QuestionI recently purchased a 83 Nighthawk 650 and everything seems to be in good working order... Except. When the bike is cold it idles fine, right around 12-1400 RPMs... however as the bike warms up it continues to idle at a higher and higher RPM the highest I have seen it is all the way up in the 4500 RPM range.
The guy I bought it from said he had the carbs cleaned and synched and hadn't ridden it since. I am wondering if the people who did the carb work didn't properly set the pilot screws...
That brings me to another question, I can only find 3 pilot screws on this thing... shouldn't there be four?
What are your thoughts?
AnswerI love that bike. The '83 Nighthawk is an one of the best ever made.
You are correct in your theory that the pilot screws haven't been set but it could also be an air leak in the intake system.
The cause of this idle surge problem is always due to not enough gas in the idle mixture. What happens is that a motor has an ideal mixture (the ratio of air to gas) for a given temperature. When its cold it requires less air and more gas. As it warms up it requires less gas and more air.
When the motor gets the optimal mixture it runs efficiently and at higher RPMs. As the RPMs climb, the motor becomes even more efficient. This creates a feedback loop which causes the RPMs to climb even more until it reaches a point where there's not enough fuel.
So in your case, the motor starts out cold with a mixture that has too much air but manages to run at a low RPM. As the motor warms up it gets closer to its optimal mixture and the RPMs start to increase.
So here's what you need to do.
First find all the mixture screws. I believe you have Mikuni CV carbs on that bike. I believe you want to adjust the screws which are marked #8 on this diagram:
http://www.bikebandit.com/houseofmotorcycles/showschematic/m17497sch558424
You are probably looking at the carb sync screws (which are only 3 screws).
Note that they sometimes have a plug which covers the idle adjustment screws. You may need to remove this plug. Sometimes you have to even have to drill a tiny hole in it to remove it. If the previous owner has never removed the plugs then you have to wonder if he really cleaned the carbs.
In any case, once you've identified the idle screws you need to turn them counter clockwise a bit. You'll notice the idle will increase as you do this. You then use the idle RPM set knob to bring the idle down to a normal level. This will enrich the idle (adding fuel and removing air).
Repeat this process until the motor finds that range where it doesn't have enough air to go to a higher RPM but it has enough to not foul the plugs.
At this point the idle screws may still not be optimally set but you should be at a point where the idle is steady across different motor temperatures.
You can now make fine tune adjustments to the idle screw to get the optimal operating mixture for a warm motor. This is done by finding the midpoint in the adjustment between the setting where the plug is just starting to foul (too much fuel) and the mixture is too lean (not enough fuel). In both cases the RPM will start to drop so you need to turn the screw in either direction until you notice a slight drop in RPM and find the midpoint between those points. You do this for each carb.
I like to have a fan blowing on the motor while I do this to prevent overheating. I also use a small external fuel tank so that I can have easy access to the carbs with the main tank off the bike.
