'05 Brute Force 650 4x4 Running on one cylinder - MudInMyBlood Forums
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post #1 of 2 Old 04-23-2017, 04:41 PM Thread Starter
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'05 Brute Force 650 4x4 Running on one cylinder

Hey guys.... new here but I've spent the last several weeks reading forum after forum looking for the same symptoms. I'll try my best to describe mine and everything that I've done so far. But I have an issue with backfiring through the front carb/significant loss of power/no 4wd. I have downloaded the service manual, and searched, searched, searched the net for info.... But still having this issue and lost!

2005 Brute Force 650 (KVF650E) 4x4 with no significant mods.

Bike has not been run hard in several years (or so I'm told). At the time I got it, the belt indicator was flashing. Got that reset by following the reset procedure in the manual and found online. So "Limp Mode" has been reset. May need to actually replace the belt but I don't think it has anything to do with my symptoms. It doesn't show much wear at all, but looks to be a tad loose.

Original symptom was backfiring through the front carb/diaphragm slide not moving like it should. I removed the carb assembly, broke them both down independently, thoroughly cleaned both carbs, and installed a new "rebuild kit". Jets were 2 sizes up on the front and rear respectively. I "tested" the slide movement with compressed air, and both front and rear act similar (smooth movement, typical suction sound, and a "slow" return).

I read about the carb boots being a culprit, so without taking the boot off I inspected all the way around it. I even tried spraying starting fluid and carb cleaner around the boot as the bike was running, just to see if there was suction from around the boot indicating a leak. No change in motor. So I don't BELIEVE there is damage to the boot.

Next, I moved to compression. Manual calls for 40'ish to 80'ish psi in both cylinders on start up. (sorry cant recall exact numbers)
Front cylinder initially had 75 psi, and the rear had 180 psi. Way too high on the rear, so I tore into the cam and found the weights sticking on the decompression valve. Cleaned that up real good, and cut 2-3 rings out of the spring to tighten it up. Saw an immediate change to the rear cylinder on retest. Both cylinders hovering around 70'ish psi.

I next started to think about valve issues. Checked the gap at the rear cylinder valves, and they definitely needed adjusting. Could not "check" the front valves due to it being so tight around the front cylinder top end, so I assumed they may need adjustment as well. Loosened them up, got the gap tool in there and adjusted to manual spec's. I also turned the crank over by hand, watching the valves for a "sticking valve" and saw smooth fluid movement in all valve sets. I was concerned after reading so much that people have adjusted valves on the wrong compression stroke(ie. 180 degrees off), so I actually checked this 3 times. Absolutely no change in behavior. Still popping back through the front carb. And intermittent backfiring through the exhaust. Mostly through the carb though. Unfortunately, I do not have the proper tools to do a leak down test, so currently no way of knowing if a valve is hanging open slightly.

I began to wonder about an electrical issue. The fact that there is no 4wd got me thinking (it does not flash, switching into 4wd on the thumb switch displays no change on the display, stuck in 2wd). So I began to test many different electrical components per the manual. Installed new spark plugs. Before deciding to order new coils (for a spark issue), I swapped the front and rear coils with each other, no change, still issues with front cylinder.
4WD thumb switch
Speed indicator sensor
rollover safety switch
Power to the "igniter" (cdi) and ECU actuator controller
Checked all fuses/relays
Searched around for the "buss connector" many people have talked about with the '05 (there is not one!)
Everything I tested checked out fine. In doing my searches, I read a lot about other people upgrading to the Dynatech CDI. I was sold on the belt indicator elimination, the reverse limiter elimination, and so forth. Thinking this may have something to do with an electronic issue, or even some other type of "limp mode" not indicated by the belt light, I went ahead and ordered one. Plugged it in and IMMEDIATELY noticed that it was no longer backfiring. I let the bike run for about an hour, idling and every so often reving it up to various rpm's. The front carb slide still was not moving, but it was not backfiring. So I reinstalled the air cleaner box, and decided to take it down the road to give it a good test run. Got up to about 28mph MAX! As soon as I let off the throttle to decelerate..... POP! Blew the front carb right out of the boot. Back to square one.

I have played with various settings on the pilot screw. 1 turn out all the way to 3 turns out in 1/4 turn increments. NOTHING changes. I have tried to adjust the carb sync, and can get the backfiring to taper way down, but at that point the bike wants to stall. I have tried small adjustments with the sync, and incorporate different pilot screw settings...... NOTHING seems to work.
For poops and laughter, I unplug the front spark boot and it runs like a dream. Except it only goes 28mph. So essentially, the bike is running on the rear cylinder alone.
It seems a little harder to start when cold, but nothing out of the norm. Need to pull the choke lever and once it fires up, turn the choke off. after it's warm it actually runs really well on the rear cylinder.

Here's where I'm at now..... and I'm looking for any advice I can get. I'm about to order the ECU. I believe this is the culprit for the 4wd issue, as it controls the actuator. But I can't bring myself to spend $200 to fix 4wd when the bike itself doesn't even run right. However, I've read that the ECU works in conjunction with the CDI for spark and timing advances. Does anybody know about this? Maybe offer a better explanation of how these work together?
Could there be an internal timing issue on the front cylinder causing valve timing to be off? I know they are timing chains, and typically would not "jump timing" without possibly breaking the chain or blowing up a sprocket.
Is there another "limp mode" that would not be indicated on the display? I took care of the belt light reset, but can find no other information about any other sort of "limp mode".
I'm really thinking it is a mechanical timing issue having to do with the valves, or an electronic issue not providing proper spark. But I AM LOST! Unfortunately, the spark tester I currently have will not fit between the plug and boot, so I can't check with an inline. But if I pull the plug and ground it outside the engine, it SEEMS to be sparking properly. Which is leaning me more toward an internal issue between timing or valve problems. I'm losing hair with each day that I put into this project!

Sorry this post is long but I tried to supply as much info as I could. Any help to kick this off again would be greatly appreciated!
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post #2 of 2 Old 04-24-2017, 07:35 AM
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Man I wish I could help but, you already know more than I do about engine issues so.... lol... I won't be much help.


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4wd engage , backfiring , cdi , power loss

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