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Thursday, 21 April 2011

Synchronizing the Carburetors (Part 3)

Ok, here's the last part of our series on synchronizing the carbs that you've been waiting for. I apologize for the delay, been busy with work.

First, I'd like to address a couple of things. The manual recommends that you warm up the bike fully before you start to plug in your carb sync tool. I don't think that's a very good idea because everything's going to be hot, and it's hard not to burn yourself trying to remove screws and installing the fittings to sync the carbs.

I mean, life's already hard enough as it is. Why make it harder on yourself?

So I installed all the fittings for the carb sync tool while the bike was cold. Stone cold. No way of getting burned that way.

First I attach my carb sync tool to the right handlebar grip.
Next I hook up the tool's vacuum hose to nozzle #1.
And hook up the other end of the hose to the carb sync tool.
Hose #1 done.
Here's another shot.
Do the same for hose #2 to nozzle #2.
All hoses hooked up and ready to go. As I look at this picture, I realize that I might have done something wrong. The hoses are supposed to go over the grips and behind the sync tool. Not sure if it makes much difference though. Probably not.
At this point, I re-checked all the connections, especially the fuel hose, to make sure everything's tight and firmly fitted. Then I started the bike and let it warm up.

Here's what the initial setting looked like. The vacuum in carb #1 is the lowest, followed by #4. The level is blurry because it's oscillating as I took this picture.

I started by adjusting #1. Remember, #2 is the reference level and it's not adjustable. So adjust all levels to match #2.

Once I got #1 balanced with #2, I proceeded to #3. When you adjust #3, you will affect #1, but just leave it for now. Once #3 is close to #2, I move on to adjust #4 to make it match #2.

Turn the screw in very small increments, then blip the throttle, look at the sync tool and wait five seconds. If that's insufficient, then turn the screw somemore, and repeat.

It takes a little while for the new setting to register on the gauge, and if you keep turning the screw very quickly without waiting, you'll overshoot the correct setting. That's probably the most important thing to remember if you're doing this for the first time.

Once that's done, go back and do them all again, starting from #1. You probably have to do this a few cycles to get it right.

Here's a shot I took while adjusting the carbs.

And here's the final state. Looks pretty balanced.

After that's done, just put things back in the reverse order from how you took them off, and you'll be all done.

What are the advantages of doing this? There are a few that you can feel right away.

The bike is smoother and there are less vibrations, and the bike warms up faster. I believe the fuel consumption also decreases as all four cylinders work equally hard now, more or less.

Seriously, the most challenging part of this whole project is getting the carbs sync'ed up. If you have your own garage and are able to get a fan in front of the bike, use it. Point it at the oil cooler and turn it full blast so the bike doesn't overheat.

I didn't use a fan though. The key is to be patient, take your time, and keep at it till the carbs are properly balanced. Changing the settings on one carb will affect the other carbs, so you'll likely need to go back and do it several times. This is normal and it's nothing to be frustrated over.

If I remember correctly, turning the screw clockwise causes the vacuum (and the level) in a particular manifold to rise. So if the mercury level is low, turn the screw clockwise. If it's too high, turn it counter-clockwise.

Here's another tip. Once you're done sync'ing the carbs, turn the fuel tap off and let the bike run till it dies. This will clear up all the fuel in the long hose, and you'll have less cleanup to do. Remember to turn the tap on after you hook everything back up.

I've heard that experienced folks can do this whole operation with just the fuel in the float bowls. Which means they don't have to go through the whole process of fixing up an auxillary fuel tank. If you can do it, please send me a video. So far, I have never seen even a professional mechanic who does it that way, so I'll believe it when I see it. :)

Well, have a good Good Friday, use your tools, and happy wrenching! Will talk to you soon.


  1. Pepper Crab, nice series. Thank you for the work! Two questions:

    1. Brand name on the 4 cylinder carb sync tool and how do you rate it?

    2. Looking at the pics I wasn't sure but are your final vacuum readings 16 to 18 range.



  2. Hi Peter,
    1) I'm using the CarbTune. I can't compare it with others, because I've only ever gotten one. It's pricey though, but the build quality looks pretty good.

    2) According to everything I've read, the readings aren't important, and I didn't note them either. What's important is that the levels are balanced.

    Hope this helps.

  3. Just in case you really need the readings, yes its between 16 and 18.

  4. Pepper Crab and other readers:

    Here's some data for comparison. I used my vacuum bleeder gauge (1/8" air hose, small crescent wrench and vacuum pump) to read the various port vacuums. The gauge reads in inches of Mercury (inHG).

    Carb 1 - 8.25 inHG
    Carb 2 - 8.5 inHG (Base)
    Carb 3 - 8.5 inHG
    Carb 4 - 8.0 inHG

    Specs say: 1.2+- Base Carb, so I'm okay.

    (By the way to settle the gauge needle from oscillating wildly, use the small crescent wrench and thumb screw down the air hose to restrict the vacuum. Keep closing the wrench until the gauge is still. Blip the throttle to ensure you haven't completely closed the vacuum line to the gauge.)


  5. So when i put everything together do i still have to use the srynge just before i connect the little hose in the petcock? can someone one be kind to help me on this issue?

    Thanks in advance!

    1. Sorry for the late reply. Just managed to get a new laptop and transfer the data from my crashed hard disk over.

      The purpose of the syringe is to keep the vacuum operated valve in the petcock open so that fuel can flow to your carbs and engine. That's the only purpose. Once you turn the bike off and don't need the engine to be running, you can remove the syringe. So short answer to your question is "No".

  6. Great write up. Was exactly what I was looking for. This made everything the manual said crystal clear. Only difference I have is the CA model. Their emissions standards suck

  7. Excellent explanation. I just synced my 1992 750 this weekend and this was invaluable. The biggest issue that I ran into was keeping fuel flowing from the tank. I had a syringe to create a vacuum on the diaphragm valve, but it kept fuel flowing too slowly. I would suggest rigging up a simple fuel line + funnel arrangement make life easier.