4.1 mpg?

[Ajax19]

Put a q jet back on problem solved

 

[JBswth]

I know the fuel mileage shouldn’t be that bad. Every time I take the truck out I’m stopping at the gas station at least twice, which isn’t making the wallet happy with gas being over 5 bucks a gallon. Timing is set to 12 degrees initial, carburetor is fine tuned with vacuum gauge, idle mixture screws are about 1.5 turns out. Accelerator pump on second notch. Edelbrock 1406, hei distributor with accel super coil, none of the valves seemed loose or out of adjustment. Holds timing steady no jumping or anything. 35” tires lifted 6 or 7 inches, turbo 400, 350 engine, Holley contender intake, stock mechanical fuel pump, no emissions, hooker headers. I’m running about 1/3 throttle going 65 on the highway. Is there a way to tell if my vacuum advance is messed up?

If your new tires are bigger around, then in the city your engine is being burdened, as you have the effect of much taller gearing. Good at high speeds on the freeway, but in the city, requires a lot more throttle just to move off. That hurts gas mileage. Also, if you haven't changed your speedometer gear, it is underreading. You are actually going considerable faster than it shows on the dash. Ditto for the odometer.

J.B.

 

[JBswth]

If your new tires are bigger around, then in the city your engine is being burdened, as you have the effect of much taller gearing. Good at high speeds on the freeway, but in the city, requires a lot more throttle just to move off. That hurts gas mileage. Also, if you haven't changed your speedometer gear, it is underreading. You are actually going considerable faster than it shows on the dash. Ditto for the odometer.

J.B.

Also, are you sure that Edlebrock carb is jetted correctly for your engine? You might want to get some additional jets and experiment, and check the float levels at the same time.

J.B.

 

[Ricko1966]

Okay everybody keeps missing the obvious,he needs 45 -50 degrees of advance,to get anything that resembles milage. With vacuum advance inop. He has at most 34 degrees and that is probably up around 3500 rpm so most of the time he's running around with about 20 degrees. With the vacuum leak from the vacuum advance his carbureator is seeing a low vacuum signal and adding fuel,because that's how carbureators work the lower the vacuum the more fuel is added because the carbureator thinks you are in the throttle. Now it's going to Rev good at full throttle,because no load revved until the distributor weights are out is the only time his AFR and timing are even close. He needs to get ignition timing right before anything else can be diagnosed other than running compression.

 

[Dixie Pride]

Do not feel too bad my 85 K20 with stock 350 and de-smoked only gets 5 or 6 mpg.3" body lift ,5 suspension with cut out fender flares and 44" super swampers When it was new it got 16 mpg.Pushing them tires mpg went way down.

 

[Ricko1966]

Okay got a few pics for you and some explanation. Pic 1 is a just few different vacuum cans, there are dozens. First 2 numbers are what vacuum starts vacuum advance moving. Second number how much vac adv. the can adds 3rd number when the advance is all in. 2nd pic is a FEW of the different factory centrifical weights and eccentrics specs. 3rd pic is a few after market centrifical springs. 4th a pic of a few Original eccentrics and weights. Look at the difference in variance of how many degrees of vacuum advance and when it starts and when it is all in, and that is a very small sample Look at difference on weights and eccentrics, just on that chart we have a possibility of 7 degrees difference and it can be more than that. On those few vac. cans there is a 4 degree spread. So just with 2 variables there is as much as 11 degrees difference, lets throw in another variable.Lets say you use that top vacuum advance but your engine only pulls 17 inches of vacuum, that top advance can isn't going to give you that 11.5 degrees Look at the springs, thats just 6 of them but many combinations there, 1 light spring 1 heavy, just 1 med spring etc.There is no one size fits all application. All engine combinations don't even need the same amount of total advance some need 38 some like 32 There are the this is as generic as we can make it distributors. That will work meh, in some,good on others, and terrible in others. The best thing in your, I bought it this way, have no idea what parts came from where,and what's been done to anything. Is learn how to tailor your distributor to what your engine needs.

 

[75gmck25]

Make sure you are reading the vacuum advance specs correctly.

Example - VC700 83 8-10 11.5@19-21
This means it
- starts to advance at 8-10 inches of vacuum, and
- maxes out at 11.5 degrees of distributor advance when it reaches 19-21 inches of vacuum
Crankshaft timing is double the distributor advance, so this advance adds up to 23 degrees at the crank
This is probably for a mild economy car tune, since most performance engines never get to 19-21 inches of vacuum at cruise.

Most off the shelf replacement distributors I’ve seen seem to have a 9-10 degree advance, so they add 18-20 degrees at the crank.

 

[84prspctr]

Here’s an update, I put a new vacuum advance on hooked up to manifold and reset the timing and the thing is running a lot better. I was at about 1/4 throttle on the highway before, now I’m barely even touching the gas to keep it going down the highway. Starts better, and is running cleaner. I re set the carburetor too and was able to drop the idle significantly and turn the mixture screws in. Runs a lot better. We determined my old canister was pulling 15 degrees of advance, and the only one I could get my hands on at the parts store was one that pulls 20 degrees. I made a restrictor plate that bolts up to the vacuum advance so that it pulls about 15-17 degrees of advance. No pinging under full throttle in all gears.

 

[T-roy K10]

I just went through this on my 85 k10 with 350cid. Right after I bought it, I drove it from Mississippi to Louisiana and got an insulting 7mpg. I was sick. I had a Dodge back in the day with a 400cid and 8” lift with 38s that got 11mpg. This k10 has 4” lift and 35s. This is what I’ve done to it Since the 7mpg ride.
New front brakes with rotors, hubs and bearings. (Both inner brake pads on front were out of place and maybe dragging)
New fuel switching valve and switch.
Rebuilt trans (th350).
New wheels and 33” bfg ats.
Tossed the chrome air cleaner and put stock back on to pull in cool air.
Fuel pressure regulator.
New larger radiator.
Also had a buddy (excellent mechanic) look at it and changed some vacuum lines. It had the front main vacuum line that comes out the front base of carb (edelbrock) tee off and connect to both valve covers with pcv. Put a new hose and connected to only the left cover. I plugged the one on the drivers side. It did change the idle and we adjusted that.
Well today I drove it 130 miles (actual not odometer) and stopped to fill up. It took right at 8 gallons. That’s 16.2 mpg!!!
I’m so shocked that I’m going back to the station in the morning to make sure it didn’t use and gas from the other tank. While at the station, instead of trying to put fuel in the other tank I just hit the switch and it read full, but with these numbers I want to make sure because it’s hard to believe. I’ve had much newer fi 4x4s that wouldn’t get 16.
I’m it sure what repairs/mods made that much difference but guessing it was a combination.
I don’t know what the timing is set at because it’s a 1996 vortec 350 that doesn’t have the timing tab. Might look into adding one.

 

[T-roy K10]

Update. Tried to add gas to the other tank and it took about 1/2 gallon. I’m sure you can always get something in there. So this carb 350 4x4 actually got 16mpg.
Impressive.

 

[84prspctr]

Took the truck on a 200 mile trip, the first 100 miles I filled it and only needed 12 gallons. Still not the best at 8.3mpg but it’s a heck of a lot better than 4.1.

 

[Ricko1966]

Took the truck on a 200 mile trip, the first 100 miles I filled it and only needed 12 gallons. Still not the best at 8.3mpg but it’s a heck of a lot better than 4.1.

Now get the distributor set up correctly,and it will go up even more.