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Thread: 1976 R75/6 Warm-up

  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.

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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.

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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.
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    '78 R100/7 & '69 R69S & '52 R25/2
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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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    Joe -

    As I said, the issue is primarily with the spark energy finding it's way to ground. If you are just causing the spark to jump a larger gap, that's not nearly so bad. But pulling the cap completely off a running bike is generally not recommended. It doesn't really matter if its points or EI...the energy in the system is looking for ground...if it can't find it, some harm can be done...or may be done...it might be a cumulative effect.
    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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    If strengthening the spark by making it jump an extra gap makes the engine run smoother during warm up, how does that translate into a problem with the carb? It would seem to me that that was a problem with a weak spark which would also cause the sputtering / popping during warm up. Would that also give me the sooty plug on the cylinder with the weak spark after the engine was warm enough to run (seemingly) fine?

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    Quote Originally Posted by cseltz View Post
    If strengthening the spark by making it jump an extra gap makes the engine run smoother during warm up, how does that translate into a problem with the carb? It would seem to me that that was a problem with a weak spark which would also cause the sputtering / popping during warm up. Would that also give me the sooty plug on the cylinder with the weak spark after the engine was warm enough to run (seemingly) fine?
    Last time I bought plugs at Motorrad Elektrik, I thought I was getting the old bosch style and wound up getting resistor ones. Stuck with the NGK non-resistor ones because ME wanted to charge a restocking fee and I wasn't interested in all the nonsense of changing my plug wires for compatibility with the new plugs. Have you checked your plug gap, alot of these plugs don't come pre-gapped for our application? I'd still check the carbs (float height, even gasket condition - I recently replaced my gaskets and had to completely readjust my float height as the old gaskets were too compressed and basically shot), after that worry about the spark (plug, wiring, coil, etc.)
    // 1975 BMW R90/6 (cafe'd) // 1957 BMW R60 (in pieces) // 1967 Aermacchi/H-D Sprint 250 SS (custom special) // 1973 VW Type 181 Custom SOLD )

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    Quote Originally Posted by typ181r90 View Post
    Last time I bought plugs at Motorrad Elektrik, I thought I was getting the old bosch style and wound up getting resistor ones. Stuck with the NGK non-resistor ones because ME wanted to charge a restocking fee and I wasn't interested in all the nonsense of changing my plug wires for compatibility with the new plugs. Have you checked your plug gap, alot of these plugs don't come pre-gapped for our application? I'd still check the carbs (float height, even gasket condition - I recently replaced my gaskets and had to completely readjust my float height as the old gaskets were too compressed and basically shot), after that worry about the spark (plug, wiring, coil, etc.)
    I talked to Rick over the phone when I placed the order. I got the plugs that he recommended and they are NGKs and I checked the gaps prior to installation. That is what was in the engine when I got it. The carbs have just gone through a complete overhaul and I have gone to some length to settle the float heights. With the new ignition and everything else new, I guess the only thing left is the coil(s). Whatever it is, I believe, is something that has to be isolatable to one cylinder; right? Heads were just torqued and valves adjusted. Thanks for the feedback.

    Chip

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    Registered User godfather's Avatar
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    I am not a mechanic by any means, but did you try swapping the coil and plug wire, one at a time, with the right side to see if the problem moves? That would tell you if it is indeed the coil or wire.
    Attitude is everything!
    1978 R100/7
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    Have you timed the bike with a strobe?
    I would check for a double image. Firing can be way off form one side to the other if the Dyna is not adjusted properly.

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    Quote Originally Posted by photorider View Post
    Have you timed the bike with a strobe?
    I would check for a double image. Firing can be way off form one side to the other if the Dyna is not adjusted properly.
    You can also check that without a strobe because the Dyna can be "static" timed - each side individually.

    But I think that if this were the problem, it would show up all the time, not just during warm up.

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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

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