80 XS1100 running rich

[vintagemcyclegu]

Which way do I turn the air jets to lean out my carbs. Thanks.

[skids]

Which way do I turn the air jets to lean out my carbs. Thanks.

The pilot screws are really fuel adjustments. Lefty-loosy lets more fuel in, righty-tighty closes down the fuel feed. Keep in mind that this adjustment if not for midrange rpms and higher. It has the most affect near idle and a little higher...

[scott0757]

Another Rich Running 1980 Special

No to hijack this post but my 1980 XS 11 Special is giving me fits. It has run great until the last rebuild I did this spring. I found the pilot jets and the needled were wrong when compared against those I removed. The main jets (120s 1 & 4 and 110's 2&3 ) are a combination of old and new with the 110s being original. The floats seats/pins were also replaced and the floats adjusted to the books spec of .906 +/- .020. I have the pilot screws out about 1.75 turns.

In this set up, the idle seems a bit rough and I get a near constant burble on neutral throttle up to about 4K RPM after which is seems to scream per usual. My plugs (all 4) are coming out very black and I can smell a difference in the exhausts (unburned fuel and very dark color). My gas mileage is terrible ...almost 1.2 where it had been. I can barely use the enriching (a.k.a. choke) even at the first step as the bike bog down.

Is there anything that could cause this beyond the float levels? I have no leaking gas so that must be low enough to prevent that.
Are there certain pilot passages that I should be checking? I hate taking off the tank/seat to get the OEM airbox screws out to be able to pull the carbs only look it over and put 'em back on.

I have rebulilt these many time before and this time I'm having nothing but trouble. Maybe a bad/wrong set of rebuild kits...Help!

[3Phase]

Hey, Scott, what was wrong with the carburetors that made someone, apparently not you? change jets, needles and floats?

I guess it's possible the new parts aren't genuine Mikuni or they're the wrong type.

[bikerphil]

Do you have the rubber plugs in the pilot jet towers? Without them you will be sucking too much fuel. Most 1980 carbs require them.

[cywelchjr]

Actually only VERY early 80 carbs have the cross passage and require the plug. Most of them don't need the plug, and if they need the plug and don't have it they will idle, but will barely run at all, not even under heavy throttle, or any throttle setting.

[B.B.]

rubber plugs

What and were are the rubber plugs that you are talking about? My 80 special does not have any rubber plugs on it.

[bikerphil]

On the 1980 carbs, if there is a passage connecting the main and pilot jet towers, you need the rubber plugs in the pilot jet towers. No passage, no plugs needed.

[natemoen]

The rubber plugs would be used in the pilot jet tunnel. The plug is only needed if there is a cross passage drilled from the main jet tunnel to the pilot jet tunnel.

No cross passage, no plug.

[scott0757]

II have a 19801100 SG Special. It had the pilot screws capped with a aluminum plug that I removed so they could be cleaned and never put them back in as other models did not have them and they blocked access to the needles. I am seeing no rubber plugs, but see several spots in the carb body where the factory looks the have plugged passageways during manufacture. Not of these have been touched to my knowledge or since it had been running well. If by rubber plugs you mean rings around the body of the needle screws, yes I have those. When I got the bike and put new pipes on it, they blued quickly so I needed to richen things a bit. Reading the posts, this carb had 110 mains in all four positions, but other posts indicated the center two should be a set larger to keep the center cylinders a bit cooler. That the set up i have now: 110's on 1 &4 and 120's in 2&3. Had seem to be fine for the past year or more so don't suspect that is the issue.

Why did I mess with things?
I was having issues with rough idling and then a race condition that could only be stopped by slipping the clutch to pull the RPMs back down to normal. With the ethanol gas (even though I use startron additive most fill-ups), I figured I was getting gelling up in a passageway over the winter months (even though I drained the bowls) so I ordered a rebuild kit that came with jets, float needles and seats, pilot screws and gaskets. When I first got it together, it would not idle well at all. Awfully lean. Then I found a post in this forum about the pilot jets varying by specific carb models within the same year so I took them off and sure enough what had been there had eight holes and the replacements only 6. Swapped back the originals and it got better at idle but still had a hesitation between the pilot and main circuits about 3500RPM and the high idle issue was still there.

Another suggested adjusting the cam chain tensioner for the high idle,, which I did, and that seems improved but not gone.

There were other suggestions about checking the float bowl heights which I did and they appeared to be a set too high, I am now thinking they were better when set over the .906 /- .02 level. Not sure if the float needles are different lengths but they appeared to be the same as the old. It also did not seem to be an issue as long as the floats were set properly so the new needles were closing at the right float height.

