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MG MGB Technical - Failed Smog

I did everything that Eric Wilhelm said:

Increased valve clearance by 0.005
Increased spark plug gap to 0.032
Retard ignition by 3 degrees
Adjust carbs lean
Remove air filter elements
Made sure all smog equipment was present and working

Still, my '74 MGBGT is a gross polluter at idle. It cleans right up at 2500 RPM. Can anyone think of what I might be missing - aside from buying new carbs?

~Dan
Dan Neu

P. S.

This engine has 33,000 mi since complete (long block)rebuild
Dan Neu

Check the thread titled "Fuel Stabilizer and Smog Check", regarding clearing the crap out of your air injection rails (this is VERY important)...your '74 doesn't have the cat converter, so the air injection rails are even MORE important). Consider relocating to a less hostile state...once you've spent a few nights in the desert and stepped on several sandy beaches with too many rocks and way too much pollution (they used to be better when I was a kid), you've seen it all anyhow. I spent four+ decades in that particular regime...there IS life elsewhere (I just love reminiscing about CA after a couple of beers ;-) Hey, if you can hold out for one more year (and CARB doesn't get it's way by eliminating the older car exemptions), you'll be able to non-op it this year and register it free and clear in '03.

BTW, I'm not sure what you mean by removing the air filter elements...not a good idea. If you're replacing them with new, then great (this IS recommended). Increasing spark plug gap? If they're firing, then that's it...using a "bigger match" doesn't make the fuel burn any better, assuming you have sufficient spark to bridge a larger gap anyhow.

Have fun,

Barry
Barry Kindig

Are both the CO and HCs high? This might give
a clue as to the problem. High CO with low HC
means too rich. High HCs and low CO means
too lean.

Maybe check compression on the off chance there
is a burned exhaust valve.

Ronald

This is odd - I just went through smog on one of my vintage ragtops and the tester said idle isn't measured anymore - he only tested it at 15 MPH & 25 MPH whilst on the dyno . he said they do not look at idle anymore , good thing too as mine was too fat
@ .05% CO and 500 PPM HC , I'll go tweak it leaner next time he's slow .

-Nate
in So.Cal.
Nate

The new smog law has two different sets of rules. One is for "enhanced areas", which is where pollution levels are higher, I think. If your registration zip code is in one of these areas, you have to do the rolling road measurement. If you're not, then the old style measurements at idle and at 2500 RPM apply. Where most cars fail is at idle, tuning discrepancies are more apparent as there is less volume of exhaust gases. So if you have a small vacuum leak, it may not be seen at 2500 RPM, but it will fail you big time at idle.

Does this answer the question on the other thread about why Barry is happier in NJ than San Diego?

Dan,

Post your complete results, it will help to diagnose the problem, as Ronald notes. Also, let us know where you live, we may know a shop or mechanic locally who can help you.

Nate,

It does look like you're rich, but sometimes high HC is an indicator of a tuneup problem (incomplete combustion). My 77 had failed on high HC at idle, but passed everything else before I bought it (for $100!). A cap/rotor, plugs, and wires later it passed w/flying colors.
Paul K

Install a pertronix Ignitor. And, if all else fails, use a straight pin to punch holes in a vacuum hose to admit air. Drive the car to the inspection station with unpunched hoses. Swap the hoses to the punched ones before the process begins. Swap again after. You'll have to experiment to find the correct number and diameter of the holes to punch. If you can purchase or borrow an air/fuel ratio meter it can provide guidance.
Charles Edwards

Dan,
As Paul indicates, high HCs (unburned hydrocarbons) don't necessarily equal too rich. If you imagine a graph with HCs on the vertical axis and air-fuel ratio on the horizontal axis, the graph will be somewhat bowl-shaped. (http://www.sjmautotechnik.com/20vo2.html)
The lowest HCs should occur around 14.7:1 (the ideal ratio). Best power will occur around 12.x:1 (don't remember exactly -- would have to go dig to find it) at the expense of slightly higher HCs. As you go richer you get higher HCs because there's too much fuel. As you go leaner you get higher HCs because the fuel becomes to lean to support complete combustion. So being out of adjustment in either direction can cause you trouble. Also things like leaking rings or valve stem seals, although at 30k mi I wouldn't think that a likely culprit.

