QuestionHi, Bill. We picked up a baby Nighthawk last year for my wife and it's done nothing but foul plugs since we bought it. I pulled the carb and tore it apart...nothing appears amiss, the inside is clean enough to eat out of (if you don't mind the taste of unleaded). The bike has a HUGE flat spot off idle, so much that it's dangerous once you push it past that RPM and it clears its throat, unleashing that monster 18hp! If you pull the choke a tiny bit, everything gets a lot better.
I'm wondering if the idle jet screw isn't adjusted incorrectly. It's the one (on that carb) with a little flag on it to prevent it from turning with the bowl attached, but I Dremeled the flag off so it can be adjusted on the fly. Do you know the number of turns out that should be? Any other advice would be wonderfully appreciated. I've rebuilt dozens of 80's and 90's V4 carb racks, but this little guy's kicking my butt.
AnswerDavid, if you have done a lot of V4 carbs, then I am not too sure what else I can say about this problem. The carb does have an accelerator pump, which you can usually tweak a little bit to compensate for off idle lean conditions and you can try to put a very thin shim in under the needle head, inside the slide. Diaphragm has to be good, of course, for the slide to lift properly.
You could bump the idle jet up to #38, from the #35 size, but the real concern has to be what is causing the lean condition? DO check the valve adjustments, as they are only .002" and can cause weird running conditions when the clearances are too tight.
Make sure that the fuel cap is venting properly so it can draw in air to compensate for fuel going out. The fuel screen in the tank/petcock inlet should be checked for restrictions, too.
There is a vacuum operated canister diaphragm which could be checked for an air leak or disconnected line. If the carbon canister is full of fuel, that would cause some fouling problems, but that isn't a lean condition.
Check the intake manifold gaskets and connections for air leaks. Usually, the idle mixture screw stopper is keeping things lean within EPA regulations, but an extra 1/4 to 1/2 turn out is all that is needed to get the flat spot out on an otherwise stock bike.
Is the air filter element still in place? Stock mufflers? Any changes to either side will cause calibration issues.
I have seen some bad spark plug caps on higher miles bikes, which causes the spark to break down/weaken under load. Check them for good connections on the wire ends and about 5k ohms resistance.
Go through all the electrical connections, including ground paths, to ensure that there are no vibration related failures interrupting the CDI ignition system, which starts at the flywheel stator and goes through the CDI controller on the way to the coils.
Bill Silver
