My 1969 XR7 351W 4V with FMX came with what I believe is a Holley 4160 Carburetor (List 9834-3157). Engine is running extremely rich, exhaust reeks of unburned fuel and dumping black soot on the ground below the exhaust tips. Engine shakes at idle and has some intermittent hesitation during excelleration. Starts okay and seems to run well down the road. Carburetor was rebuilt one year ago according to reciepts.
I checked the float level on primary and secondary - they both dumped fuel when the sight screw was removed. I adjusted both correctly. Did not make any noticeable difference in performance. The mixture screws were both set at less than a half turn out. Tried moving out to 2 turns - barely idled and seemed to dump more fuel. Finally moved back to original position. Vacuum advance seems to be working correctly. Vacuum line is connected between carb and manifold tree.
Not sure where to go from here. Any thoughts would be helpful.
I see that is a 600CFM Vacuum secondary carb. It should not be a hard problem to find and fix. How is the choke working? Is it opening up? And are the secondaries closed at idle?
Choke opens and seems to work correctly. I just tried turning both mixture screws all the way in and the engine still idles - I read that that is an indication that the power valve is faulty. I will take a look at the secondaries to confirm they are closed. There is a Holly dealer just down the road. I am going to pull the carb and bring it down to get the correct parts as needed.
What is the expected vacuum range at idle off the intake manifold?
So, I took the carb into the Holley dealer and discovered that the power valve had two gaskets, the old one was adhered to the plate. It had a 6.5 valve which I replaced with a 5.5 (my vacuum was between 10.5 -11 in gear) and adjusted the mixture screws to 1.5 turns to start. The best I could get it tuned to was 3/4 turn on the screws and increasing the idle. Going the other direction just made things worse. Still seems super rich (it reeks). The tech at the Holley shop inspected everything and comfired everything was open and shutting correctly. Do I have a problem elsewhere?
I do have significant exhaust leak at (or through) the exhaust manifold. I have not vacuum tuned the mixture screws yet because I do not think it will matter at this point.
Over rich idle has always been a Holley issue that’s why a few have converted over to the edelbrock. Since you have said a Holley tech checked over the adjustments then the secondary throttle shaft stop must be ok. Slightly opened secondaries can increase the richness. I have found over the years the adjustment screw for this has become corroded and possibly keep the plates open more. Adjusting the idle speed low enough to lessen the stink results in stalling. The primary plates cover more of the idle openings when closed resulting in less flow through the openings. There are two good articles for trying to solve the ‘stink’. One was in Hemmings Musclecar Monthly about a year ago and it gives the procedure for drilling and plugging the idle passages in the metering block. Gives the mixture screws more control. A Holley racing technical bulletin from the past says to drill a small hole in the each primary plate to allow for air bleed at idle. Somehow with a vacuum gauge you keep drilling bigger until the mixture screws have a greater effect.
I spent some good money on a 735 Holley rebuild from my 390 Mustang partially because of the ‘stink’. It didn’t make a difference so I put that carb aside for a future owner and put on a Holley streetavenger which mostly cured the ‘stink’.
I had the same thing with my 735 carb. on my 428, rough idle, air/fuel screws didn’t make much difference, black smoke out exhaust, strong smell of gas, tried the power valve thing, didn’t work. Holley tech told me to check flatness of the carb. body where the metering plate fits, take a big flat file and file the surface until the file strikes the center, what happens is, the corners get pulled up where the screws hold things together and will not let the gasket seal, this causes vacuum leaks and rich condition. It took a lot of filing to get mine flat, but now it works like a charm, no smoke, no smell and the air/fuel screws are doing their thing.
How much of a raise was visible at the carb body corners? If I put a strait edge on the body will it be obvious? I would assume the secondary base should be checked as well.
A straight-edge from corner to corner should show low in center, but a file would work better, filing the body will not hurt it, and yes I would do both, but the back doesn’t need as much to level. After you finish, spray down with carb cleaner, blow it off good ,new blue re-usable Holley gaskets, and good to go.