I took my 72 GS to a chassis dyno last Friday. Didn't get much info since the hardware stack crapped out after 2 pulls. My first pull had an air/fuel reading in the 18-19 range (Really lean). I had 93 jets all the way around. I then put 99's in front and 98's in rear. It got down to 13-13.5. It did dip into the 12's beyond 6000 RPM. What is going on here? Why would I have to run 98's and 99's in a 1090 dominator carb? Combination is as follows: 464 cubic inches, 12.69-1 compression, heads flow 275 CFM peak, TA intake, TA308S cam, 2" headers, MSD distributor, MSD 6AL box. Fuel pressure stayed constant at 7 PSI, I did these pulls with open headers. Also, my new 9" converter is off. It appears to be closer to 5000, then the 4200 it was suppose to be. Once the converter fully locked up, the power curve fell off a cliff. It had 417 HP at 5200 and dropped to 350 HP at 6000. Any suggestions on my fuel issues? The dyno guys also suspect a faulty coil as well....based on the lack of power once converter locked up. Thank you. Tom Jenkins
Something sounds funny. At 18-19 air fuel ratio you would experance lean pop condition and burnt plugs. You could be leaking air somewhere in the exhaust system which would give faulty readings. If your headers don't glow red then you are not that lean. As far as jetting goes I'm not sure 98s are to big depends on the booster. I would concider bigger main air bleeds to lift more fuel in the mid range to fatten this area up (larger air bleed does not lean, it makes the emulsion less dense and lifts more!). If you where not as lean as there wide band indicated then you just went very fat which will kill power under heavy load. This is also hard to ignite. I would say thier tuning help was a bust. Don't trust it, trust but verify!
David, The plugs looked very lean. I think I also have some of the lean pop that you mention. I drove the car 20 miles @ 2800-3000 RPM and thought I had some faint detonation. I was running pump gas with only a gallon or 2 of 112 with 12.69 compression. I drained the tank, put new plugs in, and added 10 gallons of 112 before the dyno. I also removed the exhaust. After the dyno, I put the exhaust back on and had the same 'popping' even with 112 in the tank. I will check the air bleeds tonight. Thank you. Tom Jenkins
Did The Dyno Operator Have Any Advise? Do You Have Known Good Carb You Can Try? I Would Contact Holley Or Barry Grant's Tech Line And Ask Them If This Seems Normal. Good Luck
I'm confused in what the hardware stack is. if it's part of the dyno, could it be the circuit board reading wrong before it finally took the rest of the system down.o No:
I will call Barry Grant and see what they say. The hardware stack is part of the dyno. It is where all the cords and lines were hooked to. They did change the circuit board. You may be correct about it reading wrong before it went down. I guess I didn't get anything accomplished during this dyno session. I did a 'gas can' test on my pump last night. Barry Grant's web site says I need to fill a 1 gallon gas can in 25 seconds or less. Since I didn't have a can that could be read easily to 1 gallon, I turned my pump on for 25 seconds to see if I had 1 gallon. I had between 1.5 and 1.75 gallons......pump volume is good. It held good pressure on dyno. Carb is the problem I guess. I hope to get this solved quick. I will be racing at the Pinks event in Indianapolis next weekend. Thank you all for the replies. Tom Jenkins
I didn't ask before, did you go thru your Demon before installing it? They make all parts from machined billet alluminum. This can leave lots of small bits of flash and left overs. Supposedly, they start tumbling to parts to solve this issue. I always strip a new demon down to parts carefully blowing thru cleaning everything. Take the jet plates off and use carb spray to blow thru from the air bleeds out. make sure the stream is consistent. Do the boosters via the large feed hole. Then the idle circuit from the idle feed hole right about the the transfer slot at the throttle blades. Get the transfer slot while your at it. The jet plate has several emulsion holes, 3 or 5, get each one of those looking for any signs of debris. Wipe out the fuel bowls. During reassembly look at how much of the transfer slot is showing. Should be about .030 which makes it look like a square showing below the thottle blade. As you add on the bowls it's time to check the pumps. The pump arm should be tight enough that the tinyest amout of movement will make some pump shot but not so much that idle vibration makes fuel bleed out. Preset the idle mixture screws to 3/4 turns out. After starting the motor set the center idle air bypass under the air cleener stud. Your looking for the smoothest idle or best vacuum. Now take the motor to 2500 and hold that rpm, your looking overly hot headers. If so turn out the mixture screws until this condition stops. Setting mixture is another issue altogether. Getting a LC1 wideband is good.
