390 Auto Question

OK, so this project took longer than it should have (I hate it when life interupts my hobbies), but I finally completed the 79 390 Auto rebuild project I started about a year ago.

Before I tore it down, I ran it briefly to get an idea of any transmission problems that it may have, and sure enough, it consistently jumped out of gear under hard acceleration. So in the course of the rebuild I had Forrest Stahl rebuild the engagement parts of the mainshaft and first gear since I had heard those parts can get "rounded" allowing it to slip out of gear.

Today I started it up (3rd kick!) and allowed it to warm up and settle into a low idle before engaging the transmission. Unfortunately, under acceleration it continues to jump out of gear. When the engine settles back to a low RPM it reengages.

I intend to give Forrest a call next week to get his thoughts, but do you guys know of anything besides the engagement parts of the mainshaft and first gear that cause those sort of symptoms (jumping out of gear under acceleration)?

Thanks for any help you can give me!

Scott

No never had that issue. I don't have a transmission to look at right here right now. That gear you speak of is way on the end of the shaft the big drum full of the not first gear clutches. Somehow I don't think the power flows through those dogs once a clutch set in that big drum pushes harder/faster than the first gear clutch on the crank. The shaft which would come out with a plate and the drive sprocket has a bunch of one way devices which if they slip might do what you describe. If/when you take it out make sure you have some sort of lage pan to work over should a bunch of pieces come out. When they went to the three speed auto some of the gears have two one way devices so I assume they could hold more power.

To take the front chain sprocket, the plate and that shaft out you don't need the pan. Not sure what happens when you start disassembling that shaft which looks like a gear cluster it has been quite a while since I played with one and might have just decided if it wasn't broke to put it back together before totally disassembling it.

I also doubt he did anything to the shaft. The gear selection lever skides a piece lengthwise on the shaft (As I recall) Neither of those engagment pieces with four dogs is actually the shaft.

Fran

I would guess one of your sprag(one way bearings)is on it's way out.Pull your trans.It's a easy job from the sprocket side.Don't worry about the clutch hub shaft,you can leave it in.After you have the one way bearing shaft out,carefully disassemable it.If the one way locking teeth start falling out of their postion on the shaft,that is a good sign that the bearing is shot.You can take apart a good sprag bearing and everything stay in place.When I say take apart I mean sliding the outer gear off the inner gear hub.I owened early autos and experanced what your talking about.I've been down the engagement gear issue road too.Welded up the dogs on the shaft and redressed them to make them work.I moved up to a 84 500 Auto that has a redesigned engagment shaft.The weakness to the auto are the sprag gears.It's a dam shame because the motor is a beast in the tight.Good luck and enjoy your auto.

Make sure that your cable is adjusted correctly, they can be real finicky if not adjusted right, as for the gear and shaft Forrest has done a bunch of them for my buddys that ride Autos and they all work well when set up correctly. You can change to the 420 shaft and gear with a little fabrication and its a much better set up.

Guys -

Thanks for your comments. I've spoken with Forrest and Craig and everyone seems to agree.. it's likely one (or several) of the sprag bearings is on the way out. I've invested in replacing the set ($$!!) and hope to have it back together in a few days. I'll let you know how it goes.

Thanks again,
Scott

390 Auto Question - Problem Solved!!

Guys -

I replaced all of the the one way sprag bearings in my 390 Auto and the problem of jumping out of gear is solved! I rode it around for about 15 minutes this evening through all four gears with no episodes of "unintended neutral".

BTW, because of cost I originally only replaced the sprag for first gear since that was where I thought the problem was located. After replacing that one sprag I found I had the same issue (jumping out of gear under acceleration). Roger Allen hooked me up with Craig from Beloit, WI who clarified that when in first gear, all sprags must work properly. Once I replaced the sprags for second and third gears it worked like a charm (fourth gear is solid with the shaft).

This thing is a beast! Gator Back here I come!!

Thanks for your advice.
Scott

Glad you got it running! I will be selling my '78 auto, Craig keeps my 420 for me so I am liquadating all of my auto items(bike,a few motors, parts etc.

Hello Trailridernut
I'm interested by the auto motors and parts you're selling.
Can you list them and send me pics ?
Thanks
Michel