Pilot Air Jets

What exactly does the pilot air jet do. This is the jet under the chrome top. I think it gets it air feed from the kidney shaped orifice on the carb air inlet. I've worked alot on the older duel BS38 but I'm new to the quad BS28's. Mine is a 170 from a seca. I have the 165,s. The cam was changed to a seca cam so the pilot air jets were changed to. I got with xj4ever and I'm in the process of getting this thing to idle and run correctly. Got air leaks from the seals. thanks Dirty

 

it feeds the pilot jet/mixture screw set up which runs the bike up to about 2000 rpm then the the main fuel jets kick in.

it adds air to the gas the way you stated

here is a few links for you to read
this link tells in detail how to tune the carbs
http://www.zrxoa.org/webpages/techinfo/carb/carbtuning.html


this one covers rejetting basics and lists stock jets
The Information Overload Hour


this one tells how they work
THE SECRET LIFE OF CARBURETORS

IN THE CHURCH OF CLEAN

http://www.xj4ever.com/inside your carbs.pdf

TALKIN' TECH: Various thoughts on various issues

for parts and more info

http://www.xjbikes.com/forums/forums/xj4ever-supporting-vendor.23/

 

So if i decrease the size on the pilot air jet i would more get fuel to the air/fuel mixture

 

or increase the size of the pilot jet.
this should be adjustable by the mixture screw and you should only start changing air jet or pilot jet if you can not get the mixture screw to work for you.
I think this is a question better answered by our carb experts as to which way you want to go and why.

untill you fix your air leaks rejetting is a waste of time
stock exhaust and air box???

you say you have gone to seca cams have you checked to see if your carbs are jetted to a seca as opposed to a maxim motor?
on 550 seca the only thing diffrent motor wise is cams and carb parts, and rear tire size. (frame and forks too)
so seca cams seca jets

28mm carbs you talking 550?
XJ550 Models:
Years & Models: 1981-83 XJ550 Maxim North American models
Carb Manufacturer: Mikuni
Carb Series: BS28
Carb Model ID: 5K500
Main FUEL Jet Size: #112.5
Pilot FUEL Jet Size: #35
Main AIR Jet Size: #70 (non-removable, in carb throat)
Pilot AIR Jet Size: #165
AIR COMPENSATOR Jet Size: not used.
Starter FUEL Jet Size: #35 (non-replaceable, in float bowl)
Main jet NEEDLE ID: #4GN
Main needle JET Size: #O-8 (the main needle JET is also known as the "power valve" or "emulsion tube").
Fuel Level in float bowls: 2mm +/- 1mm (.08" +/- .039")
Idle RPM's: 1,200 rpms

Although not part of your carbs, your valve clearances should be considered as part of your "intake system", so here are those specs, too. Note that valve clearances should be measured with the engine "cold", meaning 70-degrees Fahrenheit or less:

Valve Clearance Intake: 0.11 - 0.15mm ( = 0.004 - 0.006")
Valve Clearance Exhaust: 0.16 - 0.20mm (= 0.006 - 0.008")

Years & Models: 1981-83 XJ550 Seca North American models
Carb Manufacturer: Mikuni
Carb Series: BS28
Carb Model ID: 4U800
Main FUEL Jet Size: #112.5
Pilot FUEL Jet Size: #35
Main AIR Jet Size: #70 (non-removable, in carb throat)
Pilot AIR Jet Size: #170
AIR COMPENSATOR Jet Size: not used.
Starter FUEL Jet Size: #35 (non-replaceable, in float bowl)
Main jet NEEDLE ID: #4GZ11
Main needle JET Size: #O-8 (the main needle JET is also known as the "power valve" or "emulsion tube").
Fuel Level in float bowls: 2mm +/- 1mm (.08" +/- .039")
Idle RPM's: 1,200 rpms

from
http://xjbikes.wikidot.com/carb-specs

also if its an 1984 there are different jet specs

 

