Jet type for 1978 XS1100

[brharris27370]

I am wanting to change the jets a bit in my 1978 XS1100 and I see under mikuni jets on ebay for carb styles RS/HS large round, BS series CV, SJ and I am sure a couple other styles. I don't want to pull the carbs just to look at the main jets. I know that 1978 used a big jet and its way too fat past quarter throttle the more throttle the worse it gets until it starts miss firing and jumping and carrying on. If I run the thing with just the stock velocity stacks out of the air box with no filters the thing runs great but of course I don't want to run it that way all the time. So looking to try a main jet a tad bigger than the 110 that cam in the 1980 xs1100. So if I can figure out what style carb I have, I can snag a $12 set of 4 jets off ebay and drop those carbs and see how it runs. I want to run 4 emgo 54mm pod filters so that is still very easy to drop the carbs instead of reusing the original air filter box setup.

[WMarshy]

Stock Yamaha carbs on the XS11 are BS34.

Before you start changing jets you might want to verify what style carbs you have. The 110 mains are correct size for the 80 and up carbs (which does not have any bowl vents). Is it possible the previous owner but on a set of 80+ carbs and thats why it has 110 main jets in it?

The earlier carbs used 137.5 main jets and have bowl vents (2 additional T's above the fuel T's) that connect to the air box. If you do have the earlier carbs that came stock with the 78-79 model then you do need to jump up to the 137.5 main jet. Changing the jetting without knowing what year/style the carbs are will only mess things up further.

What are you running for an exhaust? This is another consideration when jetting your bike.

[brharris27370]

mine has the 137's in it which I thought was too big when I cleaned the carbs when I pulled this thing out of a barn ALL original. After riding it its obvious that 137 is too much. I want to run closer to about a 117.5 in the thing but need to order for the correct style carburetor.

[brharris27370]

as for exhaust it has a black or used to be black years ago $75 header that I bought off ebay that I sanded and repainted. Not for a xs1100 as advertised as it required some tweaking to get it to bolt to engine and doesn't follow frame lines well. has baffle and sound better than running the stock rusted out exhaust it had on it when I pulled it out of barn 100% original.

with stock air box and header installed the bike was barely ridable. fouled plugs and anything over 25% throttle and it ran AWFUL! pulled bottom of air box and removed filter and bike ran 100%. Ordered 54mm pod filters to replace air box assembly and now about 50-75% throttle and the thing runs awful. So I know I am too rich, just not sure just how far to drop the jetting. Thinking 117.5 on those mains.

[crazy steve]

You have to know which type carbs you have.... look here:
http://www.xs11.com/forum/showthread.php?t=35964

The early type need the 'big' jets; if you try running the later small jets the bike will be waaay too lean. Jet them to 'type', you don't need to go more 3-4 sizes bigger for each type, never smaller as Yamaha jetted these very lean from the factory.

You have cleaned/rebuilt the carbs, right? The slightest bit of crap in these will cause all sorts of problems, almost always resulting in too much fuel getting somewhere...

[WMarshy]

mine has the 137's in it which I thought was too big when I cleaned the carbs when I pulled this thing out of a barn ALL original. After riding it its obvious that 137 is too much. I want to run closer to about a 117.5 in the thing but need to order for the correct style carburetor.

You can change them to anything you want but it doesn't make it right. How did you come up with 117.5 is that an educated guess?

BTW, typically when you add a free flowing exhaust and intake you need to bigger jets (not smaller) as those changes make the bike lean. Doesn't make sense to me your bike would be opposite. That is why I said you need to validate what style carbs you have (and make sure they are clean clean!). They may be the later (80+) style carbs which take the 110 mains and the PO put in 137.5 and now it is way over rich... in which case 117.5 would be a good size.

Knowledge is power, find out what type of carbs you have (earlier version with carb bowl vents vs later style) and what should be in there from stock. Then make only 1 or 2 step changes from stock. You run the risk of burning up you engine (if ran too long) with 117.5 main jets in the earlier carbs that are 137.5 stock...

[motoman]

You can change them to anything you want but it doesn't make it right. How did you come up with 117.5 is that an educated guess?

BTW, typically when you add a free flowing exhaust and intake you need to bigger jets (not smaller) as those changes make the bike lean. Doesn't make sense to me your bike would be opposite. That is why I said you need to validate what style carbs you have (and make sure they are clean clean!). They may be the later (80+) style carbs which take the 110 mains and the PO put in 137.5 and now it is way over rich... in which case 117.5 would be a good size.

Knowledge is power, find out what type of carbs you have (earlier version with carb bowl vents vs later style) and what should be in there from stock. Then make only 1 or 2 step changes from stock. You run the risk of burning up you engine (if ran too long) with 117.5 main jets in the earlier carbs that are 137.5 stock...

May be a good idea to heed the warnings.....for your bikes own good. Ventilated pistons are NOT good.

