'81 H main jets

[bikerhawk]

I'm getting conflicting information on stock main jet sizes for an '81 H. The factory manual says that they are 110s all the way across, while the Clymer manual says the outer 2 cylinders should run 115s and the inner 2 120s. Any input?? No mods on this bike, stock exhaust & airbox.

[John]

On my 80 SG the jetting was 115 on #1-4 and 120 on #3-4 . I read that this was done for a little extra cooling on the inner cylinders. I never had any problems with that set up and stock air box and pipes. What is in your bike now?
I recently installed a Jardine 4-2 header and the carbs from a 78 e. They have 137.5's all across. The internal passages are different on the older carbs as compared to the later ones. That is one reason for the different jet sizes. On the other hand, The 82 XJ100 uses 112.5's all the way across. I would go with the 115-120 combo. Go figger.

[bikerhawk]

IIRC, I've got 115s on all 4. I haven't had them off the bike in a while, but I need to do some other things anyway, so they will be pulled soon.

My whole reason for this is to try to improve mileage...only getting around 28 mpg, so I figured I'd start with putting the mains back to stock. The carbs were set up as they are now when I got the bike...no clue why, except that the bike was in Colorado.

[chevy45412001]

jetting

I was told that the two center carbs were jetted differnt because the center carbs did not get the same amount of air as the outer carbs.Kawasakis i heard did this quite often. Just a thought.My xj had 112.5's all across. Now running 137.5's across.4-1 kerker,4 no name filters.

[TopCatGr58]

Hey there Tom,

Well, running 115's across is using the lower/leaner jets vs. the mixed 115/120 as per the book. How is your bike setup, does it have stock airbox and stock pipes? Is your filter clean? Are you running fresh plugs?

The 81 carbs came with the pilot/idle screws capped off, have you opened yours up to allow you to adjust them. The main jet needle attached to the vacuum slide isn't adjustable. However, you can get to your idle screws and turn them in a bit to lean things out. You could also lower your floats.

What do your plugs look like, are they heavily black carboned, or a nice gray/tan, or even white? How many miles on your bike, how is your compression? The mileage you spoke of, was that in town stop and go or mostly highway? Are you running any windshield/fairing? Are your tires a softer sticky compound vs. a harder touring, are they properly inflated? Are you brakes dragging?

Sorry for the 20 questions, but so many things factor into mileage. I've got a big bore, 3 sizes up on my main jets, 4-1 pipes, Indy filters, and I get only about 30-33 MPG on highway with just a handelbar mounted windscreen!
HTH.