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Yamaha Tech Originally the Yamaha XT600 Tech Forum, due to demand it now includes all Yamaha's technical / mechanical / repair / preparation questions.
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  #1  
Old 19 Jun 2013
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99 XT600 Severe popping under acceleration

Last couple rides the bike has been popping severely through the carbs, starts around 3500 rpm and up and is getting worse, it pops five or six times each time I start getting on the gas and with each pop it momentarily kills the power, not like a fuel starvation bog but requires gearing down to get up hills, no popping on decelartion, bike starts and idles perfect, valve clearances are all up to spec, plug looks fine, compression is 159, fuel flow is good, I'm thinking it might be electrical since I dismantled/cleaned the carbs only 2500km ago when I installed a new top end. my question is if it's electrical which is the most likely part to cause this ? don't have a lot of cash so I'd really like to get it right, any suggestions are welcomed and appreciated !
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  #2  
Old 19 Jun 2013
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The small diaphragm on your lift side side carburettor possibly.

Mezo.dont
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  #3  
Old 19 Jun 2013
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Have you checked the rubber intake? and there is a filter inside the carb did you clean this? sounds to me like the problem is when you need more fuel and its not getting the right mixture.
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Old 19 Jun 2013
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A very long shot but a sticky inlet valve could explain it -it might allow the mixture in the inlet/carb to ignite under certain circumstances. This would clear them of combustible air and account for the bog down.

Perhaps a cheap start would be some redex and an oil change?

Timing is electrical and very hard to get wrong. You are much more likely to get no spark than one at the wrong time/place.
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Old 19 Jun 2013
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I agree with Bandit127, to get the popping through the carbs might require a stuck valve or timing way off. Slipped timing chain?? The cdi defaults to the early advance stage I believe or quit outright. Are you sure the poping was from the carbs?
I had a V8 car engine with high miles that the valve springs were getting weak and as the rpm hit 4500, the things would start to pop, crackle, flames out the side pipes, from floating valves. Broken spring a slight possibility, but definately check the "normal" stuff, like Mezo's suggestion, hate to overlook the simple
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  #6  
Old 19 Jun 2013
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The link below shows how the carbs looked when I rebuilt the top end about a year ago. the only thing I didn't pull apart was this diaphragm valve cause it was pinched in place.

Motorsports Forum - Carb Inspection/Cleaning

Mezo do you mean the coasting enricher ? what should I be looking for ? I remember the diaphragm was in good shape last time I pulled it apart but I didn't remove the brass pin below it since it was pinched in place I wasn't sure if I should play with it.



Shouldn't be a sticking valve I just a did a valve job 2500km ago and the two exhaust valves are new.
extrock the rubber intake boots were starting to seperate from the aluminum but those have been cleaned up and resealed with silicone and are still looking good, when I had the carbs apart the only filter I remember is the course foam filter for the air coming into the secondary diaphragm (top of carb) otherwise the fuel tank has screen filters on both petcocks and an inline fuel filter between the tank and the carbs.
It does sound like it's coming from the carbs and sounds a lot like the pop you sometimes hear when first starting the bike, the one that blasts out the left side of the carb, I'm getting lazy I don't wanna pull the carbs out/apart again if it's not an obvious electrical part going bad but I guess I might have to.
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  #7  
Old 19 Jun 2013
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Yamaha XT600 1990 1992 Carburetor Rebuild Kit 00 082 | eBay

Dont know if you have same but as you see on pic there is a small filter, and i had same problem when this was full of dirt.
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  #8  
Old 20 Jun 2013
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Quote:
Originally Posted by pursang View Post
Mezo do you mean the coasting enricher ?
Sorry mate i should have read the title better (not drunk)

That makes it pop un de-acceleration, not acceleration.

Mezo.
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  #9  
Old 21 Jun 2013
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Quote:
Originally Posted by xtrock View Post
Yamaha XT600 1990 1992 Carburetor Rebuild Kit 00 082 | eBay

Dont know if you have same but as you see on pic there is a small filter, and i had same problem when this was full of dirt.
Thanks for that link xtrock, I can't recall if my carb has that screen or not, I noticed some of the rebuild kits include the screen while others don't, I'm not sure if it's a fuel starvation problem just yet but I'll check if my carb has that screen and see what it looks like.
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  #10  
Old 25 Jun 2013
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Well the search is back on, I pulled the carbs and gave them a thorough cleaning, inspected every part except the coasting enricher and didn't notice anything obvious in the secondary carb, threw the bike back together and went for a short snort only 5 minutes into it the bike was popping like crazy, it seems to be getting worse with each ride cause on the previous ride it took half an hour to begin popping this bad, I'm wondering if it's an electrical issue ? would appreciate any suggestions.

I noticed the carb was sucking unfiltered air and discovered the original hose clamps even when bottomed out were not clamping tight enough, didn't take much effort to pull the rubber boot off the carb, will use another style of clamp that tightens more.

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  #11  
Old 25 Jun 2013
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All that dirt , it is likely that some has plugged up at least one of the air intake passages. That secondary can get stuck down with all that dirt or if the diaphragm isn't getting vacuum to pull it
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Old 25 Jun 2013
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What did the did filter look like inside the carb?
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  #13  
Old 25 Jun 2013
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Filter looked good fairly clean, it's only got 2500km on it.
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  #14  
Old 30 Jun 2013
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To eliminate a few more possible problems I drained the fuel tank/carb, pulled both of the fuel petcocks off the tank, removed the screen tubes and blew everything clean with compressed air, the screens were surprisingly clean and the tank only had a few grains of sand inside, refilled with fresh premium and installed another spark plug, hit the magic button and no spark now I'm wondering if this issue is related to the original popping under acceleration problem ? any and all help would be appreciated with what to do next !
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  #15  
Old 1 Jul 2013
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Be logical and don't jump to conclusions

Quote:
Originally Posted by pursang View Post
no spark now I'm wondering if this issue is related to the original popping under acceleration problem ?
It rarely is, IMO.
I had a similar experience years ago with a TTR600 and that turned out to be the spark plug wire connected to the coil - I had pulled on the plug so many times, checking the spark, that the connecting wire had developed an air gap inside its connection.

Basically, I suggest you check the high tension side of the ignition for continuity.
If not that, then it will be something pretty simple associated with the latest work you've done on the bike.
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