In a message dated 96-09-01 00:53:06 EDT, bleckstein@monmouth.com (Mike)
writes:
<< For over a year the TF has a strange problem. After redoing the head ( I
had a burned valve and wanted a lead free head) the engine would break up on
long straight runs of 4000 rpm when you increased the gas to make slight
rises in the road. ..... What happens is a suuden loss of power and a chainy
pinging noise. >>
I hate to be the bearer of bad news, but I had almost exactly this same
symptoms with my MGA 1500 about a year ago. I had just had a no-lead head
job done, and the car ran great until I pushed it hard when it was hot. I
first noticed it during an autocross with a particularly fast and long run.
First thoroughly warmed up, then run very hard for about a minute, at about
4000 rpm it would start tapping and missing on one cylinder.
The short story is that it turned out to be a sticky valve caused by tight
valve guides. The shop had just pressed in the new silicone-bronze valve
guides and reassembled everything without checking clearances. The new
guides have a very close fit with the valve stems, and pressing them into the
head sometimes shrinks the bore a bit. Any new guides should be checked for
size after installation and reamed or honed to the correct clearance if
necessary before final assembly.
My first try for a fix was to run the crap out of the car for about 1000
miles in the next week before the next race date. It did help to loosen
things up a little, ran a lot better on the street but still hung up badly
when racing. Having seriously lost the second race event, I knew I had to
pull the head. After removing the valve springs I found that nearly all the
valves were pretty snug in the guides, hard to move and to remove with my
fingers. One valve in particular was so tight in the guide that it took a
punch and hammer to remove it.
The solution was to hone the guides. I did this by cutting the head off of
on old valve, chucking the valve stem in an electric hand drill, applying a
little valve grinding past to the valve stem and having at it down the guides
until the valve stems would fit and move freely. This took less than one
minute of honing for each guide, except the really tight one that needed
about three minutes of honing. When the valves would move freely up and down
with the slightest pressure of the finger tips, I thoroughly cleaned and
reassembled everything.
The good news is that the problem has been banished. After another 20,000
miles, including a lot of trailer hauling and autocrossing, the problem has
not reoccured. It cost me for a new head gasket and about four hours work,
but if you have to you do.
Barney Gaylord -- 1958 MGA
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