I've got a question for all of you carb gods,
I placed a set of dual HIF4s (from a 72 MGB) on my spitfire. I finally was
able to balance and tune them (using the directions from the VTR on tuning
SU and ZS carbs). And once I found that there was a vaccuum leak on the
vaccum advance (which I fixed using heat shrink tubing on the leaky end).
Now that I have the car running, I adjusted them so the car'd idle at
~800 to 1000 rpms, but every few minutes the idle all of a sudden goes up to
around 2000rpms!
The carbs also become "rich"...so now when I push-up on the dashpot piston
lifting pin, the car's rpms increase even further.
If you give it another minute or 2 (just stare at it intently), it drops
back down again to 1000 rpms, and the Unisyn carb tool continues to say the
carbs are equal. And it appears to be cyclical..happening in short
intervals.
At first it would seem as though it would increase RPMs when the
electronic fan kicked on (which is a push fan that pushes the air toward the
block).
One of the problems with the HIF4's, from what I understand is that if the
float chamber becomes excessively hot, the bi-metal plate (which levers the
jet via the mixture screw) will flex and alter the fuel jet to compensate.
Does anyone know if the bimetal plate opens the fuel jet or closes it?
I'm thinking the heat coming off of the radiator and the header might be
the problem.
Finally, there is some throttle shaft vacuum leakage (if I spray some carb
cleaner down where the throttle shaft assembly peaks out of the carb
housing, the idle will drop). I pressed the throttle shaft gaskets firmly
into the throttle shaft, which reduced the leakage a good bit. Could that
vacuum leak be the problem? (I know, I should have replaced the throttle
shafts and disks anyhow, but they looked fine!)
I'm starting to feel like this car is never going to run right again!
-Terry L. Thompson
'76 Spitfire 1500 (Man that monza exhaust is loud!)
Maryland
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