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Cannot move shifter knob into 3rd or 4th positions when bus is moving

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I have a 1973 bus that has a 2000cc engine and 6 rib 091 transmission that I assume were transplanted into this bus at the same time and from the same donor (engine code GD).   I bought it DOA and it's had it share of problems but the engine is finally going pretty good, replaced transmission fluid (same as I've done several times previously on other buses) and engine oil filter/strainer-clean, etc.  Transmission fluid was a fairly darkish brown.    The clutch cable looks quite new and I'm assuming it was longer than needed because there is something like 5 large nuts between the clutch lever and the large wing nut.   Ok, enough background.

So here is the problem.   Engine off, it easily shifts into each forward gear and reverse.   Engine on and bus sitting on driveway, I can shift into any forward gear and reverse and can feel the shift
 knob "click in" just like I would expect it to when it finds the right place.   Letting out the clutch oh-so-slightly engages in all gears and starts to move the bus (i.e. I did not want to stall the engine trying 4th gear sitting on my driveway) just as I would expect.  Now the fun begins.  1st test drive.   1st gear is great and I'm onto the road.   Speed up a little and into 2nd gear no problem.   Speed up some more and I can feel the excitement........until I try to go into 3rd.   It does not grind or anything, it just refuses to move the shifter knob into the 3rd gear position that it clicked into when sitting on the driveway running (bus stationary).   Back to neutral and try again.  No go.  Just for the heck of it I try 4th gear.   No go.  It won't allow me to put the shifter knob all the way to where I would expect it to feel it click in.    I pull over as I now am building up a small line of vehicles behind me.  I come to a full stop and as I sit there on the side of the road, I try all the gear positions again (vehicle stationary).  Yep, 1st through 4th they all nicely "click" into position (clutch pushed in, of course).   So I head out again:  1st gear great.  Moving on down the road.  2nd gear great.    3rd gear, nope!   4th gear, nope!   So I stop again.   Went through this exercise 4 times just to truly convince myself I was not just missing where this bus particularly needs it's shifter to go for 3rd and 4th and each time I was stopped, they went in fine.   It does not grind.  It just won't let me get my shift knob into the right position for those two gears while the vehicle is moving.   Only when it's not moving.  If the vehicle is moving, the shifter knob will not move fully into the position I expect it to.  When the vehicle is stopped (engine running), it goes into each no problem.  Again, if I'm sitting still and put it into 3rd and super-slowly/gently let up the clutch pedal so as not to suddenly stall the engine, the clutch engages and the car begins to move forward, just as I would expect.

I'm still researching lots but figured I'd through it out there in case one of you transmission-knowledgeable folks read this and say "hey, I know exactly what that is, you have to do "this".  So far I have not found another example of this situation I can read up on.       


I'll guess it's a bad shift coupler, misadjusted shift stop plate or maybe a bad engine mount or three, something is moving while under way enough to change the position of the shift rod in relation to the trans.

That 091/2.0 unit is longer than the 002/1700 by a hair, how were the front mounts and shift rod or shifter modified to make up for that?

I'm also wondering if my clutch cable is set right and if maybe it needs tightening just a little.   Possible while driving (moving) that the clutch is just barely touching/engaging and that is stopping me from getting into those two gears?   It's only when the wheels are turning that the problem occurs.   I'm having some trouble getting my head around exactly what is different with the wheels turning and not turning to cause this to happen.   I've read a bunch of posts and still am but not any yet that are quite the situation I'm experiencing. 

Modified the subject line to make it a little clearer and added a little clearer description of the problem and how/when it occurs.   

Are you absolutely sure you have a 73 Bus.  If I am not mistaken and from the pictures you posted I see a round external gas cap on the rear right and no flapper door for access to the 73 gas cap.  Further, the ECU and air box mounts look awfully original and not something cut in or welded to the rear inner area above the battery tray area.  Do all 4 VIN numbers on the body match?  Might help to resolve the issue of mismatched parts or worn originals.


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