So back out to test and things are running super rich in all four cylinders. My normal tank would go over 100miles and I ran out at 60 on the first tank..luckily uphill from a gas station!) This leads me to believe the float settings are way to low and I am spilling/eating gas. Before I dive into the mess again, looking for suggestions...

[natemoen]

The correct pilot jets are the jets with 6 holes (in a 2-1-2-1 pattern around the jet). The 8 hole jets are wrong.

The rubber plug we are talking about is inthe early '80 carbs and plugs the pilot jet if there is a cross passage between the main and pilot tunnel. The pilot adjustment screw has the brass plug.

Even yamaha get ride of the staggered jet size idea because it didn't do what they thought really. Most get ride of it some don't.

[Bellerophon]

My '79 had many of the same symptoms Scott describes. I finally ended up taking it to the dealer who noticed that my carbs had the wrong pilot jets. I needed the non-bleed type with no holes. The bike idles and runs great now. I wrote a full description here.

http://www.xs11.com/forum/showthread.php?t=33852

[3Phase]

Scott, nice troubleshooting finding the wrong pilot jets!

Nate's got you covered on the EPA anti-tamper caps in the mixture screw towers, the rubber plugs for the carburetors with pilot jet wells that have a crossover passage drilled between the pilot jet well and the main jet.


Okay, assuming that you have a stock late-model XS11 Special with no exhaust or intake modifications then what's happened is that you've put the 'wrong' main jets in the #2 and #3 carburetors. I don't know what you read or where you read it but the stock main jets in the late-model Special carburetors are all size 110, not staggered. It won't hurt anything and if the engine is in good shape the top end power might actually go up a little with larger main jets but the fuel economy will definitely go down.

Basically, the #2 and #3 carburetors are slurping fuel through the larger main jets. To get your fuel mileage back you should put he stock 110 mains back in the #2 and #3 carburetors, then resynchronize the carburetors and adjust the mixture screws.


The scoop:

The carburetor slide needles are called jet needles in Mikuni-speak. The brass emulsion tubes with all the holes in them that the jet needles slide into are called needle jets. The main jets are screwed into the bottom of the needle jets and the jet needle partially blocks the main jet metering orifice when the carburetor slides are all the way down at rest.


The 1980 Special carburetors have relative short, pointy jet needles with a steep taper and the main jets are size 110 on all four carburetors. The needles work best with the 110 main jets used in the Special carburetors.

The 1980 Standard jet needles are a little longer, not quite as pointy with a slightly more shallow taper and they work best with the larger, staggered jetting used in the Standard carburetors.


If you put larger-than-stock main jets in the Special carburetors without changing the jet needles then here is what happens:

The sharp point and steep taper on the needle will allow a little more fuel from the main jet at idle and when the carburetor slide begins to lift it will provide even more fuel sooner than it should. Engine power goes up, gas mileage goes down.

The short length will allow the needles to lift clear and fully open the main jets a lot sooner than it should. Engine power goes even further up, gas mileage goes even further down.


Again, this is on a stock 1980SG. If you've changed the exhaust or removed the stock airbox to put on pod filters then you may or may not have to change the jetting.

[scott0757]

Yes I still have the OEM airbox but have MAC 4-2 pipes, another reason I had pushed the jet size up a notch, but this may not be necessary. I also installed an XS850 oil cooler unit to keep things a bit cooler so maybe going back to stock jet settings is ok.
So, based upon all the above, looks like I have three things to do: Put back in the 6 holes pilot jets and replace the 120's in #2 & #3 with the stock 110s. Then sync and adjust the pilot screws.

Stay tuned for more carb adventures.

[scott0757]

OK. Epilogue: Main jets back to 110 in #2 &3. Pilot jets w/6-holes installed. Hooked back up. At first I was nervous as I got lots of pops and sputters when at idle, but backing the needles out seems to have rectified that. I am at 2.5 - 3 turns, which is more than I'm used to, but with the 6-hole vs the 8-hole pilot jets this makes sense. I initially had a racing idle that would jump up to 3500RPM, but as I backed down with the idle screw and opened the pilot screws, thing seem to be good now.

Seems to chug a bit about 4,000 RPM and may need a bit more pilot screw opening, but I'm going to see what happens after a bit of riding as I think I have some carbon to burn off the valves and plugs. So far very encouraging. Thanks to all above for the info and assistance.