Car surges forward when starting in N

[Civilizedape]

I am finally getting my 73' 240z (L28 S/U's 5 speed) running after many many modifications and reassembly. The problem I have is that the car surges forward when cranking even when the transmission is in neutral. With the clutch pushed in it does not do this. The transmission was rebuilt by a shady backyard mechanic, me. Could it be clutch adjustment?

[Dave WM]

check the oil level in the gear box. did you replace the brass balk rings?

Edited July 20, 2019 by Dave WM

[Mark Maras]

I'm leaning toward the trans, input shaft binding in the pilot bearing (bushing) in the flywheel. The fact that you can defeat the surge with the clutch would tend to rule out a clutch adjustment.

[Dave WM]

just to clarify, car in N, sitting on level ground and you crank, it will start rolling?  so when it starts and the engine in running the car will move forward again in neutral? I don't like terms like "surge"

the 5 speed is a sychromesh transmission, so all the gears on the counter shaft and output shaft are always meshed. The output shaft is locked to ONE set of gears at a time by the hubs. there is always some fluid drag that will tend to turn the output shaft since all the gears are spinning on it. Now if you have some kind of binding between the output shaft and the gear needle bearings, or if the brass baulk rings are not properly fitted, there could be some drag that is excessive. turning the input shaft by hand will turn the output shaft just by the slight drag of the oil.

did you pay attention the correct orientation of the 3/4 sychro hub? I don't know what would happen of even if you could install it backwards, but it does have a correct direction.

[Civilizedape]

Thanks for the replies. I have not gotten the car running yet. If the car is in neutral and I try to start it w/o the clutch pressed it will try to roll forward. Never heard of the balk rings? Is that the brass?? rings that sync the gears? If so yes I replaced them. 

[Patcon]

3 hours ago, Civilizedape said:

Thanks for the replies. I have not gotten the car running yet. If the car is in neutral and I try to start it w/o the clutch pressed it will try to roll forward. Never heard of the balk rings? Is that the brass?? rings that sync the gears? If so yes I replaced them. 

tries to roll forward? or bucks forward like it's in gear? or sort of like an automatic transmission sort of creep forward?

 

[Civilizedape]

I let about a 1/4c of fluid drain out from the fill plug. Tries to roll just a bit and cranks slow. I am hoping once I get it to idle it will free something up.

[Civilizedape]

On 7/20/2019 at 1:50 PM, Mark Maras said:

I'm leaning toward the trans, input shaft binding in the pilot bearing (bushing) in the flywheel. The fact that you can defeat the surge with the clutch would tend to rule out a clutch adjustment.

What would you suggest? Pull trans and look for what? 

[siteunseen]

Do the clutch slave cylinder adjustment and the correct distance from floor to pedal.

You can google "240z slave cylinder adjustment classiczcars.com" and find the info.

Good luck, don't want to drop the transmission if you don't have to.

[Zed Head]

On 7/20/2019 at 2:14 PM, Dave WM said:

jNow if you have some kind of binding between the output shaft and the gear needle bearings, or if the brass baulk rings are not properly fitted, there could be some drag that is excessive. turning the input shaft by hand will turn the output shaft just by the slight drag of the oil.

If he's getting actual movement of the weight of the car he has some serious binding inside.  Sounds like the shady mechanic.  You can turn the input and output shafts by hand, easily, in neutral, with the transmission out and on the ground.  There's no way it should move the car at all, unless there's a serious bind-up.

[Mark Maras]

59 minutes ago, siteunseen said:

Do the clutch slave cylinder adjustment and the correct distance from floor to pedal.

You can google "240z slave cylinder adjustment classiczcars.com" and find the info.

Good luck, don't want to drop the transmission if you don't have to.

 I agree. Rule out all other possibilities before pulling the trans. The pilot bushing lies in the center of the flywheel. The trans. input shaft nose fits into the pilot bushing. If there isn't sufficient clearance between the two the bushing will grab the trans input shaft and turn it even when the clutch pedal is depressed. Lack of lubrication on a new tight bushing could cause it to grab.

[Dave WM]

would that account for why even when in neutral he has problems? If it is the trans, I guess the take away is to always spin the input shaft while in N and see if you can hold the output shaft still. I assumed this would have been done, as a check out of the rebuild (shift thru all gears and confirm nothing it binding and gears are all working). Does the OP recall doing that?

Edited July 27, 2019 by Dave WM