Large bang - gearbox?

[Mothy]

Recently bought a '83 911SC and not long after discovered it had need of new headstuds (6 broken).

Just got the car back from the engine rebuild and have been enjoying taking it for a few long runs with all the horsepower back the way its meant to be. Also, the gearbox linkage seemed to be so much better and smooth when its adjusted by someone who knows what they are doing!

However, yesterday when starting off hard from lights up a medium hill there was an enourmous BANG from the rear as I shifted from 1st to 2nd. Since then, its been hesitant to shift between gears and the gearlevel is certainly a lot stiffer. Also, the car seems to be down on HP again. I'll probably take it back to the mechanic to see what he thinks but thought I'd try for some opinions here first.

By the way - I'm still getting used to the long travel of the clutch and the gear stick after the WRX STi's 'micro' gate - its entirely posible I pulled it out of first before I had fully pushed the clutch.

Hope I have not done any damage ...... wife barely talking to me after the engine rebuild!

Mothy

[Crowbob]

I guess my first question is why did you swap a relatively brand new Sti for an old SC? And second, it sounds to me like either your helper spring busted or the clutch finally gave up the ghost since it's 22 years old if original, is it?

[hkspwrsche]

helper or "omega" spring... Should be easy to see if you look underneath

[jdowty3]

Welcome to the SCWDP. Sounds like you inherited PO issues.
I just a shadetree mechanic but will put forth a few possibilities.
Like JIMCRY said, could be your help spring but you don't say that the clutch pedal became extremely hard to depress.
It could also be the original rubber centered clutch self-destructing. You don't mention how many miles the car has. Do you know if it has a rubber-centered or spring-centered clutch? If you mechanic did the engine pull, I hope he would have advised replacing a rubber-centered clutch while it was out.
If you have LSD, you might have snapped the CV bolts on 1 side at the wheel.
I'm not sure what you mean when you say "hesitant to shift between gears and the gearlevel is certainly a lot stiffer".
The helper spring and CV bolts are fairly easy to replace. The clutch requires the engine to be removed again.
Good luck and let us know what the problem is.

[Paulporsche]

Are you sure the bang was from the gearbox? Could it have been a backfire through the exhaust(probably a too rich mixture) or through the intake (probably too lean)? Do you have an airbox popoff valve?

[jdowty3]

Paul is right. You should check to see if it backfired through the airbox and if you have a pop-off valve. A cracked airbox could account for you stated loss of HP.

[Mothy]

Thanks for all the suggestions.

I'll check the helper spring today. As for the clutch, it is brand new as is the clutch cable - the mechanic did the right thing and advised a new one since the engine was out and the old one was original (150,000km).

The engine also had a pop off valve fitted during rebuild. It was not a backfire - I know what they sound like. The bang was much louder and sounded more mechanical.

However the engine is backfiring more often now under low revs load - even when warm. Used to be only when cold.

Mothy

[Mothy]

Bit of an update.

The low power was just the car settling in after the rebuild. The mechanic did an exhaust gas test and adjusted the mixture and its running well again and has been for weeks now.

The big bang has happened once since - and it was more obvious the second time. I was pulling the gear lever our of first before I had fully depressed the clutch.

I'm getting much better at it now - learning to take my time and not trying to speed shift. After the minute throw and short clutch movement of the STi, it will take time.

Thanks for all the input - much appreaciated.

Tim