Early in August, I wrote about having changed my carburetor in my 1986 F150 with 4.9L and Duraspark conversion. The Duraspark conversion had been done previously and I was changing the carburetor [1982 Carter] due to a worn throttle shaft. The "new" carburetor is a factory remanufactured Carter YF, originally specified for a 1970 F350. THe previous thread was about having difficulty with the vacuum advance, immediately after the change, even though nothing else was adjusted??? I finished the thread with reported MPG, while driving on a 400 mile round trip which was almost all 55-60 mph with hilly terrain.

The general response was that the mileage was very good [20.54 out and 21.62 return], but would have been better with the vacuum advance working. I agree and everything I have ever read supports this.

Well, the plan was to replace the vacuum advance cannister, but things got in the way and before I knew it, I had filled up two times. I normally drive ~200 miles before filling up. My daily commute to work is 26 miles of urban type driving. Maximum speed of 45-50 with many stop signs and residential areas. [Quite nice, actually!] The 1st tank returned 17.29 mpg and the 2nd 17.40, so I thought, "do I really want to change this?" These two were followed by 17.31, 17.24 and yesterday was 17.82! The average for these 5 tanks is 17.41 mpg [w/o vacuum advance].

I record every tank, so I pulled out the records and went back 5 tanks prior to the carburetor change and found: 17.34, 15.77, 16.73, 16.39 and 15.95 mpg for an average of 16.44 mpg [with vacuum advance].

I am the sole driver, the basic routes are the same, the weather has been similar and the mechanics are the same except for the carburetor change.

I KNOW the vacuum advance is supposed to be hooked up and that it should perform better and get improved mileage... but???

What are your thoughts?

By the way, the timing was set by using a vacuum gauge, some time ago, not with this carburetor change. At idle @ 650-700 rpm, the engine can pull right @ 21", but was adjusted to 18.5". The elevation here is ~550'.

 

That is excellent mileage! Sure wish I could get that.
So, looks like with advance and bad carb you got ~1mpg less, but have you got any numbers with advance AND new carb or did I miss read it?

I've been debating doing the DSII and carb changeover myself. I have the TFI running great but still not very good mileage. So for the records can I ask what trans/rear gears/tire size are you running?

 

Answer to your 1st question... I am still running without vacuum advance.

4 speed manual OD [SROD]; 3.08 LS and 215/75X15 Michelin LTX M/S

 

Thanks for the reply. Just wondering if you wouldn't be back up to the 17.4mpg, or better, with the vacuum working. Could always unhook it if it hurts anything but really think it will help.

Thanks also for the specs on your truck. That's what I wanted to know. We have really have close to the final drive ratio, however I have the 4od/3,55 and 31's, but close to same final drive. But mine is only doing 13-15mpg. So 1 more point for the DSII swap.

 

Might as well try it with the vacuum advance. I used to get 21 with mine. You may jump into that boat with it.

 

You are getting good mpg now, now doubt. But I'm with AbandonedBronco - I'll bet you can get a few more mpg with vacuum advance. My '86 4.9L has 3.08 rear and 4 speed OD manual transmission like yours, 235 75 R15 tires and stock EEC-IV feedback ignition. It gets around 18 mpg.

All other things equal, advancing ignition timing with engine speed should increase your efficiency. You can always remove and cap the vacuum advance hose if you find otherwise!

 

Guess I just thought my 85 EEC-IV with feedback card was running great! Found and fixed all the vacuum leaks and runs smooth but can't even get close to that.

 

Your 3.55 rear and 31x10.5's probably account for the difference in mpg between our trucks. Mine has 3.08 rear and tires that are ~ 2" less in diameter and width as yours. That is a lot more unsprung weight and road friction.

 

That is a REAL good possibility. Plus there A/T and not road tread tires.