Join Date: Nov 2018
Location: Los Angeles, CA
It's my understanding that popping through exhaust is caused by a lean condition, but a rich condition can result in its more violent cousin, backfiring. Not sure if this can — or does — happen with the KLR, given that most of them run super lean, and then many lean them out more by opening up the intake and exhaust, but it's perhaps worth a mention because in closing up the hole in your airbox you restricted the airflow, and less air flow = richer mixture. If you're way, way rich on the jetting, perhaps that could be related. I'd be curious to hear:
• What pilot jet you have
•*Your idle fuel mixture screw setting
• What needle you have
• What main jet you have
• Whether your carb is clean (what's the recent driving history? Consistent driving with good performance and this just cropped up?)
• How does your bike run in general? Does it feel powerful/smooth/responsive?
- Does it idle at the same speed always or does that move around?
- How does the bike react when it's warm and you open the "choke?"
I will say that I recently experienced a super rich condition when taking my "tuned for power and smoothness at sea level (i.e. damn rich)" bike to 7000+ ft. I don't recall getting backfiring, but did have a host of other symptoms (bike dying at idle, laggy feel on the accelerator, general rough running, super smelly exhaust) that I addressed by lowering my KLR needle and stepping the pilot jet down. I would suggest taking inventory of other factors like these — are there any other changes in how the bike is running overall?
Here's my favorite tip to keep in mind when thinking about fuel mixture. Temperature of the engine has a huge impact on how the carb works. Obviously, OK. Well for me it was news to learn the reason engines need an enricher circuit: because fuel evaporation is a big part of carb operation, and because fuel in a hot environment dissipates into smaller particles more readily than in a cold one. Increased dissipation allows a given volume of fuel to burn more readily, owing to increased surface area of the fuel itself when it's spread out amongst a bunch of small droplets as compared to larger ones. This is why we have to dump extra fuel in a cold engine to get it to start, because the normal amount doesn't burn as well due to the fact that it's "clumped up" — i.e., because the mixture is effectively lean.
Alright skip all that and consider this: At startup, because it's cold, your engine is effectively running lean. Then as it gets warm, the mixture effectively enrichens. As it gets hot, the mixture gets effectively richer still. So, if you're wondering about jetting and where you are compared to where you want to be, you can get valuable clues by taking note of how the bike runs as it warms up. If it's running better (by your judgement, whether "better" means more power, less popping, smoother operation, etc.) cold, it's likely that your jetting is rich because your engine is effectively running lean when cold. Same vice versa: If your bike runs better in stop-and-go traffic, your jetting might be lean, because the bike is effectively running rich. I say "might" because, surely, there are other things that go into how the engine is running at different temperatures (things I really don't know about), but I personally found this to be a very handy way of working with carb settings (and not blurring the picture by messing with more than one thing at a time).
The same thing applies to air temperature. Good cold days can indicate that the jetting is rich. Good super hot days can indicate that the jetting is lean (if the goal is to run well on "normal" temperature days, anyway). Altitude and humidity also go into it. For all three of these factors (temperature, altitude, humidity), "higher" results in "effectively richer."
2017 KLR in black