Join Date: Jul 2018
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engine backfires and carb issue
2011 Brute Force 650 4x4 KVF650DBF
I had to change out a locked up engine (overheated) for my father in law. Since I have changed it and due to time constraints at that time, I didn't fully disassemble the carbs. Instead I disassembled without splitting the carbs, cleaned and blew out passages, and checked/adjusted float level. After installing rebuilt engine and adjusting carbs I noticed that the front cylinder would backfire while at mid throttle and sometimes at idle. It would backfire less frequently after the engine gets to operating temp. I also noticed that I had a heck of a time getting the carbs adjusted correctly while engine was running and always seemed to be running lean. Any ways after finding/replacing both leaking intake runners. I finally got it running good enough to ship back to him so he could use it on the dairy farm. Granted it was running good, but I never could get the backfire issue lined out and still seemed to run lean.
Here it is about a year later and I get it back to do some other maintenance. While I have it I'm trying to fix a lean condition and get the occasional backfiring through the carb to quit. So I adjusted the valves, all were within specs, and fully disassembled/cleaned the carbs. What I found there is hard and cut o-rings on the tubes between carbs, so I'm thinking "awesome, there was my lean condition". So I finish cleaning, static adjusting, and re-assemble carbs. After re-installing and adjusting the carbs with engine running, I've still got the occasional backfire thru intake of both cylinders and notice when I hit the throttle the CV slide between carbs do not move symmetrically. The rear one barely moves in the bore while the front moves freely. So I remove/swap the CV slide and diaphragm from front to back and re-try. Now the front barely moves and the rear moves freely, also while doing this I notice that it looks like the diaphragm of the CV valve that doesn't move very much has shrunk ever so slightly. The outer sealing lip does not fit in the groove of the carb very well, possibly causing a vacuum/air leak.
So I'm thinking no big deal just replace the CV diaphragm. I look and find that it'll cost about $130 just for that small piece of rubber and aluminium. Before I replace this, will this fix my backfire problem? What could I expect on engine running if I don't replace the valve and have that one CV not moving very much through the rpm band?
One other thing, while messing with the CV valves, I noticed a trace amount of fuel coming from the port where the rod of the CV valve goes into the side of carb. This is happening with both carbs. Is this normal or is my float level off/fuel level too high?
Thanks for the help
Last edited by Daniel Wilkinson; July 12th, 2018 at 06:07 PM.
Reason: left out verbage