How much compression is too much compression?

Discussion in 'Airheads' started by misterpepper, May 25, 2017.

  1. misterpepper

    misterpepper Been here awhile

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    I put some SuperTech 20w50 conventional oil in yesterday, along with the 140 jets to replace the 145's that were in there from the previous owner. It seems to run stronger, probably more due to the jets than the oil. I'm starting to get just a little popping under deceleration if I cut the throttle with the revs high, but no pinging under load. Hopefully the rings will still seat okay and things will keep improving. I will say that riding in a hot climate is borderline painful on my feet due to the heat coming off the heads and the exhaust. I think I'm going to wrap the headers, just for my own comfort. Don't worry, the headers are already pitted and in need of replacement, so I'm not risking destroying anything irreplaceable. Is there anything else I can be doing to minimise the heat on my feet?
    #41
  2. Airhead Wrangler

    Airhead Wrangler Long timer

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    Wear boots. I dont notice it at all.
    #42
  3. Beemerboff

    Beemerboff Long timer

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    Yes, boots, or as a last resort you could have the engine tuned by someone who knows what they are doing.
    150 mains would be a start, and then setting the timing on a dyno------.
    #43
  4. misterpepper

    misterpepper Been here awhile

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    Why 150 mains? The LS came from the factory with 138's. Unfortunately I don't have access to a dyno willing to adjust timing or even change my needle height, only give me a readout of the current hp/tq and air/fuel mixture at wide open throttle.
    #44
  5. Beemerboff

    Beemerboff Long timer

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    If you can feel excessive heat it is running too hot, and going up from 140's to 150's will help cool it down.
    Forget what the factory fitted to a stock bike 30 or 40 years ago years ago, probably for emission control reasons , and concentrate on what works with what you have now.
    Gas is also different, which might help explain why the stock timing mark can be quite a long way out. Hot is usually retarded, a long way to far advanced doesn't help either, but you would probably have pinging too.
    And remember the easiest way to tell if you are running lean on a air cooled motor is if you have the jetting which makes the most power!
    #45
  6. misterpepper

    misterpepper Been here awhile

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    That's what I'm trying to do, figure out what works with what I have now. Unfortunately, until my rings seat, things are still changing. I wish I had a baseline to know if my bike was running too hot, or if the heat I'm feeling is just normal. What I do know is it's getting uncomfortable. I have an IR thermometer and can take readings of the head and the pipes if that helps. Currently the bike is timed by the book with the bean can working correctly. I plan to adjust the timing once I settle on a main jet. At this point I know that the 140 mains run a little better than the 145's, confirming my suspicion that the 145's were too big, but again, that may change when the rings seat.
    #46
  7. Uke

    Uke visualist

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    What's your altitude, that may, in part, explain why your bike is running better with a leaner carb configuration?
    Day after tomorrow is the first day of summer for those of us north of the equator, that may also explain more felt heat.
    #47
  8. misterpepper

    misterpepper Been here awhile

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    Altitude is only about 3000'. I run pod filters and have somewhat gutted my stock exhaust -- I shortened them about 15". I also deleted my crossover tubes because they were rusting out. Oh yeah, and the temperature here will hit 113 this week. The 140 mains are still a bit richer than stock, but I'm waiting for my compression to come back before playing with the tuning too much.
    #48
  9. Uke

    Uke visualist

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    The aviators with their relative altitude tables will tell you that 3K' above sea level and low humidity is effectively a much higher effective altitude due to the air thinning @ 113* F compared to perhaps a mere 93* F.

    Your engine may well have been running rich for those conditions.
    #49
  10. misterpepper

    misterpepper Been here awhile

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    I took a baseline measurement on my way home from work today. Ambient temperature was 105 and the head and headers were 330F and 600F respectively. I don't think it's running excessively hot, based on the numbers. It was pretty uncomfortable and I found myself riding the outside edges of the footpegs to minimize the burn. I'll wrap the headers tonight and try again tomorrow. If it doesn't improve things significantly I'll cut it off and change my approach.
    #50
  11. Rucksta

    Rucksta SS Blowhard

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    For comparison 1000cc motor, twin plugged large valve heads, shallow sump, no oil cooler 95 f ambient sea level to 2500 ft

    heads 245 f, measured at the exhaust.
    port headers 525 f, measured halfway along the curve of the down pipe (hottest spot)
    pipe temperature near the gear shift is 220 f
    #51
  12. misterpepper

    misterpepper Been here awhile

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    I was measuring between the fins near the exhaust valve on the head, and about 2" from the exhaust port on the header, engine still running. Both measurements are of the hotter cylinder, but they were fairly close. Thanks for the comparison numbers. It seems I may be running a little hotter than ideal and hopefully getting it tuned/timed correctly will help even more.
    #52
  13. Beemerboff

    Beemerboff Long timer

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    Just returned from 50 km ride last 2 km 60 km/hr limit.
    Crisp clear winters day , flat calm , 68F ambient.
    One side 225F, other side 227F , headers near footrests 100F lower but probably just cooled down faster than the heads.
    Mine is deliberately set rich to keep it cool on 100F+ days and slow dirt track riding.
    600F in the exhausts back at the footrests suggests that the fuel is still burning on the way down the pipe, an indication of retarded timing, and the lack of power at higher revs suggests the same.
    Forget the factory marks, which only get to within 3 degrees either way, and try advancing it a tooth or two and see how that works out.
    I mean use the teeth on the flywheel to judge how for you are advancing it-----.
    #53
  14. misterpepper

    misterpepper Been here awhile

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    The 600F is measured just after the finned nut, not down by the footrests. One reason I haven't messed with the timing yet is that I don't know how rich or lean I am running and I figured it would be easier to get the jetting correct first. Since both too advanced timing and too lean can lead to pinging it made sense to me to only adjust one at a time.

    Are you setting it rich by just raising the needle, or are you changing the main jet as well?
    #54
  15. misterpepper

    misterpepper Been here awhile

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    Header wrap helped a little, but the heat was still pretty uncomfortable on the ride home today. Ambient temperature was 110F with the head at 325F and the header at 460F. If I can get the heads down in the 250F range like some of you other inmates I think I'll be okay on even the hottest of days. I'll advance my timing and maybe raise my needles a notch to see how that helps things.
    #55
  16. misterpepper

    misterpepper Been here awhile

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    I advanced the timing roughly 2 degrees over stock. Not enough to ping, and I wasn't trying to push anything. It's my understanding that airheads were timed from the factory at 34 degrees advanced before adding the pulse air system. Then timing was retarded to 32 degrees before top dead center. I deleted my pulse air hardware almost day one, so I'm probably back to where I should be. I may try pushing it further at a later date. The same ride home, again in 110 degree heat, netted 310F at the heads and 445F at the headers. A small result, but probably well within the margin of error. It looks like adjusting the carbs a little richer is the next step. Between changing the timing and the rings (hopefully) starting to seat my idle has gone up 200rpm so I need to adjust them anyway.
    #56
  17. motu

    motu Loose Pre Unit

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    If Supershaft was still here he'd say go leaner with an R65. I did, down to 123 mains on my R65 Mono. Seeing as SS isn't here anymore, I can finally admit he was right.
    #57