Originally Posted by Mike Gayner
The carb is as clean as a whistle inside with brand new rubber hose after the fuel filter. Noticed another weird thing - when I undo the lock screw to adjust the float level, fuel pisses out of it! What would cause this? I also noticed that to have it set so that the fuel only just reaches the bottom of the sight screw, the float valve is set really, really low. The float valves are brand new, and it's running a stock mechanical fuel pump so I wouldn't suspect excessive fuel pressure. I will buy a vacuum guage tomorrow but I don't see any reason why the vacuum would be below 6.5". I'm pretty sure there are no vacuum leaks, and the cam is very mild.
I suspect a power valve leak. I initially had a paper gasket on the power valve, but when that was leaking I replaced it with a steel gasket, which is probably shit and is maybe causing fuel to leak past. I'll try to suss out a gasket tomorrow, but all of the speed shops are shut on Sundays so I'm not holding my breath. The power valve itself is brand new so I have no reason to suspect it has failed. I also did the oh-so-reliable put-you-lips-around-it-and-suck test and it seems okay. The idle circuit is obviously getting fuel from somewhere it shouldn't be and I think the power valve is the main suspect.
What is the proper method to install a power valve? Do the windows on the valve actually have to be aligned with the holes in the metering block? The instructions say to tighten to 50 inch pounds, but I don't have a torque wrench that small. How tight is that?
It isn't the power valve. It is an internal fuel leak caused by too much fuel in the float bowls...which can be caused by a sticking open needle seat. Those buggers seem to always get some little bit of crap in them that hangs them up.
There isn't a very easy way to test this (with a mechanical fuel pump, if equipped), but it can be done. Once the bowls are full of fuel, cut off the fuel supply and run it. If it runs really good for several seconds after removing the fuel supply, you can easily say that you've got too much fuel coming into the carburetor. Fix the leak.
PV failures are a thing of the past. People keep coming back to them as the source of bad carbs, but they are very rarely the true culprit. Incorrectly sized PVs can produce problems, but if it was running well earlier, then it probably isn't a wrong-sized PV.
As for measuring the torque, it is a bit silly IMO anyway, since who has an inch pound torque wrench with a half-inch drive necessary to fit the hex size of the PV? I have an inch-pound torque wrench in a 3/8ths drive and a 3/8s-to-1/2 adapter, but I just pull the PV snug by feel.
50 inch pounds is 4.167 foot pounds. That should give you an idea of how snug it needs to be.
The fact of the "deteriorating operational characteristics" of your engine suggest to me that you've got some crud stuck in the needle seat. Clean it/them out and try again. The "take the fuel pressure out of the equation" test is an easy one to do if you have an electric fuel pump on a switch. If you have a mechanical fuel pump, you need a large fuel container and long hose to reroute the fuel into the container and away from any potential ignition source.
What usually happens is that the engine will start out running rough and get increasingly better until it finally dies of fuel starvation. If that happens, you need to clean your fuel system. Take your filter off and blow it out with compressed air or replace it. Also blow out any rubber lines. If you do have an air compressor, try blowing air into the line before it comes to the fuel pump (if mechanical) until you can hear bubbles blowing in the tank. You may want to remove the fuel filler cap so that the air doesn't come back out at you when you release the pressure...depending on how your system is designed.
Also, who rebuilt your carb? Did they replace the needle seats? Are you running an electric fuel pump or a high capacity mechanical pump? Do you have a pressure regulator inline after the fuel pump? You want a maximum of about 5psi of fuel pressure for a street engine and no more than about 6.5 for a race engine...of course, you can't get accurate pressure readings without a load. But if you do have a regulator, an easy thing to try is dropping it down to about 2psi. If the problem goes away, you need to adjust things more carefully and consider how you might be able to monitor pressure under load.
Be sure to reset/recheck the fuel levels in your float bowls once you get it so it runs properly.