The reason TC bombed emissions wasn't a bad power valve, it was because the power valve wasn't getting any manifold vacuum, so it was on all the time. I fixed a broken plastic T and tried again and bombed... because apparently I pulled a hose loose. *sigh*

I did a few tweaks to make the truck run and idle leaner and advanced the timing a little.

The third try was decidedly not a charm. The inexperienced driver had to run the test three separate times before it was accepted. First try, the hose slipped off the exhaust... right at the end. The second time the guy still couldn't match the required speed so the test wasn't accepted. I guess driving a carbureted vehicle was a new thing for him. His third attempt was accepted but the truck was over the limits on CO and NOx.

I thought it'd be interesting to try and analyze the IM240 graphs to see what might be wrong with the truck that's causing the emissions test failure.



Oxides of Nitrogen (NOx) was just over the limit of 6.000 at 6.5254. When NOx is high that is usually a sign of high combustion temperatures, either due to a malfunctioning EGR, a device that lowers combustion temps, or because of too much spark advance for the mixture.

Because I increased the timing, I suspect I went too far. According to the printout, acceleration contributes 57% to the NOx readings, where cruise is contributing around 15-20%, suggesting the problem isn't isolated to part throttle acceleration (vacuum advance is controlled by ported vacuum on this truck). Base timing is probably too advanced. When vacuum advance is added under part throttle load (acceleration), even more NOx is generated. You can see from the chart that the driver was punching the gas way too frequently causing NOx spikes. It's possible that if the mixture is too rich, less advance is needed. So the mixture contributes to the problem.



The HC (hydrocarbons) chart shows spikes on acceleration indicating unburned fuel because of an overly rich mixture, undoubtedly. HC spikes on deceleration too; typical for a carb'd vehicle (computer controlled cars lean the mixture severely during deceleration). Maybe I can do something to improve that issue, I don't know. The numbers on the printout show a 58% contribution by acceleration to HC and 23% by deceleration.

It's also possible that the AIR system isn't diverting (enough) air to the catalytic converter to aid in converting the exhaust gases under acceleration. I might see about working on the ignition system, too, in case it helps more completely burn the air-fuel mixture.



Finally, Carbon Monoxide (CO) is pegging out on acceleration, contributing 80% of the overall CO output. The power valve clearly works only when it is supposed to, but is delivering way too much fuel. I'm not sure if a new power valve would fix this, or if I just need to adjust base jetting of the carburetor for thinner, hotter summer air.

Last time I passed it was pretty cold outside. I am leaning towards switching to #50 jets or smaller, then adjusting the aneroid again. I might need to replace the power valve after all, though. If there were a way to adjust the amount of fuel delivered in power enrichment mode, that'd make this easier to solve. On a Quadrajet, Edelbrock Performer, Motorcraft 4350, and others, the carb uses rods to meter fuel in power mode; one can select rods with wider diameters for power mode to deliver less fuel.

The 2150 is stuck with two-stage power valves. Supposedly 1-stage valves can be used but when I tried this last time, the valve wouldn't fit; not enough threads to engage properly. If it worked, one could select a power valve that engaged at a lower or higher vacuum to suit the application. This is the same concept as changing rod springs on an Edelbrock Performer.