I am going to be headed to collage in june and was wondering if there was a good carb for highway driving with 3.55 gears. Right now i have a holley 390 with the secondaries tied shut because it will bog when they open. I get about 14 mpg, i would like to get better mpg because i will be putting almost 10,000 miles on the dart next year. If i could do it over i would put a eddy 500 on it instead.

when i used to do alot of highway driving with my valiant (3.23 gears,holley 390) i would regularly see around 24mpg. sounds like you just need to re-jet your carb. i cant imagine that little difference in gearing is taking 10mpg from you.

My Dart has big cam, headers, ported head, oversized valves, 9.5:1 compression, low idle vacuum, 904, 355:1 Suregrip, 24.5â€￾ tall tires, and a Holey 390 that took a long time to dial in, that now gets 18.5 mpg. I have 512 jets, stock secondary metering block accelerator pump & orange cam, and short yellow secondary spring. No bog, slight lean feel on small throttle openings cruising at steady speed. This engine just begins to pull at 3500 rpm and gets its legs over 4500 rpm pulling hard to 6K+, and at those high rpm it is able to max out the 390's cfm delivery.

Install an o2 sensor, to aid in setting correct A/F mixture, and using a vacuum gage on dash to see how engine reacts to throttle opening, and different A/F mixtures. Too much transition slot exposed at idle will cause a bog, secondaries opening too soon will cause a bog, mal adjusted accelerator shot will cause a bog, wrong power valve & leaking power valve, and an engine with stock head, compression, and manifolds will cause problems with too large of a carburetor, and poor fuel economy.

This stuff may help:

http://www.holley.com/data/Products/Technical/199R8108-2rev2.pdf

http://holleytv.com/?cat=13

http://www.bob2000.com/carb.htm

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

What I would give to get 24 mpg! Right now I am running a comp cam with .440 lift and 264 duration. I milled the head .055 to bring it up to 8.6 static compression. I'm running .050 primary jets at 7200 feet above sea level, a 8.5 PV, orange cam, and I have no clue what is in there for the secondaries. I tried .046 jets in it and it leaned out way too much so I might try to get some .048s. I have the lightest secondary spring in there because I only have 10 pounds of vacuum at idle.

there's the clue...something isn't right with your cam phasing...unless you have a very radical cam or too big of a cam for less compression a small street cam or even a properly phased 270 degree cam with 10:1 compression should weigh in at 16-20" of vacc.

The 3.55's also aren't helping...since you don't have an overdrive gear to put the ratio into the 2.x range during highway cruise you will be limited in mpg at best tune on a good dialed in build to about 18-19 mpg tops (my 1973 Duster with the 10:1 motor, 3.55 and an A-230 manual 3 spd topped out at about 18.5 mpg like wjajr with the Holley 390, and my idle vacuum as 19".

This same combination was originally in my Feather Duster but with the A-833OD and would get tops of 24 mpg... (same set up at 9.2:1 CR and the comp cam 252 would top at 26-28 mpg depending on state of tune) once you figure out why your engine is too low on idle vacuum, next would be to change your gears to 2.94's for best highway use, or 3.21 as a compromise for some street pep...

I do live 7200 feet above seal level and from what I just read, vacuum should be between 19 and 21 unless your over 2000 feet. That said for every 1000 feet after 2000 feet to take 1" off. Well 7200 minus 2000 is 5000 so that means it's supposed to be between 12 and 16. Today I set the mixture with a vac gauge and I now have 11-11.5 which would probably improve with .048 jets.

I would be shooting for the best vacc. and anything less than 14 isn't going to get it...above that the vacuum pod is only going to give partial advance so you are losing essential economy timing. I suspect if you take it to college and the elevation changes you'll be recalibrating everything anyway, and you'll get to see if your vacc. reading changes as well.

Well I was hoping for that but I'm going from monument, CO which is at 7200 to laramie, wy which is also 7200 feet. I guess the only option which is the best option is to tune it. I put the light yellow sec spring in and it didn't bog when they opened. I'm shooting for at least 17 mpg.

I have dual 2bbl webers, 3.55 gears, a-904 auto and get 22 mpg highway. I think the folks telling you to change your jetting are likely onto something.

Ok I'll jump down to .048 jets. I was just wondering too, I saw on here a while back to remove the crush washer on the plugs. Is this a good thing to do, I feel like it wouldn't seal for $@!÷. Also how long should rtv hold up on the manifold? I've got about 9,000 miles on it already and it may be leaking.

where on the manifold? the carb to manifold? i dont think that would be a problem. manifold to head? surprisingly i had just pulled my manifolds, i rtv'd both intake and exhaust last time because i didnt have a fresh gasket and messed the one on there up a bit and the rtv was still good on both. i thought for sure the exhaust would have burned it up but it was still there and squishy.

If you have a drool tube head then the spark plugs will seal for $@!÷, otherwise nobody would recommend doing it. I have done it on many motors and is seals just fine. if you have a head without the spark plug tubes then you need to leave the crush washers in place.

RYV will hold up for a few minutes on an exhaust manifold, unless you use the super high temp variety. RTV will hold up on a non-overly hot part of the intake manifold for a while.

Manifold to head. I do have a head with the tubes. By removing the crush washers, what does it do? Does it make a better seal, is the reach further or both?

Yes, exactly.....read SL6 Dan's instructions and posts on this. Better seal....and it keeps from wrecking the tubes and creating leaks.

_________________
Aggressive Ted

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74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG