The increased top-end you describe is due to the Rochester's ability to flow 165cfm compared to the Zenith's 137 or so. If you are running the stock oil bath, you may get even a bit more responsiveness and top-end by going to an aftermarket (K&N) paper or cloth air filter.
Also, there is a vacuum port on the bottom of the Rochester that should not be blocked by the gasket, else the carb thinks, due to the lack of vacuum, that you have the accelerator floored, activating an enrichment circuit, which flows additional fuel into the venturri. This may be the cause of your low mileage figures. In my experience, Rochesters deliver a range between 13-16 mpg depending on habits and in-town or hiway miles. You may need to cut the gasket to expose that port. Rochesters have gotten some unfair press for running rich and washing down cylinder walls w/excess fuel simply due to the installer's lack of knowledge with regard to that port.
Regards,
JT/ric
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
John Baker said:
The other day I was travelling back from central Bangkok (ugh) on an elevated
expressway and found that on a level stretch with nil or very slight tail wind
my Rochester-equiped 109 could do 120 km/hr. With the Zenith I had installed
my max speed was 100 kms/hr. I have a 7:0 head and was told not to expect any
improvement in speed. Perhaps 120 is normal and my old carb was that bad, I
don't know, but at any rate I like the results. My mileage is exactly the
same, a dismal 11.25 mpg, but much of my driving is stop and go so perhaps
highway would be a bit better. I may be running a tad rich (51jet) so I will
be experimenting with leaner jets some day. I have an Ashcroft and I am happy
to cruise at 90-100, but it is nice to know there is some more oomph there if
needed.
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