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Thread: starting issues/rough sound 1975 R90/6

  1. #1
    Registered User
    Join Date
    May 2008
    Central VT

    starting issues/rough sound 1975 R90/6

    My '75 recently has been having a bear of a time starting... A little background.. It was having an issue with a fuel line, the line would begin bleeding fuel as soon as petcocks were open on one side. I replaced the fuel lines and got rid of some small inline filters, though I don't believe those were the problem, because I switched them to other side just to see if I could isolate issue. (Petcocks were not the issue either, and the fuel lines were new, so I never actually figured out why the one side would "bleed")... but anyways..

    So, took out the inline filters.. replaced both sides with fuel line from petcock down to the junction with other line.. for a couple weeks all was good..
    Now, it starts with great difficulty, backfires, etc. (which I realize is probably not related) but it just doesn't seem to be getting fuel in the beginning..finally will start, with choke, etc. but it turns over a bunch...

    Then when actually riding, it just sounds rough from what it has... not skipping, but just not right...
    Carbs recently tweaked by my master mechanic at Nickwackett in Pittsford.

    Anyways.. thoughts, suggestions?
    Getting rid of inline filters perhaps a bad idea? Anyone else ever experience this odd gas bleed issue?


  2. #2
    Registered User krpntr's Avatar
    Join Date
    Jan 2010
    Mt Vernon WA

    Hard Starting

    Lots of choke. Sounds like timing or valve adjustment. Probably not. Further down the line after the fuel gets into the bowl, it is further directed to the starting and idle circuits. My guess is the idle jets and/or the enricher jets are clogged. The passage-ways may also be clogged and will need to be cleaned with a small copper wire, carb cleaner and compressed air. The out-let for the idle circut is a tiny hole behind the butterfly valve. Air must be able to pass through, and this is an easy spot for lacquer to build up. (Don't ask how I know this.) The enrichener (choke) jet is in the corner of the fuel bowl. Check and see if it is open and free flowing first. Good luck.

    '82 R100cs '92R100RT

  3. #3
    Don't forget about the jet in the float bowl that regulates the fuel for the uptake tube for the enricher.

  4. #4
    If it were me, I would put new, clean, in-line filters back in, and be sure to check all locations where dirt, etc. can clog your carbs.

    Could you have ingested some water in the tank?

    I have seen bikes with water in the fuel bowl do the backfiring, etc. as you describe.

    Also, do you have a back-up set of known-good working spark plugs. I have seen them get fouled where they still work, but poorly - I had one, one time, do the back firing thing. Thought it was carburetor issue but turned out to be only a plug.

  5. #5
    Registered User lmo1131's Avatar
    Join Date
    Jan 2010
    Pismo Beach, CA
    All of the above.

    Starting with:

    Valve adjustment
    Ignition gap and Ignition timing adjustment
    Spark plug inspection and gap - I had a spark plug with a crack insulator that caused a miss under heavy load (uphill) - on the flat it would run fine. The plugs were new, but I dismissed it as being "improbable" that a new plug would be defective - lesson learned - a new plug solved the problem.

    Drain the fuel tank - pull the petcocks, and inspect the "strainers" for rust or other material clogging them. Is the rust in the bottom of the tank?
    Drop the bowls - check for grit, water, etc.
    Inspect the floats - Do they float? Are they set at the correct level.
    Pull the main jet and inspect for foreign material - this is a tiny orifice and easily blocked (although when it is, the cylinder generally doesn't run at all).

    Once you're up and running again, take it out for a twenty-mile + spin, bring it back in and synchronize the carburetors.

    There's nothing like knowing what you've got. You may want to go a few steps further and check compression (preferably with the carbs removed) and do a leak down test.
    "It is what you discover, after you know it all, that counts." _ John Wooden
    Lew Morris
    1973 R75/5 - original owner
    1963 Dnepr

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