Ok I have a jetting question about 120 jets on my 79 Special

[JeremyL]

I traded for this 79 XS11 Special last week and I didn't really get to ride it but up a 75ft side street and didn't go over 30mph. Well I buy it and when I pull on the Interstate it backfires and pops so bad it blew the baffle out. Sounded awful while it was happening but I didn't realize I lost the baffle until I got home. I just kept thinking damn this thing is louder than I remembered. And it backfired non stop almost. Its got a 4-1 megaphone so it's loud. Anyways now I've had some problems that I think are carb related but keep thinking maybe it's just the lack of a baffle causing it to run bad but the tach didn't work so it's hard to test it much. It's just like hitting at wall when you open the throttle. The harder and faster you open it the worse it kinda stalls then surges then starts bucking until you let off. Combine that with an exhaust so loud it hurts and it's really annoying.

Anyways back to the jetting thing, I pulled the carbs anyways thinking ok they are just stopped up. I've cleaned carbs on three or four other 70/80s bikes so it wasn't a big deal. What I'm concerned about is that my main jets are 120s so I've probably got the 80/81 carbs unless someone put 120s in the 79 carbs for some reason but I don't see that. I checked the plugs and they are white. So my thought is that I need to install at LEAST the 137.5 stock for 79 jets and would probably want to go bigger for the straight pipe but I have ordered a baffle. I would think if mine had a 137.5 stock then going down to a 120 would be bad anyways but the add in a straight pipe and it's worse. I'm just not sure if it'd be bad enough to make it run as bad as it does now. Id be ashamed to sell it this way.

I'm gonna get some 137.5s and put the carbs back together when the baffle arrives and see what happens. Good idea?

[crazy steve]

If you have the '78-79 style carbs, those 120 jets are too small. But swapped carbs are pretty common, so you need to look. It's easy to check carb types; the early carbs have exposed idle mixture screws, the later ones don't (they're down inside a 'tower'). These are also circuited differently internally, so you jet by the year of the carb, not the bike. If you put the 137 jets in the late carbs, you be hopelessly rich, even with aftermarket exhaust.

But also check for broken pick-up wires; your symptoms sound like a classic case of bad wires. Pull the left front engine cover, and look for the little wires that go to the timing plate. Pull on them (don't be too gentle); if they stretch, they're broken and when the timing plate moves from the vacuum advance, they'll separate and kill the ignition to two cylinders until it moves back. There's several threads about repairing these, do a search for more info.

[JeremyL]

Are the idle mixture screws the screws on top of each carb that, when removed, have a little spring on them and then the screw themselves have a sharp point to them? The ones I'm thinking off have the head exposed so you can either twist them or use a slotted screwdriver to turn them? If so I remember taking those out when I cleaned the carbs.

Thanks for your help!

[JeremyL]

Also I was searching around and do the 78-79 carbs have the rubber plugs over the pilot jet? Mine has the threaded screw covering the pilot jet. But after reading about the wire problem it sounds like my problem too.. I guess I could have multiple issues at once.

[foreda]

Are the idle mixture screws the screws on top of each carb that, when removed, have a little spring on them and then the screw themselves have a sharp point to them? The ones I'm thinking off have the head exposed so you can either twist them or use a slotted screwdriver to turn them? If so I remember taking those out when I cleaned the carbs.

Thanks for your help!

Those should be 78 / 79 carbs. Stock jet is 137.5.

[natemoen]

By your two previous posts it is obvious you have the early carbs. get the 137.5 jets and.start there.

[JeremyL]

Thanks guys! I am still gonna check the ignition just to make sure someone hasn't jb welded something together. It ticks really loud at idle so I read to check in there. I know it was hooked to the carb from the vacuum advance and not the boot.

[yamahansolo]

That's odd for someone to put that size jet in there?... also you can make sure it is a 79. I have hears horror stories of mixed and matched parts... check your motor Von number or it be faster to see if you have the the kick starter connection sticking out or if yours is capped off. I have seen people sell 80s as 79's and vice versa. But definatly jet according to your carbs! I think it would just be a teensy bit harder if you have an 80+ motor with 79 carbs might have to research for a good starting point.

