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Thread: power drop-off?

  1. #1
    aapasquale
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    power drop-off?

    (1994 R1100RS) I have had this bike for 1 1/2 years now and I'm noticing lately that the power on acceleration in the 4500+ rpm range drops off abruptly. Does this indicate that the valves need resetting? It has 41,000 miles and I haven't done the valves for at least the 15,000 miles I've had the bike. It runs beautifully at lower rpm's.
    Tony

  2. #2
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    Quote Originally Posted by aapasquale View Post
    (1994 R1100RS) I have had this bike for 1 1/2 years now and I'm noticing lately that the power on acceleration in the 4500+ rpm range drops off abruptly. Does this indicate that the valves need resetting? It has 41,000 miles and I haven't done the valves for at least the 15,000 miles I've had the bike. It runs beautifully at lower rpm's.
    Tony
    My favorite topic. They were probably checked at 24,000.
    I'm still not clear on why the lash would ever change once set. But it does.
    Anyway some folks intentionally run more lash in order to shift the torque peak lower in the rpm range.
    So it does that. It has a "detuning" effect.

    BMW recommends 0.006" intake, 0.012" exhaust.
    I can recommend reducing that by 1/3 (on the 1100) to 0.004" and 0.009" - so far I have had good results, although I am cautioned by others that it can cause the exhaust valve in particular to run hotter than normal.

    I check mine once a year. This past year that was only about 3000 miles, so yes it's ridiculous.
    Must be some kind of sickness.

  3. #3
    Registered User lkchris's Avatar
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    Well, it's 19 years old. There could be vacuum leaks by now.
    Kent Christensen
    21482
    '12 R1200RT, '02 R1100S, '84 R80G/S

  4. #4
    MOA #24991 Pauls1150's Avatar
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    It doesn't "necessarily" indicate that the valves need resetting, but yes, tight valves will often show up as a reluctance of the engine to spin freely at higher rpm.

    Why does lash change? Because things wear, especially things that operate under adverse conditions (like high heat and constant battering). Worn valve stems or rocker arm parts will increase the lash; worn valve seats or valves (on the rear side of the face) will tighten the lash.

    Abrupt power loss at higher rpm may also mean that a coil is failing (remove & replace), or you're starved for gas (replace the filter on the pump intake AND the main one after the pump); at your bike's age (not necessarily mileage), you may also need to R&R the hoses inside the tank.

  5. #5
    Nickname: Droid
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    My 94 RS has four times+ the miles of yours, and it has always come on stronger from 4500 to just over 6000 rpm. My RS easily runs strong right up to 6500+. I have done all the wrenching on my RS since I got it in late 94. I do check the valves but rarely have I had to adjust them. Other than the fuel filter, all fuel system components on my RS are original.

    4500 rpn on the RS is right in the "meat" of the power curve. BMW twins like to spin and operate better above 3500 rpm. When you say the power drops off abruptly I find that kinda hard to believe. Valve adjustment will have little or no effect on this unless they are way off or the vavles are burned/damaged/recessed. You sure the air intake isn't restricted? How about the fuel injectors being clogged up? Fuel pump maybe? 41k on a 19 year old bike is only 2150 miles per year, not much. To me its a signal the bike has sat idle and unridden much of its life, and unridden means potential fuel system related problems due to old/bad gas. I'd sooner assume that than the valves.

    I'd also assume the throttle bodies have not been synch'd

  6. #6
    aapasquale
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    Quote Originally Posted by ANDYVH View Post
    My 94 RS has four times+ the miles of yours, and it has always come on stronger from 4500 to just over 6000 rpm. My RS easily runs strong right up to 6500+. I have done all the wrenching on my RS since I got it in late 94. I do check the valves but rarely have I had to adjust them. Other than the fuel filter, all fuel system components on my RS are original.

    4500 rpn on the RS is right in the "meat" of the power curve. BMW twins like to spin and operate better above 3500 rpm. When you say the power drops off abruptly I find that kinda hard to believe. Valve adjustment will have little or no effect on this unless they are way off or the vavles are burned/damaged/recessed. You sure the air intake isn't restricted? How about the fuel injectors being clogged up? Fuel pump maybe? 41k on a 19 year old bike is only 2150 miles per year, not much. To me its a signal the bike has sat idle and unridden much of its life, and unridden means potential fuel system related problems due to old/bad gas. I'd sooner assume that than the valves.

