1. Dismiss Notice
  2. Hey Facebook people... We've created a group for XS650.com members to connect. Check it out!
    Dismiss Notice

XSJohn's right side hotter theory - explored

Discussion in 'The Garage' started by Muckroot, May 22, 2020 at 8:11 PM.

  1. Muckroot

    Muckroot XS650 Enthusiast

    In another thread on this site XSJohn mentioned something that would lower cylinder temps by 20+ degrees.
    I am REALLY interested in finding out exactly what John was talking about here, so I did some digging.
    XSJohn's comments are in blue for clarity.

    This is a start, but there has to more information.

    Found these posts, which thicken the plot considerably:
    http://www.650rider.com/index.php?name=Forums&file=viewtopic&t=3075&highlight=one side hotter
    "OK, xsjohn, you've got me curious enough to get serious.

    For those who haven't been following the issue, John checked his cylinder temps with an infrared thermometer and found that the right gets hotter than the left by about 50* F. Every time others (myself included) have checked, they've gotten the same result. Ignition system isn't a factor--happens with points, OEM TCI, and Boyer. Happens with OEM and aftermarket exhausts. Happens with OEM and aftermarket cams. Happens with BS38's, BS34's, and VM roundslides. Happens with both stock and ported heads.

    John's theory is that it's causing (caused by?) a power imbalance between cylinders that can be corrected by pouring more fuel to the right. What puzzles me is that I've neither seen nor heard reports of a higher incidence of piston holing/detonation, failure, valve burn, or premature wear on the right than on the left.

    So--John's agreed to look into testing his theory on a dyno, and since I instigated him to do it it's only fair that I put some time and money behind the research too. Combustion temperature is what signifies, so in a few weeks there'll be a pair of probes hanging in my headers and a Westach double-needle EGT gauge in the instrument cluster. I think the cause is probably reduced right side heat dissipation due to the hot closed clutch cover, but we'll have some hard data to digest pretty soon.

    John, we'll run this down if it harelips every cat in the country."

    "Dang Griz you didn't have to do that but I truly appreciate you looking at it....dumping more fuel to the right is a bit of a stretch.....a bit of fuel is more accurate......Would be interesting to see it the exhaust temps are higher also.......

    put 100 miles on it yesterday 60 degrees or higher and could not find any temp discrepencies anywhere.....measured plugs.....heads.....cylinders......and intakes.....at least 5 times......I feel its ready for the dyno when it warms up enought to survive the trip.........."

    From a different post:
    "I always step in Dog Doo when I mention this…….With one carb there will be no balancing the heat with the right hotter cylinder.....I would be darn sure this was not occurring……..With 25 degrees higher temps on the right using the same carb for both would be a mess with no way of reconciliation ….No body believes this but that sure isn’t from lack of my warning…..I know people are doing it but have they checked….In the summer when temps are very marginal the right would be a furnace……Not for me…"

    "From what I have found if you have a hot cylinder and both carbs are set the same once you warm it up completely to operating temps in essence the hot cylinder runs leaner than the cooler cylinder which in turn becomes a richer cylinder......Richen both cylinders the same and the hotter cylinder becomes happy but the cooler cylinder becomes to rich.......Having two carbs sure makes it easier to tend to each cylinder independently giving each cylinder what it needs when it is at operating temperature.........Sure has smoothed mine out and hopefully make it last longer.......More useable power also......I have kits for the BS34 and others will be coming soon.........."

    John again mentions his carb needle kits here:
    So, Near as I can figure XSJohn, and a few other people who bothered to test cylinder temps found that after a good hard ride the right side cylinder was hotter than the left by anywhere from 25-50 degrees Fahrenheit. One person speculates that this is due to the clutch cover being filled with hot oil and therefor not wicking away as much cylinder heat as the left side alternator cover. Apparently the carbs used did not affect the temp discrepancy, nor did the type of exhaust, aftermarket or otherwise.

    As best I can tell John's solution to this was to ever so slightly richen the right side carburetor so that when the engine reached operating temps the right side would not run as lean, and therefore would not get so much hotter than the left. This, in his experience, resulted in smoother running, more even power, and and overall happier engine.

    -Does it make sense from a mechanical and physical standpoint? yes.
    -Does this sound like a bunch of ghost stories to upsell one man's carb needle kit? Maybe.
    -is it worth enriching the right side carb a wee bit to see if it results in more consistent emissions/temps from each cylinder? yes, absolutely.

    At this point I'm skeptical, but there is certainly enough information to prompt further investigation. XSJohn and some other people mentioned Dyno'ing a 650 with the carbs adjusted as John implies to verify his theory, although I couldn't find any mention of this ever actually happening.
    I think the best way to verify the validity of John's "right side hotter" theory is to measure engine temps after various rides, both aggressive and relaxed, and see if the temp discrepancy is present. If it is, the next step would be to enrich the right side carb, although by what amount I am not sure. the easiest way I can think of is to adjust the mixture clip on the needle, although this won't enrich the cylinder mixture across the full spectrum of carb function (idle to wide open). However, I believe it would be enough to test John's "hotter because leaner" theory because a majority of riding is done in the range directly affected by the needle. cylinder temp data should again be collected after a variety of rides. We can then compare the data from the "balanced carb" rides to the data from the "right carb richer" rides and see if the cylinder temps were affected. Finally, emissions data would be helpful during both tests to see if the "balanced carb" bike's right cylinder does run leaner at operating temps, and if the "right carb richer" bike's cylinders run at the same mixture at operating temps.

    If John is correct, and the right cylinder runs 25-50 degrees hotter than the left, and enriching the right carb fixes this temp discrepancy then we can all rather easily extend the longevity (and to a lesser degree performance) of our bikes. Please expand the information on this topic by linking to John's old posts or to posts others have made regarding a temperature discrepancy between the two cylinders of the XS650.

    Lastly, a shameless plug; I upload all the progress I've made on my 1975 XS650B hardtail project to instagram (Muckroute). Give it a look if you're bored, or if you think you'd enjoy the self teachings of an amateur mechanic building his first motorcycle.
    Last edited: May 22, 2020 at 8:17 PM
  2. Superjet

    Superjet XS650 Guru Top Contributor

    I thought he used plexiglass wings that directed airflow towards the head thus cooled it? Lemme look for that.....

    *edit* looks like they pointed down to his oil cooler.
    Muckroot likes this.
  3. grizld1

    grizld1 Grumpy old man Top Contributor

    I ran the question past Michael Morse once, and he mentioned that race tuners for Brit twins often "stagger jetted" the mains with a richer jet on the primary side. It stands to reason that the right cylinder of the XS650 would run warmer for the same reason: the primary side case cover is sealed, and the drive side is not, so that there's less cooling at one cylinder base than at the other. John's solution was well thought out. Since he was tuning for street use in the cruising range, not for full-throttle racing, he fabricated richer needles for the right side. My personal take was and is that the temperature difference wasn't great enough to worry about. Still, in every dead XS650 motor I found with a holed piston, the ventilated item was on the right, except for one. Note that all of those motors except the one with the holed left piston also had breaker point ignition that had been dialed in to the "F" mark at full retard and was advancing well past 40*, and one had stock jetting and no mufflers as well.
    GLJ and Mailman like this.

Share This Page