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Te570 2002 carb backfire

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Hey guys, new to this side of the forum so be nice ;).

My friend is currently having some issues with his bike, or well had had nothing but troubles since he bought it.

The problem is that it keeps backfiring out the carb making the carb pop out! Also it's a pain in the ass to start when it's warm. 

We have checked the valves and they are set per manual spec (0.004 and 0.006 inch), looked through the carb and found someone had put in a #40 pilot and from factory it's a #60 so we thought we had found the issue since running lean can cause carb backfires. Put the #60 in and it ran fine for one day but then the issue came back. 

Jetting is currently stock but we changed the jet needle. 60 PJ, needle top position because it runs fat otherwise and 175 MJ. When engine is cold it starts first or second kick all the time and runs fine, but if you shut the engine off when it's warm it will backfire 1st or 2nd kick all the time. Few weeks ago we were out riding in the woods when we shut our bikes off to take a break. Then when he kicked it over it started to smoke from the airfilter area, turns out it actually burned at hole through the airfilter!

Then we thought it were the ignition, but tried turning stator both ways to no avail. Magnets look fine, glued and tight and bike is hardly run since new. Cam chain is 4-5 clicks out so chain should be fine too and we took it apart some to look for the timing marks on the cam sprocket vs the crank and it also seems fine. 

I am a 2stroke guy and running out of ideas now, anyone had similar problems with their thumpers? Maybe the intake valve isn't closing properly or something? 

Weird though because when it's up and running there is no issues, only when trying to kick it warm. If you bump start it there's rarely any issues to. 

Appreciate some ideas guys :thumbsup:!

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If it runs fine, as in idles perfectly and revs out, it's not carb or valves or cam timing. If it blubbers anywhere in the rpm range under load, that is a problem.

Most likely there is a problem with the ignition that is causing it to fire too soon.....as the only way you can get carb back fire is when you get igntion with the intake valves open.

I would bet it's cam timing...

 

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3 minutes ago, THE KRAN said:

If it runs fine, as in idles perfectly and revs out, it's not carb or valves or cam timing. If it blubbers anywhere in the rpm range under load, that is a problem.

Most likely there is a problem with the ignition that is causing it to fire too soon.....as the only way you can get carb back fire is when you get igntion with the intake valves open.

I would bet it's cam timing...

 

He tells me it blurbles some when you hold steady low throttle then whack is open fast but not if you are a little bit more gently on the throttle. Also it does have some hanging throttle sometimes where it idles high after riding but then goes down after a few seconds. If it were a smoker I'd say it's running lean but I'm so lost on thumpers. 

Tried reading plug and it looks kinda sooty.  

We had the valve cover off though and there's a mark on the cam sprocket which should align with the top of the head and then the flywheel mark and it did? Seems weird af. Also bike has not been ran much so I doubt anyone had it apart since new. Can it really slip when chain is still tight? He bought a dial indicator to check ignition timing but the manual is useless and doesn't spec it. Just tell you to put the stator on the marks, damn Italians :banghead:

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You guys are guessing all over the place! 

The second hand info is no good either. He should be posting about his own bike...

Get a factory service manual, or take it to a mechanic....

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36 minutes ago, THE KRAN said:

You guys are guessing all over the place! 

The second hand info is no good either. He should be posting about his own bike...

Get a factory service manual, or take it to a mechanic....

Yeah kinda, I know. My post is probably a little more chaotic than reality though since we tried all we can think off. 

With this I totally agree with you, I am telling him that all the time but he just won't do it for some reason. 

We got the factory service manual but it's kind of a joke really. We had a car mechanic we know who also rides look at it but he didn't find anything weird either. 

He is scared taking it to a mechanic I think because if they don't find the issue fast it's gonna be $$$. Just thought I'd ask and see if anyone had same problem. Guess we will have to double check that ignition timing ☺️

Edit: Just feels weird when it's only during hot starts with the kick.

Edited by Christoffer Brandel

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You can easily prevent the starting backfire by 'clearing' the cylinder before you start it: WOT, kill button held, kick it over a few times. That will remove unburnt fuel.

It sounds like it's running very rich, and those carbs are notorious for early wear on the emulsion tube (needle jet), so replacing it, and probably dropping the needle two clips, would help with the richness.

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9 hours ago, THE KRAN said:

You can easily prevent the starting backfire by 'clearing' the cylinder before you start it: WOT, kill button held, kick it over a few times. That will remove unburnt fuel.

It sounds like it's running very rich, and those carbs are notorious for early wear on the emulsion tube (needle jet), so replacing it, and probably dropping the needle two clips, would help with the richness.

Thanks, I think you might be onto something with the emulsion tube. Would explain why he has to run the needle clip in top position for it to run. Will look into that asap!

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