If you find you still need to tweak the mixture , but just a little, you could try modifying the carb’s fuel bowl level a tad. Higher == richer, lower == leaner. That’s a good chart. Appears an o2 of around 0.2 - 0.3 % is what to aim. The 0 - 0.1% I get on my Corolla suggests it is running a little too rich.
I believe the air injection pump supplies air to the exhaust manifolds and the rear bed of the catalytic convertor, depending on conditions. The tailpipe oxygen value is not reliable for assessment in this case.
Yes it does have an air pump that injects air into the cat. Sounds as if this would increase O2 out the exhaust pipe, after combustion. The actual O2 coming out of the combustion chamber would be lower, and this points to a rich mix.
Had to edit from original.
I had to replace a blown head gasket many many years ago and took the opportunity to remove as much of the carbon as practical. I also dressed the valve faces and seats. Today, with the idle set high I trickled a gallon of H20 down the carb throat. Took about 20-25 minutes.
The tailpipe oxygen value would be higher than the value exiting the combustion chamber. Would this not point to a rich mix?
Had to edit from original.
I suggest you pay for a pre-test now
That way, if you fail, you’ve got plenty of time to figure things out
The point is that the added O2 provided by the smog pump makes just looking at O2 levels unreliable. And assuming that the catalyst is functioning properly the NOx and HC readings lead me to believe that the combustion chamber is too hot, which can be alleviated by adding fuel.
Also, the chart you show is not only for vehicles without air injection, but for the exhaust before the catalyst.
My emissions testing shop gives me a second test no-charge, if the first one fails. Pretty common at one time, but lately I have to negotiate this deal in advance.
I’m looking forward to OP passing this test. First try! 
After digesting what you point out here, I realize that I don’t have a clue whether it might be better to use the jets that were in the truck when tested last time, or to use the slightly bigger jets that I put in today.
Here is a pic of a plug after trickling a gallon of water down the carb throat, and then driving on the freeway for about 10 miles. Looks good. This is with slightly larger main jets than the ones that were used in the test I posted earlier.
Speaking of the freeway, how long do you drive your Dakota before going in for the emissions test? Maryland uses the California emissions standards and test preparation recommended practice here will likely benefit you too. MD recommends driving for a half hour on the highway then driving immediately to the test station to get the best results. You may do this already. If not, it could help you pass.
Good idea in theory, but there are usually people waiting in line to get tested. I suppose the best way to go about it is to time one’s arrival just as the shop opens to have a shot at being first. If someone is already there, blow it off and keep doing this until either you are first or the deadline passes.
I don’t mean this to sound snarky, but looking at a spark plug to see if you will pass a smog test is like feeling your own forehead to check your blood pressure. It’s not a meaningful test.
Why not just have the smog test done, and if you fail then you can base your next repair on the results of that test. You can look at all the gas readings (not just one), figure out how they all affect one another, and choose the appropriate course of action.
On the other hand, it seems you enjoy researching coil winding specs, rejetting your carb, and taking half a dozen emissions tests. If that’s the case, keep having fun.
That is what I have always done. However, it has not been until this time that I learned two things.
- Retard the timing to improve emissions, not advance it.
- The coils I have been buying at the chain stores are not up to factory specs.
Before California twice reduced the ppm level needed to pass I was able to get by OK with a weak coil and there was no need to change the stock timing. Yes, I enjoyed researching resistance on coil windings, but only so far as it enabled me to find and purchase the right coil among those available, then I stopped.
The condition of a spark plug can yield a good indication of the air fuel ratio, so why not use it before taking the test? The point is to use all available info to make adjustments before spending money on a test. It seems as good a way as any to determine the richness or leanness of the air fuel mix, or at least to use in conjunction with other indicators. Of course the air fuel mix is just one factor to look at to determine if a vehicle is ready to be smogged, not the determining factor as you suggest is what I believe. If I believed that, I would not do anything else as far as parts upgrades, timing setting, air, and oil filters, and oil changes, etc.
