Technical advice - running rich

[Larry T.]

I had my carb restored at the end of August. After installation I went through the process of making all of the adjustments. The car has run fine, but it has a strong gasoline odor coming through the exhaust. It has never seemed to load up or try to flood and stall. Before I decide that is just the way it is I wanted to get some input. The engine is an L-72 with the correct 3247 carb. I have R43xls plugs that look perfect after about 600 miles of running. I have adjusted the idle mixture screws so that I have the maximum vacuum while still running smooth (16"Hg at 850 RPM idle). Initial timing is set at 10* BTDC and I am getting 14* from my vacuum advance at idle. The only other thing I can add is that the carb has 72 primary jets and 76 secondary (I know nothing about jets, or the rest of the parts inside the carb).

I do not know what above is irrelevant or what is helpful, but I would like to know what I should be looking at as the culprit or if this is just the way it goes with this set-up.

[Michael J.]

Re: Technical advice - running rich

Are you getting any smoke out the tailpipe or any signs of liquid gasoline? Usually a car running very rich will foul the plugs pretty quickly as gas pouring in washes into the cylinders. The timing and adjustments sound perfect. I would suspect the jets or a leaking bowl in the carb if you are seeing signs of very rich running.

[Duke W.]

Re: Technical advice - running rich

Check your CSM to see if those are the OE jet sizes.

Setting the mixture as you did is the proper way for pre-emission cars, and it should result in an A/F of about 12.5:1, which is 20 percent richer than stoichiometric with about 6% CO and several hundred ppm HC at the tailpipe.

That's the way it was back then. A strong gasoline odor in the exhaust was not that uncommon back in the day, but it can catch us by surprise since modern cars have had none for at least 20 years. It can also come from fuel percolation in the float bowls and can be particularly noticeable after hot shutdown if you close the garage door immediately.

Modern cars essentially have sealed fuel systems, so the only time you should ever notice any odor is on cold start.

You could try leaning the idle mixture screws in eighth-turn increments until you pick up a little stumble or the idle gets unacceptably rough.

You should also get a handle on oil consumption as high oil consumption can considerably increase noticeable exhaust odor.

Check at idle if any fuel is seeping out of the venturi nozzles. If so, the float level is too high.

Also, your plugs are too cold. I'd suggest running the 45XLS for normal street and highway driving.



When describing a problem you can NEVER give too much information, and your description/test data was very good!

Duke

[Timothy B.]

Re: Technical advice - running rich

Larry,

Are you saying at IDLE the exhaust smells rich? I don't know how you could smell exhaust at cruise plus if the plug color is good that's a indication the cruise A/F mixture is good. If it's at idle, make your curb idle adjustments (side screws) after the car is warmed very well like after a drive.

The engine timing sounds good along with the vacuum advance and idle vacuum..You may consider moving timing to 12 BTDC but that's got nothing to do with your concern here. Keep in mind a lean idle will smell funny also, like it's rich and want to burn your eyes.

[Joe C.]

Re: Technical advice - running rich

I had my carb restored at the end of August. After installation I went through the process of making all of the adjustments. The car has run fine, but it has a strong gasoline odor coming through the exhaust. It has never seemed to load up or try to flood and stall. Before I decide that is just the way it is I wanted to get some input. The engine is an L-72 with the correct 3247 carb. I have R43xls plugs that look perfect after about 600 miles of running. I have adjusted the idle mixture screws so that I have the maximum vacuum while still running smooth (16"Hg at 850 RPM idle). Initial timing is set at 10* BTDC and I am getting 14* from my vacuum advance at idle. The only other thing I can add is that the carb has 72 primary jets and 76 secondary (I know nothing about jets, or the rest of the parts inside the carb).

I do not know what above is irrelevant or what is helpful, but I would like to know what I should be looking at as the culprit or if this is just the way it goes with this set-up.

Holley listing shows 3247:
primary: 122-70, secondary: 122-76
If you have 122-72 installed on the primary side, they will be just fine.
How many turns are your idle mixture screws backed out in order to get the best idle?

[Larry T.]

Re: Technical advice - running rich

You could try leaning the idle mixture screws in eighth-turn increments until you pick up a little stumble or the idle gets unacceptably rough.

You should also get a handle on oil consumption as high oil consumption can considerably increase noticeable exhaust odor.

Check at idle if any fuel is seeping out of the venturi nozzles. If so, the float level is too high.

Also, your plugs are too cold. I'd suggest running the 45XLS for normal street and highway driving.

Duke

I can lean it out a bit without much change in the idle, but I do lose some vacuum. I do have a 360 VAC so even at 14-15"Hg I should be fine, but could I experience any performance change, i.e. a stumble on initial acceleration?

What is considered high oil consumption?

No seepage, also when I remove the inspection plugs the level is always just below the holes. If I rock the car slightly I can get a little bit to overflow from the holes.

I have some 45xls's I will put in. If the 43xls's were not performing well I would have expected the plugs to have some discoloration or even carbon building up.

Are you saying at IDLE the exhaust smells rich? ...If it's at idle, make your curb idle adjustments (side screws) after the car is warmed very well like after a drive.

Yes, it is at idle and the carb adjustments have been made when hot.

How many turns are your idle mixture screws backed out in order to get the best idle?

1 3/8 turns on each. I can get a good idle turning them in another 1/8th to 1/4 turn, but I lose some vacuum.

Thanks for any more responses and follow ups here.

[Larry T.]

