I have a 750CFM stock rebuilt Qjet that is idling ROUGH, super rich. The needle and seat tests good. The float is adjusted properly power valve and primary rods functioning normal. Idle screws set 1.5 out. well plugs are epoxied. One thing I notice is that when I remove the air horn, the gasket is very wet around the primaries and fuel bowl. Abnormally wet in my opinion. Is it possible that the air horn could be warped and thus make a bad gasket seal which draws fuel from the fuel bowl? Im not sure if this is a condition that a warped air horn could cause. Another Idea I had was fuel getting past the primary jets via their threads, though this seems unlikely. Any other ideas? Thanks!
Look down the primaries with the engine running. See if you can detect any nozzle drip. This happens when the throttle is open too far, and the engine starts to run on the mains. The triple venturi of the Q-jet are very efficient. It doesn't take much to initiate fuel flow.
Top could be warped from incorrect tightening through the years. You would have to check that with a straight edge. Call me old school (most do) but I don't like epoxied plugs. I have never pulled a Qjet apart and seen the sealer still in tact. Not a single time in my years of pulling them apart. It would have to be a pretty good leak to be your issue on this, and the real test would be cold start up. If you have to crank to fill the fuel bowl, before it will start after sitting overnight, then it could be draining down. Good luck. Hope you figure it out and let us know the solution when you find it. Always willing to learn.
I agree with the comments about sealing up the well plugs by "dabbing" epoxy over them. In my book I describe this process as similiar to "cutting your arm off and putting a bandaide on it!" Common for airhorns and main bodies to show some warping. In most cases this isn't a problem as both parts typcally warpm together at close to the same amount. The two inner screws usually assure the gasket seals tightly in the critical areas that it needs to. It still helps on heavily warped units to flatten the front of the airhorn a bit, and use a thicker gasket. "Rough" idling is usually associated with a lean condition, which leads to "nozzle drip" as mentioned above, and really "stinky" exhaust if we don't put enough fuel in at idle speed for complete combustion.....Cliff
I checked the air horn, and it was mildly uneven. I filed it flat a bit, but I dont think it was the problem. It was very mild unevenness. I used soapy water and pressurized air to check for leakage at the epoxy and got nothing. Most likely the nozzle drip is the issue then I think. Do I understand correctly that backing out the idle speed setting is the fix for drip? The exhaust smelled a TON like gas was why I thought rich. Black smoke out the pipes. Thanks guys I will update as I proceed.
The more initial timing you run, the smaller throttle opening you can use for the same idle speed. You can get more advance at idle by running the vacuum advance off manifold vacuum, or you can temporarily advance the timing at idle and back out the idle sppeed screw to see if it corrects the problem.
Is this where the engine has best vacuum? Or just where they happened to be; and you haven't verified that they're adjusted properly?
that is the "starting point" for adjustment. I monkeyed around with them with no noticeable improvement. I am at 10 deg initial advance setting. mechanical comes in @ 1000RPM. Vacuum advance disconnected. Will advise.
Ken, if you can screw down one of the idle adjustments all the way and it doesn't die, you have some vacuum issues.
Potential for nozzle drip or other fuel seepage might be aggrevated by a bad phenolic float as well. Might meaure fine from an adjustment standpoint, but if not floating so well any more it might result in a high bowl level. The drip condition is easy to spot by looking down the primary side while the engine is idling. Devon
I went through the carb a last time before reassembly. The only thing I found was the secondary throttle plates werent EXACTLY right. they were positioned such that they might have held open just a smidge at idle. So I readjusted them. Started the car up with a vacuum gage installed. No black smoke, no stinky exhaust. I could not see any "drip" at all. The secondary throttle plates and flattening the sir horn were the only things I changed. In fact it idles pretty nicely right now. I adjusted the idle screws and throttle screw, and have a nice 750RPM idle with 13InHg vacuum, which is right on the money for my cam. At least its the same as MY carb - the one I drive that has no problems. I do have one last problem with this new carb though that was not evident before. When I rev the motor with a WOT stab from idle, the carb backfires occasionally. Not all the time, and generally not the first time. When it does backfire, the motor takes a sec to rpm itself down. ?? VroomVroomBANGoooooommmmmmmm if that makes any sense. If I ease the carb into higher RPMS by hand, you can hear a slight unevenness to the exhaust, not quite right. Excepting as noted before with the backfire, it returns to idle nicely. I am going to go look at the 72 manual. Any thoughts?
It's going lean then. Make sure the power piston is popping up. You can actually rest a small screw driver on the power piston (through the carb vent), and when you crack the throttle, the piston should pop the screw driver up. Also make sure the secondary rods are being lifted when the air valves rotate. That little plastic cam that lifts the hanger and secondary rods likes to crumble when it is old.
Thanks Larry, good info. The cam was solid when I inspected it, looked like it had been replaced previously. Now the power piston I am sure is free to move ie not bound up BUT the carb was actually missing its spring, so I threw a spare in there not knowing its worth. I will check this out.
Hard to say how good the springs is. Vacuum should hold the piston down. Crack the throttle, vacuum drops, the spring lifts the primary rods out of the jets for a richer mixture. If the spring is too weak, it may delay your enrichment timing. Gessler sells a spring assortment: http://www.gesslerheadporting.com/m...c9a8cca4441b7b76852574fd005f5932!OpenDocument
Tried the "new" carb again today after a few small changes. 2ndary Cam good as new. good primary enrichment spring, good action running/screwdriver test No detectable vacuum leaks on or around the carb I still get backfires thru the carb. only when the secondaries are opened/closed quickly. I even tried changing out the CZ 2nd rods to a richer set (CH maybe) but it made no difference. Also tried another hanger. no difference. Heres the kicker though When I put my "good" carb back on, the one I use and drive all the time, I stabbed the throtle quick a few times and BANG it backfired thru the carb too. My "good" carb had never done that ever. I could not get it to repeat either. I have been thru and thru the "other" carb looking for the cause of the backfires and I cannot find anything. My thinking is now that I may have something engine related causing a problem. Vacuum leak, leaky valve, who knows. Maybe the "new" carb with its very lean stock calibration is more susceptible to the backfiring condition than my modified richer carb. My car with my modified performance carb is running better now than it ever has. I just dont know.o No: Time to take a break from it and just drive awhile. Thanks for all the help thus far.
LoL yea my distributor crapped out on the way home from the GSGN meeting so I had to go through all of the ignition system. I am positive it is all in top condition and power tuned too. edit: I am pretty sure I have a vacuum leak somewhere, probably the manifold. I read ~wavering~ between 13-14 with an occasional dip to 12 inHg at idle, regardless of which carb is on the car.
