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MG MGB GT V8 Factory Originals Technical - Help ... V8 running poorly: flames in intake

My V8 recently started running like crap. There's a loud rustling sound above 3K rpm and no more power, and the amazing thing is that when the air cleaner is removed, you can actually see -- peering down through the carb venturis -- a bright orange flame inside the intake manifold. (!) Of course it gets pulled back by the rushing air and doesn't come up out of the carb, but it's totally visible. Yikes.

Car has had a compression test and leakdown test, and all cylinders are good to go. Cam checked for indexing; right on the money. New plug wires. New coil. New plugs. Almost new diss & ignition box. Sparks OK. Valves appear to be opening and closing as one would expect. No coolant in oil or vice versa. Something unspeakable is apparently occurring here; any wild-ass guesses as to what? ANY ideas, no matter how farfetched, gratefully received.
David

I would think something is wrong with the igniton timing!
Bob

Ignition timing is set to 10 deg BTDC @ 1K rpm w/vac disconnected. Max advance is limited to 40 degrees (10 initial + 24 mechanical + 6 vacuum). TDC on pulley mark -- while perhaps not dead on -- appears to be quite plausible judging from the valves on #1 cyl. But you could certainly be right. Formerly very fast car seems to be having a serious migraine.
David


Okay, David, here's my wild-ass guess. You have a broken valve, and the combustion is coming back up past the valve seat into the manifold. The rustling sound is the piece of valve ratting around in the combusion chamber.

Good luck,
Joe
Joe Ullman

Many thanks Joe. But wouldn't it have failed compression/leakdown in that case?
David

Hi David, Flames in the intake at high RPM, Loss of power
Compression good, time on and cam indexed.

Lifters pumping up holding the valves open at high RPM sound like the problem to me.
Check the preload.

Kelly
Kelly

David
Most commons causes of this, is weak/broken valve springs or flat exhaust lobe on the cam. It is possible to have an exhaust lobe that is starting to go flat, but is still opening the valve a small amount. It will usaully idle ok, but when you bring the rpms up there is not enough lift to allow full exhaust scavenging. Then when the intake valve opens the combustion gases go back though the intake.

Bill bill_jacobson@hotmail.com
bill jacobson

Oh yeah, I almost forgot. Be careful when looking down the carb, when its acting up...it can be an eyebrow toasting experience!! So I've heard!!

Bill
bill jacobson

How about crossed plug wires or crack or water in the distributor cap? Blown head gasket???

Wayne
Wayne Pearson

It turns out Bill is *exactly* right. Way overly strong valve springs ate one of my exhaust lobes! Time for a new cam, and new springs. Many, many thanks Bill.

Hopefully I changed the oil enough to get the metal shavings outta there!

David

Yes, David, sorry for that obvious oversight. I read the part about the good compression but overlooked it trying to come up with how that could happen. Luckily, there're people with more experience to contribute.

Joe
Joe Ullman

This thread was discussed between 20/08/2002 and 21/08/2002

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