New: 1988 Mighty Max, CA emissions, Mikuni Carb

[ford254]

Hi everyone, I'm new here.



I just got this 88 Mighty max, with the 2.6L, G54b, original Mikuni carb and California emissions.
Its in great overall condition and seems to run fine, plenty of power and the engine sounds great.

But there is a bit of indication that it might be running slightly rich, Black sooty exhaust and failed CA smog.
Here are the results:

Test at 15 - passed 15.5% Co2, 0.3% O2, 36ppm HC, .07% CO, 854ppm NO
Test at 25 - failed 15.2% Co2, 0.0% O2, 31ppm HC, .80% CO, 96ppm NO

The only offending measure is the test at 25mph - .80% CO and the allowable amount is .60% CO.

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My initial research led me to a potential issue with the Mikuni Carb bowl vent diaphragm or solenoid. Blowing into the vent hose kills a running engine.

How should I proceed ?
Could the bowl vent diaphragm cause the failed smog or should I look elsewhere ?
Do I have to remove the carb and dissemble it to troubleshoot this Bowl Vent diaphragm issue?
Is there a test to distinguish between the a bad bowl vent diaphragm and a bad bowl vent solenoid ?

Thanks in advance.
Mike

[FMS88]

Welcome to MR50. Your truck looks like it’s been treated very well and should be pleasant ride.

The Bowl Vent Valve (BVV) of your truck’s carburetor is vacuum driven so there’s no solenoid to worry about. Mikuni changed from a solenoid to vacuum valve in 1987. There are two other solenoids on the Mikuni (Feedback and Decel), but they’re unrelated to the BVV. A failed BVV is one of several possible causes of elevated CO and rich running. This article from 1998 describes symptoms of a BVV failure and how to test it: https://www.motor.com/magazine-summa...ter-july-1998/.

If it fails testing, it may be repairable. The valve (P/N MD617185) has 7-8 pieces including two diaphragms. Looks like a complete kit is available from Japan (Amayama, Megazip) but it will cost $275 or more. You can get the most failure prone components for less on ebay from a seller in Australia if the other parts are okay:
https://www.ebay.com/itm/19445629425...Bk9SR_yOuaj_ZA

Replacing the valve requires partial disassembly of the carb so you’ll need a rebuild kit ($55 on ebay) for the base and top gaskets. Disassembly and repair of the Mikuni is a challenge not to be taken lightly as it has many small parts that aren’t in a rebuild kit and aren’t replaceable.

Is there a shop near you that’s versed in carbureted vehicles? They may be able to adjust the carb enough to get the CO to a legal level as well as give you an assessment of the BVV and the Mikuni in general. If the news is bad, then consider getting a rebuilt Mikuni that’s Calif compliant, or repairing your Mikuni.

For more info about the BVV and the Mikuni in general, look at Post # 8 in this thread:
http://www.mightyram50.net/vbulletin...m+hose+diagram
Part 1, Page #13 of the embedded PDF discusses the BVV design, function and testing.

Cheers

[xboxrox]

Check Mike's for Mikuni parts https://www.carburetor-parts.com/

Nice ride & the safe color