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  1. #1
    CB750 New Member markguev's Avatar
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    Spark Plugs fouling if I don't use bike for a few days

    I have a CB750 K4. It runs great. But if I don't use it for a couple of days one of the spark plugs will foul. This does not happen if I use it every day. What could be the problem?
    I'm running K&N pods, 120 main jets, 42 pilot jets. 4 into 1 exhaust.

  2. #2
    CB750 Guru amc49's Avatar
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    Older engine not in top shape then makes you hold choke on a bit longer to foul plug. Every engine on the planet is subject to fuel evaporation of the normally slightly moist intake tract in a normal running at the time engine that then gets shut off to have the tract begin to dry out. The longer the vehicle sits the more the drying and then choke has to stay on a bit longer to make up for it and then you risk plug fouling as the amount between normal wet and fouling is so slight you cannot meter it exactly with the choke alone.

    Look at the ignition condition and the 42 pilots are not helping, the norm is not to increase pilots as even pretty big engine mods do little to affect pure idle, they only come into play with bigger throttle openings. I've modded countless carbs with added jetting but it's extremely rare to have to mod pure idle jetting, can't remember ever doing it, the mixture screws are enough. I'd be dropping back to like 38 on those pilots. Look at the compression on that one cylinder if it stays being the same one too.

  3. #3
    Moderator dirtdigger's Avatar
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    What cylinder is fouling, I had cylinder one foul once because the needle wouldnt seal fully and the overflow in the bowl was plugged causing it to flood cylinder 1...when on the kickstand that carb is the last to empty by evaporation etc.

  4. #4
    CB750 New Member markguev's Avatar
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    Thanks for the tips, I'll try these out!

  5. #5
    CB750 Guru amc49's Avatar
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    FWIW if your local fuel has ethanol in it then that day to day evap issue gets worse, ethanol greatly speeds up fuel evaporation.

  6. #6
    CB750 New Member markguev's Avatar
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    it does, 10% I think

  7. #7
    CB750 Member Geezerbiker's Avatar
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    Try parking it on the center stand if you're going to leave it for a day or two. The tilt from the side stand can cause older worn carbs to flood a little over time...

    Tony
    OregonMotorcycleParts.com

  8. #8
    CB750 Guru amc49's Avatar
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    What the petcock is for-turn it off.

  9. #9
    CB750 Member Geezerbiker's Avatar
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    Quote Originally Posted by amc49 View Post
    What the petcock is for-turn it off.
    There's still gas in the lines. It's enough to cause the carbs to flood but not a lot. It's still worth trying to park the bike on the center stand to see if it makes a difference...

    Tony

  10. #10
    CB750 Guru amc49's Avatar
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    Next time you buy a diet coke in a cup with straw, cover the end of straw up with a finger and lift it out of cup and see how much coke comes out of the bottom of the straw.

    Virtually none at all and the small opening at the needle will pretty much prevent any more with the top of the hose closed off. It's a surface tension vs. pressure thing.

    Why turning petcock off all the time is a biker learned skill.

  11. #11
    CB750 Member Geezerbiker's Avatar
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    Try holding that straw for 24 hours. Eventually the liquid will run out. Also gas has much less surface tension that water. One more thing to consider. It's rare but it can happen sometimes when an engine is tilted to the side, puddled oil in head will get over the tops of the lave guide seals. It's a cheap diagnostic step to try parking the bike on the center stand...

  12. #12
    Moderator dirtdigger's Avatar
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    Guess we will never know, this is an old post and the original poster never wrote if the problem was fixed. The most common cause in the old sohc is the metal on metal needle and seat. Took hardly anything to make the needle and seat leak, I have put bean new needle and seat in and woulld still seep fuel past. If you turned the petcock off it seems to help with the issue. Now days I use needles from the keyhole carbs as they are rubber tipped and seal almost 100% reliable in the old carbs. But since the OP never got back we dont know what happened, consider it case closed

  13. #13
    CB750 Member Geezerbiker's Avatar
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    I didn't realize it was an old thread. I try to avoid replying to them... Anyway I agree with you on most of that.

    Tony

  14. #14
    CB750 Guru amc49's Avatar
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    Here in Texas and ethanol used in all fuel I wouldn't be surprised if the extra evap speed the ethanol imparts to the fuel kept the bowls at the same level even with a partial drip into them. You almost can't leave a bike one week and come back to find the needles stuck from the fuel drying up in the bowls. Big problem with older carbed stuff.

  15. #15
    Moderator dirtdigger's Avatar
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    Luckily up here only out mid grade is ethanol....10%

  16. #16
    CB750 Guru amc49's Avatar
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    Pray it stays that way. I approached ethanol with an open mind but it is crap. Here in Texas since it's cheapest way, I wash many parts in fuel but on a humid day the stirring up of it has water appear because of the ethanol in about five minutes. Because of the ethanol it DOES have a good side, any parts fuel cleaned now wash off after perfectly with water without any oldschool oily residue but you have to dry the parts FAST or they rust like lightning. I pretty much use a high pressure blower anyway when done, you absolutely cannot let the parts sit. Even sitting them in ethanol laced fuel and the fuel covering them will layer separate upon hitting air and anything low down in the fluid then corrodes to damage in 24 hours or less. So letting parts sit to soak in it is a mistake.

    The stuff has a few good qualities but the bad highly outweigh them. It's transparent as long as the vehicle or tool (say chainsaw) gets used every day but let it sit and then problems begin quick. Anything old school with lots of tank and carb venting it tears up fast. The car evap systems stop a lot of it but once fuel hits a certain point even that can bite. I just pulled a fuel pump module out of one car that sat a year and the brand new fuel pump back then looked like it came out of a 20 year old vehicle, and that was after in the past doing the same one year sit before the old pump went bad due to other things and zero issues doing it. You could tell where the fuel separated, no corrosion in the higher gasoline only layer but the corrosion went crazy in the ethanol lower layer. I suspect the last fuel batch of having some water already in it, if water gets into the fuel then sits it turns into acids that eat everything. I saw loads of that with farm trucks when I was in parts, the modules just looked like lumps of red stuff when pulled out of them. I drive older cars and one of three sits a lot to be used when needed and now on that one I pay far higher to store it with pure gas only so the car sitting is much safer. Bike the same and the sticking needle issue disappeared as soon as I did it.

    It can be some funny stuff, either wrecking parts or doing nothing. For sure the amount of air that gets to it has much to do with that. I used a same big batch of chainsaw premix for 3 years before running it out and no problems with it at all and it wasn't treated with anything. I just made sure it capped back up instantly after use everytime I used some of it.

    The law says 10% here but commonly it's over that and up to 20%, the problem being that no one checks the % at all.

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