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Thread: Hot idle, cold idle - I'm missing something!

  1. #1

    Hot idle, cold idle - I'm missing something!

    Maybe it's just the nature of the flat top CV carbs on my '79 R80...

    I've rebuilt both carbs. Top to bottom with all new guts. New air filter, no leaks at the inlet tubes. I've checked to make sure the chokes are all the way off and resting on the stops when disengaged.

    It starts first tick hot - even very hot - or cold and runs great. Both carbs are balanced perfectly at 3500 rpm and at idle after running the engine for about 10 miles or so. Hot idle is 1000 rpm. At speed all is good, smooth as silk.

    My issue is that cold idle does not exist unless I crack the throttle a bit - and it gradually warms up and settles down into a "lumpy" idle. Once hot everything is again nominal with a nice smooth idle at 1000 rpm.

    The only thing left I can think of is to replace the cables, but at 21K miles I would find it hard to believe that they are toast. I did notice a slight bend in the left cable due to the PO getting too aggressive pulling out the dipstick...

    Any insight would be appreciated - this one has me totally stumped.

  2. #2
    Registered User mneblett's Avatar
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    You're not doing anything wrong -- it's the nature of the Bing CV's. In my experience, cold idle is ~200-250 rpm below hot idle, so you basically set the idle "range" about the point you want. If you can't get a stable cold idle, bump it up to where it is stable, and decide whether you can live with the higher hot idle -- the bike will idle fine hot at 1100 rpm; it's only a question of whether you find the noise/busy-ness of the engine at the higher idle liveable. Some folks have no problem with that in exchange for the stable cold idle, while some don't like it and are willing to live with a few extra minutes of warm-up throttle monitoring in order to have a 1000 rpm hot idle.

    FWIW, my RT is currently set at 980-1000 rpm hot, and the cold idle is 750-800. I might bring it up to 1025-1050 hot to get 800-850 cold, but I'm in no rush, as I don't like a "busy" idle. On the other hand, my Mystic seems to prefer to be at 1100 hot idle to have any resemblance of a stable cold idle -- I just accept that as the nature of the beast.

    HTH!

    edit: ps: You will likely receive advice to keep the idle up to maintain oil pressure and avoid hammering the mains. While this is good advice to avoid damaging the motor with a 600 rpm idle, up in the range of normal idle this isn't a prime concern.
    Last edited by mneblett; 06-03-2010 at 08:23 PM.
    Mark Neblett
    Fairfax, VA
    #32806

  3. #3
    Well, alrighty then - thanks Mark! No more sleepless nights.

    Sounds like the solution is to leave it where it is, and install some sort of mechanical cruise control/lock for cold idle - or my old standby - an "O" ring between the mounting block and rubber flange of the grip. Yeah, a bit dangerous I suppose but I don't intend to ever fall off.

    I've seen some all metal ones in the past with a nice thumb lever - can you recommend a brand? I've used the plastic ones but have been less than impressed.

    I mean, I've got to get this thing running perfect when you take it on it's first ride.

  4. #4
    James.A
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    First off, your "choke" is an enricher circuit with a variable jet. The varible jet is a disc about the size of a quarter with different sized holes near the edge. When the "choke" is fully engaged the big hole is metering fuel to the carb. Then as the motor warms and you move the "choke" lever to off, successively smaller holes are in play until the "choke" is on it's stop and no hole is in play. Check to see that the enricher function is even on both sideswith respect to the cable pull. An un-even enrichment one side against the other might make for a choppy idle. Other than that an obstruction on one of the enricher discs might be a possibility as well.

    I wouldn't think you'd need much enrichment in a warm climate like Arizona. I don't need much enrichment here in the mid-west when temperatures are over 80F.

  5. #5
    #4869 DennisDarrow's Avatar
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    No problem with the idle being even a bit lower. Timing chain may stretch a bit over a 100K or so; but how many folks sit there and idle for that many hours?

    Start that puppy, flip up the choke to perhaps 1/2, (YOUR throttle lock screw is used for right now...tighten it just enough so that it lightly hold the throttle setting)......crack your throttle a bit, turn off the choke while you get your gloves, helmet and jacket on........and rock and roll.......Whole thing should take no more than a minute or two.......

    For me the so called HOT idle is 800 to 900. Just above where the red light alternator comes on. Any higher than that is just busy and hides other tuning problems.........Perhaps it's bragging rights to have low idle, letting everyone that hears it that YOUR'S is so sweet and maintained......Good Luck......Dennis

    YOU do have a throttle lock screw don't you......There just under your throttle. A large knurled screw that when tightened holds tension on the throttle. More or less a friction lock...........

  6. #6
    Liaison 20774's Avatar
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    Quote Originally Posted by RonKMiller View Post
    Both carbs are balanced perfectly at 3500 rpm and at idle after running the engine for about 10 miles or so.
    One thing you might do is to check the balance at around 1500 rpm, just off idle. At this lower rpm setting, the vacuum and any differences will be more noticeable. The butterfly in the carbs has just come off its stops, whereas the butterfly is open much wider at 3500, so any slight variations that might have existed at idle will be masked.

