Carb Jet Sizes

[roach25681]

I have a 1980 Standard with pod filters, stock 4-1 exhaust that I will be turning into a straight pipe. What main and pilot jet sizes do you suggest I switch to?

[goldcoastguy001]

Personally i wouldnt use a str8 pipe the engine needs back pressure to be tuned right......I have also heard rumors of burnt exhaust valves from lack of back pressure.....maybe some one here can confirm or clear that up....Cheers

[roach25681]

The reason I say straight pipe is because this bike is done up bobber style. King Sportster tank, pod filters, drag bars, painted wheels, custom rear fender, custom metal side panels, model a tail light and liscense plate custom side mounted, harley 11.5" Sportster shocks, etc. Straight pipe would be my first choice, but I may put a GSXR muffler on. Audijunky runs two straight pipes with bigger jets and has no problems and I like the look of it. I would of course do the washer/bolt mod in the pipe if I straight pipe it.

[TopCatGr58]

Hey Roach,

If you know or have seen Patrick's Succubus, then you've been around the site, but apparently you've missed this tech tip in the Mods,Carbs section?
T.C.

[roach25681]

Actually, I just plain forgot about that thread. I read it a while back before I was even thinking of doing this. Thanks for reminding me and posting a link!

With my bike having a factory 4-1 exhaust, do I need to overlook the 2 jet sizes it calls for there, and just add the 2 size for the open headers? Or do I need to add for the 4-1 AND the open header?

[TopCatGr58]

Hey Roach,

Regrettably, it's just a guide, not a bible. It states that when 2 mods, you add both but then subtract one, so 2+2+3-1=6!?
That seems a bit much to me, but as he says........

you'll still need to monitor your plugs and make adjustments accordingly! Good Luck!
T.C.

[pggg]

I'm only talkin' 34mm 78' carbs here, the 80 carbs are possibly different, but even with all the 'breathe easy' goodies, going up one size on pilot jets and one or two sizes on the main jets is all you'll ever need on the normal 1100cc motor, I experimented with 147.5's and even 150's and over, but they simply run too rich even though the plugs may look 'clean' at a high rev plug chop. Actually high speed plug chops are really more of a 2 stroke thing. Chopping the plugs at midrange on the eleven gives a much better picture of what's going on in there. Go just one tiny size over on jets - and fuel economy starts taking a big dive

[roach25681]

In the 80 carbs, the stock sizes are 115 for 1 & 4 and 120 for 2 & 3. I figured Id go up 2 or 3 sizes on the mains and one on the pilots. Thanks for all the help.

[Orion]

Tagging along here....I am unable to find accurate figures on my stock jet sizes. The range of jets included in rebuild kits greatly deviates from one of the lists in the tech tips section.

Anyone know what stock jet sizes are for an 81 XS1100 SH?

I figure once I get the sizes I can begin to adjust for aftermarket air filters, exhaust, and the 5000 ft above see level thing.

[roach25681]

Orion, assuming your carbs are the stock 81 Model carbs, your jets should be 115 for #1&4 and 120 for #2&3. Pilot jets should be #42.5

[Orion]

Excellent, thanks for the reply, and apologies for hijacking the thread. One last thing, anyone think I should go up jet sizes per "the guide"? I'm running pretty rich with the current set up due to the altitude.

[wingnut]

Do not be suprised if when you take the carbs apart and you find 110 mains all the way across on a set of stock 81 carbs. Mine were from the factoy and that's what they still have in them.

[somesuch]

Wondering what emulsion tubes did you have with 110 all the way across?
--Nick

[wingnut]

Sorry Nick I do not know. I haven't been in the carbs in 2 years and did not know enough back then to even look at the tube numbers. Do remember lots of little holes had to be cleance out though

[somesuch]

No problem, I think the tubes with less holes go in the middle, I just wanted to make sure. But now that I am reading all the posts about GSXR carbs, maybe I'll skip my carbs alltogether
--Nick