Rebuilt 350 - Abysmal Fuel Econ

[Nalanthi]

Good morning, y'all...
Long time creeper, first time poster - and I'm at a loss for what to check next.

The backstory is that I picked up a '79 K10 for one hell of a good price - it had the 250 straight 6 in it with a monojet, SM465 trans. After cruising it for a few days, I found that it was giving me 6mpg - WAY below what it should. Like any self-respecting keeper of the swingers, I decided that this simply would not do and went onto rebuilding a first gen 350 I had in my garage with all new comps.

Everything was specced and checked, plastigauged and double-checked. My build (for now) is mostly stock exception of flattop pistons, a double roller chain, and a high-flow oil pump. I rebuilt an early qjet, and proceeded to rejet 8% leaner for cruise and 4% leaner for power due to my altitude (5300ish feet).

Upon testing, I found that I was pulling about 4.3MPG. Convinced that the quet was junk due to a minimally warped air horn (although it sealed up tight), I swapped for a known working Edelbrock 1406 from the Wife's currently running K5 that was just purchased last fall (got her a NIB 1406 that day) along with replacing the fuel line from carb to tank. Upon testing, I was pulling 3.9MPG. I guess that qjet wasn't junk like I thought after all.


Here's my current setup:
The 1406 is jetted 8% lean for both power/cruise.
Initial spark is set at 10 degrees BTDC.
Vac holds steady at 15Hg.
Compression holds around 140 per cylinder with no more than a 5psi deviation per cylinder.
I have not checked the gear ratio, highly likely that the truck has been re-geared due to the age of the previous owner being in his 70s. - rolling on 31s (for now), so... stock. - maybe 3.73 or 3.55 - not enough to cause such a detriment to econ by THAT drastic of a measure.
I know I'm not pissing fuel out of any lines.
The engine can be wrench-cranked at the balancer without using major force, so I know that I have no hangups (I shouldn't anyway - it's all been built by the book).

Carb idle mixture screws are nominally effective if at all, which tells me possible vac leak, but... shouldn't that pull me below 15Hg? I also had the idle set too high (1000RPMs), so I may even be out of the idle circuit on the carb, which would explain the screws...


I'm at a loss.
If any generous soul could push me in the right direction, I'd appreciate it greatly.

 

[marks86]

prob a dumb question but have you replaced the fuel filter?

also is the truck a full time 4x4?

 

[MadOgre]

Idle should be 550 rpm in gear so 600ish in neutral.

Vacuum leaks and lean jetting as well as clogged fuel filters should cause less fuel consumption.

1000 rpm for an idle will cause significant fuel consumption. unless driving in the upper rpms. Basically your not getting any benefits from the idle circuits. Its like driving all day with the choke on.

Also if you jet lean you may notice that you have to apply more throttle to obtain equivalent power needs thus in reality you are burning more fuel to obtain the same power results.

It is best to jet to stock unless you are able to color tune your carb. It is a bad idea to run an engine lean as it can cause undetected pre ignition which will shorten the life of your engine.

 

[MadOgre]

Also a sm465 is 1:1 in 4th gear so if you have low gearing and short tires at 1:1 you will be churning some high rpms which is not good for fuel economy. If always in high rpms your secondary's will be partially engaged. So your cruising down the highway for say 1/2 an hour with your 4 barrels open.

Better hope there are plenty of fill up stations along the way!

 

[rich weyand]

I have basically the same vehicle, with a TH350 which should make mileage worse, and 3.73:1 gears, and have the truck tuned to the gnat's patoot. I am getting 12 hwy/10 city, so it is certainly achievable. So I have some questions.

Is it full-time 4WD, or has the NP203 been part-timed? Manual hubs?

What are the jets and rods you currently have in the 1406?

Are you sure the choke is opening? Is the electric choke connected? Is it getting voltage?

Do you have the stock thermostatic air cleaner mounted, or are you running one of those two-chrome-pie-plate things?

Do you have the vacuum advance connected correctly to the driver's side manifold vacuum port, or incorrectly to the passenger's side timed vacuum port?

Base timing should be more like 17 BTDC. Try that, and try to pull up a hill at low rpm; if it knocks, back off two degrees. More advanced timing, up to the limit, increases torque, horsepower, and fuel economy.

What cam is in the engine? Give me the model number, I'll go lookup the specs. A high-overlap cam will absolutely kill mileage. BTDT.

 

[Nalanthi]

There's no such thing as a "stupid question" on a car forum. I've seen people miss the most blatant of things before.

But yes... I do have a new fuel filter, and I drive with my hubs unlocked.

