engine backfires and carb issue - Kawasaki Brute Forum
 
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post #1 of 3 Old July 12th, 2018, 05:43 PM Thread Starter
 
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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
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post #2 of 3 Old July 12th, 2018, 06:19 PM
 
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Well, it's broken..so it has to be fixed. It will operate correctly when fixed..but keep in mind the CV slides have nothing to do with the idle circuit, only the main so what ever problem you have below 1/4 throttle is still there. I would look first for a vacuum leak. These most always get splits in the carb boots at or under the head-side clamps. Also check if the port on the head that supplies vacuum to the fuel pump..don't know if yours is electric or not, but the port is there, is leaking or the line going to it is leaking. Because it changed when you worked on the carbs it's likely the valves are OK but never hurts to do a leak test. Recheck the float levels and fuel supply volume and pressure. Loosen the gas cap. If it goes away, replace the vent check valve as it's stuck..common problem.

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post #3 of 3 Old July 13th, 2018, 10:55 AM Thread Starter
 
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Ok, so my inexperience with this type of carb (cv, atv/motorcycle style) is probably the main part of my issue. All of my career experience is in aviation and automotive maintenance/repair, not atv/motorcycle maintenance.

So after I posted this yesterday, I found a youtube video that better explained the installing of cv valve on a harley motorcycle. What I found is that it was indeed an installation error on the diaphragm of the valve, and I found out about a trick/tip that help me to get it to seal and operate properly. So both carbs CV valves are now of a similar full and free movement and travel response at any given throttle position or acceleration.

After getting that fixed and triple checking the synching and adj. of idle/air screws it ran better than it ever did with no occasional back firing into the intake while at operating temp.

This morning I restarted it to double check the cold start/running condition and the rear cylinder still has the occasional backfire into intake during slow to moderate throttle accel at roughly 1/4-3/8 throttle range (give or take a little). Give the throttle a fast accel from idle to higher rpm and no backfire. This is the opposite carb that I was having the issue with on the diaphragm. Any ideas on this or should the occasional backfire be a normal expectation?

While going back through the carb and other maintenance, I had already found and fixed multiple vacuum hoses that had split/cracks on the end of the hoses before getting it running again.

Also you mentioned a vacuum port on the head for the fuel pump etc. On this one the carb boots supply the vacuum for the pump and fuel shutoff valve. Both carb boots were indeed cracked and leaking when I replaced the engine. I never noticed any other port on the head. Is there another port on the physical metal head that I should be looking for, and what general area (lower, upper, fwd, aft, left, rt, etc) would it be located? I will double check by spraying carb cleaner and watching the tach, but I believe I have found and fixed all external air/vacuum leaks.

Thanks for the input.
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