XJ12 vapor lock? i.e. Frustration.

[Dave Oxenreider <daveox@av-imagineering.com>]

OK, here's one for you V12 owners out there...

My V12 runs VERY strong...right up until about 4500rpm, then it promptly
shuts off, coming back to life some several seconds later and only after
coming completely off the throttle.  I added new electronic ignition,
made sure the tanks were venting properly to the charcoal canister, made
sure the bowl vents were doing the same and that the vacuum line going
to the side of the ZS carbs was fitted properly.  The PO added
aftermarket fuel pumps and two of those dial-a-pressure regulators. 
Adjusting them makes no difference.

I have found that, at idle, if I blow *very* lightly into the cross tube
that connects the bowl vents from the air cleaners, the motor
immediately shuts off.  So...I checked to see whether the vacuum line
that connects all four carbs together was drawing vacuum.  Nothing. 
Well, almost nothing.  There seems to be a very small amount of vacuum
which I figured would increase with engine speed, but it doesn't seem to
(under no load).  And, I figured at wide open throttle there's probably
no vacuum at all.  So...I figured maybe the fuel tanks were producing
pressure back through the canister and into the bowl vents and
preventing proper fuel level in the carbs.  So...I unhooked the bowl
vents and let them vent to air.  SAME PROBLEM!!!  HELP!!!  I wanted to
take her on a short trip to the coast tomorrow and I don't want this to
turn out to be something bigger than I thought.

The questions...

1.  Is it possible that the one-way return valve on the line back to the
tanks is weak and therefore not allowing enough pressure to the carbs? 
My TR6 had the exact same carbs and never had a return to the tank.

2.  Can someone with this year V12 check to see whether their large
vacuum line (the one that connects to the sides of all four carbs) has
strong vacuum at idle?

3.  What should the proper fuel line pressure be for the ZS carbs?

BTW, this problem seems to wane slightly with cooler outside temps.

Thanks folks.

(Thanks, also, to everyone who responded to my headlight and leaper
questions)

-- 
Dave Oxenreider
'73 XJ12
'75 TR6 

AVI's homepage http://www.av-imagineering.com
daveox@av-imagineering.com
76764.274@compuserve.com

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