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Old 02-08-2011, 09:46 AM
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RicardoJM RicardoJM is offline
Rising Sun Member
Join Date: Jul 2007
Location: Centennial, CO
Posts: 2,396

Originally Posted by subzali View Post
Have you been tracking your gas mileage since you did the rebuild?
Yes, I keep a log book and calculate mileage with each fill up. After the rebuild, I average 12-14 mpg. Up until recently, I was not running vacuum advance. As soon as I started running vacuum advance, I had to deal with the cracked block and just after getting the rebuilt engine in my odometer stopped working. I got it fixed this past weekend.

I'm looking forward to seeing what my mileage is now. Then, I am going to increase my jet size.

Originally Posted by subzali View Post
Also, I've been reading that there is a 38/38 and a 38/40. Is the 38/38 the 2 barrel carb for F engines while the 38/40 is the 2 barrel carb for 2F engines? Do you happen to know the venturi size difference between the two? I think on a 38/40 the venturis are 31/35. And from Ed Cook's writeup I think the 38/38 has 28/31 venturis. Do those numbers flange up? I think I'm going to tackle my carb at some point and figure out what I have. I know it's a '78 Federal spec, just don't know how it's jetted and if everything is working properly. How is your secondary actuated? Is it linkage actuated?

My research indicates the DD38 was used from 69 through 74. After that, I have not really paid much attention - but it would make sense that the carb specifications (jets, venturi size) would have changed with the "more powerful" 2F.

I'm not sure I understand your question about the "numbers flange up".

My secondary is mechanically actuated. I can tweak the linkage arm to get it to start opening earlier or later. I have not really worked with it to see how adjusting it would affect performance.

I am not a carb guru/expert but have come to understand through experience that the optimum setup (tuned for power or tuned for mileage) is something that will vary for each truck.
Ricardo Maestas - Bio Page
1999 LX 470
SOLD - 1971 FJ-40
TLCA #18941
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