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MXT-L

Part 3 of the new carb, working on tuning.

Let me start by saying, clean every passage in your carb. before you install it. I've been going round and round with some problems and finally worked out most of them. It looks like my carb. was clogged in a couple places, this resulted in poor running and very much wrong Air/Fuel ratios. But let's start with the A/F. I bought an Innovate MXT-L wide band oxygen sensor based A/F meter. Chose a location about 24 inches away from the exhaust port as indicated in the direction, drilled and welded the bung.

welded bung

My rudimentary welding skills shown, but it went OK despite the large gap I had to bridge around the sides. Show here with the sensor installed.

O2 sensor

Now back to the tuning issues... Since no one has ever done this, I had no way of knowing what size jets were needed. And since I didn't clean my carb. properly before installing it (it was new, should be clean???) I had some initial difficulty. The carb. comes with 20 pilot and 130 main jets, there is a wide range of sizes available. And since I'm running on PowerMist UL RFG unleaded fuel, no guide points. After having an initial extremely lean idle with the stocks jets, I jumped up a size and had something more rich than I could handle. After going round and round with this one night after work, I stopped and contacted SudCo who is the importer for Mikuni carbs. in the USA. It was suggested to eliminate the cable "choke" and put the stock "choke" knob back in. This is where things really got weird. I left a large main jet installed so that it couldn't effect the needle/needle jet and started back after the pilot jet. I have from 12.5 up to about 30 pilot jets. No matter which jet I installed I would get black fouled plugs and a 10:1 or so on the meter. Went through from 22.5 pilot back and forth all the way to 12.5 getting un-reproduceable rich or lean results. After a couple hours I quit again. Worked on it yesterday and finally I'm making progress. Here's the problem:

 

Beware the "choke" knob!

 

What I was finding was that when I pulled the choke knob out to start it, the rubber seal around the shaft would ride up out of it's groove. When I pushed the knob back down, I didn't notice that the rubber was holding the "choke" valve part way open (sorry no picture). So after several hours of going at this task, I finally started making progress and things started working correctly. After sitting with the fuel in the carb., everything cleaned up. And pushing the seal down after using the start knob got e mostly working. I'm currently at a 17.5 pilot and about 10.4:1. Needs a smaller jet but I think my 15 pilot is defective as it goes about 18:1 lean when it is installed. With the 17.5 jet I'm more than 3 turns of the idle air screw out (indicates one size smaller jet normally). I'll need to order a new 15 jet and try again. Once this is all finished, I'll put the cable "choke" back on and forget the problems with the knob.

With all the mods. I've done to the intake, I expect either 15 or 17.7 pilot, needle in the middle or one notch higher (less needle), and probably about a 125 main for average temperatures and pressures at home. the track is a little lower elevation but shouldn't need much change.