[TR] TR4A mid RPM miss update

[Dave Connitt]

About a week ago I posted a message about my recently restored TR4A developing
a mid range misfire.
The symptoms were that the engine idles perfectly and runs strong and steady
as long as I keep the RPM's under about 2,500 RPM. I can run through the gears
and get the engine up to 4,000 RPM and it runs absolutely perfect. If I drive
for more than a few minutes over 2,500 RPM's it develops a misfire and there
is a distinct clicking sound from under the hood. If I push in the clutch and
let the engine idle, it goes away until I get the engine over 2,500 RPM. I
suppose if I never went above 40 MPH that would be fine.
>From what I have been told by a few people the problem has to do with the
bronze valve guides I had installed along with the hardened valve seats. They
think there isn't enough clearance between the valve and guide. I talked to
the guy who did the engine machining and he doesn't think it's a valve guide
issue. His thought is that if the problem was a sticking valve it would stay
stuck! Two other machinist's I talked to think at least one of the valve
guides is a bit too tight. One of them thinks that the clearance on at least
on valve guide is just slightly on the tight side. He suggested I put a bottle
of Marvel Mystery oil in the crankcase to help lubricate the valve guides. I
tried that and I think it might have been a little worse if anything. One
other thing is that I had changed the oil  a week or so ago and the oil looked
great.
Here is what I have done so far..
First, I removed the rocker arm shaft thinking I may have a broken valve
spring but they all looked good. I also wasn't able to find any evidence of
excessive heat or galling of anything. I didn't see any unusual witness marks
or anything like that. There was plenty of oil sitting on the head and when I
moved one of the rocker arms I could see oil seeping out of the small oil hole
in it so there is plenty of oil up there. Speaking of oil, I am running Brad
Penn 20-50 weight oil.
Today I drove it to the local Cars and Coffee (not on the freeway) and it ran
fine to and from keeping the speed about 40 MPH. I met a friend there who owns
a TR-6 and is a pretty good aviation mechanic/pilot and he thought I should
check the timing so I did. The initial timing was set at 10 BTDC and total
advance all in is 20 degrees. I backed the initial timing to 5 degrees BTDC
but that didn't help.
I am just about resigned to the fact that I have to remove the head and get it
checked out. I found another machine shop who comes highly recommended and can
turn the head around in a week. While I have the head off, I am considering
getting some material milled off since I have a BPNW 270 degree camshaft. I
had .005" removed just to get it flat but am thinking that raising the
compression a bit will help the cam. Any suggestions from people with this cam
regarding milling?
Well, I have put it off enough.. I am off to the garage to start removing the
head.
Crap...
Dave Connitt