Pilot Air Jet...?

[Jontheguru]

I've been trying to do some final jetting on my 83 GS1100. I've have a question about the pilot air jet. Keith Krouse put up some great photos of the GS carb set-up that clears up alot. But I've heard people and dynojet imply that putting in a smaller pilot air jet will lean out the bottom end.

It appears in the drawing that the pilot jet has a metered amount of fuel that mixes with air in the upper part of the pilot jet itself. The air is drawn from the pilot air jet. The fuel/air mixture than leaves the pilot jet and is divided up through secondary ports and the pilot air screw port. So it would appear to me that if a larger pilot air jet was installed it would lean out the mixture since you would be letting in more air and the fuel from the pilot jet is fixed.

When I purchased the Stage 3 jet kit from Dynojet, they said that putting in the 160 instead of the stock 180 that it would lean out the bottom end....???

I made some exhaust changes so I have to run through the jetting once again, but I figured it would help alot if I knew what everything is doing.

Currently I have single K&N filters and an older MAC header. The stock header did not run well so I modifed a really nice straight through 2" baffle. Wow, it is totally different. I had no idea my bike had so much potential. I can roll on to full throttle in first gear and the wheel pulls right off the ground before I reach 7000 rpm, if I don't let up I could flip it right over!!! Do most GS 1100's do this under simillar conditions? What have I been misssing!!! Anyway, my top end is great, mid range is a wollup, but the bottom is choppy and I'm at 5 turns out on my air screw (it runs best there...???). I know it can run better.

Any help or advice is more than welcome!!! Thanks.

[salty_monk]

If they are CV carbs which I think they must be then those are fuel screws not air screws as far as I know..... my amateur guess is if you are at 5 out then you are at the limits of your pilot jet & should go up a size, leave the air jets alone.

I'd wait for some more opinions though...

[chef1366]

DJ jets are not the same as Mikuni jets.
Try going up to 47.5 on the pilot jets. I think your stock pilot air jet is 170. If you have a 180 you should be good to go with the larger pilot jet.

[saber1971]

I am sooo confused...... i also thought going from a 180 pilot jet (air) to the 160 would be richening.... since the 160 is smaller it would let less air in..right????? i know the canadian spec carbs run 47.5 and 160 jets and theyre richer than their us counterparts which run a 180 and 45.0

[Jontheguru]

Well it seems I can only try different configs because I can't even get the right answers from Dyno Jet. Going from the stock 180 pilot air jet down to the 160 (I can visually see that the dyno jet DH160 is smaller) actually leans out the bottom end...I know, I'm not sure why either. I put the 180 back in for laughs and it was very boggy and lot's of loud popping out the exhaust on decelleration. It seems very rich. I believe there is a secondary pilot configuration where fuel is picked up in the long stem that protrudes down into the bottom of the float bowl as well as the pilot jet. I'm uncertain though. If I knew how these variables affected eachother, going in the right direction would be easy. But for now I have to figure it out by trial. Thanks for the response guys.

[mac1170]

gday mate, that loud popping on overrun when the throttle is shut off , means it is running lean,not rich, try screwing out the mixture screws 1 full turn, ie, anticlockwise, go for a ride and see what happens, do 1 adjustment at a time, then check how it acts, also check your exhaust is not leaking air, from around the headers,as this causes popping,best of luck,regards.

[Jontheguru]

Well I've heard it both ways. The loud popping on decelleration (I've heard) is unburnt fuel combusting in the exhaust. I have tried all configurations with the air screw. Starting at about 1.5 turns out and working my way out to 5 at either a half or full turn at a time. It popps more at the fewer turns out. The more I open the air/fuel screw the popping lessens and seems much less boggy. I will pull the plugs next time to verify what it is actually doing. Thanks for the input.

[mac1170]

gday mate,that screw is a needle that if you take the carbys off, and look inside the throat, you will see a little hole in the top of the throat, just in front of the butterfly valve, if your winding it out and the popping is decreasing, then it is richening up,this needle jet is only being used at nil to very low throttle openings, your pilot jet comes in after this but it finishes around 3 grand, if your pilot is lean, you should find if you ride along at about 2-3grand, you willfeel the bike hunting, ie revs going up and down slightly on constant throttle, as the pilot drops away you come onto the needle, regards,

[Jontheguru]

Alright, I'm gonna have to rescind my guru status. Obviously, I don't know as much as I thought I did; that along with some poor help from dynojet. You guys set me on the right track, I should've figured. Well I finally got back to jetting a couple of days ago. This is what I came up with. My main jet is a 138, my needle is on the 4th clip from the top, I was running lean on the bottom as indicated by the popping on deceleration and the fuel screw at 5 turns out. The fuel screw idea instead of an air screw threw me a bit. I put in a 47.5 pilot jet as recommended here. I'm still out a bit at 3.75 turns out, but it runs like a champ. It's very smooth on the bottom, and the mid and top end is awsome. In first gear, I can roll on to full throttle and the wheel comes right up. Thanks for the help guys and my appologies for thinking I knew better. Rock on.

