12 mpg Help!!

[dammitmitchell]

mine gets 12 mpg hwy....
simple question have you vaccum tuned your carburator????
that made a huge difference on my truck

[88 Dippy]

also mine has a overdrive transmission and that helps alot on for the milage when i drove in "3" by mistake for the first week i had the truck (the pointer is off) i got 11 mpg

[OrangeAnimal]

I vacuum tuned the carb but I only did the idle so I don't see what difference it would make when you are cruising.

[Desert1957]

Originally Posted by OrangeAnimal

I drove my truck this weekend to houston and back. About 220 miles round trip. Burned almost 2 tanks of gas. 16 gallon tank. The engine is pretty much stock. 350 4 bolt main, RV cam (wish I had specs) Flat top pistons, 1.94/1.50 valves stock heads. Edelbrock Performer EPS intake, Edlebrock Performer Carb. Timing is set base 8*, Vacuum Advance hooked to full vacuum limited to 14* at idle, My full mechanical timing is 32* My full vacuum timing no load is 42* degrees. I spent alot of time working on it and have not gotten the mileage any better. My rear end is a 3.07 open diff. I am lowered 5/7 running 20's with a 28 inch total tire diameter. At 65 mph I am running 2300 rpm could this be the culprit? I didn't think is was too high rpm.

I Figured this thing would get better mileage than this. I could be wrong though. I guess it could be a jetting issue as well. It's a 1406 carb. Anyone have a good starting point for the jetting or should I ditch this for the Fuel injection if I am looking for mileage? I also noticed one other issue. It is too rich once it's warmed up good floods easy when try to start it. I hadn't ever had this problem b4 the trip to Houston. Any thoughts or ideas would be great.

Ok, Here is my info or (.02 cents) I have 1987 heavy 1/2 ton , With "TBI" which is the old style fuel injection on top of a 1992 350 , Edelbrock TBI intake , Comp-Cams performance recomended cam for this engine , K&N filter , 2 1/4 Comlpete dual exhaust with cats , performance/towing computer Chip , fresh Turbo 400 (3 speed) , 2.73 rear with 31 x 10.5 x 15 tires.

I changed the 3.73 rear when gas went over $4.00 per gallon a while back. Now I know Throttle Body Injection is ALOT more gas friendly then Carbs but the mileage went from 15 to a best of 19.7 best. Here is the problem , the truck is now a complete STONE! , I mean DEAD until about 30 Mph . Its now a HIGHWAY RACER , anyway I think you will get better with more miles on the engine , but I would still check that carb out . You mentioned something about running rich at speed , that's no good , after all that highway driving tailpipes should not be super black inside , could be too much fuel pressure causing it to load up or flood , maybe the float level is also to set high, also recurve your distributor so the advance comes in sooner.

I would love to hear the more about the Camshaft (post Cam Card) if you can obtain one , if it has to much duration/overlap the engine may have a weak vacum signal at cruising speed , which causes the carburetor to dump more fuel in through the power enrichment circuit , remember at 2300 rpm its possible the secondaries could be partially opened causing even more fuel to be used , more timing in earlier can help this if its the problem. Less gear ratio like I did=Less RPM or a taller tire.(31")

Sorry about getting to technical , but I have been through this with my Hot Rod ,1406 Edelbrock performer carb on another 350 , with too much camshaft. I fixed that problem by leaning the carb down , more ignition timing coming in earlier , Motor responded well less lazy and the mileage went up 3 mpg. even with 3.73 gears...

Also My truck gets WAY better mileage at 55 vs 65 , try it you will see and after we fix your problem.

Desert

[OrangeAnimal]

Desert1957, On your issue with the truck being "dead" before 30mph was this as soon as you installed the TBI unit? Also who calibrated the chip? Alot of those chips aren't correct and if you give a company like Howell EFI the info on your setup they can create a chip that will work for you. I am going to switch to fuel injection. I just don't know when I will get around to it. Thanks for all the info.

