Terrible fuel mileage and black soot coming out of exhaust
 

I am thinking something is not right with my carb. Im getting terrible fuel economy, like around 7mpg mixed city and highway. I’m trying to keep my foot out of it and it’s still pretty bad. Also, a lot of black soot coming out of the exhaust when I start it after it has sat for a while. the condensation in the exhaust clinging to the black soot in the pipes and blowing it out on my driveway which is extremely annoying.

I had Harry at HK Motorsports tune the carburetor about six months ago and it seems like I was getting around 12-14 mpg afterwards.

I have noticed the car has been running rather rough even when warm so it dawned on me to check the choke. Sure enough the choke was partially closed when the engine was warm. I adjusted it slightly so choke is open when warm and it runs much better. I will check it tomorrow morning when cold to make sure it working properly. I have no idea why this changed? Maybe because it cooler out now and the engine stays around 160?

I am no carb tuning expert. Far from it. Do you guys have any ideas on why so much soot is blowing out of the exhaust? The exhaust smell is really strong when sitting at idle which makes me think it running rich at idle. What should I be checking for?

Engine is a 468 stroker, iron 670 heads (ported), factory intake (ported), and a Quadrajet from cliff (17059201).

Does it still have the issue after you fixed the choke?

Good question. I’m not sure. Haven’t really driven it since this morning when I fixed the choke. Do you think that was my problem?

It was a problem. You now must drive it to see what difference it makes.

With Choke open or wired open, go for a 15 minute highway run, with some hills and see if the plugs burn clean, if not then time for fresh plugs.

Do not run the motor any more, as running while something is wrong ( like with the Carb) to that degree of richness will only serve to wash out the rings and ware out the cylinder walls in short order!

If you ask me your Carb needs to get gone thru , if your lucky it's just today's gas that has gone bad and is blocking some passages and or hanging up some part.

I have also seen new brass floats go bad at times, not a lot ,but a few.

Sean,
How about taking a ride back over to Harry's shop and give him 1st opp to fix it?
Maybe the dist clamp is allowing the dist to slowly move...
Jeff

If all else fails,get the car over to Sean at SMI in Huntington Beach!There is another board member here that was having carb problems with his 69 bird and he got it straightened out.Sean does great carb work and drives a pontiac.Tom

I had a 462 6X8 ported, 041 cam, 9.3 compression, Cliffs Quadrajet. I was happy to get 10 I usually got 8.. I wouldn't expect to get any kind of decent gas mileage out of a ported motor with a long duration, wide over lap camshaft..

160 is kind of cold, maybe try a 180 thermostat..

The function of the choke is to enrich the mixture, when cold. If the choke is still partially closed once the engine is hot, the choke will still enrich the mixture. You state you have fixed this.

Check the oil. If the mixture was so rich as to cause engine damage, the oil will smell gassy, and the level on the dipstick might show overfull.

Since you have corrected a potential major issue; I would: (A) change oil and filter, (B) change spark plugs, and then (C) go drive the vehicle and see what, if anything changes.

Engine state of tune is one determinant for fuel economy, but others include final drive ratio, tire diameter, and driving habits. 7 MPG is a number, but is not necessarily bad, depending on all of the parameters.

EDIT: And I would echo the comment of 389 above to install a hotter thermostat!

Jon

Thanks guys for all the info. I’m planning on driving it today to see how things go.
I did change to oil yesterday and it didn’t seem overly smelly or overly full.
If the issue persists I will be giving Harry a call on Monday.

Just got back from a 30 min round trip drive. Mostly highway. Car ran great. Idled very smooth on surface streets.
I did fill it up.... 92 miles 10.9 gallons = 8.44mpg. Obviously the mpg is subjective depending on how I drive it. I do think the choke was screwing thing up some. We will see how this tank goes.
Here is a picture of the plug for #1 cylinder.

What RPMs were you turning at highway speed?Tom

Around 2700

Please provide a picture of some of the plugs such that we can see the full ground strap?

@sjdiesel

I am the member Tom is referring to. My Q-Jet electric choke would warm up, then move, making the carb run rich all the time. The only fix was to reset the electric choke, which I did five times since May. I finally got fed up with it and took it to Sean Murphy (SMI, Huntington Beach).

