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Old 07-11-2013, 12:47 PM   #1
jk$ OP
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Question Carbon Fouled Plugs '74 TX500

Hi,

Im somewhat new to wrenching my bikes and I'm loving it!

I have a 74 Yamaha TX 500 that keeps carbon fouling her plugs :(. I've put on slightly hotter plugs (D7EA from D8EA), narrowed the jets from #132 to #120, and put on K&N filters right on the throttle body.

She STILL fouls (black soot) the plugs almost to unusability within 15 minutes of hard riding.

What could it be? I've heard it could be an ignition issue or that I need an entire new carb.

What are your thoughts?
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Old 07-11-2013, 04:05 PM   #2
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Are you sure it's not OIL fouling? Looks very similar, not always wet and shiny like textbooks say, but can be fluffy black like fuel. Have someone ride behind you, observe smoke.. blue/white? = oil. black? = fuel. How is the oil consumption?

The K&N rockscreens make existing mixtures leaner, so that's not the issue. Before you go changing lots of calibrations (jetting) lets find the problem and solve it. A misfiring ignition will leave the plugs black, but you'd see ridability issues first. As for needing "an entire new carb", unlikely, seeing as that it's running. LOT's of Misinformation/understanding about carbs floating around. Tell us more about what's been done to the carbs so far... cleaning, I assume. New carb kits with gaskets? A leaking enrichener ("choke") seat can cause fuel fouling. Have you done a compression test?
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Old 07-11-2013, 05:18 PM   #3
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Thank you for the reply!

There's black smoke coming out of the back. I have not checked oil consumption but will look at that (when i can ride her for more than 15 minutes without killing the plugs).

The carbs are stock (except for the new stock-sized main jet I just put on). They have been cleaned within the last 50 miles of riding.

I'm suspicious of the ignition, I just talked to my mechanic and he told me to examine the points. I plan on diving into this tomorrow. Any thoughts?
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Old 07-11-2013, 06:34 PM   #4
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Sometimes on older bikes, that have had the carbs "cleaned". someone took a steel wire and cleaned the idle jets and gouged them larger. I had a small bike for my son some years ago and that was the problem there, the number on the jet made sense, but the size of the jet hole was way too big.
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Old 07-12-2013, 03:49 AM   #5
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Quote:
Originally Posted by jk$ View Post
Thank you for the reply!

There's black smoke coming out of the back. I have not checked oil consumption but will look at that (when i can ride her for more than 15 minutes without killing the plugs).

The carbs are stock (except for the new stock-sized main jet I just put on). They have been cleaned within the last 50 miles of riding.

I'm suspicious of the ignition, I just talked to my mechanic and he told me to examine the points. I plan on diving into this tomorrow. Any thoughts?
Ok, how old/young are you? If you inspect the points, do you know what to look for? How to file/clean/replace and reset the TIMING? Before that, answer the question, "is it running well, no misfiring"? If the points are bad enough to cause fouled plugs, it would barely run.
The black smoke is without doubt, OVER FUELING. The enrichener plunger(s) is most likely stuck (up off the seat) or the rubber seal damaged, causing the over richness.
Also possible the air passage leading from the intake horn into the main mixing well is partially plugged. Do BOTH sides smoke black?
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Old 07-14-2013, 09:46 AM   #6
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Any news?
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Old 07-14-2013, 11:23 AM   #7
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Sorry for the delay! I was out riding yesterday and didnt have time to take a look at the bike until last night.

Quote:
Originally Posted by concours View Post
Ok, how old/young are you? If you inspect the points, do you know what to look for? How to file/clean/replace and reset the TIMING? Before that, answer the question, "is it running well, no misfiring"? If the points are bad enough to cause fouled plugs, it would barely run.
The black smoke is without doubt, OVER FUELING. The enrichener plunger(s) is most likely stuck (up off the seat) or the rubber seal damaged, causing the over richness.
Also possible the air passage leading from the intake horn into the main mixing well is partially plugged. Do BOTH sides smoke black?
It is running well with no misfiring. You're right, I'm a little on the young side and do not know quite what to look for on the points. I took the cover off and everything looked right (though I do not know what to compare to). The bike only has 4300 miles on it, so they would probably not be bad from normal wear and tear.

I am pretty sure that both sides are smoking, but will check tonight to confim.

Both carbs have very recently been cleaned, but how could I check to see if the enrichener plunger is working appropriately?
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Old 07-14-2013, 11:24 AM   #8
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Any news?
Sorry for the delay on getting back. I took advantage of a few hours of Sun yesterday to ride. I borrowed a compression diagnostic tool from a buddy and will check the compression tonight.
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Old 07-14-2013, 07:52 PM   #9
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Quote:
Originally Posted by jk$ View Post
Sorry for the delay! I was out riding yesterday and didnt have time to take a look at the bike until last night.



It is running well with no misfiring. You're right, I'm a little on the young side and do not know quite what to look for on the points. I took the cover off and everything looked right (though I do not know what to compare to). The bike only has 4300 miles on it, so they would probably not be bad from normal wear and tear.

