383 running rich, using gas like crazy! Help!

[Triple Black 73]

My '73 Charger is back on the road after 20 years. Woohoo! The good news most of the bugs worked out.

The bad news is the engine is running very rich. I'm getting like 4-5 mpg. The car used 1/4 of a tank just going 25 miles today! I've been trying to read up on tuning the Thermo-Quad but it's just not clicking.

The engine is a 383 bored 60 over, 8.6:1 comp ratio, a new Summit Racing rebuilt '72 400 Thermo-Quad, stock '73 400 4bbl intake, stock '71 342 heads, CompCam XE256 cam, new OEM distributor, 8mm MSD wires, and Bosch 4236 Platinum plugs gapped at 0.035". My dad gave me a vacuum gauge, a timing light, and told me to have fun. Problem is, I ain't got the foggiest clue how to tune this thing. He said "ask the internet" Thank pops.

What I do know is that I'm pulling 20 inches of vacuum and timing is set at 10 degrees advanced. The engine idles between 650 and 700 rpm. I turned the idle mixture always the way in, then turn each 2.5 turns out. It now idle mostly smooth and revs decent. However, the engine produces stumbles at WOT and drinks gas like a drunk.

What are some good resources I can read to get the engine tuned? What are some of the things I can do to adjust it?

 

[GetX'd]

No help here - but I'm watching this thread closely bcause im having the exact same problem with my six pack. A bit different specs on the rebuild - but running super rich and drinking fuel like a drunken sailer.

 

[Lionized]

Try 1 to 1.25 turns out on the mixture screws.
Your vacuum is great. So is your timing.

 

[Triple Black 73]

Try 1 to 1.25 turns out on the mixture screws.
Your vacuum is great. So is your timing.

Makes sense. It was 5 turns out from the Holly. I guess Holly like to set them rich for Summit Racing. I doubt I'll be able to get it tuned right until I get a A/F gauge. What about adjusting the jets or this tune kit:

http://www.ebay.com/itm/221386284044

 

[texas69bee]

http://www.mopar1.us/engine2.html
http://www.4secondsflat.com
YouTube for carb tuning as well.
You already have good vacuum but check for vacuum leaks, especially around the base of the carb. Read your plugs to see if you can lean it out a bit. An O2 sensor made it much easier for me.

 

[Dennis H]

A friend tunes my 850 Demon by ear. Takes 4 screws and turns till engine drops rpm to a stumble, then backs off. Running sweet, clean, and you can tell by the exhaust smell no longer rich.

 

[Dibbons]

Just for a reference, driving my parents new factory stock '71 383 Road Runner with automatic and 3.23 ring and pinion to high school and back got 12.5 MPG no matter how conservative or crazy I drove it. Had the factory Holley with vacuum-operated secondaries.

 

[Dennis H]

Think I get 10-12 jerking around. Short trips, warm ups. WOT etc. On the highway with gears got 14ish. Mostly driving foolish.

 

[Triple Black 73]

Just for a reference, driving my parents new factory stock '71 383 Road Runner with automatic and 3.23 ring and pinion to high school and back got 12.5 MPG no matter how conservative or crazy I drove it. Had the factory Holley with vacuum-operated secondaries.

I use to get 14-17 mpg back in '98 with the same block and heads but with a Holly 2 barrel. Now thing is blowing soot all over the garage and driveway. It even flooded the other night moving it around the garage. Something is way way off.

 

[jimt1971rr]

Double check that the metering rod " crossbar" is moving properly. I had a problem with mine being slightly bent and it was preventing it from dropping down when engine vacuum was applied.

 

[Jackpot50]

how much fuel pressure?

 

[Lionized]

Makes sense. It was 5 turns out from the Holly. I guess Holly like to set them rich for Summit Racing. I doubt I'll be able to get it tuned right until I get a A/F gauge. What about adjusting the jets or this tune kit:

http://www.ebay.com/itm/221386284044

If it's running good other than the mixture, why bother getting into the guts?
Get that A/F gauge and let us know what is really going on.

 

[Triple Black 73]

how much fuel pressure?

7 pounds the last time we checked.

 

[Cornpatch MO]

I don't know anything about those carbs, but would expect float level set to high. I don't think jet size would make it run THAT rich........................MO

 

[Jcharger]

I am assuming you have already used the vacuum gauge to set the idle mixture. What I do is to set to just slightly richer than (1/8 turn) or at the value that gives you the highest vacuum reading at a idle RPM (the vacuum reading with the vacuum advance disconnected). I have only used Holley's but this should set your idle mixture. I have used an A/F to check the main circuit as well as the cruise circuit. My guess is that the MPG is coming from the cruise setting. For that you will have to look up how to adjust that with your carburetor. My guess is your main jets are probably not too far off. Main jets start about 3000 RPM give or take. I would then see if there was a way to work with the cruise. FWIW, I used to get 8-12 (max) with the stock set up and a 3.23 rear. When I changed to a different set up (2 x 4) (I was a kid), the in town dropped to about 5 mpg. With an OD on the highway, I could get up to 16 mpg. There is a reason we bought pinto station wagons in the 1970's.

 

[miller]

My dad gave me a vacuum gauge, a timing light, and told me to have fun. Problem is, I ain't got the foggiest clue how to tune this thing. He said "ask the internet" Thank pops.

Your dad is doing you a big favor!

First R-2800 I had to build, came up with a question. Went to the shop inspector, who happened to be the shop foreman, at the Pratt-Whitney plant that built the R-2800s, during WW2. He also held #3 A and P ticket...third man to get one.
So, asked...and he reached to the book shelf, pulled the 4" thick manual, slapped it on the desk...and said the answer is in there!

 

[miller]

I'm thinking something in the carb wrong. Maybe a float/valve hung open.

 

[kiwigtx]

Likely to be a needle valve stuck open and just dumping fuel through the carb. Helped a friend with the same problem in a Ferd up a mountain a few years back. Blowing soot and running rougher than a bears ***. Gave the carb a sharp thwack on the bowls with a heavy screwdriver handle.....immediate difference. Then set about adjusting the valves - primary & secondary - turned it into a different car.

 

[Frank Mopar]

Not to turn this into a carb primer thread but the carb has an idle circuit. Once you open the throttle blades the idle circuit no longer control anything. So you turn the screws, idle changes that indicates the blades are closed and you on idle circuit. Sounds like your good there so your can quit turning them.

What do the plugs look like ?

 

[moparnation74]

Pics of the plugs.....