I installed triple 40DCOEs on my GT6 about 10 or 11 years ago. This was
a bone stock engine, later I added a Crane XR-700 ign. There was no
reversion - there was NO FUEL SMELL an hour or two after shutdown. Six
years ago I replaced the 2.0L with a warmed over TR6 2.5L engine (9.3:1
CR, minor valve pocket/gasket match porting, headers, AL flywheel and a
270 deg cam from BPNW). SINCE THEN, my carbs display the so called
"Weber reversion".
My initial stab at solving the reversion was assuming that fuel was
boiling out of the carbs due to elevated underhood temperature after the
car was parked. To that end I lowered the float height by 3/16 in.,
installed a 1/2 in. phenolic spacer between the carbs and the intake
manifold, installed an insulated heat shield under the carbs/air
cleaners and had the headers ceramic coated. Net result was a
fractional improvement (10 to 15% at most).
My current thought is that since the reversion started after the engine
swap (w/270 cam), that the cam / Weber interaction is the probable
cause. The cam was described as having an asymmetrical profile with
short duration opening/long duration closing on intake & long duration
opening/short duration closing on the exh. THere was no traditional cam
plot on the spec sheet so I have no idea as to the overlap on this cam.
I am assuming that there is too much overlap. BTW, the engine idles
well w/out loping.
I am willing to give up some performance in order to have a car that I
can park in the garage immediately after arriving home - as opposed to
letting it sit outside for a few hours to outgas (sometimes I forget to
put the car away, once or twice it has rained and it sucks to lay towels
on a wet seat the next morning to drive to work). I am not willing to
replace the Webers - I put considerable effort into bumping out the
bonnet for carb clearance. Naturally I returned my original GT6 cam
years ago for the core refund so I can not do a "simple" swap and see if
my hypothesis is correct. And since a cam replacement is not a simple
endeavor and the engine only has 25K miles on it I am not anxious to
pull the head if I do not need to. I spoke with BPNW to find out if
there was a lesser performance cam. The 270 is the mildest performance
cam that they sell. They suggested considering a TR250 cam regrind -
according to them this was the most performance oriented stock cam in
the TR vehicles exported to the US. I have acquired a stock TR6 cam of
unknown vintage but it has a pitted lobe and is not useable as is.
A final thought is that I could simply back off on the rocker adjustment
a few thou reducing valve lift and duration at the expense of increased
valve clatter. This thought came up during my phone discussion with
BPNW, wish I had thought of this during the driving season and tried it
out last fall.
Your thoughts?
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