When technicians with exhaust gas analysers set up a fuel system, they try to find the optimal balance of HC, CO, CO2, and O2. You may want to find someone with a four- or five-gas bench to help you.

Rob Edwards

Dan,

My advice is to bite the bullet and buy a gunson gas analyser. I got one years ago just for passing smog. For me, it paid for itself first time out, in less wasted time and frustration. It can be a bit fiddly to set up, but if you're kinda anal about reducing variables, it does work well for optimizing the mixture on each carb. And then after you pass, you can use it to reset the carbs the way God and Cecil intended....which is for maximum power. When I got mine, they cost about $125. Don't recall how much they go for now.
Terrence Goodell

Dan, before you buy new carbs have you replaced the needles and jets. This is essential in getting the emissions low at idle. Even with poor and old needles and jets it will normally pass at 2500 rpm.

You may as well get the carb rebuild kit with all new gaskets and seals and ensure that there are no leaks. I just had mine rebushed and new shafts installed at Apple Hydraulics. About $100.
Good Luck,

Andy 74 MGB GT.

See my problem with "fast idle tick over."
Andy Preston

Yes Paul but you neglected to look at my CO level , @ .05 % it's running _way_ to rich !
I'd not like over .02% . I realize the other points about high HC readings but you also need to look at the CO , what _I_ do is to watch the meter whilst running the CO down untill the HC begins to rise - I like to set my carbies to the lowest CO I can with also the lowest HC levels , usually runs .02% CO with under 300 PPM HC .

Most times there's 'wiggle room ' where I can fatten up the CO just a teense after bottoming out the HC level .

YMMV .
-Nate
Nate

Nate, I didn't miss it. You know what you're doing. The comment was meant to say that high HC by itself isn't necessarily an indicator of too rich a mixture. Thanks Rob, for an excellent explanation.
Paul K

Thanks Guys,

Here's the results of my unofficial test. Meaning, the results didn't get sent to the state. I live in Palo Alto.
HC: 1687 ppm, NO: 49 ppm, CO: 0.26%, CO2: 7.6%, O2: 10.4% Idle @ ~900 rpm
I didn't get a print of 2500 rpm readings, but they're within spec.

The things I can think of are:
A) Starter Valve - When I blow into the fuel inlet with all the all the other outlets plugged, some air comes out of one of them.
B) #2 spark plug - It had some carbon on it when I took it out to clean it and increase the gap to 0.032
C) Air Pump - I don't have a gauge to measure the air flow as per the Bentley manual.
D) Air nozzles - (I don't know what they're called) They fit in the head where the air injection manifold connects to it. I only have two of them. (Moss has none.) I cleaned out the holes with a drill (Finger powered), and left out the two nozzles I had.

I'll fix what I can this weekend and order new starter valves and gaskets. The needles and jets are almost new, but I'll check them again before I order parts.

Thanks Again,
~Dan
Dan Neu

Eric Wilhelm wrote a column in Moss Motoring years back. The mods he recomended were temporary measures to get the car to pass - not as long term fixes. I would never run my engine without air filters long term. Only during smog check.
~Dan
Dan Neu

I'm no technician, but the HC and O2 look high. One thing to consider though is when tuning the car and using exhaust gases as a gauge, you must disconnect the smog pump. If you're blowing fresh air into the exhaust stream, it will throw off the readings....

CO doesn't look too bad. Ideally CO should be 0%, but you'll never achieve that. NC allows 2.5% for older vehicles. You might try richening the mixture a tad and seeing of that makes the HCs come down.

Here are some more resources I found (some of it fuel-injection specific, but good explanations):
http://www.type2.com/library/exhaust/comix.htm
http://www.type2.com/library/exhaust/emissio.htm
Rob Edwards

Dan

Sounds like you have done everything needed to get a good emission performance. But don't lean out the carb so much if you have an air pump.

If you have a working air pump properly connected then adjust the carbs for smooth running exhaust (a bit rich). When using an air pump adjust the CO (with air pump disconnected) to about 1.5 - 2%. If you don't have a CO meter, then lean out each carb until it starts running rough, and then back it up 1/4 turn. If it does not start running rough (shakes a bit or misses) as you lean it out, that means the carbs need a rebuilt (the jets/needles are worn).

