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Mon Apr 16, 2012 7:38 pm

Just wondering what may cause valves to be loose. Just got back from a 400 miles round trip and adjusted the valves. Found #2 and #4 to be loose. She ran great on the way there but on the way back seemed to run hotter (so says the cht gauge) and seemed to have a slight but noticeable loss of power. Also developed a slight backfire every now and then. I suspected tight valves not loose. I haven't driven it since but will see how she drives in the morning. Any thoughts?

Mon Apr 16, 2012 7:41 pm

Wear on the valve stem and adjusting screw are common if made from cheap metal.

Mon Apr 16, 2012 7:44 pm

Would that cause these symptoms?

Mon Apr 16, 2012 7:53 pm

Cheap parts are cheap parts. A lot of aftermarket stuff is just not up to OM standards.

Mon Apr 16, 2012 7:57 pm

I had a pushrod that I thought was fully in the rocker cup when I adjusted my valves, but I was wrong. It popped in once the engine was started and the adjustment went out.

And, as WT said, spalling on the adjusters or valve stems results in a temporary adjustment only. I've had that issue too.

Mon Apr 16, 2012 9:04 pm

the can be loose for several reasons already mentioned. Another is you got them wrong - or nuts holding the shaft can be backing off. Also you did not say what you considered loose means. .0005 change on 500 miles could be simply how you hold the feeler gauge whereas for example going from .006 to .015 would be really significant. Backfiring can also be the head nuts working loose and sucking in air between the cylinder and head which can cause valve clearances to increase.

Tue Apr 17, 2012 6:17 am

As I mentinoed, I adjusted the valves last night and then drove the bus to work this morning. Definitely seemed to be running better, but not really a long enough drive to get a trend on cht's. I always adjust the valves to .006. Last night when I checked them the ones that were loose were probably at .0075 (guessing). It had been at least 1000 miles since the previous adjustment but less than 2500 miles for sure.

I was still able to get a backfire when downshifting into third at decent speed. Doesn't seem to be an exhaust leak. I've dealt with those in the past and almost always notice some extra noise from the exhaust in addition to the backfiring.

The engine was a rebuild and installed when we finished our restoration about five years ago. We've probably put about 20,000 miles on the engine since then...maybe. Is it common for the head nuts to work loose after that amount of time? I would have thought it would have happened earlier on like during the break in period.

Tue Apr 17, 2012 8:01 am

they seem to work loose oevr time. I used to retorque my head nuts any time the engine was out of the car. One can also buy the special locking head nuts.

My guess is that it is time to retorque the rocker shaft nuts and then replace the lifter screws. Look at the ends of the valves to see if any are being worn abnormally.

Tue Apr 17, 2012 8:03 am

The valves seem to vary by 1 or 2 thousandths from adjustment to adjustment, sometimes tighter, sometimes looser. Maye two to four valves will be off by that amount. Don't know why, that is just the way it is.

Tue Apr 17, 2012 8:10 am

I think I'm going to keep a log here over the next couple thousand miles to see if any kind of trend develops. If things appear to get worse I will probably go ahead and yank the engine and retorque the heads and rocker arms and such. I'll probably be looking at a clutch replacement by then anyway, so at least I could accomplish a couple things while the engine is out.

Tue Apr 17, 2012 11:01 am

jpeters wrote: I think I'm going to keep a log here over the next couple thousand miles to see if any kind of trend develops. If things appear to get worse I will probably go ahead and yank the engine and retorque the heads and rocker arms and such. I'll probably be looking at a clutch replacement by then anyway, so at least I could accomplish a couple things while the engine is out.

Keeping a log is the best way. I will mark in the log .006 tight, .006 and .006 loose. That way if one is close but tight (.006T) and the next time .005L I don't worry too much. They do vary a tiny bit - just the spot on the lobe can change it a tiny bit.