression is created at the point of opening. Look for an opening just above or below the leading edge of the air valve. This passage feeds fuel on opening only. If this is blocked, the lean out can cause surging on opening. The loss of air flow at that point can slow the valves opening, leading to power loss and surging. Every one makes a big deal about Holey (spelling intentional) double pumpers. At least the Qjet's works reliably, without the constant fiddling Holey's need. Anything affecting the fuel delivery to the secondaries can cause your problems. Let us know... Ray
is it true that quadrajets have an internal fuel filter? the guy at the local speed shop told me they do (i've never heard of it before) and I was wondering if thats true? If so I probably need to replace that filter but to be honest I think the carbs just shot. Still since most backfires are caused by timing issues I am worried that there might be another problem latent here.
holy moly.... when I got rid of the Carter 635 carb this spring and the long rubber fuel line w/inline filter, I totally forgot about a filter with the 750 Qjet I put on, with a steel fuel line! ou: Where would you put an inline fuel filter when you have original-style steel line from the carb to the fuel pump? In the hose before the fuel pump, on the tank side? Or maybe I should just get a stock filter to put into the carb inlet? It's just a street-driven car anyway.
If you have to put a in line filter in you would be best off before the fuel pump . I have seen a few engine fires due to rubber fuel lines cracking open and fuel hitting the engine and starting a fire. The filter in the carb. is all GM needed , Why change it ?o No: p :TU:
ou: ...just ocurred to me, I wouldn't have seen a filter unless I unscrewed the inlet fitting... which I didn't do. Maybe I'll email B/S Bill to confirm whether there was a filter when I bought it. I know he had run the carb for a short time, so I'm guessing there is, unless he used an inline. BTW, I do have a stock filter here that I picked up a couple years ago just for fun.uzzled:
I always heard that it isn't a good idea to put a filter on the suction side of the mechanical pump. I still run the stock paper filter. David, I'm sure there is a filter in yours.
Nope, I'm afraid not. Bill tells me he did not re-install the stock filter after he pulled the carb. He was using an inline filter. So I've been running w/o a filter for a couple months... guess I should have checked before I installed it. I better take care of that before I head out to the Twin Cities this weekend.
I'm working out the problem...I replaced the quadrajet internal fuel filter with a new one (it was slightly gummed up). I then borrowed a new timing light from a friend and found that the timing was at about half of what it should be (later I disected my old timing light and found a short that was causing the light to misfire - I went out and bought a new light), The backfires themselves stopped. However the car when at wide open throttle still feels like the secondary butterflies are opening and closing. It will take off from the line strong but will then slow down, surge, slow down, surge. I was going to check for vacum leaks and also the accelrator pump but I got in a motorcycle accident on saturday and as a result I have a sprained sholder and cannot work on anything for a while. When I heal I think I am just going to tear the carb down and rebuild, to see if that fixes the problem.
Sorry to hear about your accident. I would look closely at the secondary metering rods and cam, before rebuilding. I suspect problems here. Get well soon.
...and of course there's no spring in the filter housing anymore either... checked a few parts stores here, no luck. I know a repair/resto shop here though that will probably have spare springs, but was too late tonight to go there. Noon tomorrow I guess, or I won't be heading out of town with the car Friday
