How-To: Diagnose Motorcycle Vaccum Leaks

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What is a vacuum leak?

A vacuum leak is caused by extra air introduced into the fuel/air mix produced by the carburetors, which will lean out the mixture and cause poor running conditions. Vacuum leaks occur between the cylinder head and carburetor slide/butterfly, where the low pressure will draw in air through the leak area. Vacuum leaks are very common in older motorcycles.

 

What are sources of vacuum leaks?

The most common area of vacuum leaks are the carb holder (rubber boots). The rubber degrades over time, dries out, and can begin to crack or become brittle. Sometimes the boots look fine until you bend or stretch them, which can reveal cracking. The images below show a carb holder from a Polaris that looks fine until you bend or stretch the boot. While this particular boot was not cracked all the way through, it was replaced as preventative maintenance. You don’t want to lean out a 2 stroke due to vacuum leaks! Kiss that engine goodbye!

Carb holder vacuum leak boot

 

Carb holder vacuum leak rubber boot stretched cracked

 

Throttle shaft seals can also cause vacuum leaks and are the most difficult to replace. The leak can be caused by either worn throttle shafts, bad seals, or both. Some carbs rely on the tight clearance between the throttle shaft and carb body to minimize air leaks. Below, a Keihin carb from a Nighthawk which has felt installed on the throttle shafts, is not to be mistaken for a seal. I’ve replaced the felt and only reduced the vacuum leaks by about 50%. If you run into a situation like this, I would recommend replacing the felt with an O-ring.

Throttle shaft vacuum leak felt seal keihin CB

 

Fuel pump diaphragm and/or tubing leading to intake can also be a source for a leak. The fuel pump diaphragm can also leak fuel through vacuum tubing causing the motor to stall on deceleration. The video below will show you how to check a fuel pump diaphragm.

 

There are several other areas where vacuum leaks can occur, I mentioned the most common above. Please refer to diagnosing methods below to thoroughly check your motorcycle.

 

What are symptoms of vacuum leaks?

• Loss of power.
• Runs better with choke on, and in some cases the only way it will run.
• Erratic idle. You’ll never be able to set the idle. Sometimes it will idle higher or lower.
• Runs better at higher RPM.
• Sounds “boggy”.
• Do not mistake vacuum leaks for out of synch carbs. Never attempt to synch carbs without verifying that there are no vacuum leaks.

The video below takes you for a test ride on an 81’ Honda CB750 with severe vacuum leaks in the carb holders and throttle shafts. This will give you an idea of what symptoms to look for.

 

Diagnosing vacuum leaks

Luckily, diagnosing vacuum leaks are pretty easy, at least when the leak is fairly large and greatly affects performance. Listed below is what you’ll need to find your leak (listed in order of preference)

• Carb clean or starting fluid (these will remove paint, wipe it up immediately & blow dry with compressed air. Also keep fire extinguisher nearby)
• Propane (take the tip off and attach a long rubber hose)
• WD-40
• Water

To find your vaccum leak, choose one of the above and spray or point in the suspect areas while engine is idling. Any change in idle RPM, whether up or down indicates a vacuum leak. Below is a video on how to find vacuum leaks.

 

Has this information been helpful? Tell me your story below!

 

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  • Dave

    Hey Matt,

    Great site…

    I own 4 older bikes and all have symptoms of vacuum leaks and or have been improperly jetted or not jetted at all to compensate for the change in exhaust of air box to pods…

    Thanks for all the great info…really learning alot.

    I’m working on a website to share my bike repair adventures…

    http://mybikes.ca it’s still under construction…

    Regards,

    dave

    • Matthew

      Hi Dave, yes vacuum leaks are to be expected with older bikes. I’ve bookmarked your site, looking forward to some posts!

  • bill

    I have done this test with starter fluid. I can not the leak. I did just change the #4 mixture screw hoping that would help. it did a lil bit. when attempting to synch the carbs. #4 comes and goes. when coming to a stop the rpms idle high then level out. Any idea as to what I might be missing? o’yeah when I temped the exhaust #4 was a easy 60 degrees hotter than the other three. I am nervous to pull the diagram cover but is this the next step you would take?

