High RPM

Discussion in 'Airheads' started by gvganser, Apr 1, 2013.

  1. gvganser

    gvganser Adventurer

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    Just did my first carb rebuild. I took my time and made sure to only do on carb at a time and when I took a piece off I put it back the same way. I did o-rings, enricher gasket, needles, needle jets, floats, springs and diaphragms. It is a 1988 R100GS and the diaphragms are the type with the 4 screws to remove the top piece, not the press on kind. I also replaced choke and throttle cables. I used Snowbum's web site to assist me in this process and think I did all the steps and set up as prescribed.

    Bike started fine. Choke worked. Light blip on the throttle and maybe RPMs didn't come back all the way down? After idling for a minute with the choke off I turned bike off to check air in the tires and restarted. Idle a little high but that is what adjustments after the ride are for.

    Two easy miles into the warm up ride I come to my first stop... RPMs stay hung up about 3000 RPMs. Throttle control snaps back nicely on both sides. Choke is off. A gentle tug on each throttle cable at the carb causes RPMs to raise. Cables are seated the collars wolithout too much play...

    Has anybody else had this happen and can tell me what I did wrong? Or at least know what direction to start?

    I do not live where my bike is. So if I need check or adjust something physically it may take some time to come up and do that.

    Thank you in advance for any advice.
    #1
  2. gvganser

    gvganser Adventurer

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    After letting bike sit for 15 minutes to right that post, I started it up. Idling at about 1500 RPM and they dropped back to that after a slight and short rev. Don't know if this info is pertinent or helpful.
    #2
  3. Kai Ju

    Kai Ju Long timer

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    This is why it is recommended to adjust your carbs following a ten mile testride.
    20w-50 takes it's sweet time to fully warm up and will effect how much drag there is on the engine internals..
    So, go for another ride and reset your carbs.

    In case you haven't already done this, make sure your intake stubs are tight in the cylinder head. Easy way to do this is to make sure the engine side hose clamps are tight while the airbox side clamps are loose. Grasp the carb body and see if you can turn it around it's bore axis. If you can you need to either tighten the intake stub in the head or replace the rubber sleeves between the carb and the head.

    If you're not running a stock airbox anymore, all bets are off. You may have to rejet to get the results you're looking for.
    #3
  4. brocktoon

    brocktoon Been here awhile

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    I had a similar problem recently after a carb rebuild. The culprit was found to be off-center butterfly valves, which let some air leak through. After rebuilding did you hold them up to a strong light and look through the carb body to check that no light was leaking through the when the valves were fully closed? Are you certain the valves were oriented properly on reassembly with respect to the beveled edges?
    #4
  5. disston

    disston ShadeTreeExpert

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    It is time now for the adjustment and balance of the carbs. According to the Bing Manual these carbs should be the 64/32/357 & 358 carbs. Please verify these are indeed the numbers on your carbs?

    According to the Bing Manual the starting position of the idle mix screw is 0.5 turns out.

    Turn the idle speed screws down so you get a proper idle. After the ten minute ride try to adjust the mix screws. First one side then the other. Tuyrn screw in till the engine stumbles then out, it should run fine, till it stumbles again. the proper place for the idle mixture screw is halfway between these two positions. It is really a judgement call I think.

    You are not allowed to remove the plug wires while the engine is running to gauge the balance of the cylinders. Some method of reading the vacuum or shutting off one side at a time is needed to properly balance the cylinders.

    What method to achieve balance are you using?
    #5
  6. gvganser

    gvganser Adventurer

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    I think it may be in the idol speed screws? I guess that I read it as turn until closed then turn in 1 and 1/2 turns. When the butterfly's come to a close the idol speed screw disengages the throttle lever at that point. Maybe I thought that because I couldn't see how disengaging the idol speed screw would then allow it to adjust the idol...

    The idol mix screw is currently at 1/2 turn as recommended in the book. And once I am able to complete a ten mile ride I will be using a twin max carb balancer.
    #6
  7. gvganser

    gvganser Adventurer

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    And yes. Those are the numbers that I have on the carbs.
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  8. disston

    disston ShadeTreeExpert

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    As mentioned by Brocktoon the butterfly plates must be centered in the throat of the carbs otherwise you will never get these carbs to balance or run right. Let's assume for now you don't have the throatle shaft/butterfly plate problem? That is if you know what this is in reference to?

    It is a common problem for riders to get info about their carbs from a friend and so they adjust the mix screws to what ever their friend used on a completely different bike. So to begin you are using the correct start spot on the mixture screws.

    The idle speed screws are the bigger screws that set the stop place of the throttle plate opening. At this point an even setting should be used. That is the same amount of opening per side. Hopefully this will give an even enough balance the bike can be ridden. What I do is try to find the exact spot the screw contacts the lever that moves the shaft. This will be the zero point. I then use One (1) turn in. If this is too low the 1 1/2 might work better. But it sounds like what ever you have tried so far is to far open. No mater what I say or any expert has written on this subject you still have to find what works.

    The cable adjusters should also be preliminarily set by eye. Try to have some slack on each side. The same amount of slack is supposed to work but sometimes kinks in the lines effect this and what is needed more than the same amount of slack is that the cables start to work at the exact same moment.

    I hope this helps with preliminary set up. Balance is not my favorite subject so I'll bow out for a bit.
    #8
  9. gvganser

    gvganser Adventurer

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    Idle screws are at 1 and 1/2 turns. The cables have about 3 mm of play and the best I can tell via eyeball and feeling when each cable starts to turn vs throttle resistance...