David, I will check everything you mentioned. I did send the carb to Barry Grant a few years ago, and they went through it. I actually got this carb off eBay. Once I got it, I sent it to BG with all my engine specs. I will be checking everything you mentioned. Thank you. Tom Jenkins
Your engine would not run @ 19:1 A/F. Sounds like you may have experienced a misfire, which will cause a lean reading because you are measuring O2%.
Where is your O2 sensor mounted, are you running full exhaust? the sensor can't be to close to the outlet or air entering from the outlet can throw off your readings. Also exhaust leaks or misfires can cause false readings.
An O2 measures oxygen not fuel. If the mixture is unburned it will detect a high amount of oxygen in the exhaust. where the sensor is placed is also critical. If it is placed near a leak, then @ low speed you will see hi O2 readings @ low speed.
The sensor was placed inside the end of the headers. It wasn't one that is installed on the car. I was running open headers on the dyno. My car has a history of running lean. I was shocked by the high numbers. The highest numbers were at lower RPM. At higher RPM it was 13.5 @ 5500 and dipped to 12.1 @6100. Seemed fine 5500 and above, but still can't figure out why it took a 99 jet to get this. A 93 jet had me in the mid 14's at high RPM. Hopefully I solve this tonight. I'm leaving for Pinks in Indy early tomorrow AM.
You are of course correct. The innovate website is full of lots of information on a a/f ratio and wideband tuning. "1) An exhaust leak will allow oxygen to enter the exhaust stream and therefore will measure leaner than the engine is actually running. For correct measurement, air-leaks in the exhaust MUST be prevented under all circumstances. 2) Missing ignitions (where the air-fuel mixture does not ignite) also pump unburned oxygen into the exhaust and cause the LM-2 to measure lean. 3) The only circumstance where the LM-2 will measure richer than the engine is running is if the pressure in the exhaust tract is excessive (and the engine is running on the rich side to begin with)."
Find out what the H:C ratio of your fuel is.Make sure your fuel is not oxygenated. If it is then the wideband you are using will have to have an offset to compensate for the extra oxygen in the fuel.
The higher the H:C the higher the A/F to reach stoich. EXAMPLE: Fuel with an H:C of 1.5 will have a stoich number of 14.06:1 Fuel with an H:C of 2.1 will have a stoich number of 14.92:1 Fuel with an H:C of 1.87 will have a stoich number of 14.60:1 When you add O:C to the fuel (oxygenated) you get H:C 1.5, O:C .037 = 13.285:1 for stoich H:C 2.1, O:C .037 = 14.14:1 for stoich So if H:C is higher than 1.87, then you may not be running as lean as you think.
The O2 sensor doesn't really detect the amount of oxygen in the exhaust, it measures the tempeture of the exhaust. Hot exhaust = lean ... cool exhaust = rich... The sensor sends info to computer to keep the temp at a range that may be preset into the sensor or ajustable by a F.A.S.T system or any other system capabile of controling fuel mixture.
Don, with respect, the comment about an O2 sensor being a "temperature" sensor is incorrect. It's actually a dual-electrode electrochemical zirconia element that produces a varying voltage depending on the amount of oxygen present in the exhaust. The ECU can decide what to do depending on this voltage and other inputs like manifold pressure, air/engine temp, rpm and throttle position. Devon
I was going back to the early days of O2 sensors and the CCC systems of the early 80's working as a tech at a Pontiac dealer..lol.. having a old guy moment...lol ...well that isn't is really that funny...