[mac1170]

gday mate, good on you, one other little thing, if your needle is on the 4th pos. from bottom of slide, its still lean, now we all know mean is lean, but, drop it to the middle notch, and see what happens, as you tend to ride around the street in this area of carby, you might find its a lot more tractable, regards.

[FJman]

Let's play some.

First off, I'm going to state a disclaimer. If you work on your carbs and bork it all up, it isn't my fault. According to a person on the FJ mailing list (micaspeak) I'm a retard who has no clue. (Nevermind that person's rep, heh.) That being said:

Dynojet jets are numbered by orfice size. Mikuni jets are numbered by flow rate (by what standard, no one really knows).

The conversion is 1:.9375. This seems to hold true for most. My experience is that a DJ114 is the same size as a mikuni 122.5, but some will argue until blue in the face that DJ numbers are bigger on the same sized jet. Again, use your common sense and eyes when selecting jets.

The next big bit of info is that this is your bible:

http://www.factorypro.com/tech/carbtun.html

Read it and obey!

You sound like the mains are pretty sorted. Good.

The needles being 4 clicks down isn't a bad thing. Whatever the motor wants is right, period. I would heartily suggest a plug chop to check them, but if it's running smoothly and pulling nicely, who cares? That's the goal of good jetting, making things work well.

Your mix screws at 5 out seems a bit excessive. Get yerself a new set of pilots and try a little bigger ones. Going up one size and putting the mix screws in to about 3.25 turns should get you back to where you are now, should be close.

But hell, if it's running well, RUN IT... if it makes noise on decelleration, who cares? The motor isn't for slowing down, it's for speeding up

[chef1366]

Let's play some.

First off, I'm going to state a disclaimer. If you work on your carbs and bork it all up, it isn't my fault. According to a person on the FJ mailing list (micaspeak) I'm a retard who has no clue. (Nevermind that person's rep, heh.) That being said:

Dynojet jets are numbered by orfice size. Mikuni jets are numbered by flow rate (by what standard, no one really knows).

The conversion is 1:.9375. This seems to hold true for most. My experience is that a DJ114 is the same size as a mikuni 122.5, but some will argue until blue in the face that DJ numbers are bigger on the same sized jet. Again, use your common sense and eyes when selecting jets.

The next big bit of info is that this is your bible:

http://www.factorypro.com/tech/carbtun.html

Read it and obey!

You sound like the mains are pretty sorted. Good.

The needles being 4 clicks down isn't a bad thing. Whatever the motor wants is right, period. I would heartily suggest a plug chop to check them, but if it's running smoothly and pulling nicely, who cares? That's the goal of good jetting, making things work well.

Your mix screws at 5 out seems a bit excessive. Get yerself a new set of pilots and try a little bigger ones. Going up one size and putting the mix screws in to about 3.25 turns should get you back to where you are now, should be close.

But hell, if it's running well, RUN IT... if it makes noise on decelleration, who cares? The motor isn't for slowing down, it's for speeding up

03-09-2008, 08:00


Interesting fact about Dynojet versus Mikuni jets.
.9375 is the magic number.

Dynojet to Mikuni/ Dj 138x.9375=129.375 so a DJ 138 is close to a 130 mikuni

Mikuni to Dynojet/ Mikuni 130 divided by .9375 = 138.666 A mikuni 130 is close to a 138 dyno. This works throughout all ranges.
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04-29-2008, 01:59 PM
I was referring to the o-rings inside the carbs.
The DJ 110 will be too small. Try a 117.5 Mikuni main. The DJ 110 is like a 102.5 Mikuni which is smaller than stock.

Mikuni to Dynojet
Mikuni divided by .9375 = DJ size

Dynojet to Mikuni
Dynojet x .9375 = Mikuni size
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You're preaching to the choir.