Basket Case Carb 101 - Dirt Rider Magazine

We've all been there: In the garage late at night wondering why the heck our girlfriend's dad's KTM won't start worth a darn, idle smoothly or have the common decency to resist backfiring on decel. OK, maybe that's just me. But what many of you can likely relate to is a carburetor that just won't work right.While the inner workings of these magical mixing containers are a mystery to most, it's really only a box with holes in it. So I decided to stop being scared, put some serious daddy issues on the line and give one doozy of a carb rebuild a try. Before I went to work, I called JD Jetting where James Dean himself recommended a short list of upgrades to the older FCR carb. If you have a carb question, just call or email James Dean ( It makes life easier. JD recommended we upgrade the float bowl to a modern unit with a leak jet, install his needle and jet kit, replace and modify the accelerator pump diaphragm and linkage and stick one of his easily adjustable fuel screws in. Total, we are into JD Jetting for about $180. Add in $8 from Moose Racing for new carb vent hose and we're looking at around a sub-$200 parts bill for this fix.

The result? Well, considering I stuck old gaskets into a nonserviceable chamber of an FCR carburetor that was basically seized up from bad gas gone gooey, the result is amazing. The bike ran great right away, and during John's trip from Tecate to Mike's Sky Ranch and back it didn't miss a beat: "It was crazy! I took two-thirds the gas my son did with his KTM 450 and a guy on an XR650 did! Some could be riding style, but not all of it."

1. The owner of this carb, we'll call him John, is an intuitive soul brimming with haste. He's not one to sit and wait for a problem to fix itself so he unbolted this whimpering pile from his 2003 KTM 450 and tore into it. He found some problems and created a couple of monsters in the process.
2. I knew I was in for a world of hurt when I compared the old accelerator pump diaphragm with the new one from JD 2. Jetting. This bike had seen some dirty, dirty gas. And since John has a day job, his trusty steed sees its fair share of parked time. I really couldn't have guessed how bad rotting fuel could be.
3. This chamber (the one with the fuel line fitting stuck in it) is the big problem here. I call it the intermediary chamber of death. It is definitely not supposed to come off the carb's main body, and even though they paint the bolt heads shut that hold it together, people like John still try.
4. In this case, John's mechanical rapidity was rewarded with a bounty of suspicious seals. These grotesque gaskets were melted, mashed and misaligned. Could it be cracking the seal on the intermediary chamber of death was just what this carb needed? It certainly wasn't going to work better without a cleanup.
5. After inspecting the mating surface on the bell housing, um, housing, it became clear there was no turning back. Mostly because half of one gasket stuck to it during disassembly. We needed to find new gaskets and we needed them fast. John was set to Baja his bike in Baja in a couple of weeks-no time to waste.
6. It turns out you can't buy gaskets for the intermediary chamber of death. It's true. We checked everyone we could think of. Sudco ( even said, "That chamber isn't supposed to be removed." Or something like that. Hence the bolts full of paint. While we searched for new gaskets John went out and eBay-purchased an FCR carb of the same bore for around $50 hoping it would bolt in and take him through Mexico. When you buy carbs off eBay, sometimes they're weird. John's new carb was from an ATV, and the cables were too short causing it to rev to the limiter whenever he turned to the right. A cable swap was out of the question (ATVs are weird, too).
7. So the decision was made to steal seals from the new carb to fill the gaps in the old carb. Yes, I took the paint out of these bolts (again). Heating them up with a soldering iron and digging with a pick works great, by the way.
8. Before I disassembled the new carb I cleaned out all the orifices on the oldie with a strand of wire from a frayed throttle cable. This is the pilot jet port.
9. Next, I grabbed some of John's dental tools (he's not a dentist) and went to work carefully removing sticky goo.
10. Satisfied, I turned my attention to the carb's bell-housing body and spiffed it up with carb cleaner and 800-grit sandpaper.
11. This is the new carb's intermediary chamber of death gasket-much better! Except, it's built as a one-piece seal incorporating the perimeter O-ring and linking the inner seal with a tab. Time for more innovation...
12. The gaskets are obviously the same size (except for that stupid tab), so I sliced it off with an X-acto knife and placed it on the old carb to check fitment-it was pretty close, and when I pressed it down it locked into place perfectly.
13. I also robbed these square seals from the eBay carb.
14. The last step to the resealing of this section was to use a light speck of grease to stick down the swapped seals. This held them in place while I slapped the chamber of death back into the main carb body.
15. These bolts got a good dose of thread-locking compound. We never want to open this again!
16. After patching the carb back together, I went to work assembling and installing JD Jetting's accelerator pump mods. This little spacer taps onto the diaphragm to tighten the tolerance between the pushrod.
17. An O-ring is also installed over the pushrod linkage arm and actuator to eliminate pump squirt delay.
18. The last piece to the puzzle was the JD Jetting Adjustable Fuel Screw-it really adds a nice touch of adjustability and shine.