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  1. #1

    Default '86 EVO FXRS Carb & Engine Issues

    My '86 FXRS has a mikuni 42 carb added by the previous owner. I don't know how old it is, but it requires the old style tabs on the clutch cables. Lately, the bike has had some trouble getting started and idling. It was running extremely lean and would die with some throttle after warming up, or at a stop light after a while on the freeway. It's always given me a ton of backpop and fired often. I knew I needed to adjust the carb and had never really cleaned it, so I got some redline fuel system cleaner and added to my tank after draining the settled gas out of the carb.

    I also adjusted the air mix screw to make it richer. It started up first try, idled healthier and the popping was all but gone. I went out for a ride and listened closely for any difference in anything since carbs are pretty uncharted territory for me. At the end of the ride something strange happened: the sound of my bike completely changed. The Supertrapp makes it pretty loud usually, and all sound at idling was gone. What I could hear was much throatier and deeper sounding than i'd ever heard. There was some loss of power, but the bike didn't die. There was some kind of rattling sound that sounded like something loose, but I couldn't find anything. I was about 3 miles from home and it was 100 degrees today so I rode it home . When i parked it, it gave off two of the loudest, most distinct backfire pops I've heard. Less wet sounding than normal and more like a gunshot. Not when I let of the throttle, but when I turned the bike off.

    There is nothing visibly wrong with it. No more leaks than normal, nothing visually out of place, can't find anything loose. I gave it about 15 minutes and started it back up again. 2 more of the same pops, but the bike sounded better. But there's no way whatever that was fixed itself. Has anyone experienced anything like this?

    Tl;dr: rode my bike after fucking with carb and its sounding weird and acting up

  2. #2

    Join Date
    Dec 2018
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    Most of what you are describing sounds like a vacuum leak, or maybe a fuel delivery issue.
    The intake manifold in all probability is secured to the heads by two flanges and two intake seals which are a modified oring which are triangular in section. The carb then plugs into the manifold and is sealed by a rubber collar, or has a rubber adapter with a clamp to secure the carb. Any of these rubber parts can deteriorate and cause a vacuum leak. You need to disassemble all of this, down to the heads, and inspect all the pieces, and replace the intake seals at the heads, part #26995-86A.
    Next, replace the fuel line, using a 5/16 ID hose. And while you have the fuel line disconnected, check that you have good flow from the petcock in both main and reserve positions, and that it shuts off completely, without leakage.
    On your FXR, I would remove the header and inspect, or better, replace the exhaust gaskets, and inspect the pipes for cracks. Also be sure the mounts are serviceable.
    And finally, the ignition can cause most of the symptoms you describe, and so a new set of plugs at the very least should be installed. Be sure to gap them.

    That should get you started toward solving your problems. Running a motor with a vacuum leak, especially in hot weather, can cause internal motor damage like you wouldn't believe, so get after it.

    Jim

  3. #3
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    Beginning at the begining you had issues. Sounds like it became worse after richening and adding fuel system cleaner. I would re-set the carb to the way it was and clean your carb. Change your plugs.Then check for vacuum leaks at intakes. Then check the timing. Check your ignition sensor module to see if it looks melty. Is the inside of the gas tank good or rusty?
    Of course anything JB says is always good advice.

  4. #4

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    I changed the intake seals and exhaust seals; both were in dire need. In doing so, found a significant crack in the exhaust and got that welded back up. Based on the carbon surrounding the crack, at least part of the crack had been there a while. My plugs and cables are ~6 months old. Fuel line looked to be in good shape and there is a good flow of fuel through it. Once I got everything buttoned back up, I adjusted the carb again to account for all that work.

    It was still backfiring worse than I'm used to until i adjusted the idle to about 2000 rpm, but it got to a normal sounding idle, so I took it for a test ride. The new sound persisted. It sounds like a car that's missing its catalytic converter or a blown speaker. I'm thinking my next move is a compression test?

  5. #5

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    Yes, by all means run a compression test (warm motor, throttle wide open). You may have some valve damage from running it with a vacuum leak.
    Your ignition module may be going bad and causing your symptoms as well.

    Jim

  6. #6
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    What do your plugs look like since they are 6 months old???????????? And the bike has been running bad........

    Post a pic...........

  7. #7

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    Pulled plugs. One showed signs of running lean so I replaced both (ngk gapped to .40).

    After lots more looking, found that the ignition rotor was bad. One of the magnets had dislodged and was out of place causing the bike to misfire and run all kinds of weird. It’s one of those things I would have never known to look for, so I’m glad I’ll know now.

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