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Thread: 1976 R75/6 Warm-up

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  1. #1
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    1976 R75/6 Warm-up

    My bike starts immediately upon pushing the button; within a revolution or two with the choke on. It runs fine at idle (choke still on) and after 30 seconds or so, I push the choke off and the idle slows way down. At that point, the left cylinder (the one with the sooty plug) drops in and out. I can work the throttle to keep the bike running and once it is warmed up, it runs just fine, idles fine and acceleration is strong (still that sooty plug though). Is this more likely an ignition problem or a carburetor problem? I replaced the points with a Dynatek Dyna III electronic ignition, The coils are original. Plug wires and plugs are new (from Motorrad) Any thoughts?

    Thanks,

    Chip

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    I honestly don't remember how my '76 R75 was, but neither my '84 R100 nor my 76 R90S would/will idle off the choke until they had/have been ridden for at least 10 minutes.
    61 Gold Star, 76 R90S, 03 CBR600RR, '13 690 Duke, '14 Street Triple R

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    This will idle on the right cylinder and sputter and pop on the left. I can keep it running by tweaking the throttle. By the time I get to the end of my lane (1/10 of a mile in first gear), it will sit at idle. I am wondering if this condition is related to the sooty left plug.

  4. #4
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    Most likely the left carb. I have the same problem. I pull the cap off of the plug and just let it rest in the top of the plug to facilitate the firing of the left cylinder. I, too, replaced the plug wires and caps - no difference. I know my carbs need to be either cleaned or rebuilt because I bought the bike new (1975). Yes, once the left cylinder begins to fire, it's OK.

  5. #5
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    Quote Originally Posted by Weasel View Post
    I pull the cap off of the plug and just let it rest in the top of the plug to facilitate the firing of the left cylinder.
    I'm trying to envision what you're saying. Is the cap not physically attached to the top of the plug and that you're forcing the spark to jump a gap from the cap to the plug?

    This practice is frowned on for coil-type bikes. What's happening is you're causing quite a build up in energy which can echo back through the system. It's far worse if you remove the cap and don't attach it to anything. I've heard that this is an old dirt bike trick to clear plugs, but it's not a recommended thing on our bikes. Even a worse situation with an electronic ignition as this bike has a Dyna III installed.

    It might work one, twice, a dozen times...but it has the possibility of weakening the coil to the point of failure, or to a point where if you have a marginal ignition system, the bike won't run correctly or fail to start.
    Kurt -- Forum Administrator ---> 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!

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    Gosh, I've been doing this for about 20 years - doesn't seem to hurt anything at all, however - I do have points, so I guess my advice is not for those new-fangled electronic ignition devices.

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    I take it back. After much work on the carbs, it now idles after just a few minutes of warming it up.

    Quote Originally Posted by ccolwell View Post
    I honestly don't remember how my '76 R75 was, but neither my '84 R100 nor my 76 R90S would/will idle off the choke until they had/have been ridden for at least 10 minutes.
    61 Gold Star, 76 R90S, 03 CBR600RR, '13 690 Duke, '14 Street Triple R

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    Quote Originally Posted by ccolwell View Post
    I take it back. After much work on the carbs, it now idles after just a few minutes of warming it up.
    I'm going to try swapping the coil wires and see if that moves the issue to the other side. Maybe this weekend.

    Thanks,

    Chip

  9. #9
    Registered User lkchris's Avatar
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    I'd vote for carburetion problem.

    1st, understand why "choke" is required--it's required because fuel portion of the fuel/air mixture will "stick" to cold manifold and head passages and not enough make it past intake valve. When manifold and head passages are warmed up, this doesn't happen. The way to combat this is pretty crude, i.e. send more fuel. Note that an actual choke does this by creating more vacuum to draw more fuel through jets. Bing enrichener justs adds more via tube into float bowl, no extra vacuum required (or available).

    So, when you shut the enricheners off, the left cylinder goes lean. Think idle jet problems, vacuum leaks, etc. While you're at it, make sure the right side enrichener indeed shuts off.

    And yes, it's pretty obvious the intake tract on an Airhead is pretty short and some of it is rubber. Enricheners still needed anyway.

    If you've got ignition strong enough to not cause missing under high speed load, it will run the motor at idle, too.
    Kent Christensen
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    '12 R1200RT, '02 R1100S, '84 R80G/S

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