Pros & Cons: Pilot Jet Increase

[IanDMacDonald]

Observing Nate's signature, I saw he has a #45 pilot jet (stock). For some reason I was thinking #42.5 as I have '79F, guess not. Anywho, I'm wonfering what advantages/disadvantages I would see bumping-up the pilot jet size. I already have a 140 main.

[natemoen]

I think it just adds a little more kick on takeoff. A lot of people complain about the low end lag and I think that it helps with that. But that also might have a little to do with the significantly larger 210 pilot air jet as well, all the others have a 180 or 185.

[DGXSER]

The 78 model came stock with the 210 pilot air jet, 45 pilot jet, and 137.5 main jets.

The 79 and up models came with the smaller pilot air jets, and smaller pilot jet at 42.5.

Note that the 78 also had a different timing curve, and I believe different cams as well. So, it is not as simple as changing jets to get the same performance Nate has from your 79.

Larger jet with no increase in air jet adds more fuel richer mixture. Increasing both changes the mixture, probably still a little richer.

[D0wn5h1ft]

On my '78 I have 45's for the pilots and it's too rich taking off (with pod filters and dual mac exhaust). From what I remember, we had the same exhaust and mine is running wayyyy too rich on the low end so I'd keep the 42.5.

On my '79 (with the midnight special carbs), I'm opting to keep the 42.5 pilots (just went with 135 on the mains) because I'm transferring the dual macs over to it and the pod filters. I'll let you know how it turns out once I put the ignition back in (just changed the head gasket you gave me).

I'm under the impression that the dual macs we have make for an overall richer mix at least compared to the 4 to 1 I had in before. But if you're convinced that you're running too lean and not too rich on the low end I'd bump UP the pilot air jet. I'm not sure where everybody else is getting their numbers but the manual and this (http://www.xs11.com/xs11-info/tech-t...rburetors.html) say that they are 180s for both the '78 and the '79 so I have no idea where this 210 number is coming from.

[IanDMacDonald]

Yeah, I completely removed the stutter and bog issues I had with a good sync. I'll consider dropping down to 137.5's in the Spring. I kind of miss the modified advance I had, that vacuum advance makes take-off kind of boring.

[DGXSER]

On my '79 (with the midnight special carbs), I'm opting to keep the 42.5 pilots (just went with 135 on the mains) because I'm transferring the dual macs over to it and the pod filters.

If you have MNS carbs, they are the newer style, and 135 mains will be HUGE!! They came stock with 110s.

[D0wn5h1ft]

If you have MNS carbs, they are the newer style, and 135 mains will be HUGE!! They came stock with 110s.

What do you mean "newer style"? I was told they were the exact same carb bodies as before (BS34) and according to the tech tips it's the BS34 carb body too.

The reason I upgrade the mains because I'm trying to jet it out closer to the year of the bike ('79). With pod filters and 4 to 2 exhaust it should be just about right at that seeing as the '79 actually came with 137.5s

[DGXSER]

There are two styles or models of the BS34 carbs that came stock on the XS1100. I was looking for the carb identification guide, but I can't seem to locate it. The earlier model XS1100, 78 and 79 model years, came with the early style carbs. Most easily identified by the idle mix screw being visible on the engine side top of the carb, having vent Ts above the fuel Ts, and only 3 holes on the inlet bell. Likewise, the later models, 0 and 81 and the XJ came with a different style BS34 carb. These have only fuel Ts between the two sets of carbs, the mixture screw is down inside a tunnel on the top engine side of the carb, and four holes in the inlet bell (fourth being a bowl vent, eliminating the vent Ts).

You want to jet the carbs for the carb model, not the bike year. So you do not put the big 137.5 mains in the later 80-81 XJ carbs even if they are on a 78-79 engine.

So, you will want to go with maybe 120 mains if you have upgrades like POD filters or better flowing exhaust, but 137.5 will be huge.

[IanDMacDonald]

http://www.xs11.com/forum/showthread...identification

[D0wn5h1ft]

Well, sounds like I'll be making another trip back out to the yammy dealer. Thanks for the heads up. What about the "newer" style carbs warrants such a large difference in the main jet sizes?

[MPittma100]

In a Word - Emissions

Well, sounds like I'll be making another trip back out to the yammy dealer. Thanks for the heads up. What about the "newer" style carbs warrants such a large difference in the main jet sizes?

Government emissions regulations were for motorcycles too. Yamaha and other makes had to adhere to the rules as they changed. Leaning out the carbs on 1980-1981 was a result of emission regulations - cleaning up the exhaust.

I put the 1978 jets into a 1979 Special. Performance was enhanced and noticed immediately in the seat of the pants. MPG was less by about 3-4. Plugs were more of a rich color. 78 to 79 jet change was not such a big one. But if you did this same change to an 80 or 81 carb, it would be dramatic. Probably would not work out well.

Compression ratios and camshaft profiles were changed after 1978. Carb changes were in direct relation to them.

Next time I have carbs off of the 79SF, I think that going back to stock is in the plan. MPG is a bigger factor on a Special with the smaller tank.

MP

[IanDMacDonald]

This is really interesting. I regretfully will be going back to the modified advance, and seeing what will happen if I keep the vacuum advance still attached.

[DGXSER]

Part of the change was in how the pilot jet was fed, and also in the needle valve and emulsion tube. On the older style, there is a plug over the pilot jet tunnel, and a crossover to let fuel from the main jet feed the pilot jet. On the newer style, that plug and crossover were removed, so the pilot jet draws directly from the bowl.

There is also a change in the float setting between the two styles.

So you want the jets set up to match the carb.