Re: Technical advice - running rich

Are you getting any smoke out the tailpipe or any signs of liquid gasoline? Usually a car running very rich will foul the plugs pretty quickly as gas pouring in washes into the cylinders. The timing and adjustments sound perfect. I would suspect the jets or a leaking bowl in the carb if you are seeing signs of very rich running.

No smoke and no liquid fuel. No fouled plugs. When you say leaking bowl do you mean float? How would a leaking bowl exhibit itself? How would I check that? How about the jets? They were installed new six months ago. What do I look for?

Thanks,

Larry

[Bill I.]

Re: Technical advice - running rich

As already said, check float levels in both bowls. Use the 45 series plugs. Bill.

[Clem Z.]

Re: Technical advice - running rich

i would check to make sure all the bowl screws are tight as fuel can be pulled around the gaskets. i would also check the base to main body screws for tightness

[Timothy B.]

Re: Technical advice - running rich

Larry,

Everything is about right with your engine, I can suggest some things but that gas smell is the nature of the beast. My 67 300hp car is the same way and clothes smell from exhaust after you get away from tinkering with the car.

1. If you find the curb idle adjustment screws do not effect the engine idle enough, take the carburetor off and look from the bottom and adjust primary throttle blade opening (idle screw) to where you can barely see light through the bottom of the transfer slot. Try to keep the transfer slot above the throttle blade just so there is light. There will be no tip in issues and curb idle screws will control idle.

By doing this, you will need to adjust engine idle with the secondary stop screw so remove the screw and get a home depot allen head screw because the holley screw is very difficult to adjust.

2. Clem's suggestion is right on about making sure the bowl screws are tight, Holley's are modular carburetors and gasket seal is the root of most problems. Don't strip the screws..

3. You can take the primary metering block off and get a small drill bit set and verify IFR (idle feed restriction) size and air bleed size, that will tell you if A/F ratio is in factory spec and if someone improved it along the way.

4. When the metering block is off you can check the flatness of the main body and check the metering block to see if warped. Remember about the gasket seal.

5. If you are so motivated, check the base for flatness and make sure the base screws are tight. There is good information in archives as all of us have been down the same road with Holley's.

6. If everything checks out remember my first paragraph and don't take wife on a date because after your married they complain about everything and gas smell will come up in conversation..

[Jim T.]

Re: Technical advice - running rich

Larry one other area to check is the power valve. Older Holleys do not have the later modification that protects the power valve. Easy check of power valve, set the parking brake, start your car, remove the air cleaner top and put a finger to close off the front vertical fuel bowl vent. If your front power valve needs replacing, the engine will die within a few seconds of closing off the vent. If the valve is good the engine will continue to run, but will eventually die if you leave your finger on the vent for a much greater length of time.
Power valves I have had to replace ruptured with just a small burp through the carb when the engine was cold and stalling the engine on take off. A back-fire on start up can also rupture a power valve.

A stripped fuel bowl screw thread can be easily repaired with a HeliCoil 12-24 size 770-3043 thread repair kit. (part number is over 20 years old). My Holley's front fuel bowl retainer screws screw into Heli-Coil repaired treads. The new stainless steel threads will give great service for future fuel bowl and metering block removal.

[Duke W.]

Re: Technical advice - running rich

"I can lean it out a bit without much change in the idle, but I do lose some vacuum. I do have a 360 VAC so even at 14-15"Hg I should be fine, but could I experience any performance change, i.e. a stumble on initial acceleration?

What is considered high oil consumption?"


Loss of a little manifold vacuum is not a big deal as long as it's consistently above 12" and the VAC pulls to max at no more than 12". The idle might be a bit rougher and could stumble on takeoff. It's something you have to experiment with including idle speed, and I recommend about 900.

Tell me your oil consumption and I'll tell you if it's excessive.

Duke

[Larry T.]

Re: Technical advice - running rich

Tell me your oil consumption and I'll tell you if it's excessive.

Duke

Duke,

I feel like I use more oil than I should. Somewhere in the neighborhood of 1 quart every 500 miles or so. One thing I will add to this whole situation that I should have mentioned probably is that whenever I take the car out I get a slight black residue on my lower rear quarters out of my sidepipes. When I wipe it off it is powdery, not wet. I have not decided if it is fuel or oil related. I pulled 4 of the plugs to see how they look and realized the plugs I put in 600 miles ago were R45xls's. What do you see when you look at the plugs? It looks like they are burning well, maybe slightly on the hot side, but there is clearly some oil some of the threads.

Larry

[Duke W.]

Re: Technical advice - running rich

Unless that engine is an original Tonawanda-built L-72 with over 100K miles a quart every 500 is definitely high.

SHP engines consume more oil than base engines because of the loose piston clearance. A tight SHP small block with the OE forged pistons should make at least about 1500 miles per quart and maybe only 1000 for a SHP big block, so yours is high.

What's the engine's history?

The deposits on the plugs look like oil additive ash, and the oil on the threads is typical of engines with high oil consumption. The "5" heat range keeps the tips pretty clean even though a lot of oil is being consumed.

Powdery carbon deposits in the tail pipes are usually due to excess fuel. Wet black deposits are usually oil as all of the oil that passes through the combustion chambers does not burn, and it condenses in the pipes and back of the car to form a wet, sticky residue.

Set the mixture as lean as you can without developing any driveability problems like start-off stumble, but I suspect the odor is from oil consumption.

Duke

[Clem Z.]

Re: Technical advice - running rich

every new BBC i ever owned and i owned 5 never got more that 600 miles per quart when new. they used oil but the exhaust systems never rusted from the inside.