Jetting and Mileage

[wrangler615]

Hey guys! First time poster, long time reader here. I searched through the carb forum and didn't find quite what I was looking for. I wanted to get a general analysis of what kind of mileage versus jetting you guys are getting? Last season I purchased an 82 GS1100G(for 300 bucks!) It has brand new Dunlops, needed a battery, and a tank of gas. Bone stock except for properly cleaned and oiled K&N, ran good, poorish mileage(28-30 mpg highway). While prepping it for this season, I noticed a leak at the #2 float bowl(Mikuni BS34SS's). I purchased a set of K&L rebuild kits off of ebay, to go through them. This is where it gets interesting...

The kits came with two main and pilot jet sizes. #40 &#45 pilots and #107.5 & #115 mains.

Upon teardown I discovered the bike had #115 mains and #40 pilots installed. The manual I have(for an 82 GS1100E)shows a #45 pilot & #107.5 main as being stock. Undoubtedly someone was in there before, jetting for who knows what. Maybe this bike lived near sea level before now, but I live and commute at 4500'-6000' above sea level. So I believe it was jetted to rich on the main.

So, I installed the #107.5 mains and re-installed the #40 pilots(new ones), set float levels, re-installed the carbs, synched carbs, and adjusted the idle mixture. The bike runs fantastic. Good idle, great idle to rev transition, excellent top end power. Now I'm getting about 38 mpg on the highway. The question is what more can I squeeze out? I have a buddy with an 82 GS1100E, and he says he gets low 40's on the highway.

I was thinking that I could try getting #105 mains, are these even available? I have to take the carbs back off anyway to fix a small air leak at the #3 manifold and install a new throttle cable anyway. Of course I don't want to go so lean as to cause overheating, misfiring, and other damage. Keep in mind about my higher elevation, I know the jetting probably seems kinda lean to you guys on the coasts, especially with engine mods.

Sorry to be so long, just wanted to give a good overview of what I have to deal with. Any opinions or advice is very welcome

[Dave8338]

I think if you have it running as strong as you say, LEAVE IT ALONE!

Trying to eek out few more miles to the gallon isn't worth the strain on a bike that was designed to run very lean to begin with. 38mpg on a litre bike is perfect. I doubt I get that with my 650 and also keep in mind...the right hand will do more for fuel economy than any rejet you're going to try next.

Welcome to the GSR, by the way.

[tejasmud]

Just cruising, the main jets are most likely not in use, but the pilots and needles would be.

Do some plug chops at your cruise speed, and see what you have going on.

But, you say "runs fantastic," as is. I would say ride.

If you have a heavy wrist it will burn more fuel naturally.

Another thing is the G is pretty much fixed in the final drive ratio, an E could be set up on the final drive to eekkk... out more MPG.

[Big T]

Good question

So, do a plug chop, post up some pictures of your plugs and we can tell you what you want to know

[rudy]

Another thing is the G is pretty much fixed in the final drive ratio, an E could be set up on the final drive to eekkk... out more MPG.

Mmmmmmm! I LOVE chain drives!!

[tkent02]

The E's chain drive is more efficient than the shaft, if the chian/sprockets are in good shape, the eight valve is probably more efficient than your G engine. So comparing mileage between the two his should be better if all else is the same.
You main is likely close and won't effect mileage much except when you have the throttle open pretty far, mostly getting the best mileage while cruising is in perfecting the pilot circuit and jet needle/needle jet.
The low power circuits can be tweaked very lean with no harm to the engine, start making very small changes and testing until you get it perfect. Thin washers for shims on the needles, small tweaks to the mixture screw
Getting the main right is the easy part, getting the rest perfect takes time.

[BassCliff]

Greetings and Salutations!!

Hi Mr. wrangler615,

I just stopped by to welcome you to the forum in my own, special way.

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Here is your very own magical, mystical, mythical, mind-expanding "mega-welcome". Please take notice of the "Top 10 Common Issues", the Carb Rebuild Series, and the Stator Papers. Now let me roll out the welcome mat for you...

