Hi, this is my first post:

I just recently installed the super six setup on my 75 Plymouth Valiant, and the gas mileage dropped from 17 to 11, with a worst of 8. I knew it wasn't my driving habits, so I decided to retune my carburetor. I set the float level much lower than the recommended height, I dropped the metering rods all the way in, and I minimized the accelerator pump travel. The gas mileage did not change. What is wrong with my car?

Welcome to the board. Something is definitely amiss with your setup. What can you tell us about the carburetor you installed? Is it an original used one? A used one of unknown origin? A "remanufactured" unit from a parts store? How certain are you that you used the correct carburetor base (mounting) gasket and the correct gaskets between the three castings? Using the wrong gasket will prevent vacuum from reaching the step-up piston, so the rods will be out of the jets all the time.

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Too many people who were born on third base actually believe they've hit a triple.

The carburetor is a Carter BBD of unknown origin. I think it is a later model with a bowl vent tube, and the horizontal accelerator lever. I rebuilt the carburetor myself, and everything looked fine. I replaced the floats and and choke pull-off vacuum. I know vacuum reaches the step up piston because the needles are sucked all the way down with the engine running. The only thing I could think of is oversized jets?, Beside the carburetor, could anything else induce such a giant drop in mileage?

I am not sure what is wrong but I can tell you this. There is no way your gas mileage should be that low.
I put a Super Six on a 75 Dart. My mileage went from 17 city/24 highway to 16 city/22 highway.

I'll venture a guess here... if you really have checked everything with the carbruetor carefully, find out if your timing mis-set.

Wow good call newport77. Turns out the vacuum advance was not working at all. I ordered a new vacuum advance diaphram, and am waiting for it to come. The timing at idle checked out to be 5 degrees before TDC. The specifications in the engine bay say that the timing should be exacltly TDC at idle. Can mildly offset timing, and no vacuum advance really kill gas mileage? also what exacltly does timing advance do? does it stop pinging, or does it cause pinging? I will fix these things and see if my mileage goes back to normal.

5°BTC, if you can run it there with no pinging, will give better driveability, performance, and fuel mileage, compared to the TDC setting.

A blown vacuum advance will definitely hurt mileage, but I suspect there are still other issues to work out besides the blown vacuum pod.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

5 degrees BTDC is fine.
It is actually BETTER than TDC- so long as you don't have much pinging on acceleration. I have the base timing set on my 75 Dart at 5 degrees BTDC. You can try 2.5 degrees BTDC if you get pinging at 5 degrees.
The factory setting, TDC, was done mainly for emissions reasons, I believe.
I think personally that setting is too retarded. Most engines are a few degrees BTDC.
Timing that is too far retarded can play a role in causing the engine to overheat. I had a lot of trouble with overheating when my base timng was TDC, much less so with 5 degrees BTDC. I mean it would overheat slowly, building up heat slowly, especially if idling in traffic for long periods.

I think a vaccuum advance that doesn't work CAN play a role here, but I would also check to make sure the mechanical advance (centrifugal weights) are working properly.

If this does not help, go back to the carb. Make sure the float is not set too high, though it sounds liek you checked that. What about the diaphragm on the accelerator pump? Did you replace it when you rebuilt the carb? Fuel could be leaking past it if its dried out or cracked. A missing check ball in the accelerator pump system could also cause extra fuel to be drawn into the engine.

Do you have the step-up piston setting correct?

Is the choke releasing all the way?

I have not rebuilt a Carter BBD in several years so I do not remember it very well now... but keep in mind fuel could be drawn in from a circuit that it shouldn't be drawn from if there is a spring or check ball missing, if there is an incorrect gasket somewhere, or if the fuel level in the bowl is too high.

Did you check the needle and seat? Is the needle not seating properly, causing fuel to leak past? You could have set the float right and the fuel may not stop flowing in due to the needle not seating properly.

You should know if your carb is flooding. The engine will be hard to start, You will see fuel dripping inside the carb when the engine is off. You will probably smell gas also.

How do I check that the distributor's mechanical advance is working?

as for the carb, the float level is low, the step up piston is completely screwed in, the accelerator pump is new, and all the check balls are there. I can't figure it out, so I'm going to take it to a rebuild shop and have them take a look at it.

With a timing light.
Unplug your vacuum advance and measure change from in timing from idle. You may be able to see the change if your initial is at zero. The mark should move over to BTDC at least 10 degrees if not more.
To accurately tell you need an adjustable timing light. It should move over to about 20 degrees.

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74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

Ok, so fixed the vacuum advance, set the timing to 4 degrees BTDC. The car ran a lot better.......... for one day. The next day the car borke down. It starts cold, but the engine is pretty weak. As soon as the car warms up it stalls under load. It will idle in gear when warmed up, and will rev in nuetral, but any acceleration will kill the engine. I rebuilt the distributor, checked the gap, set the timing again, replace the balast resistor. Nothing changed. I thought the engine wasn't getting fuel, so i took the jet metering needles out, and raised the float level, and still nothing changed. Any Ideas about whats wrong?

Yes, a week spark. Sounds like your coil needs to be replaced. Check the voltage going to the coil too. Do you have an ECU or points?

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

Hold on a second, let's not just throw parts at it. Coils can fail, but it's not all that common.

Do you have a VOM or multimeter?

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.