I am pretty sure that both sides are smoking, but will check tonight to confim.

Both carbs have very recently been cleaned, but how could I check to see if the enrichener plunger is working appropriately?
OK, good. The points were scheduled maintenance.. clean/adjust every 2000 miles! You either spent time at your mechanic once a month, or learned to do it yourself. You must have a family member/friend/co-worker who's ancient like me, walk you through the points thing the first time. some 320 grit sand paper folded over a flat wooden splint (smaller than a popsicle stick) makes a suitable cleaning tool. Back in the day, (in fact came in your tool kit, look under the seat) a points file was standard toolbox item. BE SURE to clean afterwards, a white (uncoated) business card spritzed with brakleen will work well.
The enrichener plungers can be visually inspected in place, press gently on the rod and see if it goes down any further than it was resting, it'll only be a small amount, 1/16" or less. The person cleaning the carbs may have accidentally left out the rubber sealing disc on the end of the plunger. The floats may be sunken (shake while you listen for sloshing within) causing richness. These are the finer points that the casual carb cleaner will miss. If not performed, chances are good you're going back in the carb.
How is the fuel consumption? Did you set the tripmeter and fill the tank? 40's mpg is ok, lower than that collaborates the richness evidence.
that's a low mileage sweetheart you have, worth the effort to find the small (once you find it) problem
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Old 07-15-2013, 03:25 AM   #10
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Wow, thank you for the great response. Looking into this fully will take a few days for me (work is hectic).

Fuel consumption is abysmal (~30 MPG or less). Given limited time, do you think I should focus on the carbs/enrichener plungers? Do you think it would be worth it looking for a carb replacement/kit on ebay? Or do you think I shouldjust work to identify the core issue?
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Old 07-15-2013, 02:15 PM   #11
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Wow, thank you for the great response. Looking into this fully will take a few days for me (work is hectic).

Fuel consumption is abysmal (~30 MPG or less). Given limited time, do you think I should focus on the carbs/enrichener plungers? Do you think it would be worth it looking for a carb replacement/kit on ebay? Or do you think I shouldjust work to identify the core issue?
Ok, for sure you need to focus on the carbs, they must be CAREFULLY dismantled and inspected. Carb kits are needed for new gaskets. Any CARBURETOR you buy will need the same inspection, so move foreard with yours. You think YOU will tackle it? Or someone else?
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Old 07-15-2013, 02:22 PM   #12
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You think YOU will tackle it? Or someone else?

I plan on doing everything I can using this forum, the manual, and YouTube. There is a lot I dont know, but I want to learn as much as I can (which is why I own this bike and why I did not take the bike to a shop to diagnose). Yours and a few other's help has been (and hopefully will continue to be) awesome.

I've taken apart the carbs a few times and had a knowledgeable friend walk me through how most of the sub-systems work. I'm learning slowly. My next task will be to tackle the enrichener plunger per your suggestion.
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Old 07-15-2013, 03:58 PM   #13
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I plan on doing everything I can using this forum, the manual, and YouTube. There is a lot I dont know, but I want to learn as much as I can (which is why I own this bike and why I did not take the bike to a shop to diagnose). Yours and a few other's help has been (and hopefully will continue to be) awesome.

I've taken apart the carbs a few times and had a knowledgeable friend walk me through how most of the sub-systems work. I'm learning slowly. My next task will be to tackle the enrichener plunger per your suggestion.
Ok, cool. You can post images here easily with your smartphone, yes? When you get them off, bowls off, we'll need pics. We will VERIFY FLOW through all passages using aerosol solvent, and most importantly, verify SHUT OFF in the plunger circuit. Got you JIS #1 & 2 screwdrivers ready?
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Old 07-15-2013, 05:41 PM   #14
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You can post images here easily with your smartphone, yes? When you get them off, bowls off, we'll need pics.
No problem here!

Quote:
Originally Posted by concours View Post
We will VERIFY FLOW through all passages using aerosol solvent, and most importantly, verify SHUT OFF in the plunger circuit.
Do you have a solvent that you would recommend? Does drive chain cleaner work, or will this clog things?


Quote:
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Got you JASO #1 & 2 screwdrivers ready?
Not at the ready, I have no problem going to the store and getting a set. What sizes would you recommend? Is it critical to use JASO over similarly sized phillips head screw drivers?
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Old 07-15-2013, 06:07 PM   #15
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No problem here!



Do you have a solvent that you would recommend? Does drive chain cleaner work, or will this clog things?




Not at the ready, I have no problem going to the store and getting a set. What sizes would you recommend? Is it critical to use JASO over similarly sized phillips head screw drivers?
You can MAYBE squeek by using #1 & 2 regular, but the bowl screws can be very tight, risk bitching up the head. Actually, not available at most tool stores, AFAIK. I meant to say JIS, these are good.
http://www.vesseltools.com/hand-tool...-products.html
We can press on in the meantime..
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