Fellow Triumph-types,
I'm in need of your sage advice. A little background: I've been
putting a GT6 Mk3 back together. The motor (from a GT6+) was seized
when I got it as a result of the head having been removed for a valve
job and not replaced for 15 years. I'll spare you the details of the
motor rebuild (a long story), but I finally got the parts back from the
machinist and put it back together. Things went pretty smoothly
considering this my first attempt at rebuilding a motor. Yes, I'm a
motor virgin. ;-)
The machinist made a few performance mods on the head (port matching,
removing the rough stuff in the exhaust ports and lightly unshrouded the
valves) and took off .010 to make it flat. I put in a hotter cam (.415
lift, 270 duration) and a header and am running the stock CD150 carbs.
On starting it up for the first time, it ran pretty roughly, which I
expected, but I couldn't get it to idle at anything below 1500 rpm.
Thinking I had a vacuum leak somewhere I began spraying carb cleaner to
try to find it. Spraying around the throttle spindles yielded a
noticeable increase in rpm, so I sent them off to Apple Hydraulics for a
complete rebuild. They came back in a week (and look great), but I'm
still having a problem getting an acceptable idle. I was told by the
cam supplier that I should be able to have a smooth idle at 900 rpm. I
can't get close to that, it's more like 1200 and it hunts at idle.
In trying to dial in the carbs, I've turned the needles to the max rich
position, but if the throats of the carbs are partially blocked with my
hands there is an increase in rpm. Lifting the pistons 1/16 inch causes
the engine to die. Putting a Unisyn over the throats also causes an
increase in rpm. I think this is a case of still being too lean.
At this point, I have checked the points gap, verified the timing with a
strobe light, made sure the cam was set at the right point (#1 intake
max open @112 degrees ATDC) and sprayed carb cleaner around the intake
manifold to check for leaks (can't find any). I've plugged the vacuum
advance port on the rear carb and am venting the rocker cover directly
to both carbs via a tee-fitting with no emissions control valve. My
plans this weekend are to take off the intake and header and look at the
gasket to see if there is any sign of a leak there and check the valve
clearances again, assuming there aren't too many honey-dos from the
wife.
Is it typical to have to run smaller needles to account for the cam and
header, or am I a classic case of a little knowledge being a dangerous
thing, or both? Have I missed anything?
Thanks for any advice you can offer.
D.
Spit FM90204UO
GT6 KF20632U
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