Please click here for your mega-welcome, chock full of tips, suggestions, links to vendors, and other information. Then feel free to visit my little BikeCliff website where I've been collecting the wisdom of this generous community. Don't forget, we like pictures! Not you, your bike!

Thanks for joining us. Keep us informed.

Thank you for your indulgence,

BassCliff

[twr1776]

Wrangler615, I do not mean to rain on your parade but the 1100G two valve motor has the #115 mains and #40 pilots as stock jets from the factory for the BS34SS carbs. I would recommend putting the # 115 mains back in and tuning from there although if you run mainly at altitude I am not sure which way you should go on jetting. The E manual you have is for the four valve per cylinder motor. Some will bump the pilots one size (42.5) on these motors at normal altitudes. I have gotten up to 48 mpg with my 1100 and the low ratio rear end on a straight highway run and lots less on twisty runs.

[wrangler615]

Thanks twr1776! Jetting info like that is what I'm after! I knew the manual was for a 4v engine and I was wrong to assume that the jetting would be the same for a 2v engine vs a 4v engine. Seems odd that the 4v would have leaner mains with the increased airflow. It is a shame that the shaft drive ratio is fixed.

rudy: Mmmmmmm! I LOVE chain drives!!

Agreed, but couldn't pass up on this deal.

I can admit that I though the bike was spending more time on the mains being at highway speeds (70+ for about 70 miles/day), not so much on the pilot circuit. I was really suprised though that I got such an increase in mileage even though the jets I changed were the mains. The pilots are the factory size (#40 per twr1776), only replaced with the new from my kits. Does anyone have an opinion on the calibration of K&L jets vs. factory Mikunis? Perhaps the K&L's are calibrated leaner? Leading to a mileage increase even though they are supposed to be the same size?

I know alot/most of you guys would suggest leaving well enough alone, but I am after efficiency! I am very confident in my ability to do and understand the work and concepts. I am a auto tech, just not intimately familiar with older carb systems. Everything I work on is either fuel injected or a diesel!!!

I gotta think that it should be no harm with the #107.5's in the mains, considering my altitude. Any opinions on putting a pilot that is one step smaller in? Would this be a #37.5?

Dave8338: Trying to eek out few more miles to the gallon isn't worth the strain on a bike that was designed to run very lean to begin with.

I value your opinion. But keep in mind that at my altitude everything needs to be leaned out just a bit. I am in no way looking to push the engine past its limits. I just want to see what I can get out of it comfortably with no driveability concerns.

[twr1776]

wrangler615 It is a shame that the shaft drive ratio is fixed.

I would not worry about it. My 850 will run at even higher revs all day long without any issues. These motors love to spin. What do your plugs look like? Do a search for plug chops and you will find lots of info on how to do it. Your mileage does sound a bit low. I am not a carb expert but it is my understanding that throttle position controls which circuit you are on and they do overlap somewhat. Pilots, needles, and then the main by approx thirds on throttle position so at cruising speed you are mostly on the pilots and needles as previously mentioned.

[chef1366]

Wrangler615, I do not mean to rain on your parade but the 1100G two valve motor has the #115 mains and #40 pilots as stock jets from the factory for the BS34SS carbs. I would recommend putting the # 115 mains back in and tuning from there although if you run mainly at altitude I am not sure which way you should go on jetting. The E manual you have is for the four valve per cylinder motor. Some will bump the pilots one size (42.5) on these motors at normal altitudes. I have gotten up to 48 mpg with my 1100 and the low ratio rear end on a straight highway run and lots less on twisty runs.

I agree....................

[Steve]

Another agreement here with the stock jetting, in spite of your altitude. Stock jetting is borderline lean at sea level, should be about right in your area. Rejetting for altitude will really only work if you NEVER, EVER go lower in altitude or agree that you will rejet before going for a visit.

If you really want to improve your mileage, keep it below "70+" mph. There is something magical about the 65-70 mph number. Most of my vehicles (bikes, car, van) all get respectable mileage that really drops off dramatically if I go over 70. Example: my Wing. On the highway, cruise control on 75 for most of a tank of gas, I will get 34-35 mpg. At 70, it goes up to36-38. At 65, it's 38-40. If I can keep it in the 60-65 mph range, mileage is 42-44.

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