[3r!c84]

no i think it was when he put that gear in ^^^^^

[Desert1957]

Sorry,

Didn't mean to create any confusion, My truck came with "TBI" its an 87 model with all factory throttle body , only change to the Injection was a (305 to 350) throttle body upgrade.

No the Gear swap is what killed it , I knew it was going to be bad. Don't get me wrong if you drive it normally , which I do most times its fine. But when I want to hear my "Flowmaster's" a little , just takes longer to get to the sweet spot! As is now I would not even consider trying to use it as a back-up to my other truck (2004 Silverado Z71) to tow my Enclosed Car Trailer unless the gears go back in. I plan to reinstall them soon now that gas has gone back down to around $2.00

Desert

[Desert1957]

the Chip was a Ebay find , Here is his Ebay link..
http://cgi.ebay.com/ebaymotors/HOT-C...1%7C240%3A1318

Nice guy to deal with , he will customize anything you want , Power / Towing / GAS MILEAGE Chip , this was the add for the power chip , but he will do what you want , email him and ask questions.

Desert

[OrangeAnimal]

Well I am going to get a wideband o2 and start tuning the truck for now and save for a MPFI setup. I want something that can be easily adapted to make more power later if I change cams and heads. So for now the carb will just have to be tuned. I have used the Innovate WideBand o2 before and have alot of experience tuning. I built a turbo civic about 2 years ago and it ran high 12's. I would rather tune FI but I can figure the carb out. It's more time to get it right since I have to take the top off the carb to change things every time. I am going to data log the info and hopefully I can get it just right. Stay Tuned! (pun intended) LOL

[dammitmitchell]

well if you really really want i can trade you my truck for a week or two..
i know it's a definate change but that subaru would be a nice break for me, and you could still drive a truck evey day
=-)
he he he sorry.. im a subiejunkie too...

[OrangeAnimal]

Yeah I appreciate the offer but I would cut off my left nut before I would let anyone other than my wife or myself drive the Subaru. I don't even like to take it for service anywhere. I do most of the work myself. I have had it for over a year and only put about 4000 miles on it.

I love the STi it's the most bad ass ride. The only thing I would trade it for is a Vette. I can beat up on just about anything on the road. It's tuned and runs 22psi. It hauls serious ass. One day I might take it to the track but for now it spends most of it's days in the driveway. Here is a picture to drool on.

This pic was taken when I first got it. Now it's COBB Tuned and Has Perrin full exhaust and air intake.

[stepside-fm]

try a holley fuel injection ,just the 2 barrel 670 cfm, I have an 81 -4wd 4 spd with 273 gears and i get 22mpg (imperial gallon) at 70 mph. 350 engine ,rv cam and performer intake.

[MylilBowTie]

Imperial mpg when converted to US mpg is 18.31 US mpg vs Imperial mpg at 22 mpg.

[88 Dippy]

18.31 isn't bad at all.

[MylilBowTie]

I would be really happy with 18 mpg. It wasn't a jab at anyone. Just showing what the difference is between imperial and US measurements.

[88 Dippy]

Originally Posted by MylilBowTie

I would be really happy with 18 mpg. It wasn't a jab at anyone. Just showing what the difference is between imperial and US measurements.

wasn't ment to eather just saying that aint bad

[OrangeAnimal]

Thats pretty good considering all the issues I am having with the carb. I may give it a shot. Still weighing my options.

[TexasJeff]

I get between 10 and 13 mpg on my stock 350, 350 trans, 3.73 gear. I think the key to the better gas mileage on these trucks is the over drive tranny.

[OrangeAnimal]

Yeah the od does help if you drive on the highway but I would imagine his in town mileage is better as well.

[Adams74Chevy]

My dad warned me when I tagged his old truck: "It gets 10mpg uphill, downhill, flat, empty, or loaded" and he was right...

