[Degener8ion]

So I was getting about 12.5 mpg on my '70 Chevelle SS 396/TH400 when I first got it. Had a major issue with gummed up fuel lines and tank. Replaced all the fuel lines with high quality fuel injection grade hose (in case I decide to go to FI later). Replaced the gas tank, fuel sending unit, fuel pump, fuel filter, inlet washer, and hose from carb to fuel pump. After getting everything dialed in with the idle mix screws and timing to get her running smooth my fuel mileage seems to be down to closer to 7 mpg. There are no leaks anywhere. Vacuum is good. Timing is good. What in the world am I missing here?

 

[oldcutlass]

Apparently your new timing settings are not the same as the old ones.

 

[lucifershammer]

One possibility - things were so clogged up before that you were actually starving the engine for fuel and it was running lean. Drinking through a tiny straw.

Now that you have it opened up, you have it drinking through a fire hose and have to tune it appropriately for better gas mileage. I bet it makes more power now though eh?

What carb do you have, what is the jetting, what is your timing set at, how did you adjust the idle mixture screws, etc. If you can, post all that relevant information.

 

[77 cruiser]

What carb?

 

[Degener8ion]

It has the original Quadrajet carb that was professionally rebuilt by the previous owner, so I don't know the details of the jets, etc. The total timing is set at 38, the idle mix screws are at 2 turns each. Those settings were necessery to get a major stumble out of it. Whenever you got on it around 3000 rpm just as it was shifting back to 3rd it would fall on its face. Had to let out of the throttle to get power back. But once I got the timing and idle mix screws set where I have them all of that was gone.

Should I consider smaller diameter hoses for the fuel lines? I'm using 3/8" for the send and 1/4" for the vapor return.

 

[lucifershammer]

Oh no, I wouldn’t change the fuel lines. It just sounds like your tune is off.

I‘m not knowledgeable in tuning Quadrajets - someone else will have to help you out there.

 

[Degener8ion]

Oh no, I wouldn’t change the fuel lines. It just sounds like your tune is off.

I‘m not knowledgeable in tuning Quadrajets - someone else will have to help you out there.

Yeah, more complex carb adjustments are out of my depth too. I may have to take it to somebody I know.

 

[oldcutlass]

Is that 38* with the vacuum advance connected or disconnected? At what RPM did your distributor stop advancing?

 

[Degener8ion]

Is that 38* with the vacuum advance connected or disconnected? At what RPM did your distributor stop advancing?

It's 38 disconnected at between 3000-3200 rpm. Not sure on the second question. Didn't really go much past 3200.

 

[oldcutlass]

It's 38 disconnected at between 3000-3200 rpm. Not sure on the second question. Didn't really go much past 3200.

If you did not go until the timing stopped and set your total your timing may still be off. What distributor and whats the initial set at now?

 

[Degener8ion]

If you did not go until the timing stopped and set your total your timing may still be off. What distributor and whats the initial set at now?

I'll have to recheck. What should initial be at and how far out does a 396 usually go before the timing stops? Ballpark ideas might be helpful.

 

[Degener8ion]

I bet it makes more power now though eh?

Actually the throttle response seems a little sluggish by comparison.

 

[Harris Roc Malouda]

I'll have to recheck. What should initial be at and how far out does a 396 usually go before the timing stops? Ballpark ideas might be helpful.

total idle timing (mech+vac) should be around 24°(+-2°)
total mech. timing should be around 36°(+-2°), all in around 3000 rpm (+-200rpm)

You can achieve that with - for example - 18° mech. intial + 6° vac. (limited via stop plate).

Then you can use the black 18° MSD bushing for 36° total timing. Crusing would be +6° from vacuum.

Springs you should use 1 Heavy / 1 light silver, which bring in the 36° at 3000 rpm.

The stumble when flooring it might come from excessive vacuum advance. Limit it to max. 8° and try again..