This is a 1981 Maxim with seca cams and seca carbs. Its has a free flowing intake with stock exhaust. Its already been jetted with 37.5 Pilots, 120 mains and 170 pilot air. Just wanted to know if decreasing the pilot air jet size to 165 would increase or decrease the fuel in the mixture. Thanks Dirty

 

by free flowing you mean no air box or pods?

short answer is yes but you would be reducing the air not increasing the fuel

if when you turn the mixture screw in and can not get a lean condition you would increase pilot air jet. or
if when you turn the mixture screw out can not get a rich condition you would increase the size of the pilot jet.

i assume you are happy with the main jets and fuel level in the carbs and that is why you are focusing on pilot circuit

this link explains how to tune the carbs
http://www.zrxoa.org/webpages/techinfo/carb/carbtuning.html

 

from the link TALKIN' TECH: Various thoughts on various issues

in the plug chop portion

" The pilot fuel circuit is active from idle up to about 2500-3500 rpms, and at about 3000 rpms the main fuel circuit starts becoming involved, and by 4000+ rpms the main fuel circuit is responsible for about 80%+ of the engine's fuel supply conditions. So where you do the "chops" tell you not only what the fuel mixture and combustion conditions are like, but also, which circuits are responsible for those conditions."

 

The question is will the amount of fuel increase if i reduce the size of the pilot air jet. simple question. Dirty

 

i answered your question.
you want more fuel potential go with a bigger pilot jet, bigger hole.
why would you want to go against yamaha specs? they designed the best setup for the engine . so unless you have modified the motor beyond the cams you are not going to get a better result.

what problems are you having with the engine ?

 

I don't have any problem with the engine. I'm Still studying these quad BS28's. how to sync and jet size' s for different mods. Got it in a state of mismatched parts. Got to replace the throttle seals before i do anything else. I've been working on bikes of different makes for 43 years, these quad carbs seem to be the worst to figure out. I know stock is the best way to leave it but that aint a option here. have to work with what i got. What i have on start up is i have to hunt for idle. Once its warmed up it wont start without the choke. Then its has a high idle at +2000rpm's. Adjust idle and then i have no throttle response. hen it dies. I have to get into the mains in order for it to keep running. Got to get seals and stock air box back in and test. It has a homemade box for all four carbs with a single layer of uni-fliter in place. Dirty

 

why is stock not an option the seca jet size would be stock.
untill the throttle seals are replaced and you do a propane test for leaks you will not be able to sync or tune the carbs.
you also need to check valve clearance and get them in spec.
the sync is an easy task in diy forum there is a couple of tools that can be made for 10$
you should return the pilots jets to stock #35
what do your spark plugs look like?
have you confirmed that all internal ports are clear and free flowing

 

Valve clearance (pucks) is correct, carbs have been to church. You could eat off of them. The YICS port is cleaned. Got to get seals before i sync. This engine is a maxim with seca cams and seca carbs. All i got for pilots is 37.5 and 40's for now. 120 in the main, 170 for pilot air. Got to move regulator to make room for the stock air box,it has a K&N filter. Seems that pods and open air carbs are a devils ring when it comes to the YICS system. making progress. Dirty

 

Well i found an interesting thing, the gasket between the head and the carb boot on #4 was half way on(air leak). I figure you have to drop front engine mounts to place stock air box back into place. Checking plugs now to see their condition. Dirty

 

http://imgur.com/a/2J6ol. Number 4 is on the left. 1 and2 look like three

 

Making headway. Dirty

 

the boot will make a lot of differance.
paperbag brown is a good color

the k&n filter can also be a problem the closer to stock you get the easier it will be.
seeing your running a "Seca" motor you want to be cautious of the front end lifting off in a high rpm shift to second.
Seca has taller rear tire. a little quicker in the 1/4 mile and a few mph in the top end, than a maxim with the same sprockets oem

 

650 jets are the same for the Seca and Maxim.
Y-10 Needle
110 main jet
40 pilot
50 main air
195 pilot air

The best bet to compensate for the 37.5 pilots is to either drill them out to 40, or get the correct size jets in there. Trying to compensate with the air jets won't work very well. The smaller pilot will be the limiting factor; can't flow more fuel than the hole allows.

