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With the Finish Line in Sight, It’s Back to Square One for My XR600R Project
Diagnosing an engine that isn’t running like it should is a pain. Doing the same procedure on an engine you just rebuilt is downright infuriating. This is one of the few times where we mechanics know that all the little bits that could be wrong are right—or, at least, they were right when we assembled them. So where to start when your freshly rebuilt engine refuses to run? Well, you start at the start. It’s a reminder I needed recently, after three frustrating weeks of trying to get my Honda XR600R project to run—not even run right, just run.
Fuel, air, and spark. All of those need to be present, at the proper time and in the proper amount. An engine has ways of telling us if the holy trinity of combustion is present, but the problem is that nearly all of those indicators can be misread. For instance, because the spark plug in my 600R was wet with fuel, I deduced that the carburetor was doing its job.
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It was, sort of. The Honda-fit carburetor for this engine is long gone. (That happens when buying basketcase projects.) The OE constant-velocity carb would have likely been fine for most of my riding but since I own multiple motorcycles, and have a penchant for not leaving well enough alone, I decided to upgrade to a better flowing, more tunable carburetor. The biggest selling point to me was the accelerator pump, something that was supposed to make my life better by providing snappy throttle response.
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Instead, that accelerator pump sent me on a wild goose chase through the ignition system. For weeks, the engine wouldn’t start. Even the can of ether—typically hidden in a dark corner of the flammables cabinet—could barely make the bike cough, let alone run smoothly.
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Where do you find the solution to complex problems like this? In the last place you look. After two evenings of double-checking the electrical systems, another confirming the valvetrain was functioning and timed correctly, and a few extra evenings sitting and staring, waiting for a mental lightning strike, I decided to take a step back and confirm that the carb was indeed working like I thought it was. This bike is still nothing more than a chassis acting as an engine stand, and because I had only attached the bare minimum of parts needed to support the engine, pulling the big carb took only two screwdrivers and a 10mm wrench. The carburetor was on the bench in minutes.
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Investigative surgery began, with heavy flashbacks to six months ago when I did all of this the first time. I bought the Keihin FCR carb used, and it arrived covered in gross ethanol deposits. I swapped out all of the jets and replaceable parts with genuine replacements, leaving no doubt that the carb should function correctly. All of my machines receive fuel from five-gallon storage jugs which I regularly inspect to ensure there is no trash floating in or sitting on the bottom, waiting to be poured into a tank and wreak havoc upon an engine.
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Yet somehow I missed at least one chunk of nastiness. When I pulled the pilot jet and held it up to the light, the jet was nearly completely blocked. The spark plug was wet because of the accelerator pump, which was shooting long streams of raw gas into the intake with each frustrated twist of the throttle. An easy misdiagnosis.
The remedy was easy enough, considering it was one I’d already done. The body of the carb took a second trip through the ultrasonic cleaner while I sprayed out the jets with carb cleaner and air. With the Keihin reassembled and installed, the bike coughed to life, but only with the choke engaged—oddly, a symptom of a clogged slow jet. Fiddling and tuning the mixture screw improved the situation, but the engine was still showing all signs of running extremely lean: The very hot header, and the engine’s refusal to idle longer than a few seconds with the choke off, revealed that the engine was asking for more fuel that it was receiving.
But it runs now, and a running engine is so much easier to diagnose, troubleshoot, and fix. That aforementioned can of ether came back out for diagnostics and pointed out a vacuum leak, likely between two of the carburetor body sections. The midbody gasket on these Keihins is a known problem point, but one I had already addressed—just like I had with the clogged pilot jet. Once more, the carb will come apart so that I can inspect and double check of my own work, but that happens sometimes.
At least I finally have confirmation I am working in the right direction. If this project went together like Lego bricks, it wouldn’t be fun, and I wouldn’t learn anything. The XR600R is still on schedule to be ready for its desert run in late fall, so I guess now was as good a time as any to remember the importance of getting at the root of the problem, and to solve it before everything is buttoned up—hopefully, quite soon.
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“a running engine is so much easier to diagnose, troubleshoot, and fix” – man, what a true statement, that.
Moving ahead, Kyle, moving ahead – and glad you mentioned that you’re still having fun with it. It IS looking better and I get the sense that it’s going to look AND run good really soon. Nice work, man!
Such a tiny piece stopped the whole party. Glad you got it going.
Interesting nom de plume you’ve got there, G.B.!
Not sure what happened there.
Kyle- Funny it should be the carburetor. If I had a nickel for every guy who tinkered a little with cars too many years ago and told me – ‘Must be the carb’ – straight off, I could retire comfortably. On a similar note. My old Craftsman mower started doing the telltale sign of a dirty/clogged carb. Start ,run, stall. Start, run, stall . I pulled the bowl and gave everything a good cleaning. There wasn’t really much shellac or. All fresh gas and everything else checked. A little better but would stutter and die half way through a mowing. Finally ordered a new one even though couldn’t find anything wrong and the pinhole size jet was as clean as a babys bottom. I used a pin to make sure. Couldn’t figure out the problem and that annoyed me because that carb is almost as simple as a tomato can with a nail hole punched through it. ( see Henry and Clara Ford “Kitchen Sink ‘ engine ) I put the mower back and didn’t touch it. If you’re sweating the grass getting a bit too long for the neighbors approval. it’s time to reconsider your priorities. I kept thinking that new carb will show up and that thing will then start running on full song before I install it. That is exactly what happened. One pull no problems since. Engines can be odd unpredictable beasts.
After going back into the carb over the weekend, somehow one of the gaskets got pinched during assembly and was causing the vacuum leak. It’s always something, and it wouldn’t be any fun if I got everything perfect the first time, every time!
Maybe but… after carefully disassembling and reassembling everything. Taking that 5 – 10 minutes to run through the job in your head to make sure you didn’t forget anything. It is pretty sweet when she fires right up.- ‘We’re good!’- Instead of “Oh, what the f**k now ?’ ” I hate to write but love having written.”