Question May I begin by first saying thank you for any suggestions you may have. 6 years ago, a friend gave me his '86 Honda CB450SC Nighthawk which he had wrecked leaving it crippled with bent forks... knowing absolutely nothing about bikes and being broke as a joke, I let it sit for 5 years before I decided to resurect this thing. This winter I completely disassembled her right down to the heart, every single part scattered in the garage including the engine and carbs. Well now I have it all back together and it rides just fine but there a just a few issues with it that I can't seem to resolve (quickly discovered that the shop manual was pretty much useless except for balancing the motor.) The first problem (which may be the most simplistic one) is that whenever it rains or when I wash the bike, it's hard to start and runs awefull for the first 10 minutes/3-5 miles. The second problem (which may be related to the first) is that when I connected vacuum gauges to both cylinders, the needle for the left side cylinder sputters as if there is a vacuum leak but I have no idea where to begin looking. I lapped the valves during rebuild and the clearances were to spec. Third and final problem is that when the engine is held steady between 4k to 5k RPM, it mis-fires and I have confirmed this by checking it with a timing light. The shop manual that I am using is published by Clymer and the more I look through it, I can't help but think that either it is incorrect about my specific model or somewhere in the last 22 years someone has made modifications because it does not have components specific of a California version of the Nighthawk, for example, the fuel switch has a gravity feed line but also has a vacuum pump which it should not and there is an intake line on the left side carb directly in front of the intake manifold but I have no idea what it is supposed to be connected to... all I know is that if I plug it up, the bike runs fine but if I open it up then the bike dogs out. I sincerely appologize for such a lengthy description of problems but these are the issues that I have and I can't seem to find any information to resolve them. Please help me in any way you can, if anything, just to leed me in the right direction. Thanks again.
Sincerely,
Erik
AnswerI don't have experience with this bike but I'll add my thoughts to your excellent written description.
The rain/water problem is probably the spark plug caps. Start the bike up and spray a little water on the caps. Put a hand on the tank and touch the wires and I bet you will feel the pulses. Replace the caps with the correct type. You may need to trim a little wire off the coils to get fresh metal to screw the caps into.
I'm not sure why the vacuum gauge needle is jumping on one side. Might be an air leak. Some folks will spray WD40 around the intakes and listen for a change in RPM as the oil is drawn into the air leak. This might help confirm the air leak is there and help isolate it.
Have you checked compression?
It sounds like you have a vacuum operated petcock. There should be two nipples off the petcock (sometimes three). There's the fuel feed into the carbs, a vacuum line which hooks onto one of the intakes for vacuum, and an (optional) third line which I'm not sure about.
The nipple on the left intake is for vacuum, I'm guessing for the petcock. The bike will run fine when you plug that up as otherwise it leaks air into the intake.