    Might not solve your current issue, but it's probably something to check.
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  7. #7
    Quote Originally Posted by woodnsteel View Post
    First off, your "choke" is an enricher circuit with a variable jet. The varible jet is a disc about the size of a quarter with different sized holes near the edge. When the "choke" is fully engaged the big hole is metering fuel to the carb. Then as the motor warms and you move the "choke" lever to off, successively smaller holes are in play until the "choke" is on it's stop and no hole is in play. Check to see that the enricher function is even on both sideswith respect to the cable pull. An un-even enrichment one side against the other might make for a choppy idle. Other than that an obstruction on one of the enricher discs might be a possibility as well.

    I wouldn't think you'd need much enrichment in a warm climate like Arizona. I don't need much enrichment here in the mid-west when temperatures are over 80F.
    Yeah, I need to get my terminology down better - I always confuse alternator and generator too!

    The issue isn't a choppy idle - its smooth, and the discs were whistle clean and in the correct position when I rebuilt the carbs. I paid special attention to this.

    The issue is hot vs. cold idle. It appears I can have one or the other, but not both.

    Just one more charming aspect of owning an Airhead!

  8. #8
    Quote Originally Posted by DennisDarrow View Post
    No problem with the idle being even a bit lower. Timing chain may stretch a bit over a 100K or so; but how many folks sit there and idle for that many hours?

    Start that puppy, flip up the choke to perhaps 1/2, (YOUR throttle lock screw is used for right now...tighten it just enough so that it lightly hold the throttle setting)......crack your throttle a bit, turn off the choke while you get your gloves, helmet and jacket on........and rock and roll.......Whole thing should take no more than a minute or two.......

    For me the so called HOT idle is 800 to 900. Just above where the red light alternator comes on. Any higher than that is just busy and hides other tuning problems.........Perhaps it's bragging rights to have low idle, letting everyone that hears it that YOUR'S is so sweet and maintained......Good Luck......Dennis

    YOU do have a throttle lock screw don't you......There just under your throttle. A large knurled screw that when tightened holds tension on the throttle. More or less a friction lock...........
    Yep, I've tried that trick - and it's a good one. It's just a bit awkward screwing it in and out since it takes several full turns to hold. My current routine is just to pin the flange of the grip against the mounting block with my thumb.

    I think I'll go with the mechanical cruise control - so much easier to just flip and lock it in position - will also come in handy to relieve the old carpal tunnel issues on a long trip.

  9. #9
    Quote Originally Posted by 20774 View Post
    One thing you might do is to check the balance at around 1500 rpm, just off idle. At this lower rpm setting, the vacuum and any differences will be more noticeable. The butterfly in the carbs has just come off its stops, whereas the butterfly is open much wider at 3500, so any slight variations that might have existed at idle will be masked.

    Might not solve your current issue, but it's probably something to check.
    A good thought, but it is balanced perfectly all the way up to 3500. It may fluctuate a tiny bit side to side on the way up from idle but I attribute it to turbulence. I built one of the $4.00 "yard stick" balancers and it works beautifully - extremely accurate.

    I'm still stunned how incredibly important it is to have excellent balance. "Close" just doesn't get it.

    With that heavy flywheel this engine is as smooth or even smoother than my LT above 3K. Amazing considering it's age.

  10. #10
    Registered User lmo1131's Avatar
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    ! ! ! THREAD JACK ! ! !

    I built one of the $4.00 "yard stick" balancers and it works beautifully - extremely accurate.
    I've got a Twin-Max carb synchronizer, but would like to build a manometer type. Any one have a link to a How-To article?

    Thanks.

    ! ! ! UN JACK ! ! !
    "It is what you discover, after you know it all, that counts." _ John Wooden
    Lew Morris
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    1963 Dnepr

  11. #11
    Liaison 20774's Avatar
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    DIY manometer...this is a very popular website:

    http://www.airheads.org/content/view/183/98/
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  12. #12
    I now have a Vista Cruise control mounted ($25.00) and can have any idle I want, any time, any where. Problem solved.

  13. #13
    Rally Rat
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    Quote Originally Posted by RonKMiller View Post
    I now have a Vista Cruise control mounted ($25.00) and can have any idle I want, any time, any where. Problem solved.
    Is this that fix-em-up bike??? Where are the pics sir??? This thread is useless... well, you get the idea...

    You have one of your seats on it yet???

  14. #14
    Quote Originally Posted by UncleMark View Post
    Is this that fix-em-up bike??? Where are the pics sir??? This thread is useless... well, you get the idea...

    You have one of your seats on it yet???
    Soitantly!
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  15. #15
    It's gonna be 107 this weekend - buh bye vinyl:
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