    I'd also assume the throttle bodies have not been synch'd
    I think that I will go ahead and do the valves and throttle sync--will be doing them myself for the first time- I'm actually looking forward to it--air intake is good, air filter new and clean--replaced fuel pump recently with new one--I got the bike a year and a half ago with 26,000 miles on it--will save replacing gas and vent lines for winter when I tear her down--I will be sending out fuel injectors during winter as well--
    Tony

  7. #7
    aapasquale
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    Quote Originally Posted by lkchris View Post
    Well, it's 19 years old. There could be vacuum leaks by now.
    Is there a test for vacuum leaks?
    Tony

  8. #8
    Registered User roger 04 rt's Avatar
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    Quote Originally Posted by Pauls1150 View Post
    It doesn't "necessarily" indicate that the valves need resetting, but yes, tight valves will often show up as a reluctance of the engine to spin freely at higher rpm.

    Why does lash change? Because things wear, especially things that operate under adverse conditions (like high heat and constant battering). Worn valve stems or rocker arm parts will increase the lash; worn valve seats or valves (on the rear side of the face) will tighten the lash.

    Abrupt power loss at higher rpm may also mean that a coil is failing (remove & replace), or you're starved for gas (replace the filter on the pump intake AND the main one after the pump); at your bike's age (not necessarily mileage), you may also need to R&R the hoses inside the tank.
    +1 to these thoughts ...

    Focusing on the fuel side for a moment, it was about a month ago that you posted this:

    Quote Originally Posted by aapasquale View Post
    Victory!!!
    Got the replacement fuel pump by USPO from Re-Psycle this morning. This is a new aftermarket one. I decided to go new as opposed to used. The OEM part is $400, this one was $200. Used could be had used for around $100. Took about 4 hours (I'm pretty slow and methodical) to remove tank and get to fuel pump. Replaced with new fuel pump and...............it works! I'm so happy to be tooling around town again. Thank you to the folks that took the time to get me going.Tony
    When you replaced the pump, did you replace the hoses, filter and screen? If not, you may want to check the health of the fuel system by the one of the following tests:

    Either:
    1) Tee a fuel pressure gauge into the high pressure line and make sure that the pressure is about 43 psi.

    Or.
    2) Assuming you don't have QDs, disconnect the return fuel hose at the tank, cap of the tank inlet with a short piece of hose and point the return line into a collection tank. Confirm that at idle you get about two liters of fuel in the collection tank in one minute.

    Or.
    3) Open the tank and carefully inspect the screen, filter and hoses.

  9. #9
    aapasquale
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    Hi Roger-

    I did replace the screen when the pump was replaced and the fuel filter was changed at 36,000 miles so that should be fine. Did not change any of the hoses--I will borrow a pressure gauge and test whether the new fuel pump is up to snuff--thanks for the help--will inform all of results in couple of days

    Tony





    Focusing on the fuel side for a moment, it was about a month ago that you posted this:



    When you replaced the pump, did you replace the hoses, filter and screen? If not, you may want to check the health of the fuel system by the one of the following tests:

    Either:
    1) Tee a fuel pressure gauge into the high pressure line and make sure that the pressure is about 43 psi.

    Or.
    2) Assuming you don't have QDs, disconnect the return fuel hose at the tank, cap of the tank inlet with a short piece of hose and point the return line into a collection tank. Confirm that at idle you get about two liters of fuel in the collection tank in one minute.

    Or.
    3) Open the tank and carefully inspect the screen, filter and hoses.[/QUOTE]

  10. #10
    aapasquale
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    another victory!

    Hi All--just did a valve adjustment and throttle sync-- cleared up these issues:

    -almost entirely eliminated "popping" sound from exhaust on deceleration
    -much more power available with no signs of dropping off past 5,500+ rpm
    -surge almost entirely eliminated, absolutely minimal
    -has much more torque at lower rpm's (2-2,500 range)

    I would say these are very good results. Thank you for all your suggestions.
    Tony

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