My max won't go passed 55MPH

Ok I have an issue with my max. It is a 1985, 2.0L 2 wheel drive. My problem is the truck runs fine at low speeds (below 55 MPH), but when I get above 55MPH on the freeway it starts stuttering, loss of power and speed, and sounds like it is backfiring. I get off the freeway (under 50MPH) and she runs like a dream.

I've replace the fuel pump (about 3 months ago), fuel filter, distributor cap and rotor, plugs and wires, and air filter (all today). Still won't go above 55MPH.

What am I missing. I also insured that the distributor is tight and not moving (saw this on YouTube)?? Hopefully someone here has suggestions please.

Thanks in advance for any help you might have

Hi and welcome to mightyram. First place I'd look - the secondary vac actuator on the carb. They are notorious for failing from age. Once it fails, you lose power from mid range to WOT. Disconnect the small vacuum hose that is attached to the vacuum servo, connect a longer one to the servo and suck on the other end of the hose. If you can continuously draw air through it, the actuator has a split diaphragm. You 'can' replace it without removing the carb but it will take a little persuasion (force) to pry it free and reinstall the unit. Once it's replaced you will notice a big difference in throttle response and power delivery.

I agree with geezer, I have about 3 or 4 carbs sitting around in various states for parts etc and I think all but maybe one is bad. I think there are some on ebay that are rebuilt coming from Australia, that"s the only source I know of for good ones but these are for second gen pickups but they might be the same. its been a while since ive looked at a first gen carb.

Hmmm interesting I'll take a look thanks, however the carb is newly rebuilt (less than a year ago).

There are 3 components that get overlooked when rebuilding the Mikuni - the secondary vac actuator, the auto choke release assist and the wax pellet for the auto choke. Reason being is that these 3 parts are expensive and don't always need replacing. They will fail without much of a warning. The wax pellets are a difficult part to find and are a biatch to take out and install. Although the Mikuni is a relatively 'good' carb, it's a complex carb to strip down and reassemble, can be expensive to overhaul and is designed primarily to meet emissions standards and are not easy to adjust. For performance - nada. These carbs seriously strangle the engines they're bolted up to. If you aren't restrained by emissions inspections, a Weber 32/36 (which was the old stand-by) or a 38 DGES will make a world of difference. Not a straight swap and requires trial and error to tune, but when set up correctly will noticeably improve economy and performance.

Having same issue with my MM, got new secondary vacuum actuator. The new one has a spring off the bottom? One currently on my Max does not. Can anyone enlighten me on why and what the spring is for?

Pretty sure all it does is pull the diaphragm back to a 'seated' position off throttle.