Jet question

[Hired_Goon]

I have a small list of questions regarding XS 11's that I try and search out the answers for before I ask, but the one that most intrigues me is the jet sizes between different motors.

Essentially all the motors are the same bore and stroke,similar size airbox with similar carbs. Why does one come out with 137.5 Mains and the other 110/112's?

Is it due to the slightly different carbs? Valve sizes?

In my head, if one is running with 137.5's hbappily then shouldn't the other. Why doesn't the bike with smaller jets run lean???

I'm sure there is a simple answer, it's just eluding me at the moment.

[Crazcnuk]

Everything in the engines is pretty much the same, with the exception of the 78, 78 valve size being smaller than the 80,81.

The main reason (pun intended) is different models of carbs, near as I can tell. Everything else, with the exception of the valve size is identical between the various model years.

I have seen places that say the cams are slightly different as well, but other places say all the cams are the same.

I am sure one or more of the gurus can tell you specifically.

[TopCatGr58]

Hey Goon,

Yes, the 78-79 and some early 80 carbs have a tunnel between the main jet tower and the pilot jet tower, and the pilot jet gets it's fuel supply from that tunnel THRU the main jet, so the main jet needed to be bigger to be able to provide enough fuel flow for both jets. That's why the pilot jet tower is PLUGGED OFF so that it will only draw thru the main jet/tunnel.

Then the later 80-82 carbs closed that tunnel, and the pilot jets get their fuel directly from the bowl, so the main didn't need to be so big, so they downsized it! The pilot jet tower is unplugged in these carbs!
T.C.

[thewiz]

cam difference

I can attest to the fact that there's a marked difference in cams between 79 and later. Have never seen 78 cams that I know of. Actually ... don't even need to mic 'em to see. Obvious by comparing them with just the eyeball. In fact, I just chucked a set of 79 cams that were trashed .... don't ask me to explain.

[Crazcnuk]

I was told this too, and was interested in a set of 78-79 cams to go on my 80 with the 1208cc overbore kit, altough I am a bit worried that I may not have the clearance for the piston domes.

But I was recently reading a comparison of the models, and I'll be danged if I can remember where, which gave a complete rundown of all the changes. The mentioned the larger valves, for the later models, but siad the cams hadn't changed.

According to my manual the Intake lift is the same for all the models (up to 1980, my manual is a 1980) but the exhaust is different, 8.305mm for 78-79 and 8.8mm for 80 ( and presumably 81).

Personally, I would prefer more on the Intake side, it would make more of a performance hit. The exhaust side is far less important, not sure why they changed it this way.

[pggg]

78' cams have more duration, ie valves stay open longer, a bit weaker at low revs, specially below 4500 revs, and even more so below 3000, the power band a bit more pronounced when full power hits at around 5500, I prefer 80' cams on my bike for grunting out of corners rather than having to change down a gear when gassing it, I found modern 36mm carbs really improve things from low revs right thru to high revs with 80' cams, haven't tried my modern keihin carbs with 78' cams yet, but I'm picking the 78' cams would suit well also