Main jet - does the outside shape matter?

[Guest]

http://i.imgur.com/tFQeWBv.jpg

I just bought a set from Z1 Enterprises and they're not the same shape as the stock jets. But the thread pitch, length and inner diameter are right so is that okay then? Just weird that they're not as big.

[chef1366]

What are you working on?

[thewozza]

This is for a 1981 GS850.

I'm fitting new pipes, so I ordered these 120 main jets to replace the 115s.

[Steve]

Are they Mikuni jets?

Depending on the pipes, you might need more than 120s. I have 125s in mine, and it's a rather restrictive (quiet) system.

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[thewozza]

Are they Mikuni jets?

It looks like I bought Keihin jets.

I got the Vance and Hines 4 into 1 megaphone pipes. Their instructions indicate 120 jets.

[Steve]

It looks like I bought Keihin jets.

Hopefully you have not run it very long with those jets. Kehin's jet sizing is NOWHERE NEAR Mikuni's. A Kehin 120 is the same as a Mikuni 107.5, which is three sizes smaller than stock.

You should be able to find the large, round Mikuni jets in a 120 size, but if all you can find is Kehin, the cross-reference chart says you will be looking for a 138 to get the same size.

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[thewozza]

Hopefully you have not run it very long with those jets. Kehin's jet sizing is NOWHERE NEAR Mikuni's. A Kehin 120 is the same as a Mikuni 107.5, which is three sizes smaller than stock

I haven't run it at all, I just picked these up from my mailbox last night and they looked suspiciously small. But doesn't the 120 mean 1.2mm? Why should Keihin use a different metric system from Mikuni?

I'll get the right jets in the end, I'm not going to run with the wrong stuff here.

[blowerbike]

the threads should be different also.
they was in the past.......

[thewozza]

the threads should be different also.

I didn't turn the jet into the carb, but I lined it up against a stock main jet and the threads meshed nicely. It is almost as if Keihin makes jets specifically for these carbs, but just with a slightly different external dimension.

I've emailed Z1 Enterprises to ask about returning these (not sure if it is worth returning after shipping and time spent) and they may have some opinion on the matter.

[thewozza]

Kristen from Z1 Enterprises says they'll take an RMA return so I've just ordered four Mikuni 120s and I'll ship back these Keihin jets.

Even though they look like they'd work, I don't know enough about carbs to pick my way through something like this so I'll do it the safe way.

[JTGS850GL]

Just to clarify, Mikuni did make both small and large head round main jets that will fit a BS carb. Same thread pitch, same length and the same metering. A Mikuni 120 small head main jet has the same fuel delivery as a Mikuni large head main jet. The difference is how much extra material is around the head of the jet in order to hold the emulsion tube in place. Since the BS carbs use a brass washer the "NEED" for one over the other is moot, but if you want to stay consistent then go ahead and return the jet and get the large head version.

Here's a link to available large head jets:

http://www.jetsrus.com/a_jets_by_carburetor_type/jets_mikuni_N100-604_main_Large_round.htm

and here's a link to the small head jets:

http://www.jetsrus.com/a_jets_by_carburetor_type/jets_mikuni_N102-221_main_Small_round.htm

As you can see, they are both MFG by Mikuni. I think he simply got the small head Mikuni's and not the Kehin.

[thewozza]

This is what I ordered:

http://www.z1enterprises.com/ItemDetails.aspx?itemDescription=Keihin+99101-393+Type+Round+Main+Jets+%23120&item=SIKHM-110120

It looks so much like your small-head Mikuni jet I find it hard to believe that it is actually different. I did NOT measure the inner diameter with my calipers, but that's because I intend to return them and I don't want to scratch them.

I don't really have much reason to think that they're really the wrong size, but I've considered that I may not be the last owner of this motorcycle so I don't want to setup the next owner with something weird and inexplicable like small Keihin jets where there should be large Mikuni jets. And that next owner may be me, long after I've forgotten what the hell I did to get these carbs working.

[JTGS850GL]

Yep, those are the wrong jets and need to be returned. I'm not even sure they would even thread into the carb.

[Steve]

... But doesn't the 120 mean 1.2mm? Why should Keihin use a different metric system from Mikuni?

Not necessarily. While they do use the same Metric System (after all, there is only one), they might be measuring flow rate, rather than hole size, and numbering it accordingly. I can't say for sure who measures what and how, but the chart that I have says that a Mikuni 120 is the same as a Dynojet 128 and a Kehin 138. Three different manufacturers, three different methods of numbering jets that all flow the same amount.

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[KEITH KRAUSE]

The outside shape doesn't matter as the main is completely immersed in fuel.