Just thought I would thank you for the suggestion to check the vacuum lines. Earlier today I found a leak in a section of the vacuum line going to a transducer mounted on the spark advance computer that tells the computer what the manifold vacuum is, the electronic equivalent of the old vacuum advance on the distributor. A section of the plastic line got too close to a source of intense heat and collapsed and split. The split was on the manifold vacuum side and the collapse was on the transducer side, so I am not sure if any of the manifold vacuum signal was making its way to the transducer. Setting the base timing involved removing this line from the transducer at the computer and plugging it. I may have let it dangle too close to a section of metal pipe going from the air pump to the cat. Just downstream from this damaged section was a tee going to a valve between the vapor canister and the gas tank. It could have happened in the last two days, or two or four years ago. There is no way to tell. The only times I muck with the timing is when it is smog test time.
Anyway, thanks again for bringing up checking the vacuum lines. Somehow that stuck in my head, and after I had finished installing new parts, retarding the timing 2 degrees and setting the idle speed (amongst a host of other things) I thought I better check the vacuum hoses thoroughly. The damaged section was not visible at first. It was facing away from view.
1 Like
Hi Jack:
You’ve made several comments about California twice making the emissions test more stringent for your vehicle.
Are you implying the test standard that you have to pass is now more strict than the emission levels the manufacture designed the car to meet back in 1987?
I have saved every emission test result for this truck since I bought it new, and that is what they show. I don’t have them in front of me at the moment, but I can check later and post the details here if you like. To put it another way, the state of California twice made the test harder to pass for this particular vehicle. I do not know if this happened for any other vehicles.
Jack
The test procedure may have changed. For example the HC spec may have decreased when they changed to roller-treadmill testing for your truck.
Late 80’s/early 90’s, my VW Rabbit and Ford truck both were originally tailpipe tested (Calif) with them idling in the shop, not being driven on a treadmill. Both always passed the tailpipe test on the first test. I sold the Rabbit about the same time the truck was given emission-testing exempt status, so no experience with treadmill testing for them. I think my Corolla’s first tests were done w/no treadmill , but later they started with the treadmill testing. The documentation the DMV sent me said I was required to use a TEST ONLY station, which I presume means a roller-treadmill. the documentation however seemed to imply the reason I had to use a TEST ONLY station was b/c prior testing was borderline passing. Not that all similarly configured vehicles require treadmill testing. In any event The HC level has always been 130 ppm since they started the Corolla on treadmill testing, and it has always just barely passed, until recently when I accidentally discovered the weird air filter relationship.
I usually prepared my truck for testing by replacing all the vacuum lines, the spark plugs, and the air filter. I didn’t monkey with the carb except to use a carb cleaning treatment product. This was the type you slowly pour into the carb as the engine idles, designed to clean the carb’s passages. I’d let the truck sit unused a couple of days to allow the treatment to soak. Not sure if it helped pass the test, but ironically, it did produce a lot of smoke … lol .
Here are the test results for 2008 and 2010, same dyno test at 15mph and 25mph. HCs went from max of 134ppm in 2008 to max of 94ppm in 2010. Max at 25mph went from 106 to 74. A more stringent change also took place under the original idle/25mph test with no load.
I have to say, that’s pretty strange. I can think of no reason why the DMV (or CARB) would change the HC limit in 2010 for a 1987 engine. Some of the posters here (including @Nevada_545 , @db4690 ) have a good deal of experience with Calif emissions testing, maybe one of them will know or at least offer-up a theory.
If I had to guess myself, I’d say it was some sort of clerical error.
Ya think? Maybe I should let them know. Apparently no one ever caught it. Anyway, I am pretty much done with prepping the truck for the test. Nothing left but to take it in.
Do you think it would be acceptable for me to ask about this change in the max limits for all three emissions in a new post? The reaction here so far has been surprise that such a thing could happen.
And what good would that do ? Just go take the test . If you pass than let it rest .