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MG MGB Technical - Emissions Problem

My 72 MGB, with a '69 engine and 25d dizzy, stock exhaust, no air pump or PCV valve, but evap canister, in for emissions and failed. the test is called a "loaded idle" test, run slowly on a dynomometer to "warm the car up" and is supposed to be a tailpipe probe only at idle. Hydrocarbons were at 1457 (limit is 800 and CO2 was around 4 (limit is 8). I readjusted the carbs leaner, using a gunson Colortune, and took it back in, This time hydrocarbons were above 2000 and CO2 was around .02. I had passed two years ago with a Weber DGV on it but have since swapped back to SUs.

I was planning to readjust using an exhaust gas analyzer that registers CO2 and set it so CO2 is around 6.0. Everything has been checked, adjusted, tuned, or replaced prior to the first test so the carb adjustment is the only area to go through. This is my last "allowed" test before I'm subject to a visual inspection of my car for all of itsemission components, which it doesnt have.
william fox

These engine run best with a CO2 of around 6 so this might bring the hydrocarbons down but double check the ignition timing and all the ignition components - it could be that either unburnt fuel is getting into the exhaust through a misfire or it is oil being burnt to give the high hydrocarbons.
Chris Betson

William. Something is seriously out of whack here. In Arizona, 67-71 cars can have 5.5% CO at idle and 500 ppm unburned HC. I meet these standards with SU carbs (new about four years ago) and no air pump. (They are not required to check for equipment on cars made before 75. But, since the 68 has its VIN in the engine compartment, they get a glance at the equipment anyway. So far, they have not failed me for equipment, but this may happen sometime in the future. We also have a "loaded" requirement which is 4.2% CO and 500 ppm HC.) So, if my car can pass in a similar configuration to yours, your car should be capable of passing if everything is working right.

I agree with Chris that a total check out of the ignition system is in order. The low CO reading you are demonstrating can induce a "lean miss" which results in high unburned HC. So could a leak in the fuel induction system or excess fuel being pumped into the system, such as a stuck float. I am not fully sure exactly what a "loaded idle" means. To me, a car is either idling, and not under load, or it is under load. You seem to be describing a loaded condition at low engine speed. What needles and filters are you running? I run AAA needles for day to day operation, but find they are just a hair too rich for emissions testing. Hence, I run a slightly leaner needle (AAE, I believe, can check if necessary) as my "inspection needles". This might work for you. I would also check the valve clearances, do a compression check and change the oil and filter.

If you have a competent emissions shop in your area, you might want to have them check the engine out. I have a Gunson's CO meter and what I set the mixture at with it seems to have no relation to what the inspection station readings are. The Gunson's meter is set to a reading of 2.0 in ambient air, while the inspection station meters are tested using some form of standardized test gas. I suspect, in my area, the pollution renders the Gunson set up procedure inaccurate. Les
Les Bengtson

Bill, I really think you need to rebuild those carbs. Your car is doing the exact same thing mine was doing two years ago. I played with that Gunson's tester there at John's shop, but each time it wouldn't pass. I even stuck all the emissions equipment back on, but it didn't help. I ended up having to get a waiver that time. This time, after putting on my rebuilt HS4's, I didn't do squat before I took it in (no oil change, super unleaded, fuel additivies, adjustments, etc) and it passed with flying colors.

I was going to say that the only difference between last time and this time was the rebuilt carbs, but now that I think about it I was using HIF's and a 25D ported off the intake. I now have a different distributor (another 25D) and then the rebuilt HS4's (correct for my car). Either way, I really believe it was the carbs that did the trick.

Robert
Robert Rushing

Thanks Robert. I got the carbs I have were Drew's if you're familiar with them. Les: I have aftermarket air cleaners- the chrome MG logo'ed ones from Moss. I have no idea what the needles are. We went through the ignition-new points, condensor, rotor, cap, timing, dwell. In ran a lot better after we did all of this. I'm was thinking along the lines of a too lean mixture as I've read postings that indicate CO2 and HC's read opposite each other- one is high while the other is low. Since my CO2 dropped when my HC went sky high I was going to try that route. The hell of it is my 73 GT went through using a set of carbs I scavenged from a parts car that sat for several years. I hadnt worked with Su's before and stuck them on, set as best I could with no special tools, and it passed with no problem.
william fox

William,

I agree with the others that a tune-up should help. Including rocker adjustment (loose better than tight), spark plugs, timing and dwell. Fresh oil change is also a good idea.

We will assume the engine is mechanically sound, and you have the stock '69 HS carbs with this engine, and the air filters are clean. This was the first year for fixed needles. You may need to replace both needles and jets if this hasn't been done in a while.

Notice that leaning your mixture raised the (HC) levels, and lowered the CO. I would advise you to go in the other direction (rich). Use you colour Tune to dial in the flame until it just changes from blue to orange at idle. I recently passed our local inspection using this method with my 1969 midget without the air pump. (400 ppm HC, and 4% CO)

Les,

Here in Delaware we have (6 & 6) requirement for 1968 thru 1972 cars (600 PPM HC, and 6% CO). Anyone know we see differences between states for the same cars?

Regards,

L.C. '74 B/GT, '69 Midget
Larry C.

William,
I was easily passing all emissions tests until they added MTBE to the gas throughout the state. From that time on, I had problems meeting emissions. Every time I adjusted to lower HC, the CO would go up, and vice-versa.

I recently discovered that the rear carb had a flooded float - probably has been gradually filling up with gas for quite a while, and may have contributed to my emissions problem over the years.

There are some gas-adative products on the market that DRASTICALLY reduce HCs, but I'm not sure they would be recommended for continuous use, and it would obviously be better to "set it right." The real problem is that all too often, the emissions test becomes the diagnostic tool.

Good luck,
Pat
Pat

Bill, I would switch carbs with the GT. Fill up the tank with premium, change the oil, and then make sure everything is tuned with the GT's carbs.

I bet you'll pass.

Robert
Robert Rushing

I had given that some thought...
william fox

This thread was discussed between 16/10/2002 and 17/10/2002

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