I need help with my 72 4 barrel 800cfm Q-jet. I picked this thing up recently and it had just been rebuilt and looks like new. Now after installing it I noticed it was dumping gas into #7 & 8 . I've had it looked at by 2 different Quad gurus and everything looks correct. All passages are clear, the float level is correct and the well plugs have been JB welded. What is going on? I need help. Thanks in advance.
I don't understand how it could be dumping gas in just cyl's 7 and 8 unless those cylinders are not firing due to a spark problem.
Why do you think that it is dumping into two cylinders? If these are the only two plugs that are darker than the rest, it is necessary to determine what is wrong with those cylinders. I have seen compression issues, defective spark plugs, crossed plug wires do this...I assume that these cylinders are the last two cylinders closest to the firewall? Determine compression between two good and the two defective ones. I suspect compression issues or crossed plug wires. let's start with that.
When I first put the motor in I was doing a compression check with all the plugs were out and while turning the motor over gas would blow out the back 2 cylinders. After checking the carb and finding nothing, I reinstalled and the motor is now running extremely rough and when I shut it off vapor was coming back out of the carb. Today I plan on pulling the plugs but I'm sure I'll find at least a couple will be soaked.
More than likely, the tilt of the engine sent your "carb test" fuel to the rear cylinders. Vapor coming out of the carb after you stop the engine is normal. There is always unburned fuel in the intake when you shut off the engine. Have you adjusted the air/idle mixture screws on the carb to try to smooth it out? I agree with the others, doesn't sound like a carb problem. What you describe is normal.
Hust a thought as well make sure the secondary needles are actually on the rear hanger A long shot but it can cause problems as well ( I know it happened to me ) Ken
Is your return line on your fuel pump plugged off? Could be lifting the needle off its seat on a hot engine and draining fuel to the back 2 cylinders because of the engine tilt.o No:
I've had plugs 7 and/or 8 foul before, and my engine is 100% stock with <5k miles on it. The Q-Jet I have right now has some misaligned primary throttle plates, so it may be pulling from the main system too early. Sometimes if I pump the throttle one time too many when cold-starting, it'll shoot enough fuel to foul the plugs. I have another core I'm rebuilding which is in much better shape, just need to tap & plug those secondary wells.
Here's my update after spending the afternoon trying different things. I finally got it to where it was running better but will load up badly after the motor warms up. I can see gas puddling up inside after shut it's shut off. Running extremely rich. 1. lowered float level - no help. 2. installed new needle and seat - no help. 3. blew out and double checked all passages - no hlep. 4. double checked complete reinstall - done right - no help I can't see any cracks but I just can not figure out where all that gas is coming from. I've never seen this problem before but it's about time to break out the trusty 12 gauge. ??????
Sounds like you are getting vapor lock. What is temp on the engine when this happens? Maybe fuel pressure too high?
Like Carl said, maybe vapor locked, you never answered about the return line. I think this helps keep things from developing high pressure in the fuel line getting "heat soaked", especially when the car is just turned off.
I just got Doug Roe's book out. Better terminalogy at what I'm trying to say is in Doug Roe's book Rochester Carburetors page 101. He calls the problem I'm trying to describe as "percolation". I think a blocked off return line on the bottom of your fuel pump can also add to, or cause this problem your describing.
Q-jets Are Fuel Pressure Sensitive, I Would Not Go Over 6 Psi. Also Check For Fuel Trickling Out Of The Primary Venturi During Idle. You May Have To Shine A Flashlight Down The Carb To See It. I Would Also Consider Trying A Known Good Carb If Possible. Good Luck
I had an issue this week where the Q-jet was suddenly running very rich. Fogged the garage and left black circles on the shop floor where ever it was parked while I messed with it. Everything seemed right but was dumping excess fuel into the motor. when I pushed open the butterfly's on the secondaries while it was trying to idle the RPM's increased. After consulting with TechG8 I found the linkage between the primary's and secondaries had excess play.This allowed the engine vacuum to pull open the butterfly's and let excess fuel into the motor and at the rear of the manifold. After a small adjustment to take up the slack it now holds the secondaries closed tightly and no more flooding. You may have something similar going on with your carb.
You may have issues with what is called fuel flash. Our fuel is presently formulated for use in a pressurized, injection system. This allows the fuel, when the underhood temps rise, to flash into vapor from the hot carb, engine block, etc. This vapor will be added to the incoming air, enriching the mixtures. This fuel is coming from the float bowl. be sure you are using a good name-brand fuel, and consider using a different brand to gauge the results. This condition typically results in a fuel smell coming into the garage just after the engine is shut-down, and can make the engine more difficult to start after sitting more than a couple of days.
Thanks Briz, That is something that I haven't tried. I did put a new float as well as a new valve and seat with no change but the linkage adjustment is worth a try.
