Hello!. 35000 miles on a professional rebuilt 1970 429. Dove heads. At about 20 000 miles after rebuild was having the occasional stumble always at around 3000 rpm. Thought it was a fuelling issue. Got worse and finally loud noise from valve cover. Rocker arm let loose? Anyways took it to a recommended mechanic . Turns out was a sticky valve. Got heads rebuilt. Acted up again. They then honed out the valve guides. Today with about 1500 miles on the rebuilt heads im getting the symptoms again. A bit of a miss at about 3000 rpm. It acts funny for about 20 seconds then runs fine. Same as before. I dumped about a half quart of marvel mystery oil in the pan. And a few ounces in the gas tank. I took a temp reading today after the symptoms started. About 200 degrees on the heads and 170 at the thermostat. Apparently excess heat can cause a sticky valve and/or deposits in the valve guides. Thats why I'm trying the marvel. Common sense says take it back to the shop. But for many reasons I no longer trust them. Any thoughts? Thanks!

 

Stale gas can cause this on an intake valve. I've seen it happen lots of times and the valve stems will be so sticky that they will hang the valve in the guide. Lacquer thinner will dissolve the varnish. Right now I've got a '55 Chevy with a 235 that has two intake valves that were bent from this very thing.

 

Put some miles on it and the guides will loosen up some
Keep the mystery oil in it until the problem goes away and you can use some trans fluid as well (does the same thing)(cleans out the varnish)
Good luck

 

Thanks. Yes I have about 100 miles on it now . No issues. The real test is when I pull a small cargo trailer. Its light but acts like a sail.

 

Knock on wood. About 1000 miles pulling cargo trailer. Working great. Marvel Mystery Oil rules!

 

I Run that Marvel Mystery Oil thru my air tools mixed with trans fluid 50/50
Smells great
Tools last forever