73 bus engine problems: valve way out of adjustment

Valve seat came loose, or is now cocked slightly. That holds the valve head more in the "open" position and moves the stem "inward".

Since the valve can't seal, the cyl fires back thru the carb.

Eventually, the seat will (maybe) move some more and the valve head will bang into the piston and the engine will be toast.

As a check on my theory, remove the rocker arm shaft. Hold a straightedge across the valve stems. If #3 is further in, the theory fits.

I'll check this...

engine has made a noise ever since I bought it, a knock sound (low frequency,maybe 4X - 6X per second)... Could the valve seat be making this noise?

Thanks,

Jen

That could be the valve head hitting the piston.

Jim

valve #3 stem was approx 1/4" shorter when measured with a straight edge compared to the other valve stems.

Engine is out of the bus, head has been removed.

guess what? The speedy answer that Speedy Jim gave me is correct.

The valve seat moved all around in the head. The knocking was the valve head hitting the piston.

The piston now has approximately

Makes me think either a)connecting rod has been worn b)bearings on connecting rod have been worn or c) bearings on crank have been worn.

Any ideas or suggestions on what to do about this? replace the piston? It isn't cracked. less than 1/8" deep gouge into it.

I am looking into getting the head rebuilt, or just buy a new one. Any suggestions for this?

Thanks to Speedy Jim!

Jen

Actually, the piston/rod may have come thru OK, just dented. (No guarantess...)

I vote for a new head; pressing/shrinking new seats in is beyond the ability of any normal shop.

Jim

Something (any ideas what?) caused the valve seat come out of where it was pressed into.

The head got chewed up a bit when the seat came out. The seat is dented from the valve not hitting it flush..

I have a milling machine that is computer controlled. I would like to machine a new valve seat.

I'll have to mill the receiving hole on the head to a larger diameter, because the edges are chewed up. Also make a new seat on the lathe, with large outer diameter to fit in the widened hole.

I looked around and found rebuilt heads for $189. Ne heads is $500.

This head with the bad valve doesn't have anything else wrong with it, and I think I may be able to repair it.

I'll give it a try while I research cheaper parts.

Jen

Overheating? Pinging? Just came to the end of it's natural lifespan? Makes me wonder if the other seats would soon follow. (get you worried yet?)

I would also inspect the valve guide carefully. Especially since you need it to be straight and tight when working on the seat, because you need to use the valve guide when lining up the seat and machining it.

Is there enough room to safely do that? Without risking cracks? You will need to know fitting tolerances.

Jan