Shrapnel on carb top - what do you think?

[vntgspd]

So, I was chasing down a misfire from the last autocross when I encountered this chunk of aluminum on top of my carburetor body (inside the K&N filter area). I thought I had a dead cylinder due to a clogged idle circuit but it appears that it is a bit more serious.




I think this is part of the head where the valve guide is fitted. These heads only have about 10K miles on them since the complete valve job. If this is what I think it is, what do you suppose happened?

Here's my theory: This course had several sections where the rev-limiter kicked in. It's the factory "spring in rotor" limiter but it seems fairly accurate according to the original tach. Also, the afternoon session saw oil temps near 250*F. Both of these scenarios have occurred before with no ill-effects. So, perhaps high temps (thermal expansion) combined with sustained rpm-limit operation (and possible slight over-rev) allowed piston to valve contact? Maybe this stressed the guide and broke the head casting? Bent valve = dead cylinder. Or, is it possible that this is metal fatigue?

I should note that the engine is a "2.5L" with 911S 74mm crank and rods with 85mm JE "2.2S" pistons. Rev limit is set to 7100rpm and heads are stock 2.2E heads with no porting.

Obviously a tear down is in order when I have time. Until then, she's sadly been pushed back into the garage and covered.

[MBruns]

Sad to say it looks like a piece of your intake guide boss, may have tagged a valve with the piston, or split the boss when the guides were replaced last ?

Mike Bruns

[jimbauman]

Hard to tell from the pic, but it looks like it's kind of old and weathered... you sure it's from your car - or recent?

JB

[Steve@Rennsport]

Originally Posted by MBruns

Sad to say it looks like a piece of your intake guide boss, may have tagged a valve with the piston, or split the boss when the guides were replaced last ?

Mike Bruns

I think Mike hit the nail on the head.

Time to remove the intake system on that side for a close looksee.

[vntgspd]

Thanks for the feedback from the pros!

I guess there is some consolation in knowing that my speculation is correct and that my knowledge of these engines is growing.

I am thinking that if this valve hit, others may have also and I may need to pull the left bank of heads as well. I know that a leak-down may be an indicator but the only way to be sure that the rest of the heads are good is to remove, inspect, and measure. Do you agree?

Thanks again!

[Steve@Rennsport]

Originally Posted by vntgspd

I know that a leak-down may be an indicator but the only way to be sure that the rest of the heads are good is to remove, inspect, and measure. Do you agree?

Absolutely true!!

[Walt Fricke]

Well, as soon as you get your left side carbs off on that side you will know who the culprit is, and confirm the diagnosis. Taking the intake off with the engine still in the car may make pulling the engine a hair easier, although pulling the intake is easier with the engine up on a stand, which is where it is going anyway.

If it is what all think it is, you will be pulling that head, which means the cam and cam carrier.

While my money (well, guess, prompted by Mike) is that the last guide replacement cracked the boss, and your overrev had nothing to do with it (your springs are good for 8,000 rpm on occasion), with the cam carrier off it isn't much extra work to pull the other two heads off to see if anything is amiss.

With the engine out, doing a precautionary leakdown on the other bank will be easy if you don't find valve marks on the other piston tops.

Let us hope you were lucky, and this spit right out and never got down into the combustion chamber. It doesn't look like that happened, as it isn't beaten up enough. Valve snapping shut at just right time pinballed it up out past all the obstacles in the carb venturi?

The misfire? Alas, that may indicate the intake valve bent during all of this. But you are looking for a new head most likely, so.