Jetting question

[Firehawk]

I have a Clymer manual that claims that my carburetors for a 1981 XS1100H, that the inner two cylinders should be jetted at 120, and the outer two should be at 115.

After following the jetting recomendations, I ended up with 135s on the inner and 130s on the outer cylinders.

However, when I pulled off the bowls, moments ago, I discovered that all 4 main jets were 110s. Is this the work of some overzealous mechanic in the bike's past or could this be correct? Do I need to buy new jets based upon them all being 110s or do I go ahead with my previous plans regardless? That is my question.

The bike has always exhibited signs that it was lean since I've owned it.

Also, when I got it, it was an XS1100 with the touring package (fairing, saddlebags, and trunk) and the standard gas tank and whatnot. Is this an H, LH, or SH model or is it a G, LG, or SG model? I've assumed "H," but I may be wrong. According to the clymer, the H and G are jetted the same, while the other four have 110 on the outer cylinders.

Any help would be appreciated.

[John]

Yor bike is an H model. The SG and SH are Specials, and the LH LG are Midnight Specials. What mods have you made? My 81 Special had all 110' across the board, but I swapped out the carbs with those from my 80 Special, and they had 120's in the middle, and 115's on the out side, and the carburation is fine like that. The staggered jet sizes is said to be an attempt by Yamaha to make them run a little cooler on the inner cylinders.

[Firehawk]

According to that chart, for my bike:

Stock main jets: Inner 120, Outer 115

Stock pilot jets: 42.5

2 sizes for 4:1 exhaust (Mac)

3 sizes individual (pod) filters

2 sizes for no muffler

7 (main sizes) - 1 = 6; 120.0 + (2.5 * 6) = 135.0 for the inner, and 130 for the outer
2 (pilot jet size); 42.5 + (2.5 * 2) = 47.5

That's what I did. I'm fairly certain the bike was lean with the 110s across the board. When I went in, though, they didn't have any 130s so I bought four 135s, as I figure it couldn't hurt the outer cylinders.

[John]

On my bike, I have an 1179 cc big bore. My carbs are from a 78E, stock jetting is 137.5 mains. I have Jardine 4-2 Spaghetti pipes, and a Uni foam filter. I only needed to increase my mains to 140's, and it really runs well set that way. Have you changed the jets yet? Do you have an assortment of jets? You may want to start out 3 sizes over, and adjust from there.

[Firehawk]

Well, I bought 135s, as I mentioned, but I have yet to test them, as I have the carbs off and ready to put them in, but haven't due to the confusion with the 110s being in there instead of what the Clymer said they would be. And also it's snowing pretty hard outside, and there's currently 11 inches of snow (I think) on the ground.

So it might be a bit before I can fully test it.

It ran quite lean, and hot. The pipes are goldish. This was before I even put on the new pipes. On the highway, I was lucky to get 28mpg. The pod filters and 4-into-1 MAC exhaust with no muffler probably lean it out quite a bit as well.

So, assuming that the jetting in there currently is incorrect, and assuming that chart works for my bike, I should be good to go I think, but I just wanted to hear your guys' thoughts as you all know more about this than I.

[TopCatGr58]

Don't it make my golden brown pipes blue?

Hey Hawk,

I can't remember where you said you got your jets, or whether you can easily swap them for others!?

Mine, an 81SH also came with 110's across the board. I have Indy filters, 4-1 pipes, with simple baffle in it, not a true muffler per sae! Big Bore Kit. I put in 117.5's and it seems to do quite well, but possibly still just a touch lean according to my Dyno results...I didn't get the Air/Fuel ratio measured!

SO.....like John said, you might want to try a few sizes smaller than your 135's. According to Motoman, if your pipes start turning BLUE, then you'll know you've gone too rich!
T.C.

[Firehawk]

I could swap them, but the place is in a different town.

I might go buy some 120s, based on what you've said about yours, though the jetting guide would suggest 125s. Should I ignore that bit of wisdom?

[Firehawk]

Ok, I got it rejetted and everything, but the thing still doesn't get any gas at start. The same problem I had as before.

Why doesn't it get any gas at start, even if fully choked, yet does just fine if you can get it going?

[trbig]

Do those little jets down in the side of the bowl have anything to do with it?? I remember that they were the only thing plugged on mine... but it still sarted.. so?? Sorry, not much help.

Tod

[DiverRay]

look at the tech tips, and do a search on "carb cleaning". There is a problem with the carbs if they are NOT cleaned properly. You may still have one or two circuts that are plugged, so no fuel flow at start.
Ray

[Firehawk]

I take that back, the bike absolutely refuses to start if it is choked at all. It's about 60 degrees outside right now, and the colder it is or the longer the bike sits, the more it does this. If I crank it long enough, it will start as the engine warms/gets a rich enough intake to start.

If it's warm, it starts right up. I only have this problem below about 75 degrees.

I think there is nothing wrong with the carbs in the way of gunk having built up and blocking. I think that there is something wrong with the chokes.

Is it possible to reinstall them incorrectly after you do a rebuild? Don't they just shoot more air under the diaphragm in order to lift it and spray more gas into the air? That's how it looked like they worked anyway.

[Firehawk]

I've got the carbs off, and partially disassembled.

From the looks of things, carbs 1 & 2 are lean (gold pipes around the tops of the exhaust pipes) and carbs 3 & 4 are rich (blue pipes around the tops of the exhaust pipes).

I'm going to check float heighth, make sure those are all the same. And I'm going to see if I can't clean out the choke venturii, if they need it, as I've noticed that there doesn't seem to be anything happening when I use the chokes. In fact, it seems as though it is impossible to start when I use them.

They should still be synchronized as I haven't messed with any of that.

Can you guys think of anything else that might be causing the carbs to be putting out different air/fuel ratios?

[TopCatGr58]

Hey Hawk,

IT's usually not the "choke=fuel enrichener" tube in the carb body that's blocked, but the little jet down in the float bowl that usually gets clogged, and that prevents the flow up into the carb body venturi tube!! You may need a small wire to stick down into the hole and the jet to try to get the gunk loosened up, along with many squirts from a carb spray can! Make sure to wear eye protection!!

The choke/fuel enrichener opens up a more direct path from the float bowl into the inlet port of the carbs and more fuel is drawn up thru that venturi straw! However, if you turn the throttle you open the butterflies which actually reduces the amount of effective vacuum thru that circuit and it won't get as much fuel!! The vacuum slides don't have anything to do with the choke circuit!

If your jets are the same size, then I would suspect ineven float heights for the richness of the right 2 carbs. What are your pilot screws set at? The pilot circuit DOES contribute to the lower rpm range of running, not just idle, and so could contribute a little extra fuel!?
T.C.

[Firehawk]

My pilots are currently 47.5, up from 42.5.

[TopCatGr58]

The turn of a friendly screw!?

Hawk,

That is the SIZE of your pilot jets, but I am referring to the number of turns out your pilot screws that are on the front top of the carbs, are set at!?!? More turns out=rich, in =lean!?
T.C.