bruisedknuckles

77 Small Block

The mileage on my car is just plain awful since the day I bought it.

I get anywhere between 6-7 mpg. The first time I thought that it was an error in readings, but been recording odometer and gas pumped every time I fill up to be sure - no error . Any one have any ideas on why it could be so bad?

Changed spark plugs, installed new ignition wires, new distributor cap and rotor, re-built carb (Rochester Quad-jet), adjusted idle and mixtures, adjusted timing, tire pressure is just fine - did everything one would do to try and help mileage with absolutely no luck.

Most of my driving is local, so not highway in which case I would assume worse MPG, but 6-7 just seems crazy.

Any thoughts or ideas on why this might be or what I can do to fix it?

CA-Legal-Vette

Do you still have the original catalytic converter in it? These could clog and rob efficiency. You should really notice a lack of power though, especially at high revs.

Have you ever had the air/fuel ratio checked? That would be a very useful diagnostic measure.

Also, you didn't say how it runs. Does it idle smoothly? Good power?

Marlin

Do a compression test on the Cyl's yet?
Vacuum leak?
Cat. clogged?
Check the air/fuel ratio?
Trans slipping?

sly vette

What r.p.m.s are you driving at?
What kind of transmission do you have?
What rear gear do you have?
How does the engine seem to perform?

couperdecar

Even if your cat was plugged - it wouldn't be that bad. Your carb is worn out. Throw it away and get a new edelbrock w/ vaccuum secondary. I'm getting 19 m/g (US) with a 383.

billla

With milage that bad, I'd be looking at a number of things.

First, what do the plugs look like?

Second, you say you "adjusted" timing - how was it "adjusted"?

Third, when the carb was rebuilt were the bowl plugs expoxyed?

Finally, do you have access to a F/A meter?

bruisedknuckles

Hi All,

Thank you for the responses.

Two things I got done professionally by a corvette restoration shop - rebuilding the carb and adjusting the timing. Float bowls were changed and plugs epoxied.

Changed the spark plugs, used a set of iridium plugs, gapped at 0.045 which was recommended.

Have not checked the catalytic converter or the fuel/air ratio, maybe that's something I need to look at?? Really don't want to throw out the carb, this car is original and I want to keep it that way as far as I can.

It runs real smooth, idles beautifully and doesn't appear to have any loss in power. It takes off when I hit the accelerator with no misfiring or issues and the engine performs great.

Got the tranny serviced, no slippage and works as expected. On highway, at ~70 MPH I am at ~2500-3000 RMP's if I recall correctly.

mikep3

You should be getting in the mid teens for milage. Unsless you have a major condition like:
Poor compression
Pluged Cat
Wrong timming
Bad Carb
Odometer reading low?

Tell us all what tranny, rear end and mods you got.

billla

No one's suggesting - or should be - getting rid of anything

Being blunt, Corvette restoration shops differ greatly in terms of their ability to do this type of tuning, especially the Q-Jet overhaul and tuning.

I guess the question is are you looking to do this work yourself, or do you want to take it somewhere? Because step one is to get the ignition sorted out, which takes time, a little effort, a little learning and a timing light Then move on to the carb, which I expect is going to take sending it to one of the very few (2-3) quality Q-Jet rebuilders for a checkout.

SIXFOOTER

Man, thats pretty bad milage. Your restoration shop is jamming you. That car with that carb should be getting at least 16 with old style factory spark plugs, 77 was before they discovered Irridium I think.
If you have some skills you can get this fixed, if not your going to need to find a shop with a brain. Post up some pics and some numbers we will see what we can do to help. I would place a bet on the cat being plugged, timing slow and AIR pump not working correctly, in no particular order. Figure out what you want to do with the car and go from there. You can do some good HP mods to that car and Still get close to 20mpg

Frankenvette

Wow, I did not think anyone was getting worse mpg than me. I think I avg around 10 mpg when cruising on the hwy. I've replaced my carb, replaced my heads, had my carb tuned on a dyno , installed electric fans and still avg around 10 mpg.

cv67

How is anyone going to get almost double the mileage of them brand new amazing.

Shark Racer

What's the state of the ignition system? Are you running vac advance? (you should)

bruisedknuckles

Hi All,

Damn, got some serious responses , well, lets put it this way. I'm a rookie, but I have actually learned quite a bit and looking to do everything myself.

@SIXFOOTER

Definitely want to work through this myself and avoid shops! Infact, I have been bailing out on work and secretly working on the car. Will check timing and post sometime this week. Looks like I need to check the cat for sure and maybe even the AIR pump.

Attaching two pic's - before & after. Did all work myself, the only thing I got done through the shop was the carb rebuild and timing - mostly because I did not have a timing light, did everything else myself including a rebuild of the interior, carpeting, wiring, stereo, Guage cluster etc.

Look forward to all the help I can get, mileage is driving me nuts!

Marlin

Don't see it in your info, where are you at? If near N.Va drive by my place and I will take a quick look at it. I use to be a tech and 6-7mpg should be easy problem to find.


Its been a while since I was a full time tech so maybe this has changed, but I never ran Plat/iridium plugs in a Carb car. Copper always seemed to work best in them. Have you removed a plug recently? Whats it look like?

Frankenvette

Replaced plugs, wires and distributor cap and rotor and yes running vac advance.

CA-Legal-Vette

Something that no one has asked yet; have you changed the air filter?

It should run rough and lack for power, but it's a pretty basic thing to change before you go much further. Also, make sure that the damper in the air intake is opening up properly when it gets warm.

DRIVESHAFT

Another thing to check is the exhaust heat riser valve at the passenger side manifold.
Make sure it is opening.

bruisedknuckles

Sorry, forgot to post picture after all the work, can't figure out why it gets inverted -

@ Drive Shaft & CA Legal

I did check the heat riser, looks ok. Also changed the air filter.

ptroxx

I Was getting 15 on my 77 before the zz4 was installed and still getting about the same if not a tad more.....

I would almost bet its the carb..... Their are only a few that know the right way to rebuild a qjet carb the right way and I bet its the carb.... Just sayin...

I used Henry Olsen to rebuild mine and havent looked back..