Air Leaks and Carbs

Discussion in 'Carb/Tuning' started by gleno, Dec 5, 2007.

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  1. Dec 5, 2007 #1

    gleno

    gleno

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    The V-max carburettor and inlet manifold assembly are a most likely source of problems like an elusive rough idle, hesitation and sputtering when rolling on the throttle at low and high speeds and other seemingly fuel related issues. Before you go tearing into the carburettors it often pays to check the simpler things first.

    Something that is often overlooked by many riders (and drivers) of vehicles using carbs or EFI is the disruptive effects of a ‘vacuum leak’ on the smooth operation of your engine.

    The downward movement of the piston on the engine’s intake stroke creates a partial vacuum (low pressure) and allows the atmosphere to push (high pressure) air into the carburettors. Fuel is pushed into this airflow by fuel pump pressure, gravity feed, atmospheric pressure or a combination of these forces.

    We subjectively perceive this air and fuel to be ‘sucked’ into the engine.

    Provided you have no other avenues for air to enter the inlet manifold, all air entering the cylinder must pass through the carburettor throat at a predictable flow rate that will vary only with the rate at which the low pressure gradient can be replenished by the intake stroke of the piston. i.e. the speed of the engine.

    When you experience a rough idle or hesitation and sputtering when rolling on the throttle at low and high speeds, your first inclination is to inspect the usual suspects like a faulty plug, plug lead, blocked idle or main jet.

    This forum is full of advice on how to service carbs. There are also numerous recollections of riders doing this only to discover a cracked rubber boot, a disconnected V-boost hose, loose air box etc.

    You should check these things first. They cost nothing to fix.

    If you have an older model V-max, the rubber fittings, seals and hoses associated with the inlet manifold and carburettors will become stiff, and brittle over time. It will be difficult to get a good tight pressure seal once you unseat them unless they are perfectly refitted.

    While looking for the cause of a rough idle (after doing the ‘shotgun’ and ‘peashooter’ cleaning of the carb jets) I inspected the rubber end caps that fit over the vacuum line connections on the inlet manifolds. These manifold connections are the brass pipes you attach the hoses of your carb sync tools to. They had hardened and had started to crack. Just removing them to sync the carbs and then putting them back on over the widened pipe end had stretched them and they had no capability to shrink to back to an original fit. So they all had slight leaks. Even with the metal clips in place they can still leak.

    In spite of a big fat spark, number three cylinder was not taking part in the engine idle at all and the header remained cool enough to touch. Spark plug was dry.

    Inspection of the manifold’s rubber end cap showed that the top of the cap had split on the side facing the manifold wall. This could not be seen without rotating the cap toward the observer and was allowing air to freely enter the inlet.

    I placed my finger over the split and number three fired instantly. A bit of silicon affected a temporary repair until a few cents worth of aftermarket end caps solved the problem.

    So if you are experiencing some hard to pinpoint ‘fuel related’ problems (especially on older V-maxes), check the rubber connections and hoses on the carbs, air-box, inlet manifold, v-boost unit and pressure sensor before you tear things apart.

    You could save yourself a lot of frustration and expense.
     
    Last edited: Dec 5, 2007
  2. Dec 5, 2007 #2

    Buster Hymen

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    Wow, good find! I never even considered that as a problem area.
     
  3. Dec 5, 2007 #3

    vmaxinID

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    I smell a sticky.
     
  4. Dec 5, 2007 #4

    maleko89

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    Agreed, good post.

    I don't want to hear about your sticky. :)

    Mark
    #1098
     
  5. Dec 5, 2007 #5

    Neil-3446

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    Might be your Avatar??? :confused2:

    :clapping:

    :cheers:

    Neil
     
  6. Dec 6, 2007 #6

    hubeerjw

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    Good stuff... I know someone commented about what I am going to say next, but I thought it was worth commenting on. Along with not being able to easily see these cracks or knowing if you have everything sealed up correctly, a couple quick sprays with a can of starting fluid around potential problem areas is a great way to locate air leaks. This has been used for quite a while on 2 strokers and making sure the cases were sealed correctly as that is critical for both intake and exhaust. I have also used this method to find air leaks on my car as well. Those cheap little rubber boots rub, and crack, and dry out and can really screw up the A/F readings. A couple shots of starting fluid near a cracked hose or fitting will slightly increase engine RPM. A direct spray around the crack and you will definitely notice a higher reving engine. This is another cheap and easy way to troubleshoot.

