First of all, if your engine is stock and you aren't running at high altitude, the stock settings should work. If it doesn't, and it is set up correctly, it is a good indication that the problem isn't carburettion at all. So let's back up a little. You should adjust your valves before adjusting your carbs. As well as having your valves adjusted properly, there is no point AT ALL in attacking a carb until you have confirmed that you have a healthy, properly timed spark at all four cylinders. First charge your battery, do, or have done, a load test, then check your charging system and make sure that it is working, although a high-end miss is not likely to be caused by a faulty charging system. Check your points gap, condition of your points (pitting can indicate bad condensors, a weak charging system can do that) Check points gap first, then timing. I've found up to 2 degrees off on my 400F between static and with a light, so use a light. Also make sure the advance is working correctly. If you need new points, you may want to consider an electronic ignition system, it will pay for itself in about three sets of points time and never go out of whack on you. Throw in new plugs if there is any doubt about the health of yours.
Just to make sure, you must balance the carbs after doing adjustments like the ones you are talking about. Step one is spark, then adjust your valves., next return everything to it's correct setting, and working properly. Working correctly means clean passageways and jets, do all of the carb tuning steps in your Clymer, but forget re-jetting unless you have done some modifications like a different exaust or airbox mods. If you have some modifications, the best way to get it set up is to have someone who knows what they are doing set it up on a dyno, and yes, that will be spending some money. I have seen engines LOSE power when an aftermarket exaust was put on, so never assume that what you have is going to just work. Same goes for airbox/ pod filters. Other culprits to check: Air filter clean, and you don't have a newpaper under your seat partly blocking the air intake (From personal experience, I admit it). Vibration shook something loose? Exaust flange studs all have nuts on them and tight? Boots on the carbs don't have air leaks (spray some carb cleaner on them when the bike is running, if the engine reacts like you just sprayed it into the intake, it's because you did!) If all of that fails, then move on to compression and leak-down tests. If nothing alarming happens there, back up, make sure you have covered ALL of the bases above, and you will find the problem. Good Luck, Tom Van Auken #1269 > [Original Message] > From: Helg Bredow <[EMAIL PROTECTED]> > To: Multiple recipients of list <[EMAIL PROTECTED]> > Date: 4/3/2004 10:03:11 PM > Subject: Engine misses at high speeds > > I'm hoping that one of you mechanics out there can give me some tips. > I've got an old CB750 that runs well at anything up to 80kmph but > anything over and the engine starts missing on one or more cylinders. > > I've just adjusted the needle position in an attempt to rectify it and > the spark plugs are now a nice tan colour, instead of black so that's > done something useful but hasn't fixed the engine missing. I've also > adjusted the float levels but that hasn't helped either. Fuel flow seems > to be OK; I've opened the carburetor drain while the fuel tap is on to > see that fuel is flowing through the system. > > The Clymer suggests that the main jet might be too large but in order to > test it I'd have to spend money :( > > There's got to be something else that I can try... >