    • Matthew

      Hi,

      What year make model? Butterfly or slide carbs? Have you cleaned the carbs? What is the overall tune of bike (valve adj, timing, etc)?

  • Owls

    Awesome effort on the vacuum leaks.
    having loads of “fun” with my 250 bandit and the above will help me to look at a few things.
    I’ve already stripped carbs down and cleaned them out (they weren’t particularly dirty)
    bike will (sometimes) idle but after a while will then rev up
    I’ve already sprayed carby cleaner round the carb boots and that did help me at one point identify the carbs where slightly out of line when put back in as a block of four.
    I did notice that the little O rings that sit under where the vacuum gauge plugs in are missing. perhaps that’s my problem? will try plugging these for now to see if that helps. got me thinking re fuel lines. did have to cut 10mm off the end that connects to the petrol tank when I noticed that had a bit of a tear (maybe the whole line needs replacing?)
    anyway thanks for the tips and I will be having a play with the bike when I get home (only had it a month but already missing it after one week of not being able to use it (I’m brand new to bikes and currently on my L’s and need the bike to practice!!)
    cheers
    Owls

    • Matthew

      Raymond, don’t forget to check the vacuum actuated petcock. They can form a vacuum leak as well. What do you have your fuel screws set at?

  • tim

    Hi, I have a Honda XL 250.It’s been sitting a few years. Starts fine but over revs like crazy( even with no throttle ) then dies down to a normal idle. If engine is warm it doesn’t over rev when I start it, so this appears to be when engine is cold. Returns about 30mpg or less when it should be doing 60-70mpg. Have cleaned out carbs twice? Wondering if it could be a carb boot leak…any other suggestions very greatfully recieved!!!!

    • Matthew

      Hi Tim,

      Does the over rev happen when choke is on/off?

      What is the overall tune of bike? Compression? Point gap and timing? Check for timing chain stretch?

      I have a video on how to easily check for vacuum leaks.

  • http://tinyurl.com/yfrw4fc Godffery

    Hello,
    I have been looking for the felt O-rings for the Choke shaft and carb body of a Honda CB900 with no luck; I have also tried replacing them with rubber O-Rings but cant seem to find a size that will not cause the shaft to hang-up, leaving the choke stuck open or closed. Any chance you have a source for the felt ones?

    Godffery

    • Matthew

      Hi Godffery,

      I bought the felts from motorcyclecarbs dot com. Search for “felt seal”. They are expensive for what they are.

      I know you mentioned choke shafts. I bought new felts to seal up a vacuum leak at throttle shaft and it did OK for 3 carbs. It did not seal up #3. Next time I will try O-rings. Go to applerubber dot com, they have a ton of o-rings and sizing calculators.

      BTW, are you the same Godffrey from WI that built Cafe Overkill featured on Bike EFIX? Beautiful bike, very nice work! I did some work on that bike and need to post the vids.

      • http://tinyurl.com/yfrw4fc Godffery

        Thanks for the leads Matthew, I’ll check them out.

        Yes, I’m “That Guy”. You can see the complete build in my website as well as others.
        What work did you do on the bike and I assume you must know Richard C. then?

        Godffery

        • Matthew

          Godffrey, I don’t know why I did not see your reply until a year later. Weird. Yes, I know Richard and attempted to rejet the CB550. I should be posting the videos soon so you can see what I ended up with.

  • Sean

    I have a 1984 cb 750 sc Nighthawk just rebuilt carbs all Honda ( ouch $ ) parts. Went to sync cabs bike fired righ up with a little choke then seemed to run fine.Put vacum gauges on and all four cylinders at low but even across the board. Spray brake clean not increase in rpm , But bike runs fine?? Good throttle response? No coke needed to keep running?

    • Matthew

      Sean,

      Sounds fixed to me!