[brharris27370]

The guy that does my mechanic work is 25 year experienced Yamaha certified tech, which wouldn't really mean a whole lot but I have known him for most of those 25 years and he is VERY meticulous and does beautiful quality work. I don't trust many people to work on my stuff and I recognize good work. I can do a lot of that stuff myself but on carbs he does a better job. He soaked the carbs in carb cleaner for a few hours and they looked beautiful inside and out. The bike runs perfect as it is currently with just the black pipes from the factory air box with no filters. Any more restrictive change and immediately runs worse. I did not look to see if the holes in the mains appeared to be drilled which was the case on my 2007 Suzuki LTZ400 4 wheeler than ran much like the XS1100 with factory air box installed. perfect to 1/4 throttle and then ran like total crap like your bouncing off the rev limiter. We found the jet issue with the LTZ400 and mechanic had a stock same size jet and put in it and cured it. On my XS1100, I am just not sure if the diameter of the head pipes are as big as the originals since I don't have them anymore. No leaks or anything just know they can't be for that bike and I hate to spend MORE money and especially a LOT of it curing the thing. I also don't like the idea of running it around with no filtration at all.

The 117.5 was just a guess. If the 137's haven't been ALTERED which since the bike did still have stock exhaust and air box when I got it, I don't see why they would have monkeyed with the jetting. I didn't run it with the stock exhaust on it as the mufflers were toast and the usual rusty chrome head pipes that were rusted out at the collectors.

I will verify the style carbs by those pictures this afternoon.

[DGXSER]

I certainly believe you got a good guy looking at your carbs for you. As has been pointed out, knowing if it is the early or later carbs makes a HUGE difference. And to lean can cause it to cut out as well, starving for fuel, have to look at the plugs to see what they tell you.

Last winter I purchased an SG off a friend who had it several years, and never could get it running right, he was just done with it and moved on. Never could tune the carbs. He had POD filters, Kerker 4-1. I opened the carbs and found 45 pilots and 137.5 main jets. Floats were about 27 mm. Biggest problem being these were the newer style carbs. Once I got them jetted right, 112.5 mains, with the 45 pilots, it ran like a scalded dog! All the way to red line!

[CaptonZap]

The original head pipes were a pipe in a pipe, with the outer pipe having a diameter of 1.5 inch's, OD.
The inner pipe had a diameter of 1.140 inches, ID. (About 1.25 OD, if we assume a wall thickness of .055 inch).
The idea of the small diameter pipe is that the gas velocity in the pipe stays high to the point of merging with the second pipe and the crossover expansion tube, thereby keeping the kinetic energy high, so that the scavenging from the cylinder at the time of exhaust valve closing is enhanced.
Or so I've been told.
All of the aftermarket pipes I've seen are single wall, and 1.5 inches, to keep them looking "right", i.e. not too small. To mimic the factory pipes, you would have to use small pipe, and put up with the crowd of pundits telling you that the pipes were too small.


CZ

[WMarshy]

I am wanting to change the jets a bit in my 1978 XS1100 and I see under mikuni jets on ebay for carb styles RS/HS large round, BS series CV, SJ and I am sure a couple other styles. I don't want to pull the carbs just to look at the main jets. I know that 1978 used a big jet and its way too fat past quarter throttle the more throttle the worse it gets until it starts miss firing and jumping and carrying on. If I run the thing with just the stock velocity stacks out of the air box with no filters the thing runs great but of course I don't want to run it that way all the time. So looking to try a main jet a tad bigger than the 110 that cam in the 1980 xs1100. So if I can figure out what style carb I have, I can snag a $12 set of 4 jets off ebay and drop those carbs and see how it runs. I want to run 4 emgo 54mm pod filters so that is still very easy to drop the carbs instead of reusing the original air filter box setup.

After re-reading your first post I see now why you wanted to try a 117.5 main jet. The short answer here (which was previously stated with a link) is to verify what carbs you have visually so you can choose the right size jets but more importantly, start with a known baseline. Many people have an aftermarket 4-1 exhaust and stock airbox with stock jetting (mains), or one larger, without any issues...

+1 on checking plug color for fuel/air ratio. Any thoughts on an ignition issues? The wires going to the vacuum advance unit under the left engine cover will have a break the copper strands and reveal itself during heavier loading (more advance) causing issues past 1/4 throttle...

Have you considered an ignition issue?

[brharris27370]

plug coloring is currently a perfect light tan color. If I change and put air filters on it the plugs are much darker to fouled black depending on how close to stock I get on the air filter.

[brharris27370]

as to ignition problems, since it runs perfectly as is with no filters, I don't forsee an ignition problem. has to be a jetting issue.

[brharris27370]

Looks to be the correct style carburetor to me.

[DGXSER]

What you want to look at are the details, like, is there a T down low where the fuel line connects to, and then another T higher on the carb body where vents connect to. If you look at the inlet bell to the carb, is there three holes or four, if four, then you have no vent tees. Also if you look at the mixture adjustment screws, are the screw heads exposed, down inside a tunnel. Those are the details in that carb identification guide you want to look at.

Four inlet holes, mix screw down in the tunnel are on the later 80-81 style carbs. Those take a much smaller main jet, 110 stock.

If the inlet bell has three holes, and the carbs bodies have the upper vent tees, and the mixture screw head is visible then you have the earlier 78-79 model carbs and the mains would be 137.5 stock.

The plug coloring your seeing would indicate a rich mixture, and a need to drop the pilots down. But do not overlook the float level setting. If it is set to high, you could also run very rich.

Just trying to get as much detail as possible to get you the right answer.