Let's decide on "carb setting = fuel adjustment kit" through repeated test runs!

The procedure for restoring an old car is to find and fix bad parts as they appear, and gradually improve them as the degree of finish is increased. This is not a world where all the parts you want are available in abundance and with new parts, so it makes sense to do so. Here, I will be using a Key Star fuel adjustment kit to improve the carb setup.

It's almost in shape! The DT-1 is the king of the early days of off-road motorcycles

The Yamaha 250 DT1 is roughly in the finishing stages, starting with a frame diagnosis = checking for frame distortion before work begins, painting of all skeletal parts, painting of all exterior parts, overhaul of the engine above the waist, etc. There is a lot to do before it actually starts running... ......

The best stand for old & old type off-road bikes

This is a lift-up stand for VMX (vintage motocross) from the unit brand produced by Dirt Freak. While most products for general motocrossers have a lift-up stroke, this product has a lower lift for VMX and is easy to use.

The lift-up stand for VMX features a narrow top plate design that allows the front or rear wheels to be placed on the ground for maintenance in a more stable situation. The stand is equipped with a damper, so it operates slowly and does not fall with a thud when being lowered.

Practice stable carb setup with the Key Star fuel adjustment kit.

Fuel adjustment kits from Key Star are parts that allow for fuel adjustment = carb setting and overhaul at the same time. We purchased this kit because there are many variations for old cars and the lineup included one for the Yamaha DT-1. The kit is inexpensive and great for a simple gasket replacement.

As one can understand when actually using the fuel adjustment kit and practicing carb setup, a 2/100 mm difference in the "straight diameter" size of the jet needle makes a complete difference in throttle response and actual driving performance. All old car engines cannot be OK with stock settings.

I'm concerned about the muffled feeling at the beginning of throttle opening. ......

The DT1 was originally a motocrosser spec with no air cleaner, and since it was overhauled as is, I wanted to take this time to get the carb settings just right.

I believe this carb has been installed since new, but there were no worn marks or scratches on the outer circumference of the piston valve. It may have been used exclusively for motocross, or it may have been ridden very little!

The air cleaner had been removed, and apparently the carb was adjusted by the number of clip steps on the jet needle. The first step in the resetting process was to change the number of clip steps. We should also check the straight diameter of the stock needle.

The stock needle was not quite right for the setting, so we replaced it with a needle from the fuel adjustment kit and adjusted the straight diameter. The straight diameter was increased by one rank in the direction to squeeze the gas.

Throttle response and running is good with the jet needle change!

As a result, the stock jet settings were determined to be best at the center of the clip of the jet needle, which is one size leaner (thinner = 0.02mm thicker straight diameter) than the one size leaner (thinner = 0.02mm thicker straight diameter) of the fuel management kit.

The steel parts are yellow chromate and re-plated stock carbs. The early DT-1 was elaborately built with the main jet tightened to the float chamber drain, making it easy to change settings.

When a fellow biker who is running a DT-1 that is being finished calls me to tell me that the engine starting performance has become "bad," I ask if the points contacts are dirty or if it is the capacitor? Or a capacitor? I imagined that it was something like that. When we asked him for more details, he told us that when the engine heats up, the starting performance becomes poor, and the engine kicks like a demon. Given the current state of the engine, we were pretty sure that the ignition coil had gone.... The DT-1 owned by a motorcycle friend of mine had a history of ignition coil replacement at his garage when he was away from home and experienced poor starting performance. After the replacement, nothing happened and the bike is still running. When we asked him further, he told us that the coil was not for the DT-1, but for another model, and that it was a small ignition coil that was fixed with a single bolt. The shape of the coil suggests that it was for a mini bike.

So, if we replaced the coil with a replacement part that was similar in size to the DT-1's stock coil, "it might not look out of place" when the tank was removed. I browsed through the web catalog. At that time, I came across a 6V AC coil (AC coil from the Flamag point ignition era) for the Honda 4 Mini Custom, which is common in old cars. From among several types of products available, I purchased it based on image judgment and installed it as previously reported (previous Vol. 14), which gave me a good result and it seems to be in great shape now.

With the improvement of the electrical system, there are no more "starting problems" either when the car is cold or warm. After running in such a situation, I now began to worry about the carb setup. I checked the lineup on the web and found that a fuel adjustment kit for the early DT-1 was available, so I purchased it. After repeated changes to the settings, I arrived at the direction of making the straight diameter of the jet needle one rank larger and making it "slightly thinner" from the beginning of throttle opening to the mid-range speed range. This setting resulted in a smoother ride at low speeds than ever before.

The burnt plugs are ideal. The threaded seat was not smoked black and cracked. This is probably how most 2-stroke models should be run on the street, and idling should be stable. In fact, this great plug burn has been obtained after updating to ICBM® cylinders. Before, the threaded seat area was black, smoky, and foggy, and it seemed more stable in the idling area that way. However, after the ICBM® process, it became nicely burnished like this. We believe that the stabilized compression improved the filling from the intake scavenging at very low speeds. Now that the carb setup has further progressed in getting more oomph out of the carb, we will try to solve the "blinker blinking problem" at the next opportunity.