    Would the idle set screw allow it to run at 1300-1500 when cool and let it rise to 3000 as it starts to warm up? Could it be the diaphragms some how? I know that I put all the rubber in the right groves when installing them.
    #9
  10. 190e

    190e Long timer

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    1 1/2 turns on the throttle stop screws sounds a lot to me.

    Revs hanging up when it's carb related can be due to the throttle plates being so far open that they start to bring the transfer ports into play. The transfer ports are only supposed to flow during transition from idle as the throttle plate passes over them and creates the vacuum needed to pull the fuel mixture through. For this reason the object is always have the minimum possible throttle opening at idle.

    1300 -1500 idle is way too high especially if the engine is cool. If you back of the throttle stop screws to a lower idle speed the problem will go away.
    #10
  11. gvganser

    gvganser Adventurer

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    Ok... I have my first mission. Turn back throttle stop screws and go from their. I appreciate the input and will let you know what happens...

    May not get to it until Friday.
    #11
  12. Plaka

    Plaka Brevis illi vita est

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    mmm..you rebuilt them but didn't set them up and tune them. Why should they work right? (and if they do, go buy a lottery ticket right now)

    Why did you replace the floats?
    #12
  13. gvganser

    gvganser Adventurer

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    Tune them?? What should I have done to tune them before mounting them back on the bike?

    And I replaced the floats because the bike has 112,000 miles, they were greatly discolored and had not been replaced in about 70,000 miles.
    #13
  14. Plaka

    Plaka Brevis illi vita est

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    Not much. But while you have them in hand set the idle mixture and throttle idle stops to nominal values. For a regular tune up you do that on the bike. once mounted and the chokes hooked up, make sure off is off.

    But once they're on the bike and it's running, you balance them. personally I do that starting from cold. You set valves cold and I use go/no-gauges so it's very accurate. With the valves done I go right on to the carbs. The oil viscosity changes very little with temperature (why it's a multi grade) and the valve settings change a bit as it heats up but it makes no difference to the balance. If the idle is a little off after a good ride and full warm up I just adjust it, both sides equally, wherever I am.

    Should you have the carbs in hand again, put a smear of silicone around the bulgy place in the choke cover---at the seam. it's a weak spot and generally you do a killer glue job right there. But if you did or didn't, a little silicone is easy insurance and it's on the inside (faces block) so it doesn't show.

    You replace floats if they're bad and discolored isn't bad. If one weighs more than the other (I don't have a spec but I'd say more than 10 grams), then replace both. Done's done and the only reason I mention it is because if you get in the habit of replacing things just because they have 70k on them or they seem discolored you will waste much time and effort that could be spent on things that will make a difference. Reading and research is high on that list followed by getting an air eraser and cleaning contacts and sprucing up the ignition. I don't even replace diaphragms, much less springs, unless they have a hole (I keep the new ones on hand and carry on longer trips). There are some spare things you want around, like a clutch cable, center stand spring, cable barrels, certain nuts and bolts...these are good places to put some cash. Also a stainless carb top screw kit and that goes in now with ample anti-sieze.

    Anyway, you'll get it running fine. I'm not much of a fan of vacuum balancing but as long as you have the gear give it a go and see how it runs.
    #14
  15. gvganser

    gvganser Adventurer

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    Thanks for the Info. So I guess I did tune them. As I stated earlier I set the mixture and idle stop screws and gave the amounts.
    #15
  16. Plaka

    Plaka Brevis illi vita est

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    Those are your nominal settings. it's where you start so it'll run enough to tune them. Those are not your final settings. You always start your tune with those settings, you don't finish with them.
    #16
  17. gvganser

    gvganser Adventurer

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    Yes. I understand that. I didn't expect to not have to make adjustments. But as I said this is my first time. When the RPMs were sitting that high I thought it best to ask those that know before I started fiddling with anything and potentially making it worse.
    #17
  18. Plaka

    Plaka Brevis illi vita est

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    Nah. Just do your tune and balance. That finishes the job. THEN if you have a problem with high RPM, Ask the gurus (I think there's one over there--->)

    No worries about making it worse, just go back to your nominal settings and begin again.

    If the problem isn't the carbs, that's different. But do the carbs first.
    #18
  19. StmbtDave

    StmbtDave AKA Invisible Dave

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    You didn't say if you removed the butterflies to replace the o-rings on the throttle rod. If you did remove them, then the problem may well be you didn't get the butterflies reinstalled properly and they're hanging up in the carb body. If you didn't remove them and the idle was operating properly prior to the rebuild, then I don't suspect the butterflies.

    The diaphrams have small tabs both on the inside and outside edges. These tabs fit into depressions on both the slide and the carb body. If these aren't set correctly, you could either have a poor diaphram seal or the slide may not be installed the correct direction.

    Being an '88, I assume the carbs have the large return spring around the slide and under the diaphram. I believe this spring wasn't in earlier carbs.

    Put your finger down the throat of the carb and verify that the slide moves up and down with no drag and that it seats fully down with no hang up.

    An irregular idle usually means an air leak on the output side of the carb. As suggested, verify that both clamps are tight and the carb doesn't move when you try to rotate it.

    Dave
    #19
  20. gvganser

    gvganser Adventurer

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    Dave.

    No. I didn't do the butterfly o-ring. I did verify the tabs on the diaphragms when putting them together and I believe there are the wholes in the bottom of the slide that are on the output side.

    I did check the resistance if the slides and they seemed smooth. Won't say that I didn't mess something up. Ill start with the idle stops and go from there.
    #20