jets, Need an Answer please

[MTRob]

A friend gave a box of jets for Mikuni Carbs. Main jets from 130 to 180. Some are short head jets and some are long head jets. What is the difference in the jets? Will they work the same?
Thanks
MTRob

[Bullfrog]

While I can't specifically answer your question, I can show you the proper jets to fit the stock carbs on ANY model of Hodaka Motorcycle.

I would have to say, use ONLY Mikuni main jets which are exactly like these.

Ed

[MTRob]

The jets are Mikuni its that the head is shorter. Both say 135, Is it because they made them shorter for room reasons?
Would like to know the difference. any body have the answer?
MTROB

[Dale]

MTRob, I have encountered the shorter jets too and had wondered about using them. However, the proper main jet for the Mikuni VM carb is the 4/042 style. These are the ones that Ed has shown the measurements for. To use any other style is just asking for issues as far as I am concerned. Depending on the carburetor style, wouldn't the shorter jet mean that you are going to be altering the proximity to the fuel level set by the float?

Also, looking at a Mikuni main jet tuning calculator, the shorter style (N 100.604) shows a different percentage of difference when you change either temperature or altitude. I have no idea why that would be, but it strengthens my belief to stick with the 4/042 style jets is best.
Dale

[taber hodaka]

Short and long jets we had them both back in the day. I, we used the longer ones then and would recommend using them now. If I find out the use for the shorter ones I will pass it on. I have a adjustable main jet webco I think. Clarence

[Zyx]

Depending on the carburetor style, wouldn't the shorter jet mean that you are going to be altering the proximity to the fuel level set by the float?

Perhaps it does, but since the main jet runs immersed at the bottom of the bowl, I don't think it makes a difference. What a shorter one might do, assuming that either length jet of the same number has the same size hole, is pick the face of the jet up off the bottom of the bowl a bit more compared to the longer ones. Skipping through the various diagrams, some of the main jets look shorter, some look like the longer ones. It may have been engineer's choice which length went in, or the differences could be application specific, not flow specific. If someone with a handful of jets, specifically with the same number but different length, has access to wire gauges, he could compare them that way just to verify that the numbers and the jet bores relate as between short and long versions.

Some carbs, like the old 24mm, have a jet holder at the bottom of the bowl in a horizontal position. I can't see how using a shorter jet in that instance would change anything because it is completely surrounded by the holder and the holder chamber. Others have the vertical position mains, but these still run an inch or more below the float level, so I can't immediately see what difference it could make with regard to the float level. The jet meters fuel by the diameter of the bore of the jet. Shorter jets might have a bit less restriction in the flow as a result of the shorter length and demonstrate different flow characteristics, but the difference has to be really, really small.

On the 32mm used on the Super Combat, as an example, the face of the main jet is about 28 or 29mm below the fuel level at rest. That's a good inch of standing fuel. A couple of millimeters isn't going to make a difference there.

I don't know exactly how short the short jets are, but the only drawback I can see to using a short jet in a long application would be, possibly, with fuel slosh or frothing. The longer jet rides well down in the float bowl sump (that little round pocket at the bottom of the bowl), which holds a portion of fuel close to the jet and which dampens fuel sloshing at the jet face. With the shorter jet, the face would ride higher in the sump or perhaps even just out of the sump where fuel could be more in motion at the face of the jet. I suppose that could cause some kind of an issue in some range of performance, like wide open over rough terrain.

Try the short ones, if you have one that is numerically correct for a carb you have, and see if it makes a difference. The worst thing likely to happen is that it will run crappy wide open where the main jet does most of the meter work.

[taber hodaka]

The worst thing that might happen is you could fry your engine. Stay with the jet length your bike came with let the ones that like to experiment play. I didn't use them then and I wouldn't use them now.

[MTRob]

Thanks Guys. I'am going to try the short jet #135 and see how it runs and let you know. Like you said the jet is not that smaller than the long jet. I think they used them for room reasons on some carbs.
MTRob

[dirty_rat]

In checking some of the Mikuni sites, it appears that the longer jets are different on the inside from the shorter jets. The longer ones are made for certain jet needles (as at throttle closed to just open) the needle sticks down into the opening on the threaded end of the main jet. It even list the types of jet needles that will work with certain main jet, etc.

So, if you use one of the shorter main jets in a VM carburetor, you could possible damage the jet needle or main jet (and it might not let the slide close completely). Given the relatively low cost of jets, I wouldn't try the wrong one in my bike and possible damage the carb and or engine.

Dirty Rat

[MTRob]

The needle does not come close to the main jet. I went to put the smaller jet in and noticed that it only goes in so far. The distance it sticks out of the holder is exactly the same as the long 140 jet. Guys will see what happens. Just reset the points with the new condenser and points, kicked it oeer and have great spark. Put it back together and see what happens.
MTRob

[MTRob]

Dirty rat different carb the jet comes out of the side of the carb so the needle misses the jet. Put the 135 shorty in and it ran but not good, took the jet out put the 140 long jet in and now runs great. the 140 is better for our altitude. Thanks all
MTRob