After adjusting the timing, cleaning the fuel tank and carburetors, I was ready to fire up the old RD400.

I was shocked just how quickly it fired up and only with some minor carburetor tuning it was running.

Shortly after this time I realized that one cylinder wasn’t firing at idle…

I did a leak down test and both cylinders showed 100psi, which is too low for peak performance. That still wasn’t the issue for one cylinder not to run at idle.

Further research told me that a crank seal was leaking on the right side, allowing air to pull through the transmission case, pulling oil along with it. This leak meant that that cylinder was running lean because it wasn’t drawing fuel into the chamber. Another symptom is that the right exhaust had more smoke than the left side. That’s the problem with a two stroke that sits for 30 years. The seals dry out.

Once I determined that the seal was buggered, I decided to pull it all apart for a complete rebuilt… and make some performance upgrades at the same time.


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