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Saturday, 16 April 2011

Synchronizing the Carburetors (Part 1)

For our next project, we're going to be synchronizing the carbs. If you haven't already, please follow our blog so that you can get updates on new posts.

Please feel free to drop a comment or two and share your thoughts and expertise with me and our readers.

This bike is pretty smooth when I bought it, at low RPMs at least. But once you start to crank it up, I can feel vibrations coming on through the grips on the handlebars. So this was a job that I've really been wanting to do for some time. Well, enough talking, let's jump right in.

First, remove the seat. I assume everyone knows how to do that, but in case you don't, insert your key and unlock the seat lock.
 Next remove the seat. Just slide it back and up, and it should come off.
The next thing to come off are the side panels. There are no screws holding them on. These panels have protrusions on them that plug into rubber grommets on the frame of the bike, so you just pull them off. 

However, make sure you pull them off in the correct order or you might risk breaking the hinged tabs. I'll tell you what I mean as we go along.

Grip the panel as shown in the picture, then push with your thumbs and pull with your fingers. It should release the top of the panel.
Next grab the bottom like this, and pull gently.
 That should release the front of the panel. 

The back of the panel has a hinge, and it hooks onto the rear cowl. You need to be gentle with the panel because the hinge is fragile and it's not difficult to break it. So the rear of the panel comes off last.
Here's another view of the hinged catch.
Do the same for the other side.
Here you can see the protrusion of the panel which goes into the rubber grommet.
 A close-up view.
Here's a close up view of the hinged catch again.
The grommets where the protrusions plug into are circled in red here.
And a close-up shot of the protrusions behind the side panel.
Next we need to remove the fuel tank. The first thing to remove is the bolt holding down the rear of the tank. It's as 12mm bolt on my bike.
Wrench or ratchet, your choice. I chose wrench this time. Remember, righty-tighty, lefty-loosey. So that's anticlockwise to remove, clockwise to tighten.
This is where that bolt screws into.
Next remove the vacuum hose from the petcock. The circled part is where the vacuum hose fits onto.
The fuel tank on this bike is a gravity-fed tank, meaning the tank feeds fuel to the carbs by gravity. There is no fuel pump on this bike.

However the bike has a safety feature in case the bike is overturned in a drop or an accident. If the engine is cut-off using the cut-off switch on the handlebar, the fuel will stop flowing.

Why? Because there is a valve in the petcock that stops fuel from flowing out when the engine is not running. It's called a diaphragm valve I believe. When you start the bike and the starter motor starts cranking the engine over, the motion of the pistons moving downwards and the exhaust valves closed and intake valves open, creates a huge sucking force, or vacuum.

This vacuum goes through a vacuum tube into the fuel petcock, sucking the diaphragm valve open, allowing the flow of fuel into the carbs.

With the vacuum tube removed, it's time to prop the tank up. Here's a useful tip, especially if you're doing this alone. It is very challenging to try to prop up the tank and remove the fuel hose at the same time. So it helps if you can prop the back of the tank up with an object (I used my trusty rubber mallet) so that you have enough room to remove the fuel hose. Just shake the tank around and make sure it's stable.
Next turn off the fuel tap. If the diaphragm valve is working it shouldn't matter, but just in case it's not working, turn it off.

I'll have to admit that I always forget to turn the petcock back on when I go to ride the bike after taking the tank off, so remember to turn it back on again after you're done.
The fuel hose comes out from behind the petcock and it isn't normally visible or accessible when the fuel tank is sitting on the bike. That's supposed to be a good thing I suppose. It will prevent some low life from unplugging your fuel hose and trying to steal your gas. Yes, I've heard of that happening, what more with the rising fuel prices nowadays.

Here's a view of how the fuel hose attaches to the petcock viewing from the back of the bike. The red arrow points to the fuel hose. The blue arrow points to the attachment point for the vacuum hose that you just removed.
Here's a far out shot so you can get the context of where the fuel hose goes on. You can't really see it very clearly in this shot, so I've circled the petcock for you. Notice how the fuel hose runs towards the centre of the bike.
The hose will be tight, but you should be able to grip the hose clamp with your fingers to open it, and then pull out the fuel hose.
Once that's done, all you have to do is to lift up the back of the fuel tank, and pull it back and up, towards the back of the bike. Before you remove the tank, lay down some cloth on where you're going to lay the tank down, to prevent scratching it or the bottom of the petcock. Did I mention laying it down really slowly and gently?

The front of the tank rests on these rubber grommets.
And with that, we'll conclude part one of our series on synchronizing the carbs.

Please follow this blog so that you'll get notification of updates.

If you have ideas or advice on how I can do this better, please comment on the blog!

Thanks for reading, use your tools, and happy wrenching! Oh, and stay tuned for Part 2. :)


  1. Hi!

    Thanks for taking the time to blog about the repairs you're making to your bike. I bought a 93 Nighthawk about 7 months ago and I've been thinking about making all the repairs to my bike that you've been covering in your blog.

    Your information has been extremely helpful as I make plans to make all my necessary repairs.

    Thanks again!

  2. Hi Shawn!
    Thanks for dropping by and leaving a note. I'm glad it's been helpful. Let me know if there's anything you'd like me to cover.

    I know I'll be needing to remove the carbs for a thorough cleaning and check, because fuel is coming out of the overflow tubes when the bike gets hot. I've tightened the overflow screws so I suspect the floats need to be replaced, since I don't think ours can be adjusted.

    I've also got a noisy hydraulic lifter(s), so that needs to be checked and replaced too.

    Also, the neutral switch needs looking at, it's not working properly. Lots of stuff to do on an old bike like ours. But that's half the fun!

    Happy wrenching!