is it possible or normal for valves to go so far out of adjustment, after just 290 miles on a new engine,that the car backfires badly out the exhaust..
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I would say not normal, but certainly possible. For instance, if the nut isn't tightened-down enough. In general, they recommend a general bolt tightening after only a few miles on the entire car. After that, things settle in.
Could the rocker shaft on one side have come loose, thus altering the valve settings?
Entirely possible, not matter who builds the engine. These engines are 50+ year old technology. It's always a good idea to go over everything after 250 miles and change the oil. The oil will have fresh metal in it and some bolts will have loosened because the tolerances are nowhere close to a modern engine.
-=theron
" Is it possible or normal for valves to go so far out of adjustment, after just 290 miles on a new engine,that the car backfires badly out the exhaust..."
Normal? No. Possible? Yes.
Either the engine builder screwed up and didn't adjust the valves properly (and then lock down the lock nuts) or you have a significant valve problem or there is something else going on that you haven't yet found.
If the timing is set properly and the carbs don't have a clog problem (my first vote, since the car is new and the gas tank might be flaking inside), my vote would be for exhaust gaskets.....but it's hard to diagnose when you're not standing there listening to it.
If the exhaust gaskets test out OK, then look at the intake manifold gaskets, but I wouldn't implicate the ignition......
In all actuality, it's a ""75+"" year design
Tatra introduced the flat4 a/c engine 86 yrs ago in 1926. Karl Benz patented the boxer engine in 1896 around the same time VTwins came to be, which have been used by Harley in 1907. Versions of both obviously still popular. :-) thank goodness!
My comment was that the VW engine is a 75+ year old design, nothing to do with other manufacturers
No contest to your most correct comment Larry, my comment was just a comment, just a/c flat 4 trivia and the vtwin reference was relating to the longevity of this these time proven technologies.
So "lighten up Francis" :-)
tight valves can be caused by a lot of things, what kind of adjusters were used,stock ,swivel foot,ball bearing ford courier, valves caps used,incorrect pushrod type chromoly,aluminum? Sticking valve,rocker,tight cam follower,valve seat settling into the head. Or hopefully,just plain old incorrect valve adjustment.
With reference to older motor engineering, I was pleasantly surprised to learn of the relation ship of Olie Evinrude (outboard motors ) to Harley and Davidson way back when HD was just getting started.
It must have been a very interesting time...nothing to emulate, no internet for quick advice or reference, just some backyard ingenuity that became decades of iconic motor history.
Not sure who or what Francis has to do with this but If the "Lighten Up" statement was directed at me then there's no reason for me to lighten up, I'm not upset.
All I was doing was clarifying my previous statement by directing it towards Volkswagen and not other manufacturers.
My mistake Larry, have a fun and safe weekend, there are going to be some scary people on the roads today, lets all drive our Speedys defensively!
thanks for the Olie Harley connection David!I'll look it up.
Cheers!
I seemed to have developed a backfire as a result of all that turkey! 
It's the stuffing, you're Gluten intolerance.
Jerry,
It could be a gasket leaking between one of the intake ports and the heads.
update--valves have been adjusted-glass in line fuel filter added-jets have been cleaned out twice- exhaust backfire persists-idle races to 2000 at stops and wont kick down and now it wont go into 2nd gear-only 300 miles-did I buy a Chinese car from vintage speedsters
Jerry, where do you live?
north jersey on lk Hopatcong- cars at autohaus in peapack right now
Jerry, what size engine and what carbs, distributor etc.....
intake manifold leak.
2010 cc 40mm empis
Jerry,if you don't mind the drive,and are looking for a competent, qualified VW/Porsche Aircooled Specialist,Karl Konen in Mount Holly is my recommendation.I have known him for 40 years,and there isn't anyone else that I would have touch my car. He is an expert mechanic. Interested, contact me via dialog for his info.
he is about 90 miles away but send his info-thanks jerry
Foreign Car Services
54 Washington Street
Mt. Holly,NJ
(609)267-1515
Karl Konen-Owner