I'll drop the RPMs and re-check the timing when I get home, but my driving style has otherwise been rule-of-thumb "drive like you have an egg rolling around in the bed" - at least for now so that I can test to see what I can get in terms of econ via overly-careful driving.

I am not running in high RPMs - mostly around 1400-1600 cruising in fourth (or third) - either way the 1:1 depending on how you define a muncie's shift tree. I've never seen a common consensus on the topic lol. I'm barely on the pedal, so the secondaries are probably not open.


I hear of guys complaining about getting 9mpg - that's a dream compared to 4.


Correction: I'm three stages lean. .092 jets w/ .071x.047 rods. 89s for secondaries. Again, 5300ft of elevation. This carb was driving that 86 K5 on a 4" lift w/ 35x12.50s and giving the chick about 10MPG (not shabby).

Rich... I'll get you that number when I land on it (and highly appreciate it). Off the top of my head, I can tell you that it was the original on a 72 el camino. The cam is stock. Pie plate air cleaner. Running off ported on the carb for vac advance. I have yet to wire up the electric choke.
I also haven't really messed with the NP203 T-case yet.

I think that I may just be a bit under-tuned still.
I can't even begin to smoke the tires or peel out, which even an underpowered near-stock first-gen should be capable of.

 

[MadOgre]

He says hes idling @ 1000rpm also says he has stock engine which factory is 6 BTDC aluminum intake add 2-4 degress, so 10 is probably about right. The timed to non timed will help but I don't think that would account for 10 MPGs

The full time 4x4 is an excellent question?

I think 1000 rpm at idle is your main issue.

 

[MadOgre]

th350 runs 1:1 in 3rd gear also

 

[MadOgre]

well I have the 700r4 but I get in to the 20s for mileage so I find anything below 10 repulsive lol

 

[marks86]

check for vacuum leaks, huge exhaust leaks will kill mpg

 

[Nalanthi]

The 700r4 is an awesome trans.
I rebuilt the one that went on that K5 I was talking about (clutches were burned to all hell and the 2-4 band was gone). Hell - I got that K5 running tip top.

I wanted to stick with the 465 for the benefit of the granny gear. There's a lot out here where it is a total must.
I'll spray some carb cleaner around - see if I can make any leaks out, but again - wouldn't I see a lot less than 15Hh holding steady at this altitude if I had one?
I have a set of longtube headers bolted up nice n tight onto new gaskets, so exhaust should be fine.

I'm hoping it's just the idle among other things that are killing me...
Just chewing the damn fat with good people is sometimes enough to make the bulb come on, and facepalms occur.
I thank you gents.

 

[Christian Nelson]

Seems to me something may not be right with how youa re calculating your MPG. You say the 6 cyl was only getting 6, and now the 8 is getting 4..

What I would expect would be 16 from the 6, and 12-14 from the 8..

Are you SURE your odometer is not hanging up and skipping miles every so often or something?

Only time I got 6 mpg was when I had a truck with 4.56 gearing with small tires, and a massive squirting leak at the fuel pump. Surprised I didn't set that truck on fire! After I repaired the fuel line, I was getting 10 out of it, screaming like a banshee down the highway, feeling like I was gonna blow the engine at 65..

This was a cammed out .060 over 350 on a 3/4 ton truck with TH400 tranny..

Something really doesn't add up with that bad of mileage.

 

[Nalanthi]

Oh, no... those are the actual numbers.
I filled up last week on Sunday night (.1 mile from home i.e. up the road) and proceeded to drive to work and back which is a 17.3 mile round trip.
As I was approaching home, I went back to the station to gas up and the tank took 4.388Gal.

17.3/4.38 = 3.949 = 4MPG
Again - that was with lightfoot driving.


As for why the 6 sucked - I never looked into it. it just seemed like a tired old engine in need of a rebuild, because you really had to mash the gas to make it do anything. it was utterly gutless.

I swear to you - my MPG is seriously that bad.

 

[Skweegle89]

I haven't checked mine yet because my speedo doesn't work, but I bet you I don't get over 4 or 5 driving around town. It does ok on the highway, but its god awful in town. I'm running 33x12.5's with 3.73s and just learned to deal with it. 4 on the hwy sucks though. Hope you get it figured out soon, as I don't see the price of fuel going down anytime in the near future.

 

[marks86]

I would try and varify the gear ratio, because if your assuming the truck has 3.27s and you really have 4.10s thats a hellva difference

and I can almost see a tired old straight 6 with 4.10 gears driving like banshee getting said fuel mileage