"and am venting the rocker cover directly
to both carbs via a tee-fitting with no emissions control valve."
Dee,
Two possibilities for you:
You didn't say exactly how you were connecting the hoses - some 150CD's have a
bung on
the right hand side of the carb for an emission hose. New rings allow a lot of
blow-by until the engine is broken in properly, this creates pressure in the
crankcase
that is then "pumped" into the carbs via the emissions hose routing. If you
have this
type of 150CD and you are venting this port direct to the crankcase via the
valve
cover, combined with a vaccuum-robbing long duration cam, you probably aren't
pulling
enough vaccuum to allow the air valve in the carb to function properly, which
may
account for your increase in rpm when you restrict the airflow.
Another possibility could be torn diaphragms in the throttle bypass valves.
These are
also on the right side of the carbs and are held in place with 3 small machine
screws. Once off the carb, three more screws open the two halves of the valve
to
expose the check valve and diaphragm assembly. Air constantly leaking around
the
throttle plates could also be the cause of your high idle condition.
Good Luck!!!
Brian Schlorff
Power British
http://www.powerbritish.com/~britcars
Jackson, Dee wrote:
> 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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