    Cheers,

    Jeff
     
  7. Mar 8, 2009 #7

    lightninggrant

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    I did what the article above said. Found loose boots and a hose off.
    Made the motor run a like it should. Great Post!!:worthy:
     
  8. May 1, 2009 #8

    SHeva4ever

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    First of all, sorry for my English.
    I want to say thanks to the gleno. It is a great topic. I found an air leak on the vacum sensor hose. It has small cranks. I just cut 0.5 cm of hose from each side. My Max runs much better now.

    Thanks again gleno.
     
  9. May 13, 2010 #9

    ghostntheshell

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    What part # are the caps?
    I'd like to replace mine!
     
  10. May 13, 2010 #10

    Redbone

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    You should be able to go to a Auto Parts store and buy caps. They are just vacumn caps. My brother picked up an assortment of sizes in a blister pack last year from Napa.
     
  11. May 13, 2010 #11

    dannymax

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  12. May 13, 2010 #12

    rebar

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    Great post! I like Jeffs starter fluid trick.
     
  13. Oct 22, 2010 #13

    95max

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    Great suggestions...would be cool if someone included a list of problem areas and maybe some pics.
     
  14. Jun 6, 2011 #14

    aldo123

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    can someone plz tell me the shotgun and peashooter trick plz?? many thanks
     
  15. Jun 6, 2011 #15

    zippo6

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  16. Jun 13, 2011 #16

    likembikem

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    yesterday my 2001 vmax would not idle 'sometimes'. I later found the front left header pipe was cool. changed plug, spark seemed weak but maybe ok. removed carb cover and air box. Sprayed carb cleaner and it seemed some crap came out. It seemed to run and idle better, about 1k rpms. I looked but did not see the rubber vacuum boots as mentioned, and I looked carefully, so I tought. Even missing, the engine is balanced so it still seems fairly smooth. What lines I did see seemed good. Do I need to shoot starting fluid around carb area with air box cover on right? On a seperate note, I had a '97 max several years ago and it seemed the vboost was wicked, this 01 (i have had since last fall) doesn't have the sudden surge, but hey I'm going pretty damn fast anyway so it is hard for me to tell. Feel free to give me hell for being a novice but I'll take any advice
     
  17. Mar 9, 2013 #17

    allaraine

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    What about the rubber boots that connect the carbs to the engine? My carbs won't go back in them and stay, keep popping out, even if I tighten the all the way down. Any help????????
     
  18. Mar 9, 2013 #18

    Redbone

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    A couple questions and tips: 1) are all the carbs properly connected? If the carbs are not properly mated to each other and all on the same plane you will have issues like you describe.
    2) Are your boots on properly? The carbs have ridges that fit into a corresponding groove on the rubber boots; if they are not in their properly the carbs will work their way out.
    3) Make sure your clamps are very loose and you may even need to put some type of lubricant to ease the fitting process.
     
  19. Feb 9, 2014 #19

    antiquedave

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    Good article . Mine idles sweetly and goes like a bat out of hell when the boost hits but I have a midrange miss that I cant seam to track down . Pulled carbs down , replaced rubber (they where stuffed) diagrams on the pistons , done the usual cracked rubber checks including engine start around manifolds and carbs, no change in rpm . Balanced carbs etc . Any thoughts on the aftermarket diaphragms (bought on fleabay) being dodgy ? also starting to wonder if its spark ? Love the bike but this is driving me insane !
     
  20. Feb 9, 2014 #20

    sdt354

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    Running the E-Bay diaphragms for a couple of seasons now. No issues. They were rather stiff when new but wrinkled up nice after a while and flexible as OEM now.
    The mid-range issue can be so many things. A known good set of carbs. borrowed, can narrow the search for the problem. But try this, when you get to the rpm where the miss occurs, apply some fuel enrichment from the cold start lever. If the miss gets better, you know your running lean and it's not electrical.At least you'll eliminate one thing.
    Steve-o
     

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