Hi there.
Last year I swapped a rebuilt 350 for the original 305 in the T/A. The old setup was stock in a '83 with the quadrajet and computer. It got about 22 mpg on the hwy and around 18 around town.

The new setup has a mild mod cam, headers, roller rockers and a 600 cfm holley. I went vac advance on the dizzy and ditched the computer. MPG now is only 12 on hwy and closer to 10 around town. What would be the first things I should look at here. I expected lower MPG but that much lower seems a little too much.

Thanks for any help.

 

Advance the timing about 30°.

Yeah I know all about how it was "set" with a "light" using the "marks" and "The Book" says "The Spec" for a "83 TA" is ..... ummmm yeah, all that....

However, the engine does not whup out a light and a book and decide accordingly how it will run. All it cares, is when the spark occurs.

Experiment. Mark where it is now so you can put it back if you don't like it running better.

 

haha. ok. I can try that. I was speaking with another guy about the timing and because it starts up like a dream and doesn't have any weird run-on problems I was thinking the timing was very close to where it should be. But I'm willing to experiment. Thanks.

 

A good properly working Qjet will get much better gas mileage then a holley any day of the week.

 

ok. understood. So what kind of mpg range might I be able to expect with the holley. Like I said, I knew I would get less, it just seemed like quite a drastic change to be almost cut in half.

 

Hard to say I can only give you an example. I had a holley 870 on my stroked 500+ HP 468 Pontiac. I would get like 8-10 MPG depending on how I drove it. I recently built a Qjet for it to handle my HP output and I now get between 14-16 MPG.

You may be able to get some more MPG playing with timing but in the end while Holleys are great carbs they suck for MPG.

 

Did you set up the timing? If so how did you set it? How much vacuum advance are getting? New or used dizzy ?

 

well, another friend of mine did. it's a new dizzy, he used a light. It was set last summer when the swap was done. I don't remember what it was even set to but perhaps now that the engine is broken in completely it's time for a re-check of it all anyhow. I

 

ok. I see what you mean. I was never intending on builing a race car, just a mild mod to a freshened up 350 for a fun daily driver. But I live near Chicago (where we have the highest gas prices in the country...AGAIN) and a daily driver with 12 mpg hwy with overdrive just won't git 'r dun.

 

It could be something other than timing, maybe the choke is partially stuck, but getting in there and checking things out is the easy start.

If the carb choke opens up fully when warm I'd look at the distributor - it happens the clamp bolt on the intake can loosen and the sparkplug wires pull and turn it a bit.

The timing should be set warm, low idle with the vacuum line disconnected from the dizzy and the source plugged. Base timing of 8 or 10 might be a good starting place, though I wouldn't necessarily leave it there. Someone else mentioned 30 for total timing. The next thing to check is that the timing light shows a change toward that number when the engine revs to be sure the weights are moving. Should be no issue there since the dizzy is new. Depending on the springs on teh weights the total advance could come in near 3k or 4k rpm, you may want to find that. Just becareful of the fan when revving, lol.

 

350 HO Deluxe with 600 Holley. 14 town, 18 highway give or take.

JamesC

 

Dang that sucks man, good luck figuring it out

 

Thanks for the input guys. that's a great start for me.

Those numbers are about what I was guessing it would be close to James. Thanks for confirming. I'd even be happy with a couple less than that. With the help on this forum I'll figure it out for sure.


As I play with the timing, what would a good idle be? I think we had it set around 900 to start. As the engine broke in it seemed to go down. that was in park, btw. Is that still a good number to be near?

 

Every engine is a little different. With my 350 I like to set it fully warmed up, in drive with my foot on the brake to 500. That usually worked out to 650ish in park. That was a stock small cam. If you have a ratty cam it may not like it that low. I wouldn't go higher than 900 or the mechancial advance may be starting...

 

Did you do something to replace the computer's torque converter lock-up function?

 

not yet. The plan was to replace the trans shortly after the rebuild anyway and take care of that then too. I'm sure that's playing a part in the less than desired mpg as well. How much do you think it's affecting it? Depending on my budget over the short term, I may need to just add the kit into the current trans as it is now.

 

The torque converter lockup won't affect mileage too much, maybe 1-2 on the highway at most. However, if this is an early 700R4 (83?) the lock up is ESSENTIAL to proper cooling for the transmission. Do not drive without it functioning.

 

my 67 Camaro with a 350, holly 600 and Muncie 4-speed gets 15mpg on the highway. Not sure if that's what you want to hear, but it's a comparison. My 84 T/A with Auto Trans, CC Qjet and 3.73 gears gets 18mpg on the highway @ 70. 19 barely if I keep it at 55.

 

ok. 1-2 isn't bad, but it adds up. I'll get the lockup if I can't budget the new trans in for this spring. I wanna drive the car regularly this summer and fall for sure. At least when it's not raining.

15mpg sure isn't great but it's close to what I expected after this change. I don't know exactly what the gears are in mine but when I tried to figure it out spinning the tire and counting revolutions I remember it calculated out to less than 3:1 I think that's part of why I could get the 22mpg with the old setup as well.

I was getting ready to adjust the timing yesterday and noticed a fuel leak from the gasket in the front of the carb! that surely won't be helping me either. The carb was like new when I got it but it had sat in another guy's garage for a few years and probably should have been gone through first. I'll pull it off and get new gaskets in it next week. It's parked til then.

Again, thanks for all the help guys. I really appreciate it!