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Exhaust backfire

Yamaha General Discussion

 
 
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  #1  
Old 01-03-2011
BGKID2966
The Big Kid
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Default Exhaust backfire

I have a 77 Exciter with a 540 fan from an 80 XLV in it. When the engine was in the XLV it ran like a top with a ton of power.
It backfires through the exhaust when I try to accelerate from an idle. I have replaced the plugs, reeds, and cleaned and reset the carb. It has the exhaust from the 340 Exciter in it. It does not have an airbox on it yet. I was hoping to get it running before modding an airbox to fit.

I was told if the exhaust was not the right size it would create an issue with the performance.

This project has been one challenge after another. This is a custom I would never attempt again. I just want the mechanics right so I can get the hood done and painted over the summer. I hope to have everything done for next winter as my daily rider.

We affectionately call this Frankensled. It has parts from Yamaha, Polaris and A/C.

HELP PLEASE!!!
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  #2  
Old 01-03-2011
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You need the exhaust from the XLV, plain and simple.
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  #3  
Old 01-03-2011
BGKID2966
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Is it a problem to modify the muffler end of the pipe. The XLV pipe is way to big for the Exciter chassis. I can modify the pipe if it won't affect the performance of the sled too much.
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  #4  
Old 01-03-2011
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as long as it is after the expansion chamber, and you use the same diameter stinger it should work.
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  #5  
Old 01-03-2011
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^^^Yep he's right.
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  #6  
Old 01-17-2011
BGKID2966
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I am in the process of merging the 540 pipe with the muffler from the 340. Looks like it will work!! Then to mod the airbox. It appears to be less difficult than the exhaust!!
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  #7  
Old 03-20-2011
BGKID2966
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The pipe is finally done! Seems to work pretty well. The air box does not have a chance of fitting, but I have a plan for a custom intake tract to the carb. I am looking for a place to test run in the summer. After all is working properly I will tear it apart to clean and repair body and seat damage.
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  #8  
Old 06-28-2011
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Sounds great
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  #9  
Old 09-18-2011
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I know we have been over this before but the time has come to fix the exciter. I haven't touched a thing besides riding it around the yard this spring and summer. It runs great! Fires on both cylinders like it should(Shares a coil). But it backfires in the pipe and runs like shit i the winter!! WTF? It is not a coolant issue getting into the cylinder, it isn't a fuel issue, and it gets spark atleast in the summer. What would cause an engine designed to run in the winter to misfire on one cylinder and run great in the summer? Im thinking a stator or wire issue. What do you guys think?
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  #10  
Old 03-08-2013
BGKID2966
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I am still struggling with the backfire through the exhaust on my 540 fan I swapped into a 77 Exciter chassis. Old topic is, exhaust backfire. I put new plugs in it to see what they showed and the right was black and the left was tan. Is that strange on a single carb engine? Also when it backfires, I can see fire in the fuel pump pulse line. I am lost as to what to do next.This is a frustrating problem.

Any and all suggestions and help are greatly appreciated.


Geno
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  #11  
Old 03-08-2013
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Is the plug that is black also the one in the cylinder that the pulse line attaches to?
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  #12  
Old 03-08-2013
BGKID2966
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Yes it is. The plug was pretty new, maybe 1 mile on sled and an hour of shop run time. I am guessing you are thinking the fuel pump diaphram is leaking and that is what I thought, so I changed the fuel pump and the problem did not change.


Geno
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  #13  
Old 03-08-2013
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Have you checked that the needle valve for that carb is sealing properly.
Easy to test for, just swap the carbs and see if the problem follows.
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  #14  
Old 03-08-2013
BGKID2966
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It is a single carb. None to swap.


Geno
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  #15  
Old 03-08-2013
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Well, I don't know anything about old sleds, and I've never really heard of seeing fire in the pulse line, but hear me out. If you see fire in the pulse line, somehow combustion is happening in the crankcase. I would imagine it is in the black cylinder where combustion isn't nice like the cylinder with the tan plug. I'd pull off the y-pipe and see if there is a broken piston skirt. Maybe that would let combustion down into the crank case? A crank out of phase could possibly cause that too.

I had a backfiring problem on my sled the last couple seasons. Every time I let off the throttle I got a backfire. Sometimes loud! Finally I decided it was an ignition issue and I disconnected the throttle safety switches. Problem was completely solved instantly. There was a little delay in switch action when I let off the throttle. It turned off the spark long enough to dump unburned fuel mixture into the pipe. When the ignition came back on, the unburnt fuel would blow, creating the backfire. You might have an ignition issue like I did.
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