68 340 Vibrating idle, loss of power

kenelder said
Did it backfire while you had the clutch in? Was it a diaphragm clutch? If yes to both, remove bottom stamped dust cover plate from clutch housing. In neutral turn engine by had while looking at the back of the pressure plate. You’ll see the drag bars that attach the pressure plate to the clutch cover as shown in the pics. The bars should be straight and tight. If damaged they will be spread apart.

If you find them spread apart, like the picture Img_3120.jpg, you’ll need a new pressure plate and clutch cover.

Note this only applies to diaphragm clutches, the factory 3 finger clutches don’t have this problem. Also the bent drag bars make the pressure plate off center so it will vibrate like an out of balance engine. Or like an engine with a bad valve, etc.View attachment 1716460685

View attachment 1716460684

I dont remember exactly when it backfired at that time. I do remember that everything changed sound and performance wise after getting back on the highway, and with the clutch, it makes sense. Practically nursing it back 2 miles up the highway to my exit and thinking i was just completely out of time with exhaust practically blowing out under my door. I'm gonna remove the dust cover and hand turn over the motor and check this thing out. Not in favor of dropping the transmission from under the car, but if I find that issue with the plates, I hope that summit will replace the clutch kit. It is what it is at that point and I'll definitely breathe a sigh of relief on the motor.