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RH cylinder is too rich

Discussion in 'The Garage' started by Jay H Miller, Jul 27, 2020.

  1. Jay H Miller

    Jay H Miller XS400 New Member

    Hi All, I'm new to the "forum world". Forgive me is as I learn the process.

    So I'm working on a 79 XS 400. The LH cylinder had a real bad plug cross headed. I removed cover and inserted helicoils in both sides. Lapped the valves and put it all back together. I checked the valves with carb cleaner. They are not leaking. I'm confident things went back together with everything in the correct timing. LH TDC, came shaft and sprocket all aligned. I set the gap and the timing. As part of the rebuild I wrapped pipe tape on the exhaust and attached new mufflers. The mufflers have the same baffle. But the body of the muffler from the baffle to the front end is longer than the mufflers that were on the bike.

    Started it up. Used my color tune to adjust the idle. sync the carbs with gauges and check the idle mixture again. Not real happy with the color on the LH. Looked too lean. Idled great. When at about 4000 rpm it would miss. I should also mention that the LH side is much hotter than the RH. Missing at higher rpm so I thought maybe the timing needed to be advanced a little. I moved it forward. Still not luck. RH plug is loading up and LH looks too lean. I opened the carbs up and adjusted the main needle position up leaning out the system. (needle deeper into the main) while in the carbs I checked the jets. I wondered it by some chance they were not the same. Both are 132.5. ( I do have a set of 125) The air jets are both 45. No luck. By the way the other things I tried have been; put the old plugs back in. the new ones were different band just in case the new ones were wrong. And verified the gaps again. No luck. Remove the mufflers to see if the new ones caused some issue. No luck. I'm all out of ideas.

    What could I have done to have such an in balance between the two cylinders?

    Help - Very frustrated at this point.

    Ok. So as I lay awake wondering what was "touched" and wondered about the valve gap. Even though i set them. I grabbed a compression tester. The RH is about 45 psi and the LH side is about 80. The LH plug looks real lean and the RH side is too rich. There is nothing i could do to affect that condition. Could there be some issue with the seal of the head and cover. I put in a new gasket at the head and liquid gasket for the cover.
    Anybody have any direction?

    Last edited: Jul 28, 2020
  2. amc49

    amc49 XS400 Enthusiast

    BOTH cylinders being under 100 psi means the engine is dead if the numbers are real. You need to verify them using another tester as the test equipment as well as the techniques used there can kill your readings as to reality. Yank both plugs and throttle held wide open or the readings are worthless. Use a car battery to take several tests in each cylinder but do NOT EVER start the car up doing it. Bike battery will work but they commonly run down taking several tests.

    Helicoils are not really the correct thing to do in a plug hole, they often don't seal and something like a Timezert, will let the plug still dissipate heat correctly, the helicoil often interferes with that.
  3. Gra900

    Gra900 XS400 Addict

    Not come across a time sert before, looks really much better than helicoil, though they are probably fine for these engines
  4. Jay H Miller

    Jay H Miller XS400 New Member

    OK. So I finally got around to verify the compression. I picked picked up a new gauge and don't recall if I has the throttle open the first time. Now I'm getting 140 psi consistently. That's good. Bad think is I'm back to where I started. Why is the LH very lean and RH rich? I plan to check the valve clearances again. Could I have done something wrong with the gasket or torque that would result in the situation I have. Thoughts?
  5. amc49

    amc49 XS400 Enthusiast

    Someone a bit back pointed out a worn needle jet can do it.

    I know myself that a small pinhole or the like in a slide diaphragm can make that one slide open slower and then it tends to run rich. What slow slides do.

    You can have good compression and still leak oil a bit at a valve seal, that can easily tilt what your readings look like.

    Somewhere around here as well I thought I picked up that the engines are not jetted the same L and R and if jetted even that could be some of it.

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