Question about 1985 Yamaha XJ 700 X Maxim

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1985 yamaha maxim 700, Runs good through first half of throttle but when you try to get past that it just boggs down. Carbs have been cleaned. Spark plugs are jet black after a quick ride? Any help is appreciated.

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Hello, from your description, it sounds like some of the jets of your carburetors are in the wrong places. Your bike has four carburetors, and each one of them has a main fuel jet and an idle fuel jet inside the bowl. The main jet goes in the very middle of the bowl and is the one that screws into the needle jet. This main jet has a relatively large hole through the middle. The pilot jet goes off to the side of, and screws right into the metal of the carburetor body. This jet has a very small hole through the middle -- so small that it may be hard to see through it.

These two jets look similar at a glance, and they have identical threads which makes it easy to get them in the wrong place. You mention that your carburetors have been cleaned, so there is a good chance that whoever cleaned them accidentally switched these jets in some or all of your carburetors.

If these jets are switched you will notice the following symptoms:

  1. Your bike will start very easily with little or no choke even when it has not been run for a while.
  2. Your spark plug electrodes will be very dark black after even a little running.
  3. It will feel like you have little power from the middle of the throttle all the way to wide open throttle (WOT).
  4. If you run like this for a while you will start to get backfiring, and the bike will start to run on three and then two cylinders, and perhaps eventually not at all.
But don't worry, fixing this problem is easy. It will be especially easy if you are the one who switched the jets in the first place. You just have to take your carburetor rack off of the bike, and remove the four bowls from the bottom. Look at the main and pilot jets in each of the four carburetors. The main jet should have a biiger hole and will have a tiny stamped number that should be around 120 or 130 on the top. The idle jet should have a small hole and a tiny stamped number around 15 or 20 in the top. Be sure that the jets are in the right places, and put your carburetors back together. It is also a good idea to get new spark plugs after you correct the problems, or thoroughly clean your old ones as carbon buildup on the spark plugs may cause a weak spark.

In the attached picture the main jet is toward the bottom, and the idle jet is on the top.
1985 yamaha maxim 700, - joshorndorff_0.jpg

Good Luck,
-Josh

Posted on Mar 15, 2011

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