Update 8/9/11 Failed. Results attached to bottom of thread.

My '87 Ram 50 2.0 5spd has always given me issues when it comes to passing smog. Over the years I've changed the carb, cat, thermostat, and o2 sensor to get it to pass.

Last time it failed NOX really bad and I got a 2 year waiver.

I cleaned and tested the EGR valve, ran a bottle of redline cleaner, changed the oil and replaced the air filter. I drove it a lot for a week and got it pretty hot before the test. I attached the results.

The tech says to change the plugs, run 91 octane with a full tank before re-test, try a bottle of CRC guanranteed to pass, bring it in really hot leaving it idle and have them run it again.

It's running rich. Anyone now how to adjust these Mikuni Carbs? What else should I do? Would testing it without the air filter in help? I could retard the timing a couple degrees too.

Hi Psychbiker,

If your truck is running rich, then the engine is not getting enough air or/and too much fuel.

Removing the air filter should allow more air with the same amount of fuel, so your fuel/ air mixture will be leaner.

The higher octane fuel should burn cleaner.

New plugs will eliminate the possibly of misfire.

Don't guess at the timing. The timing and idle speed specified on the sticker under the hood should be followed to pass your emission test. Set the idle speed before you check the timing.

Make sure the accelerator pump is not leaking fuel into the carburetor or stuck slightly open.

Make sure the choke is fully open or disconnect the choke spring for the next test.

Verify the flapper valve in the air filter housing is fully opened when the engine is warm.

What is the elevation there? Does your truck have a high-altitude compensation system?

Check and/or replace all vacuum lines.

With the engine off, open the throttle and see if gas is getting into the carburetor other than from the jets.

Hope this helps,

John

Thank you so much for your response.

I'm in the Bay Area Norcal and dont think high elevation equipment is installed. This was my Grandpas truck that he bought here and it was given to me when he passed.

Changing the vacuum lines wont be fun but a good idea.

I've replaced the carb with a rebuilt one 4 years ago and will make sure it's all in working order following what you mentioned. The timing I believe is stock as I never messed with it and it passed the visual test.

Am I correct that there isnt a 'mixture' screw adjustment with these carbs?

I removed and replaced the EGR, it was plugged really bad. I replaced the hose that connects the EGR to the computer since when I sprayed carb cleaner a bunch of black stuff came out.

A float may have sunk in the card, too; not shutting off the valve will let raw fuel dump in.

There is a mixture screw directly on the drivers side toward the bottom I think but it comes blocked off from the factory, you have to drill or remove the little blockoff somehow, but I never really messed with these carbs they are too much of a hassle

I ran a bottle of CRC Guaranteed to Pass, changed my plugs, changed the thermostat, removed the air filter, inspected the vacuum lines, ratarded the timing 2 degrees, ran 91 octane through it, max filled the tires, drove it 30 miles, left it idle and had it re-tested.

It Failed

They did a Manual Mode first and the numbers were kinda hoping all over the place during the 90 sec 15mph test. The last 10 seconds the numbers dipped to passing levels.

So we tested it for real and it didnt pass. They waited and tried it again and no luck.

My numbers are attached. I failed on CO but passed on HC, and NOX. The mechanic thinks there may be a misfire. I'm pretty sure I gaped the plugs correctly.

I cannot find a schematic/diagram for the Mikuni carb. I'm going to call/email Mikuni.