Hello guys, I need your help to crack this mystery, I’ll give the info I find relevant, you are free to ask anything else.

I got this stock looking 75’ L48 Manual about a year ago, rode it for a week and detected a bunch of problems, among them it was doing 7mpg, overheating (250+) , the carb was boiling and spilling gas on the intake, and the fuel pump leaking.

I installed a new edelbrock aluminum water pump, aluminum radiator, electric fan, air damn, high flow 180f tstat, Edelbrock 2701 aluminum intake, 670 cmf street avenger holley carb, 80gph holley pump, new spark plugs gapped to .40, removed all the ac stuff, all smog stuff, all vacuum driven stuff (besides headlights and break).

• The car always started on the spot, even when I let it sit for 4 months!
• The cylinders pressures are 177,180,177,190,170,180,182,175 psi. This is the only thing out of the stock specs that I found on this car.
• Vacuum is steady at 21in-hg at idle. Smooth idle.
• Timming advance was at 6, I set to 12 and checked the vacuum adv which is at +16 and mechanical at +22, total 50 degrees and sounding great. Vacuum advance on ported outlet of carb.
• The car only runs on 92 octane gas, tried lower grade once and it kept running for 10 secs after I turn the ignition off, scary stuff!
• The temp goes quickly to 200 but never crossed it after the upgrades, the fan turns on at 200 off at 185.
• My gut-g-meter says the car is much stronger. No carb bogging or hesitation, cold or hot, idle or WOT.
• The gas seems to be boiling after I shut the car off, I saw the pumper jets spill a little and I hear bubbling on the water system. No large spills on the intake as before. Intake and carb base temp is way below 200 degrees when I turn it off but rises quickly.
• The intake manifold became yellowish on the edges close to the heads?!?!?!Overheating? gas spill??
• The exhaust mainfold is stock, no H link, no cat converter, lots of dents and small punctures on the pipes, 2” diameter (stock?)
• I drive the car every weekday 4.5 miles to work and back, 2.5 miles at 30mph, 2 at 40mph, never used the 4th gear, 9 traffic lights on the way. I like to once or twice a week I take off at WOT to stretch the legs you know…

Still 7.48 MPG!!!! Average calculated over the last 567 miles, 13 weeks. Chevron and Shell gas.
Next move should be a nicer exhaust end to end, 100 amps alternator, MSD HEI distrib, maybe someday aluminum heads and a mild cam.
I hope someone is stealing my gas… Any thoughts?

Thank you
Fábio.

 

Are you sure on the math?You should be getting about double your mileage.Maybe you're not driving far enough to get it off the choke,my advice-drive it more and your MPG should improve!

 

It's hard to analyze your problem from a distance, but i think your total timing is way to much. Should be round 38 total. I would set it at 8° of timing at idle without vacuum.
Then I would check for air/fuel ratio on your carb and then check if the second throttle blades or not sticking open when in operation, power valve, etc.

good luck

 

You might also want to try installing a heat shield between the carb and intake.

 

All that stop and go traffic as hell on gas consumption.The worst economy is when accelerating and idling.
I drive mine to and from work occasionally and get appx 14 mpg.
Thats with 370 rear headers,duals,FM 40 series mufflers,a 406,Q-jet,Holley intake and never see the high side if 190 degrees.
Appx 15 miles of that trip is 70+ interstate turning 3500rpm.
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Your fuel boiling is from the fuel line being too close to the water pump.Put a foam insulating wrap around the line where it comes up from the pump.The heat shield is a good idea,a lot of the C2s had them.

 

If it is a stock motor L-48 or L-82 it should run fine on 87 octane because the compression is so low. Run on after being shut off is only caused by one thing and that is hot spots. Generally it is a glowing plug electrode.

To much advance or way lean A/F

Is there a chance that the motor was rebuilt with other than stock pistons and cam?

 

What do your plugs look like?Why ported vacuum?
Describe how you set your timing.Did you set your total(initial and mech)with the vacuum connected or disconnected?You said you had 50 degrees max. timing...what rpm was that?
You didnt mention the heat riser....whats its story?
Have you checked your odometer for accuracy?

 

Unfortunatelly the math is correct, and the odometer accurate.
The choke stays on for just a mile or so, as I said the car warms up pretty quick.
When I advanced the timing to 12 the temp dropped drastically, I read that you should put as much as you can without knocking, most guys recomend 56 total.
Sounds like a great idea, right now i have about 1/8 of a inch clearence to the hood, might need a drop base air filter to fit a heat shield(is that the spacer kind of thing?) But don't most of you run without it? I have a mild setup shouldn't be that different.
I'm using a flexible hose, not the aluminum line, check my homepage too see more.
The compression seems to be higher than stock as the cylinders read 180psi in avg.
The advance sounds ok since there is no pinging.
Its a new holley carb, A/F shouldn be all that messed up, maybe vacuum system can be leaking, PCV looks new and rattles, brake booster holds vacuum to the next day, headlights.... they are slow opening and the right one don't close without being kicked, I hate this design and plan to replace by electronic servos.
There is definetlly a chance that this guy have pistons and cam other than stock, but let me say that the idle is the smoothest I ever heard, It's almost embarassing.
Check my homepage, I have pics of all the old plugs and the new ones after a week or so but with the old carb, I'll get pics with the new carb and post it there.
The full vacuum port was making my idle too high even with the idle screw all out, and people recomended me to use the ported one. Do you think that the idle screw set to minimum not shuting off the car indicates any issue?l. vacuum leaks?
I did set the timming with the vacuum disconnected, total timing "sounds" to be around 3.5k (I work alone )
I completelly forgot to mention the heat riser, it was another thing driven by vacuum and not working, I ripped it off and left the lever dangling, with the car running I realized that the gas flow forces it open so I just let it be. Is that ok? It will go away when I get a new exhaust.
The odometer is fine
Thanks a lot for the answers guys, and don't give up on me, lets figure this out.

