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New to me GS850 jetting questions

  • Thread starter Thread starter qwqb
  • Start date Start date
Q

qwqb

Guest
Hey guys,

Just got my hands on a 1981 gs850. The previous owner installed a 4-1 exhaust and 4 separate air filter pods, however he did not increase the jet sizing. I am wondering what I should look into buying to accomodate for the exhaust and air filters.

I also have the stock airbox... is there a performance advantage to having the 4 separate pods? or is it mainly aesthetic?

Thanks!
 
Welcome to the forum.

If you have the airbox .. use it. IMHO on that bike the performance gains from pods are not worth the hassle and added expense of rejetting. If you haven't already, check your valve adjustment and make sure your carbs are clean.
 
That may be the route I end up taking. Can the factor jet sizes be used with the aftermarket exhaust?

Also, the bike does not currently run, so I am going to be tackling cleaning and rebuilding all the carbs pretty soon and hoping that does the trick.
 
found this on another site,
We jet our sets for pods and aftermarket pipes. We use #122.5 main jets, #17.5 pilot jets and the jet needles 5DL31-3.
 
That may be the route I end up taking. Can the factor jet sizes be used with the aftermarket exhaust?
Click to expand...

It really depends on how restrictive the after market exhaust is. Your best bet is to perform all the routine maintenance on the bike (adjust valves, clean carbs..etc) and then see how it runs. If you need to make any jetting changes do some plug chops to see where you are.
 
qwqb said:
That may be the route I end up taking. Can the factor jet sizes be used with the aftermarket exhaust?

Also, the bike does not currently run, so I am going to be tackling cleaning and rebuilding all the carbs pretty soon and hoping that does the trick.
Click to expand...


If you're trying to run the bike with aftermarket exhaust you MIGHT be ok running stock jet settings but if you're using pods and aftermarket exhaust you'll be very lucky if the bike even starts up.

If you dont wanna spend the money on a Dynojet stage 3 jet kit then put the stock airbox back on the bike, with a good/new filter. Assuming its in good shape.. Be sure its sealed correctly.

Did you replace the O-rings on the intakes?
These bikes have very touchy intake systems.

Im not sure the pods actaully have any performance gains but they sure do look better that that uglyass stock airbox.(my opinion)

One more suggestion, search your questions. This subject has been beat to death.
 
#2 son's bike (see sig) has a header of unknown origin and stock airbox. I am currently running 120 mains (stock is 115), stock 15 pilot fuel jets, stock needles shimmed about one full notch. Have not done full plug chops, but it runs very well.

Note that the stock needles are not adjustable. What is necessary to lift them for a richer mixture is to get some small washers (the smallest washers in the assortment pack at Radio Shack work well), determine how many of them are the same thickness as the nylon spacer at the top of the needle, then use half as many. Usually, it's four washers for the full thickness, so just use two. If your header is rather free-flowing (no baffle, or LOUD), you might use just one washer.

.
 
what is this plug chops you guys are talking about? im assuming it is some form of examining of the plugs to see how lean/rich the mixture is?
 
Explained somewhat in detail on BassCliff's site, click here.

Basically, you mark your throttle like this:

IMG_3646.jpg

The markings are idle, 1/8, 1/4, 1/2, full

Warm up the bike, go for a ride, take your spark plug wrench with you. Hold the throttle at the 1/8 position, ride as long as you can in that position. Might need to use a higher gear on a back street with no stop signs, but you might need a mile or so. Without moving the throttle, hit the kill switch and pull the clutch when it is safe to do so. Remove the plugs and observe the color. Next, go for another ride with the throttle held in the 1/4 position. Might need a small hill to climb to keep your speed in check, but feel free to use a higher gear, too. Repeat the process for the half and full positions, but you don't need to hold those positions as long. One good long blast at wide-open throttle should be enough for that one.

.
 
I have the same modifications on my GS850 and am currently running #130 mains, with everything apart from the 4-1 exhaust and individual filters left stock. I've tweaked the idle and mixture screws to suit, but thats about it.

I'd suggest starting off with #122.5 or #125 mains and seeing how it goes from there. In my experience, the #125 mains I started off with for the first step were still way too lean. The original #115 mains only let the bike rev to 4,000rpm!

Cheers - boingk
 
The original #115 mains only let the bike rev to 4,000rpm!
Click to expand...

If your bike is stock and can only rev to 4k rpm's you've got a carburetor problem.
 
Sorry, thought I made that clear in my original post but it doesn't look that way when I read it again... second times the charm hey?

After fitment of a 4-1 exhaust (with a pretty free-flowing silencer) and then individual pod filters, my 850 would only rev to 4,000rpm because it was so lean on the stock #115 mainjets. Fitting #130 mains cured this, although I've yet to fine tune this.

Cheers - boingk
 
Steve said:
#2 son's bike (see sig) has a header of unknown origin and stock airbox. I am currently running 120 mains (stock is 115), stock 15 pilot fuel jets, stock needles shimmed about one full notch. Have not done full plug chops, but it runs very well.

Note that the stock needles are not adjustable. What is necessary to lift them for a richer mixture is to get some small washers (the smallest washers in the assortment pack at Radio Shack work well), determine how many of them are the same thickness as the nylon spacer at the top of the needle, then use half as many. Usually, it's four washers for the full thickness, so just use two. If your header is rather free-flowing (no baffle, or LOUD), you might use just one washer.

.
Click to expand...

MIne has 165 mains in it, lol.

DJ 165 mains that is..:eek:
 
Octain said:
You dont need that kit.
Click to expand...

I orderd that kit to rebuild my carbs because the bike has not been running for a while and at least one of the carbs leaks gas when I try to start it. Along with cleaning and rebuilding I want to rejet because I see that as being a potential problem.
 
As practically no one seems to post their location...
Has anyone thought to allow for altitude with these jetting recommendations? :rolleyes:

Just sayin'

Eric :)
 
Good observation. Not much altitude here so we don't generally think of it as an issue.

I'm at 2,000ft.

By the by, nothing wrong with a rebuild if you think you need it, and leaking gas is a sign of at least one (if not several) things wrong.

- boingk
 
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It has been my opinion (however mis-informed that might be) that jetting for an altitude is only practical if you will never go below that altitude. The mixture will richen up as you go up in altitude and, yes, that will reduce power a bit, but most of the power reduction is from the overall lack of oxygen, not so much the rich mixture. Conversely, as you go down in altitude, your mixture will lean out. If you had set it correctly for Denver at 5,000 feet, you will be close to dangerously lean as you go away from there on a long trip. Granted, it's likely only one or two jet sizes, but it can still make a difference. Better to keep your mixture a bit rich and be safe. On the other hand, if you live in Leadville, Colorado (just over 10,000 feet) and don't ever plan on taking your bike down to Denver (100 miles away) or Colorado Springs (6000 feet elevation and 105 miles away), go ahead and re-jet.

Racers will change jetting from hour to hour, sometimes, to keep peak performance based on altitude, temperature, humidity and barometric pressure. They can get away with all those changes because the amount of time they will be using that jetting combination is rather limited and very specific to those conditions. The rest of us should just settle for a compromise that works pretty well most of the time. :o

Or you can go with fuel injection. It compensates for altitude as well as temperature. :D

.
 
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