If you get hot start starter kick back with 18° initial, try 16 then 14°, always adjusting/replacing the according bushing size so total timing remains the same.

 

[Degener8ion]

total idle timing (mech+vac) should be around 24°(+-2°)
total mech. timing should be around 36°(+-2°), all in around 3000 rpm (+-200rpm)

You can achieve that with - for example - 18° mech. intial + 6° vac. (limited via stop plate).

Then you can use the black 18° MSD bushing for 36° total timing. Crusing would be +6° from vacuum.

Springs you should use 1 Heavy / 1 light silver, which bring in the 36° at 3000 rpm.

The stumble when flooring it might come from excessive vacuum advance. Limit it to max. 8° and try again..

If you get hot start starter kick back with 18° initial, try 16 then 14°, always adjusting/replacing the according bushing size so total timing remains the same.

Some of what you are saying is out of my depth with the distributor. I'm still new to a lot of this. But I have rechecked my timing and the total idle timing is 24° with total mechanical at 36° all in at around 3200 rpm. So if that's where it should be then what else could be the problem?

 

[Harris Roc Malouda]

Some of what you are saying is out of my depth with the distributor. I'm still new to a lot of this. But I have rechecked my timing and the total idle timing is 24° with total mechanical at 36° all in at around 3200 rpm. So if that's where it should be then what else could be the problem?

how much does the vacuum add? is it connected to manifold or ported vacuum?

 

[Degener8ion]

how much does the vacuum add? is it connected to manifold or ported vacuum?

It was 15° at idle without vaccum ---24°with vacuum added--- and 36° total mechanical ---56° total with vacuum added. It's connected to a manifold vacuum port at the carburetor.

 

[69-CHVL]

I have two guesses. First, its running better so you've been hammering it more around town and as a result mpg plummeted. Secondly, perhaps how your tallying the mpg is not precise enough (not a large enough sample size, etc). Maybe your initial 12.5 mpg was a little too high to begin with.

 

[Harris Roc Malouda]

It was 15° at idle without vaccum ---24°with vacuum added--- and 36° total mechanical ---56° total with vacuum added. It's connected to a manifold vacuum port at the carburetor.

yeah i would try to limit vacuum advance to 6-8 or max 10°.
I read many times that with excessive vacuum timing (20 is a pretty much) the engine has trouble switching load states like from cruising to WOT without bogging..
at least this goes for ordinary carburetor setups. EFI is another thing..

crane makes an adjustable vacuum canister, check that one out.
What kind of distributor do you have?
If its hei you can use their limiter plate.

If you use an non hei one, you have to cut an own plate and attach it between the vacuum body and pulling lever.. so the canister can pull the lever only against the limiting plante, not the full way.. thus only allowing 10° instead of 20°..

 

[Degener8ion]

yeah i would try to limit vacuum advance to 6-8 or max 10°.
I read many times that with excessive vacuum timing (20 is a pretty much) the engine has trouble switching load states like from cruising to WOT without bogging..
at least this goes for ordinary carburetor setups. EFI is another thing..

crane makes an adjustable vacuum canister, check that one out.
What kind of distributor do you have?
If its hei you can use their limiter plate.

If you use an non hei one, you have to cut an own plate and attach it between the vacuum body and pulling lever.. so the canister can pull the lever only against the limiting plante, not the full way.. thus only allowing 10° instead of 20°..

I'll have to look into that.

Incidentally, I drove it to the beach with my daughter and noticed it seemed to spit and sputter ever so slightly today. By the time I got home it was so bad I couldn't drive it above 50 mph or 2500 rpm. Everytime I tried to accelerate above that it would stall, jerk, and hesitate likenit was about to cut off until I slowed down to 50 or below.

When I got home I found that I had left the vacuum advance line disconnected from the vacuum advance canister but it was still plugged off. I assumed that maybe that was the problem so I reconnected it back to the vacuum advance and took another test drive down the road and it was still doing the same thing.

 

[dyno jonn]

You need to get professional help.....