Motorcycle won’t start: Carb issue

Motorcycle carb issues are the reason a lot of people get in touch with me. What I’d like to outline here is things that you can do to get your bike running. I have used all of these ideas myself for many years and with great success.

Often if a bike will turn over but not splutter into life it is a fueling issue. Lets say we know it’s not a ecu/points issue and the coils are good. Have you checked the plugs? Replacing the plugs is a cheap failsafe before you go any further. Look at the old spark plugs. These are the window to the heart of your bike. If they’re black and sooty it’s telling you there’s an issue with overfueling or oil leak issues. Alternatively if the plug is dry you aren’t getting fuel into the chamber. There’s a couple of handy tools at this point. A colourtune is literally a clear window where you can see what is happening in the spark chamber. You can also get a spark tester which sits between the lead and the plug and is a safe way of checking a constant and strong spark. Alternatively just sitting the plug on the side of the motor and hitting the start switch should show you spark.

Now do you have clean air getting into the carb? Check the air filter for blockages/dirty foam etc. Replace if necessary. It’s cheap and do you want rubbish getting into the heart of your beast. If ever I’m not sure that it is a fuel issue I will use ether at this point. A little squirt of aerostart on the filter as you are cranking the motor should get some action to bring a smile to your face. DO NOT get so excited at hearing your bike going that you empty the entire can onto the filter and come up with new and exciting ways to run the bike full time off ether. If this method didn’t at least get some action then I would go back and check for further electrical issues.

Now for the fun stuff. I won’t go into specifics here because there are so many different styles of carbs but I will give you an outline of what to try. First make sure you have fuel and it’s fresh in your carb. Most carbs have a drain screw at the base of the bowl. Find it and open it up. Try to catch the fuel that comes out. If no fuel comes out it shows that you may have a blockage or a faulty fuel tap. If fuel comes out and it smells like metho.. or in fact anything that isn’t fuel throw it away and allow your tank to fill it up again. Of course I am assuming you now have clean fuel in your tank by draining all the old fuel out.. yes ALL of it.

With fresh fuel in the bowl you try it again.. of course your battery is now flat so you put the charger back on the battery and crack a beer while watching youtube clips of bikes that actually run. Several hours later you come back and try again. Good news it runs. Your journey ends here.

Bad news: still not running. Time to pull the carb/s out of the bike. Remove the bowl and check for debris in the base. Clean thoroughly and use an air compressor to clean any little orifices you see. If things are really bad this might be time to take it to someone with an ultrasonic cleaner.. you decide no I am the master of this machine and nothing will beat me. Good for you. Find the idle jet and remove from the carb. It’ll be stamped with a number 15-25 is a pretty normal range. This needs to be really clean. You can use a needle or a proper jet cleaner and make sure it isn’t blocked. Even if you can see through it there can be build up that will definitely affect it so do this part right. Probably 80% of bikes that don’t run that have been sitting for too long will have been fixed at this point so re install jet. Check the rest of the carb including diaphragms, main jet and mixture screw. Take notes of everything you have done. A quick list like this might help.

Pilot Jet size:

Main jet size:

Needle position: ie 3 from the top

Air/fuel mixture screw: ie 2.5 turns out

This information will help you later when you want to double check your setup against a stock bike etc. Everything is installed again and you’ve checked that everything is sealing well. Again you hit the start button and after a couple of turns with the choke on she fires into life.

In the next post we’ll start talking about checking float heights and mixture screws.

Any questions Contact me here.