Stuck throttle - Overrev - Before we take it apart, what do you guess I have broken.

madda

Well-Known Forum User
What a gutter, after a excellent day bumping into fellow Z owners, I went on a spirited 30 mile drive, then 2 mins from home, the damn throttle stuck approaching a roundabout! Clutch in to stop an accident then hooked my foot under the throttle to free it.

It probably took me 1-2 second to react, and sadly she hasn't come out totally unharmed. :smash::(

Symptoms are:

  • At idle there is a rattle (up top I think)
  • She is harder to start
  • Shutters and juddery pulling off
  • Sounds fine at cruising speed and also runs fine (I only did a mile after the overrev as like I said, I was fortunately 2 mins from home)

So, what do you guess I have broken? :smash:

I'll keep this updated with pics and vids once we open up the engine.
 

uk66fastback

Club Member
Blimey, sorry to hear that. Question is, why did it stick? Sounds like it got stuck under some carpet if you were able to free it from the interior side ...

I think I'd have switched off immediately rather than put the clutch in ...
 

madda

Well-Known Forum User
Blimey, sorry to hear that. Question is, why did it stick? Sounds like it got stuck under some carpet if you were able to free it from the interior side ...

I think I'd have switched off immediately rather than put the clutch in ...

Its easy to say in hindsight isn't it.....but when it's the last thing you're expecting, I am just happy I didn't go into the back of the car in front and I managed to drive it home. :unsure:
 

madda

Well-Known Forum User
I'll investigate the sticking before she see's the road again, but my priority is to work out what's broken and get my order into the USA asap.
 

Farmer42

Club Member
What carbs have you got (SU, Weber etc.)?

The twin SUs have return springs on the throttle linkage that attach between the carb and the heatshield. One or both of those could have snapped which is stopping the throttle returning to the idle position without your intervention and the rattle could be from the remnants dangling on the heat shield underneath.

Get your head under the bonnet and use the linkage at the firewall to imitate using the throttle and see what happens and also check the linkage to the carbs to make sure it is all free & moving properly.

If you haven't got SUs perhaps someone else can suggest a solution.
 

madda

Well-Known Forum User
What carbs have you got (SU, Weber etc.)?

The twin SUs have return springs on the throttle linkage that attach between the carb and the heatshield. One or both of those could have snapped which is stopping the throttle returning to the idle position without your intervention and the rattle could be from the remnants dangling on the heat shield underneath.

Get your head under the bonnet and use the linkage at the firewall to imitate using the throttle and see what happens and also check the linkage to the carbs to make sure it is all free & moving properly.

If you haven't got SUs perhaps someone else can suggest a solution.

Thank you. Unless I'm really lucky there is definitely something ratting in the head, but thanks for the glimmer of hope that I got away with it.
 

Rob Gaskin

Treasurer
Staff member
Site Administrator
Sorry to hear of your misfortune Joe.

I've had an over-rev instance with my old trackday car and it made me realise that there is nothing to stop our cars from over-revving! I then fitted an OMEX adjustable limiter. Even an old 70s VW I had was fitted with a rotor-arm that had a centrifugal circuit breaking system.

Take the cam-cover off and you'll probably find loose lash-pads and springs. Maybe the cause was a bent valve but you might have got away with it.
 

MikeB

Well-Known Forum User
There will be the obvious damage, bent valves, marks on the Pistons, possible damage to the valve seats and head. But you then need to look beyond that, even if the rods are OK, they will need crack tested and new bolts, it's all just insurance against a potential hidden flaw
 

SeanDezart

Well-Known Forum User
Its easy to say in hindsight isn't it.....but when it's the last thing you're expecting, I am just happy I didn't go into the back of the car in front and I managed to drive it home. :unsure:

Also easy to say after the fact is to fit a rev limiter even if anyone doesn't go trackking their car....extra insurance !

Good luck.:eek:
 

SacCyclone

Club Member
Do a compression check before you pull it all apart to see which cylinder is affected. Then off with the valve cover for a visual.

I had a mechanical over rev (gear change down instead of up) on the track last year...went to 7K or a little more and it all hung together. Another track friend took his to 8K on a mechanical over rev and his was fine too.

Your probably not too bad, lash pad or spring failure...top end stuff. The bottom end is pretty stout.
 

richiep

Club Member
My car revved to the gods (off the dial high) one time when the clutch bizarrely gave way during a 3rd to 4th shift at 85mph and WOT. It was fine. The clutch was a bit funky on the drive home but there was no engine damage. Wouldn't want to do it again though!
 

nospark

Well-Known Forum User
As others have said first look for a missing lashpad. It happened to me a few months back. Was difficult to start one day from cold, then give it a bit of a rev and it answered back with a rattling noise. Found the lashpad disk loose on top of the head. Levered the spring down, refitted the lashpad and it's been ok since.
 

madda

Well-Known Forum User
Only had a chance to visually inspect it tonight. Nothing obvious, still a top end noise but more of a knock than a rattle. Bore scope and compression test tomorrow before we dig deeper.

On the plus side I was pleasantly surprised with how clean everything was in there.



 

Rob Gaskin

Treasurer
Staff member
Site Administrator
Yes clean.

The inlet rocker on Cyl 5 and ex rocker on Cyl 6 appear to have the rocker spring dislodged. However it may be just my eyesight. :eek:
 

jonbills

Membership Secretary
Site Administrator
What good eyes you have Rob. You're right, those mouse traps are off. I don't think they can account for the problems alone though surely?
 

nospark

Well-Known Forum User
madda, you say you are pleasantly surprised how clean it's in there. Implies that you have never been in there and adjusted your valve clearances. If so, try checking the valve clearances first ?

I recommend every owner doing their valve clearances. The previous 2 owners had never touched them. I had a go and the mildly rattly top end was transformed to a sweet sounding engine. A couple of gaps were way out. It says to do it with a hot engine but I did it with a cold engine and the different COLD gap specs. The first time you will faff about as you get the technique right, so for me cold was the best at the first attempt. Hope this helps
 

madda

Well-Known Forum User
Rob sir, you're a legend. Think you are right on review!!

Might as well do the clearances whilst in there later. Does anyone have the settings to hand please??

Hopefully this thread will be useful to others in the future! :)
 
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