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Pipe Dreams: turns out we’re not quite done yet

October 12, 2007

Yes, the PX has been sprightly and fuel consumption has been reasonable, but I still thought it would be good to check the plug. It was wet. And black. Ooh, too rich.

The exhaust pipe side of the rear wheel had little goopy black dots on it. Not a whole lot, but enough to indicate the fuel mixture required an adjustment. Okay, let’s try something smaller than a 103 main jet.

My thought was to get a 101 and a 102. Run the bigger one for a while and see how it works, try the smaller one if we’re still too rich. We can make a gradual transition, reducing the risk of going from too rich to too lean in one big jump. Can you say “soft seize,” boys and girls? Not a good thing.

However, nobody has Dell’orto SI 101 or 102 jets. Apparently, they don’t make them in those sizes. The next closest size is a 100.

That should be okay. The engine is nice and loose from 10,600 miles of flat-out flogging, and temperatures have settled into fall-like 50s and 60s, so the engine shouldn’t attain too high of a temperature.

Not that it was hard before, swapping out the main jet is practically a reflex. Off with the air cleaner cover, off with the air filter, unscrew the big brass fitting, out it comes.

Except the main jet was still down in the hole.

It wasn’t a serious press fit when I installed it, but the whole assembly went in in one piece. Okay, no need to panic.

First, make sure you stuff a rag into the carburetor throat. The main jet probably won’t fall through the opening at the bottom, but no point in taking chances, right?

With the aid of a flashlight, I could see the main jet at the bottom of the hole in its proper position. If you have something like a rod or a dowel that is just the right size for the jet’s flanges to fit on, that’s great, but a small Allen wrench will work, too.

If you use the latter, get it down into the hole as far as it will go… you will end up getting it inside the jet. Then use it like a lever to pull the jet out of the hole. Have a pair of small needlenose pliers ready to grab the jet when you get it to the top of the hole. If you have the right-size rod, you should be able to slip it onto the jet and extract it.

Whew! The new jet was not a super-tight fit, either, but I hesitate to put something goopy in the mixing tube hole, and wrapping the jet in Teflon tape isn’t a good idea, either. Gasoline is a solvent, and it would surely eat through this stuff and deposit it somewhere it’s not wanted. For now, let’s live with it.

I’ve found replacing the spark plug every 1,500 miles or so a worthwhile bit of maintenance. Spark plugs are cheap, and very easy to replace on the PX engine. I was about due for a new one, anyway, so we’ll get accurate readings from a fresh plug.

Checking my work, the engine fires right up, and idles just fine. Out in traffic, no hesitation or hiccups, acceleration is good. I do notice the brup-brup-brup exhaust note has diminished considerably.

I try for a top-speed run on 15th when the traffic breaks, but a stiff crosswind catches me just as I’m hitting redline in 3rd gear. Let’s not kill ourselves here. Favicon


Here is the ultimate spark plug page running the gamut from way too rich to way too lean.


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