MANUAL TRANSMISSION GEAR JUMPING
There has been a number of inquiries recently on manual transmissions jumping out of gear.
I will try to cover all the reasons they won't stay in gear.
Most jumping out of gear problems are from worn synchro teeth on the gear or in the mating synchro sleeve. They are supposed to look like a house with a hip roof. If they are rounded off or missing chunks they may not stay in gear. Sometimes transmissions jump out of fourth gear because of bad input bearings. The vibration just backs the synchro sleeve off.
Check the interlock sleeve between the rooster combs on the shifter forks in the side cover. Sometimes these get chipped and allow excess movement in the shifter forks. They are easy for a machine shop to make and the ball bearings in the ends are easy to get from bicycle shops. This little piece keeps you from engaging more than one gear at a time.
The C shaped synchro springs that keep the three dogs in the grooves in the inner hub in contact with the outer sleeve lose their tension. They no longer push the three dogs firmly in the groove inside the sleeve and the synchro sleeve backs out of gear.
Bent shifter forks are also a source of jumping out of gear. They just no longer push the synchro sleeve far enough to completely engage the teeth on the gear. This can happen even when the shifter moves the outer levers far enough on the outside of the transmission.
Bob Halverson, Carl Trunk and Kennedy American are good sources for used and new transmission parts. The input shaft, third gear, and cluster gears are getting hard to find new. Everything else is available. Run synthetic manual gear lube when you get the thing back together. There is no balky warm up period and the brass synchro rings don't seem to break as much with the synthetic lube.
I have also had problems with incorrect transmission alignment with the crankshaft centerline. This can be caused by bent and/or broken bellhousings, or the wrong snout to bellhousing hole. Broken ears on the transmission housings may indicate misalignment or warped bellhousing plate.
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