Expert: jack bahm - 7/22/2010

Question
I installed a rebuilt "touring" engine in my '29 last october. It runs well around town, but about 100 yards after I enter the freeway it starts to lose power, then miss and then a loud tapping begins. This happens at about 35-40 mph. If I slow down it will return to normal after another 100-200 yards.  I have now replaced intake/exhaust manifolds and carb as well as distributor from another running car. the problem is still there and maybe even getting worse. The builder of the engine does not want to admit it is a valve, but at this point I can't think of anything else to change. I really don't think it is fuel or spark. Compression is good. I will probably put the car on a trailer and deliver it back to the machine shop so they can experience the problem. Any thoughts before I do this? how can I completely rule out fuel or spark other than replace the parts like I did?
Thank you,
David Denning (Sonoma As)

Answer
Wow, that is a tough one.  The noise could be piston slap, but in a new engine?????  That would not cause it to lose power.  If it is timed properly, you should be able to tell.  With the spark up, it should cackle, and lope a little with it down, and  at idle.  My guess is that it is related to the tappets and the valve clearance.  Something is going on with the tappets.  What ever kind they put in.  
Take the fuel line off the carb and make sure the ferrule is not set back too far from the end of the tube.  If it is too far back, the end of the tube will be jammed against the little screen filter and restrict fuel flow.  Cut the end off a bit if it is too far into the carb.  This condition would let the engine run good until you put load on it, then it will not get enough fuel.  Could be your problem, and very easy to check and fix.  The engine shop could take the valve cover off and do a little checking of the tappets.
Good luck.  Now I am worried.  I have an engine builder doing a new engine for me.  I don't need your grief!!!!!
Let me know what ti turns out to be.
jack