1982 911 3.0L SC Sport - Number 3 cyl problem

[Tommy P]

I Just finished a session on track a few day's ago and noted as I pulled into the pits that tickover was rough (Lumpy). After trailering it home and driving into the garage it was the same. I was told that it might need the injectors re-setting, but because I was about to remove the injectors for re-placement with some IDA 40 Webers, for obvious reasons I didn't bother to check this out. After fitting the Webers tickover was the same, only now with the air-filters off I could see that the carb on number 3 cyl was blowing back fuel. The suction on all the other carbs is strong but on this carb it blows. It feels like it occilates between sucking and blowing.
I haven't yet done a compression test (just trying to find one). The spark plug is fine and sparking well. I'm no expert on the compression sound with a plug removed but that sounds quite strong. The oil pressure is fine too.
Does anyone know what the possible causes might be and what work might need to be done.
Cheers
Tom

[john walker's workshop]

broken exhaust rocker arm maybe. if it broke, the valve hit the piston to do it. another scenario would be the rocker shaft came out and the rocker fell off. that might be better, as long as the cam tower wasn't damaged in the process. pull that lower cover.

[Tommy P]

Thanks John
The bottom cover is removed and with all the plugs removed - when the engine is turned over manually I can see the rocker arm moving up and down. I could see little else as I'm working by myself. I did a compression test on number 3 cyl and the result was zero pressure. Do I need to remove the top cover. Could it be a burnt valve.
Cheers
Tom

[Tommy P]

Well the top cover came off and the problem dropped out - A broken Rocker arm, top of the valve guide broken and the large spring is in 3 pieces. I dont know what other damage this has caused, I suppose it is possible that the valve stem has broken and hit the piston. Would this cause the rocker arm to snap?
Does anyone know if this means (Just) a top end rebuild or at 27,000 miles would a complete rebuild be in order.
Cheers
Tom

[john walker's workshop]

the valve generally needs to contact the piston for a rocker to break. so the valve is most likely bent. could be the spring broke first, and allowed the contact. being that the guide is damaged where the seal would attach, the head will need to come off and be repaired. at 27K, i suppose that you could get away with just doing the one head. all new springs would be a good idea though.

[Tommy P]

John thanks for the info
Do you think that it is possible, that the piston has been damaged too? Or might I get off lightly there?
Tom

[john walker's workshop]

the pistons usually survive, but check that the top ring is still free in it's groove.

[cstreit]

Any news?

[Wayne 962]

It depends upon the pistons - the ones with cutouts for the valves do better than the CIS ones, although the CIS ones are also pretty beefy.

This an excellent argument for *not* reusing springs. If the spring is broken, it sounds like it might have caused the collision. Athough theoretically, it would be difficult for the rocker to break then, as the valve would still have clearance. Did you over-rev the car at all?

-Wayne

[Tommy P]

I guess you could say I over-reved, I was certainly pushing hard and noticed on a couple of occasions that I was well into the red zone, say 6,400 - 6,600rpm, which would have been for a couple of seconds. Also on one particular corner in second on the red line for 4 - 5 seconds.
I haven't taken the engine out yet to begin further inspection with the head off - Unfortunately I'm away on contract for a few weeks, so it could be a while before checking the piston etc.
If anyone is interested I will post my findings and progress as the work goes on.

[ChrisBennet]

It's a good argument for using new valve spring retainers and keepers as well. I wouldn't consider 6.4-6.6K "over-revving" with an SC motor. More like normal.
Are you using 964 cams?
-Chris

[Tommy P]

No just standard cams
Tom

[Tommy P]

Unfortunatey I haven't got the hang of posting images on the forum, but part of the rocker arm and springs can be seen at:

http://uk.msnusers.com/EngineImages/shoebox.msnw?action=ShowPhoto&PhotoID=1

[Wayne 962]

I would bet money on the damage coming from an over-rev. Weak springs will hurt your maximum RPM as well. CIS pistons with their domes don't tap the valves very nicely- there is a chance your valves are bent. Replace the rocker (easy to do - we sell them) with the engine in the car, and then run a compression test on the engine. If compression is good, you just escaped a major problem. If it's not, it's a bent valve, and you need to tear it down...

-Wayne

[Tommy P]

Thanks for the advice Wayne,
but I'm a little confused. I'm assuming that the head will still need to come off in order to replace the springs that are broken on the valve. If so, then I thought that the engine would need to come out for this.
Tom

[john walker's workshop]

there's a "in the car" spring replacement tool available. but your guide is damaged, and won't hold a seal, so it has to come off for that anyway. then the valve can be spun in a lathe to see if it's bent.

[Wayne 962]

Right - we do sell this tool - it might be worth trying before you remove the engine. You might be able to get by if the top of the guide isn't messed up too much...

-Wayne