I would up going with a new Q-Jet, 800 cfm, new electric choke, the Stage 2 option.

https://www.smicarburetor.com/produc.../productID/429

Previously I was getting 7.2 mpg, now I'm getting 10.3 mpg. with spirited driving. Before I wasn't doing anything but being afraid to press the gas pedal for fear it would puke out more black smoke, fumes, etc. Final mpg report when I stop testing the kickdown acceleration of the secondaries on the I-5 freeway. LOL

I got to keep my old carb, as it had enough wrong with it (electric choke, and auto pull off bad, vacuum leaks, stripped fuel line nut, etc) that it would take longer to fix all of that then just start over, which is what I did.

My 69 Bird is a 406 / 9.75 compression / approx 325 hp engine with a TH400 transmission and 3.55:1 positraction rear end, 14" BFG 225/60-14 tires.

Sounds like what you're dealing with is what I was dealing with, so it's likely time to get it sorted out. Previously my carb had been rebuilt by two different carb shops, now I'm happy with the results.

No affiliation with Sean Murphy, no commission fees, just a happy customer.

Email me for more details if you like. sales at scumbackspeakers.com

https://www.smicarburetor.com/

FWIW; My Street Q-JETs have been Choke-free since 1990.

You can wire-open to see how you like. Then remove the plate & Shaft to really go comando

A comment about electric chokes, also chokeless operation:

First, in case anyone found this thread by search rather than browsing forums, this thread is in the "street" section, not the "race" section.

The electric chokes I know are useful to help start an engine if an aftermarket intake without the provision for an automatic choke is used AND one wishes some form of automatic choke.

MAYBE, someone has designed an electric choke with a temperature sensor so that the WOC (wide-open-choke) can be tailored to the temperature of the engine, but if so, I am unaware of it.

I learned my lesson with electric chokes almost 50 years ago when my wife's daily driver had an issue with the hot air choke, needed the car, and I didn't have time during the week to change exhaust manifolds. Installed an electric choke. After having the car towed from the stop sign three blocks from our house, where the engine stalled and would not restart, the week-end appeared, and I changed the exhaust manifold and reconnected the factory choke and no more problems. Incidentally, the wife was not happy about the 3 block walk (twice) ;)

Basically, that choke was timed. The calibration was about 45 seconds from full closed to WOC. With the STREET carburetor calibration I use, the engine would not restart after stalling a minute after starting with no choke.

That brings me to chokeless operation. I use a calibration that gives decent performance, but leaves a few percent horsepower unavailable for the benefit of fuel economy. Both my GTO and my shop truck cannot be driven in town in the winter with no choke. My calibration is simply not that rich.

I have a RAIV intake with no crossover on the GTO, so the factory choke could not be used. I tried an electric, and had to restart the engine at every stop sign! Finally gave up, and threw on a carb factory designed with manual choke. End of problem.

The shop truck has dual quads. I use manual chokes on both.

Even when the choke is open, the presence of the choke plate IMPROVES driveability in town, as it acts as a straightening vane to "straighten" the eddy currents in incoming air.

Jon.

If I had all of my carb issues to do over again, the first thing I would have done would be to weld a bung into my exhaust to hook up a wideband AFR gauge. You don't even have to find a place to permanently mount it in the car (though mine is in one of the old AC vents of the TA bezel). After you get your carb dialed in, you can take it all out if you want the car looking clean and put a plug in the bung and the whole setup on your shelf.

Forget about reading plugs, that's 9 parts voodoo and one part science. You can be lean at idle, rich at cruise, and lean with the secondaries...what is that plug going to look like?? No idea? Me either.

With the wideband you can know exactly what your AFR is in every driving condition and make the necessary adjustments.

I use and HIGHLY recommend an electric choke if your carb is either a "hot-air" model or already e-choke and your intake doesn't have the heat tubes for hot air, crossover blocked or not being used, etc.

The problem with the E-chokes is that they make about 30 different ones and many being sold are NOT for a Quadrajet and will "clock" upside down often interfering with drop base and some factory air cleaner bases.

The one we sell is a USA made part, clocks correctly with the terminal down, correct release time, excellent quality and we've had very few of them go bad dating back nearly 20 years now.

There is not one ounce of measurable performance lost using the choke flap and shaft in your carburetor, tested that deal out about 10 years ago, so no negatives from using it, only positives. A correctly working/adjusted E-choke will provide instant cold starts in any weather and fast idle for warm-up. Without a choke most engines are OK in hot weather on a cold start but you may find them difficult if not near impossible to start and keep running in really cold weather as I did with mine when I wasn't using one.......Cliff