[3r!c84]

i was geting 11-12mpg on the stock gear now i just put in a 3.43 gear what do you think that will do?

[piecesparts]

Originally Posted by OrangeAnimal

I drove my truck this weekend to houston and back. About 220 miles round trip. Burned almost 2 tanks of gas. 16 gallon tank. The engine is pretty much stock. 350 4 bolt main, RV cam (wish I had specs) Flat top pistons, 1.94/1.50 valves stock heads. Edelbrock Performer EPS intake, Edlebrock Performer Carb. Timing is set base 8*, Vacuum Advance hooked to full vacuum limited to 14* at idle, My full mechanical timing is 32* My full vacuum timing no load is 42* degrees. I spent alot of time working on it and have not gotten the mileage any better. My rear end is a 3.07 open diff. I am lowered 5/7 running 20's with a 28 inch total tire diameter. At 65 mph I am running 2300 rpm could this be the culprit? I didn't think is was too high rpm.


I Figured this thing would get better mileage than this. I could be wrong though. I guess it could be a jetting issue as well. It's a 1406 carb. Anyone have a good starting point for the jetting or should I ditch this for the Fuel injection if I am looking for mileage? I also noticed one other issue. It is too rich once it's warmed up good floods easy when try to start it. I hadn't ever had this problem b4 the trip to Houston. Any thoughts or ideas would be great.

I wonder what you have for a tranny in this combo. I would gather by your RPMs at 65 MPH that it is a standard TH350. The 28 inch tire and the 3.07 gears should give you a decent mileage, but the tranny without an Overdrive is what is hurting you some. I have a 383 stroker engine, running an Edelbrock 650 AVS carb, a 700 R4 tranny, 3.73 gears, and 28" rear tires and I run at 2500 RPMs at 70 to 72 MPH. I get 14.5 to 15 MPG on the highway and it drops some for the city, since I can't keep my fat foot out of the carb. My truck does not know what the speed of 65 MPH is anymore---it is just a number that it passes through on the interstate on-ramp.

Also taking the stock belt driven fan off of the truck and going to electric fans was good for some mileage gains.

Now for the REST of the story. My son's truck had a 350/700R4 tranny, with 3.08 gears and a set of 26" tall tires. He was getting crappy mileage, due to the RPMs being too low for the truck to handle hills effectively. We changed the rear end gears to a set of 3.73s and gained almost 3 MPG. The gearing took the truck from 2800 RPM at 65 MPH to 2300 at 65 MPH. The truck now gets 13 to 14 MPG. These old bricks want some RPMs to keep the momentum up for handling hills and grades. I think, that you could increase your timing to 12 degrees max out timing at 32 degrees TOTAL advance along with a set of needles and jets for your carb, you could make it decent for a carbed engine. I caution everyone that thinks the lower RPMs will always get you better mileage---it depends on where you are diriving. I live in the Flint Hills of Kansas and that is not easy on a low RPM vehicle. Look for the optimium driving rpm in your engine.

[OrangeAnimal]

Yes I do still have the TH350 trans and even though it's a lock up trans it's not hooked up I am missing the stuff to hook it up.

I may have helped out my mileage issue today though found some booboo's in the timing. I won't know till I am done tuning some more. I need to figure out the stumble coming off of idle. Probably either not enough or too much stroke on the accelerator pump.

I also ditched the clutch fan for electric. So we will see how it goes.

[3r!c84]

how much is a 700r4 tranny cost?used ,rebuilt?
and will it bolt right in place of a th350?
same drive shaft?

[OrangeAnimal]

I don't believe that the 700r4 is a direct swap due to the length of the trans. I think IIRC the 200r4 is. THe 200r4 is a great trans too. Overlooked by many but they can be built to handle 800hp. I believe either one is around 750-1000 for a stock rebuilt one. I have seen them go for less and some go for more. I would imagine you could get a decent one to rebuild from the junk yard for around 150-250 maybe more. It kinda depends on the area you are in as well.