Without the use of a CO meter you should be able to get a CO of 1.5% and HCC 62PPM (idle) with air pump connected but no cat. convertor. I don't know if that's good enough in California but it works here for my '74.

werner

werner
Werner

Does anybody know what the resistance of the spark plug wires should be? I'm getting about 9000 ohms for the plug wires and 7000 ohms for the HT leed. My VW Vanagon Bentley manual (electronic ignition) says 8000 and 6000 respectively. That sounds high to me for our type of cars. Maybe I should get some solid core bumble bee wires.
Any thoughts?
~Dan
Dan Neu

Dan, have checked the basics? Things like vacuum leaks, etc?

Here's some info I found (this comes from the trade magazine 'Import Service', and assumes you have access to a two-gas analyser):
HC high, CO normal or low
1. Disable any air injection systems
2. Check engine mechanical condition
3. Check spark quality and timing
4. Check for vacuum leaks and unmetered air
5. Check for spark delivery at each plug
6. Check injectors (not applicable in you case)
7. Check that EGR systems are not leaking
8. Check for ruptured accelerator pump or fuel pressure regulator diaphragms (again, n/a).

"The low CO reading tells you that there's enough air for efficient combustion. Thus the problem will likely be either an insufficient spark or a lean misfire. Check for good spark delivery to a workable plug and for unmetered air or a vacuum leak."

FYI, vacuum leaks can be easily diagnosed by spraying something like WD40 at the suspect hose or gasket. A change in engine idle indicates a vacuum leak. It doesn't matter much what you spray -- it's the propellant as much as the product that gets sucked in and burned.... Try at the base of the carbs and at the throttle shaft ends. Also try any hoses that look old or cracked.

Rob Edwards

I won't know if I need them till later today, but I thought I should ask now.

Does anybody know where to get air pump parts? Those would be vanes, carbons and relief valve.

~Dan
Dan Neu

I measure 10k ohm on a 16 in plug lead of my car.

Werner
Werner

Sounds like your wires are good. I don't see them being the culprit. Just some basic thoughts: Have you tried to lean out the mixture as much as possible, change oil, and driven the car for a half hour to make sure the engines up to full temp?
J Arthurs

Does anybody know anybody in the SF Bay Area that I can borrow? I hope it's one of those things I'll only need once. Either the rolling exemption given to us by SB 43(starting 2003, 30 yrs or older exempt)is still in effect(my fingers are crossed), or I move back to Oregon.
~Dan
Dan Neu

Air pump is good.
Air manifold and holes in head are clear.
Gulp valve works great.
STRONG spark, all cylinders.

I'm not sure if the starter valves need to be replaced. Think about it. Blowing it to the carb with all the outlets pluged generates quite some PSI's. The air's got to come out somewhere. Think so???
~Dan
Dan Neu

The thing I want to borrow is an exhaust gas analyser.
Sorry,
~Dan
Dan Neu

Charles Edwards: if all else fails, use a straight pin to punch holes in a vacuum hose to admit air.
What vacuum hoses are you talking about? I have a detoxed '71, and the only vacuum hose that comes to my addled mind is the dist. advance. I am sincerely interested.
Ken R
Ken Rich

I've been going at this with the idea that I couldn't have such a high HC unless it was running rich. I tried the "spray the exterior" test that Rob suggested and, lo and behold, the idle speed increased. The spray I was using (starting ether) wasing very specific, so I figured it was the gaskets (6) between the carbs and the manifold. I put Hylomar on the gaskets and adjusted the jets till It ran better. Then when I sprayed it, the idle speed dropped. I held the piston lifting pin up and then sprayed and it was like I had an auxiliary carb attached to my carb.

I guess I found the problem. I could just adjust it rich and get it to pass while crossing my fingers. however, I think I'll bite the bullet and re-bush my carbs.

does anybody know someone who has experience doing the job local to Palo Alto, or should I ride my bike for a while and send my carbs to Apple?

~Dan
Dan Neu

Send your carbs to Joe Curto, http://www.joecurtoinc.com.
Paul K

This thread was discussed between 18/06/2002 and 24/06/2002

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