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Number1DAW

96 DR350S backfires

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I spent the winter tearing apart my DR350 to sandblast and powder coat a bunch of parts that the years had long been showing on. 

Now the bike has always backfired and the previous owner thought it was an exhaust leak which he unsuccessfully plugged with red RTV (which I just smirk at). The cause of the exhaust leak was actually the fact that he had damaged the header gasket pretty bad during installation and no gasket between the header and midpipe. 

I fixed those while I was reassembling everything and the backfires were definitely quieter. I managed to get the bike together in time to ride with a buddy one last time before I left the PNW move to VA. I knew I needed to do a valve inspection but put it off until I got here so I could ride. 

Finally moved in I got the valvea adjusted to back within tollerance but the backfires are still there. I thought maybe the fuel is running lean but the bike has a mikuni BST33 and when I was looking at how to adjust fuel/air for it I came across a mikuni page that talks about backfires. 

http://www.mikuni.com/fs-tuning_guide.html

So the exact scenario is that if I control the throttle down to idle I don't get backfires but if I just the the trottle return the carb to idle on its own real quick or pull it back real quick I will get backfires. 

Do other DR owners experience this?

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I think I have/had a similar issue to what your describe, on the very same bike '96DR350SE. I have a couple of sneaking suspicions I'm planning on trying out to fix, just too can't be bothered to pull the carb again... and I've been too busy fitting a beautiful 5.8 gal acerbis tank too. Anyhow, I digress.

For context, my bike was running PERFECTLY (starts first time, regardless of ambient on bike temp and idles fine, after warming up for 30 seconds can rev cleaning all the way through to redline, no stumbling/hesitation etc). Key word, was. I laid the bike on it's side to truck it home after getting a flat on the one day I forgot my tool kit. (working on a permanent tool tube now, lesson learned). After that the bike struggled to run, I figured something must've shifted and gummed up carb.

SO. I ended up taking the carb apart, cleaning everything, and putting it back together with new (albeit cheap) jets. I also replaced the fuel mixture screw (the head of the old one was almost completely stripped from PO). 

After getting it back together I had TERRIBLE backfiring (mainly if I came off the throttle quickly) but I soon realized I had an air leak as my vacuum cap had come loose from the carb (bike no longer has the original vacuum carb, just manual) and also the carb wasn't sealed well with the airbox. Long story short, I was running very lean. 

After fixing those things, the backfiring got a lot better, almost non-existent. However now the bike needs choke when cold (no big deal), but will stumble between idle and 3000/3500 rpm, A LOT when cold, but there's still some hesitation when at full temp. Revs fine beyond 3500 rpm. 

I've cleaned/lightly sanded my spark plug (might try a new one though too just to completely eliminate that, I've heard a dirty plug could cause backfires) I've checked my valve clearances within the past 300 miles and they're within spec. 

I'm thinking the problem is probably one or more of the following

-Still running lean from another air leak

-Fuel mixture screw set incorrectly

-Cheap main jet, perhaps the wrong size (I just compared by eye when replacing and they seemed close)

-Fouled spark plug, perhaps incorrectly gapped. 

I realize I'm not directly answering your question, but figured maybe you'd spot some similarities/think of trying something new after reading what I'd done... or maybe someone very wise in the ways of the carb will come and enlighten us... :D

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I appreciate the thoughts. The only thing right now that I can think of, which I forgot to mention is that I do have an aftermarket DG exhaust which is wide open and really doesn't restrict any flow which the mikuni page talks about. I rode the bike for some 10 hours straight once I got it together without any problems. I'm about to start riding to work as much as possible to save on fuel. So until there are actually problems I won't be pulling the carb until this winter.

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