[JeremyL]

My motor has a black plug over the kick start hole. I read about the kickstart Deal before but didn't know if "plugged" meant this black rubber plug or blocked off with metal. I didn't think the plug looked removable but I wasn't sure. My title says 79 and I think the frame vin is right (need to recheck) so it could be a 79 bike with an 80+ motor with 78-79 carbs that have 80+ jets in them. Ideally I wouldn't want mixed up parts but I dunno how bad this would be.

I guess the guy said well its a 79 but it's got an 80s motor so let's just get replacement jets for an 80+.

[natemoen]

My motor has a black plug over the kick start hole. I read about the kickstart Deal before but didn't know if "plugged" meant this black rubber plug or blocked off with metal. I didn't think the plug looked removable but I wasn't sure. My title says 79 and I think the frame vin is right (need to recheck) so it could be a 79 bike with an 80+ motor with 78-79 carbs that have 80+ jets in them. Ideally I wouldn't want mixed up parts but I dunno how bad this would be.

I guess the guy said well its a 79 but it's got an 80s motor so let's just get replacement jets for an 80+.

If it is a flush rubber plug then it is the later engine, if the shaft sticks out and has a black rubber cover over the shaft then it is the earlier engine. Any of this can be changed though so VIN is the best way to check.

Check and compare your vin here.

http://www.xs11.com/faqs/153-serial-...ry-colors.html

[JeremyL]

If it is a flush rubber plug then it is the later engine, if the shaft sticks out and has a black rubber cover over the shaft then it is the earlier engine. Any of this can be changed though so VIN is the best way to check.

Check and compare your vin here.

http://www.xs11.com/faqs/153-serial-...ry-colors.html

Yup you're right it. It's an 80 engine (3J6 code). I forgot to take my title with me to check the VIN but I'm about 80% sure the frame is a 79.

[ccogswel]

I may be wrong. But even if you do use those carbs off a 78-79 and use them on that 80+ motor you'll still have to use the 137.5 jet. Cause they flow differently then the 80-81 carbs. That is one reason why for the different jets from those years. Also wouldn't hurt to check your pilot jets too. They may have changed them out to the later pilots as well since they done the mains. JAT.

[natemoen]

I may be wrong. But even if you do use those carbs off a 78-79 and use them on that 80+ motor you'll still have to use the 137.5 jet. Cause they flow differently then the 80-81 carbs. That is one reason why for the different jets from those years. Also wouldn't hurt to check your pilot jets too. They may have changed them out to the later pilots as well since they done the mains. JAT.

Yes, you jet according to the carbs, not the engine.

But the lower jets may have been put in when the PO changed engines and thought he had to jet according to the engine.

[JeremyL]

I may be wrong. But even if you do use those carbs off a 78-79 and use them on that 80+ motor you'll still have to use the 137.5 jet. Cause they flow differently then the 80-81 carbs. That is one reason why for the different jets from those years. Also wouldn't hurt to check your pilot jets too. They may have changed them out to the later pilots as well since they done the mains. JAT.

I checked the pilot and I *think* it was 42.5 but I'm going to recheck to make sure.

Yes, you jet according to the carbs, not the engine.

But the lower jets may have been put in when the PO changed engines and thought he had to jet according to the engine.

That's what I'm thinking... this guy seemed like he knew enough to be dangerous and probably didn't research a lot of stuff on the Internet. I'm a big computer geek (do IT for a living) and am SOOOOOO thankful for this site. If I bought this bike without a site like this I'd never get it running!

[yamahansolo]

Even if your frame says 79 it can be an 80... my 79 frame says it was made in April of 78. But the Vin # on the frame and motor is 79.
You definatly have the older carbs and you have to go with at least the 137.5 my 79 came with 140 mains if you look at the parts fisch you'll see some came with 140 mains ans a 210 air jet that's what mine has.
I just thought maybe it'd be a little harder to throw in the stock jet size because the 80 motors only have a 9.0 compression ratio, bigger valves, and less advance so throwing in the stock 79 jets may not be perfect. You might have to search and see what other people have in there early Carb/later motor combo.