MFS6C carb issues

A friend of mine who is NOT a wrench tore into his 6C after not using it for 2 years. The thing literally can’t have two hours of time on it, but he let it sit in his garage for 2 years. He tried cleaning the carb but then another friend of his was examining his carb and dropped it, breaking the plastic linkage. So he purchased a new carb on Amazon, and I helped him put it all back together (he had forgotten how some things go back together and didn’t take pictures, again NOT a wrench). It fired right up, but it runs like crap. Okay maybe not like crap but not like I’d like it to. I’ve got it starting on one pull, will idle fine, but if you crack the throttle it coughs pretty bad and has a bad 1/3 throttle stumble. You can slowly advance throttle and it will throttle right up and rev to the limiter, but you can’t crack the throttle quickly and the 1/3 throttle stumble is always there. I’ve got the pilot screw at about 3 turns out, which is where it seems to minimize the cough/stumble and it’s where the idle seems to return from rev’s most normally, but it is not where high idle is. High idle is about 1.5 turns out but then the cough is bad and upon revving the engine the idle seems to “hang” a while before settling back down to idle.

So my questions...the new carb is obviously a knockoff/repro, any reports of flaky performance with these carbs? If it had been me, I would have bought OEM. I also thought it strange that the pilot screw has no spring, I was suspicious because it turns almost too easily.

I saw Paul’s recommendation in another thread to go with the A2 carb for best idle, does it make a big difference? Sure seems like I’m fighting a lean issue?

Lastly, I saw in another thread where idle on these things should be 1300 in neutral and 1100 in gear, which is higher than I would have thought and I’m sure I’ve got it too low since I didn’t have a tach on it. Could this be the majority of the problem? And can anyone recommend a decent tach, I’ve got a cheap inductive one that I think works questionably at best.

Oh and thanks in advance for helping a forum newb out...

An aftermarket carb? That's the start of issues. No spring on the pilot screw? Hmm. Possible, but a weird design. MSRP on the OEM carb is maybe $180, and they sell for less. Given $100 an hour shop labor, why bother with a questionable part? The Factory part does have a spring under the pilot screw:



That aside, due to the low moving mass of these 1-cylinder motors, they idle surprisingly fast. Otherwise, they might stall going into gear, and can stumble at tip-in, as the velocities in the carb are not right for proper flow. Yes, you need a good shop tach to set that, and the motor must be fully warm and in the water to set the in-gear speed. A good tach is https://www.esitest.com/325.html

There are 3 versions of the carb for the MFS6. The A is the earliest, and has the least restrictive EPA emissions regulations, so it tends to be the richest at idle. The C is the most restricted by EPA rules, so is the leanest. That is why the carb for the 6A2 runs the best all around.

Sounds like while you may have a carb of proper overall size, it may not have the correct flow characteristics. You also must run the two base gaskets with the plastic thermal spacer at the intake, or temperatures at the carb will be off, leading to some more issues.