Fábio.
PS:My home page's link is available by clicking on my name on the left side of every post.

 

I think you have a vacuum leak or there is something wrong with the carb if you can't run full vacuum advance without a high idle.

Also, turning the mix screws all the way in should kill the motor. If it does not you are puling fuel from the main jets which you shouldn't at idle. Look down into the carb while it is idling and see if there is fuel dribbling from the boosters.

Have you checked the float levels? High float level can cause this, they are not always set right from the factory.

 

Turning the idle mix screws all in does kill the motor. its the idle speed screw that doesn't. I have set the idle mixture to the leanest possible and maximum vacuum now 23in-hg, and set the float level using the cool transparent bowl plugs that come with this carb.
I'll check the boosters.
Fábio.

 

Yes it should idle down and then shut off.Are the secondary's being held open for some reason? I believe on the Holleys there is a screw to keep the secondarys slightly cracked open...have you checked that?

I would wire that lever down so it stays open.It seems like it should force its way open but wouldnt count on it.I remember watching it once with some rpms and it was constantly closing.

 

I would agree with the vac being an issue for your problems. If the lights are slow going up, you have vac leaks or other issue. With normal compression (stock internals and cam or mild cam) you should have strong vac readings.

 

Well, there's your problem. Take it on a 100 mile trip some time and see how it does.

 

They only get about 10 when I drive my average way. Fill it up and drive 55max like the speed limit was back then and an egg under the gas pedal and see how you do on a 100 mile trip all at one time.

 

Back in the early 80's I had a '71 with the 270/350 and turbo 400. This car would get 18 mpg on trips.

 

With a 3.08 automatic and keeping the speed 60 or below I can see you getting 18. The way I drive my 73 -4speed it gets about 10. I run it through the gears at every stop sign out of town and see 4000 rpm + often. If I wanted good gas milage I could have bought a KIA for the same money. I did not see what rear gear the original poster is running. Did he post it? A 4.11 is fun to drive but with 4$ gas you have to pay for the fun.

 

Hey guys, sorry for the long silence.
So I took the spark plugs and the exhaust mainfold out to check them out. Here are the pictures.
So if I'm seeing this correctly, it seems that my valves seals are busted and oil is getting to the combustion chamber.
Can that might be messing up the mixture and my efficiency?

In the process I ended up messing up a spark plug and the local shop didnt had the 75' spec so I took a set of 76' spec which are slightly bigger and its electrode sticks out deeper into the combustion chamber.
I suppose it's ok to use these spark plugs since the heads are the same and 75 specs are the worse in vette history with all the smog mods, right?
I turned the engine a few times to make sure the pistons weren't hitting the longer spark plugs and when I fired it up it was more responsive and could feel more power, not sure if it's because of the different specs or just because these spark plugs aren't oil coated yet

About the heat riser, as you can see on the pics, the flap was cut off so it's permanently disabled.

The exhaust seems to be assembled without gaskets or they blended with the rest and I'm not being able to see them.

The exhaust mainfold seems to be clear from thick deposits that could be clogging it.

One surprising thing that I don't know if it's related to vibration or my "rokieness" is that most bolts lost their torque!!! Intake mainfold, valve cover, water neck...

Other mistery that you guys could help me with is that yellow color on the edges of the intake mainfold. Where that comes from??? How can I clean it off?

Tomorrow I'll work on the vacuum lines.

I'm all ears

Thanks,
Fábio.

 

The old pugs are not that bad. There is some oil fouling, but mostly they show you are running rich. If you get the mixture right the chamber temperatures go up and most of the oil you see will be burned off.

I would concentrate on the air/fuel mixture for now and worry about any oil burning after you get the mixture right.

The Holleys are always jetted a little rich as they don't know what engine you're putting it on and a rich engine still runs good. Your old carb was probably a parts store rebuild which are almost always messed up.

You'll have to tune the holley to get max economy. If tuned right it will not loose any response or power, just improve economy. But it can get tricky to do them right and I highly recommend a wideband O2 meter.

Have a look here:

http://www.innovatemotorsports.com/f...isplay.php?f=7

 

Hi Fabio,
Are you saying that when you set the timing to a reasonable 10°-12° with the vacuum advance disconnected and then when you hook up the vacuum advance, you cannot get the idle speed down to 700-800 rpm with the idle adjust screw on the driver side of the carb?

If I understand you correctly, something is not right. Have you checked your throttle cable to see if it is keeping the throttle slightly open? If it's not the linkage, then start looking for a vacuum leak as others have suggested.

Good luck with this, you've done a lot of work on the car and deserve to have it running like it should.

Cheers,
Pete

 

I once had a Pontiac that the secondaries would open at about 2500 rpm in normal cruise mode. The milage went up drastically after adjusting correctly.