LT back firing - BMW Luxury Touring Community
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  • 2 Post By Farmerian
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post #1 of 8 Old Aug 8th, 2016, 3:09 pm Thread Starter
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LT back firing

I have always used the match-rev technic while downshifting on all my motorcycles, that is to say pulling the clutch, downshifting and then reving the engine sligthly before letting the clutch back. I have been doing that on my LT for several years without any issue but recently the LT started backfiring sometime after I let the clutch back. The bike works great with no hesitation otherwise so I am wondering what could be the cause of this back firing.

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post #2 of 8 Old Aug 8th, 2016, 5:28 pm
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Re: LT back firing

You might have a crack somewhere in the exhaust system.
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post #3 of 8 Old Aug 11th, 2016, 8:15 am
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Re: LT back firing

I have an 06 LT trike, that has developed a backfire after a total rebuild of the clutch and replacement of the brake assist module. Bike was apart down to the frame. Engine ran fine before the rebuild started
Bike backfires at idle but not at speed or when down shifting. When riding at about 4000rpm there is a loud howling that disappears when you increase or decrease rpms.
I suspected the exhaust sealing rings may have been pinched so I dropped the exhaust and put in 4 new rings. Still backfires.
Have taken out the fuel pump and all looks good although I have not taken a pressure reading.
I will be looking for a separated vacuum line today.
The bike ran fine before so I don't expect a valve issue. But the constant tearing apart the bike is driving me crazy.
Any Ideas of where to check?
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post #4 of 8 Old Aug 11th, 2016, 9:00 am
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Re: LT back firing

Quote:
Originally Posted by Alpac View Post
I have always used the match-rev technic while downshifting on all my motorcycles, that is to say pulling the clutch, downshifting and then reving the engine sligthly before letting the clutch back. I have been doing that on my LT for several years without any issue but recently the LT started backfiring sometime after I let the clutch back. The bike works great with no hesitation otherwise so I am wondering what could be the cause of this back firing.
Is it really a back fire (intake manifold back through throttle body) or is it an after fire in the exhaust system? They have different causes.

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post #5 of 8 Old Aug 11th, 2016, 10:23 pm
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Re: LT back firing

The backfire is through the manifold. I just spent the past 4 hours tearing it apart down to the manifold. The third manifold attachment ring is missing and the 4th has slipped off.
I suspect the manifold was dislodged when the clutch was put in. I have to get some new rings and will report back if this has fixed the problem.
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post #6 of 8 Old Aug 12th, 2016, 1:39 am
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Re: LT back firing

Check that the camshafts are correctly timed to the crankshaft.
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post #7 of 8 Old Aug 12th, 2016, 5:33 pm
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Re: LT back firing

Today I gave my LT some maintenance. I sometimes have backfire in the exhaust, so I was extra sharp-eyed for possible causes. What I found was a small crack in the exhaust. It would not surprise me if this is the cause. After high speed driving I experience backfire. Normal driving is not a problem.

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post #8 of 8 Old Aug 16th, 2016, 9:10 pm
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Re: LT back firing

After hours more dismantling, I was able to re-seat the manifold. There is almost no room to work on the throttle body. I had to pick up some more clamps since one was missing, but I got everything seated properly. There is so little room between the throttle body and the frame that when you have to work on the clutch, if the motor drops a slight bit the throttle body can be dislodged. (at least that was my problem)
The picture shows the back most part of the throttle body back in position.
I put the tank back on and gave it a try. The backfire has disappeared.
Since the backfire started after the major clutch overhaul I didn't believe that something had changed in the engine but I was incorrect. Simple fix but as I am finding major work is to get to the parts. I'm glad it was something easy and not a valve issue. Hopefully I can get some riding in this summer.
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