Ask Dr. Dirt: Re-Jet Or Carb Swap?

Dirt Rider’s Chief Physician AKA Dr. Dirt is Scot Gustafson. If you have a question about bike maintenance, set-up, tuning, or if you need help diagnosing a problem or just want to know how something works, send your question, along with your first and last name and your city and state, to We may edit your letter and might also use it in Dirt Rider Magazine’s letters section.


Dr. Dirt,

I just restored a 1984 Yamaha IT490. I’m now finding out the nightmare that it is to jet these bikes properly. I ordered a new needle jet, needle, pilot, main & 3.5 slide based on what I read in the forums that coincide my climate and sea level. It’s still not right, even after the head mod or squish. My question is if you know a plug and play replacement carb that will work with a stock IT490? I was considering a Kehin PWK 38 or 39mm or a newer Mikuni flat slide. I realize the bike is a dinosaur and would appreciate any info you could provide. If you can steer me toward an old school Yamaha IT guru I’d also appreciate that info. Love the magazine, you guys rock.


Cocoa, Florida

If you look up 1980s Yamaha Open Class bikes in the dictionary the first words are "jetting problems." It sounds like you have done your homework and tried most of the known tuner fixes. The first thing I would suggest is going back to the basics. Check the condition of your carb, the reeds, the engine’s compression, the main seals for leaks, the condition of the silencer, and the timing before you do anything. Next, I would look at my fuel mixture. Unless you are riding the bike in Baja at 100 mph, the bike is usually ridden at low rpm and best served using a 40/60-1 mixture of synthetic ester based oil. While I have heard of people giving up the old, round slide Mikunis for the Keihin carb from a KX500, I think you run into the same jetting go around as before. Finally, try giving Eric Gorr of Forward Motion a call. He has a lot of good tuning info on older 2 strokes.