And as XJ said, getting the vacuum leaks fixed will make a huge difference. Even 100% perfect and dialed in carbs won't make a bike run well if there are vacuum leaks.

 

The carbs are 37.5 pilot, 120 mains, 170 pilot air with single layer uni filter box. Seca cams. don't know what the needles are. when i get the seals in ill check. I had them out and they are not shimmed but i forgot to look at the number on them. I know they have a rubber part on the needle that I'm not use to. Got good compression. plugin away. Dirty

 

28 mm carbs I think he got a 550 or do 650 have 28mm carbs

 

Well crap. No wonder I'm thinking the jets he has installed are way off...lol

 

need to get him to put his bike in the signiture line

 

I aint to worried about the front wheel coming off the ground, seems these XJ550 dont have but about 55hp, Have to get into the mains to get any power out of it. think the max speed is 115. that's blazing. got to get all the air leaks fixed first before i can get it to run right. I'll post results after i get the seals in. I got the butterfly screws out without messing up the threads on the shaft. These old parts are expensive, didnt know what i was in for. As for the signature line, i doubt it. Dirty

 

Gary H.

 

Thanks but i use both tires, aint much for unicycles. Got a question about main jet needle.The ones that are in it are marked 4GNT5. Does anybody know what this signifies. Thanks Dirty

 

I agree...just showing it's capable.

Gary H.

 

The markings describe the taper(s) of the needle and the diameter of the needle at various points along the taper(s). You have stock needles for your machine.

The 4 denotes the needle series (range of length for the needles), in your case between 40.00 and 49.99 mm in length.

GNT denotes that you have a three taper needle, each taper increasing in angle as you go through the alphabet frontways.

The 5 is for production purposes to note revisions.

Mikuni is not known for making it easy to get all of their needle and jet charts in an easy to peruse form (particularly when it's situation where the carb was made for a specific brand of motorcycle instead of being sold directly by Mikuni), so I can't tell you much more than the basics (as I don't know any more than that).

 

So the maxim and the seca XJ550 have the same 4GNT5 main jet needle. I found one of the needles were out in alignment, the tab on the bottom of the plastic spacer wasn't seated on #1 carb. Dirty

 

maxim
Main jet NEEDLE ID: #4GN
Main needle JET Size: #O-8 (the main needle JET is also known as the "power valve" or "emulsion tube").

seca
Main jet NEEDLE ID: #4GZ11
Main needle JET Size: #O-8 (the main needle JET is also known as the "power valve" or "emulsion tube").

Original main needle size configurations are as follows:

4GN: used on all 1981-83 XJ550 Maxim carbs.

4GZ1 or 4GZ11: used on all North American XJ550 Seca carbs.

 

Is there anything you don't know?
As your agent I recommend that we get you on Jeaprody ASAP, take Trebek for as much as we can and then split the winnings 70/30.
Here's hoping the final question is about petroleum based products . . . .

 

I had to go back through my pile of mostly useless notes and look that up. No Jeopardy for me.
XJ550H (via Chacal) has the missing (and most important) bit of info; What the stock needles are.

 

When I was doing my Masters' Degree, one of the instructors drilled into us "it's important to know the answer, but it's MORE important to know where to find the information if you don't know it"

 

That's why I hangout here!

 

the question now is do i need to find 4GZ1 or 4GZ11 needles since the cams are from a seca, and the main. idle and pilot air are also. Dirty

 

Go to the following link:

http://xjbikes.com/forums/threads/in-the-church-of-clean.14692/

scroll down to the section called "Toward the Promised Land - Inspection". ( about 1/4 of the way down as it appears on my screen)

Then scroll down to the section on mikuni carb needles

Then scroll to the section on mikuni jets.

 

Just got to love political answers. it was a yes or no question. sorry i ask.

 

There isn't always a yes-or-no answer though.

In my opinion, seca cams call for seca needles and jets, HOWEVER that does not mean that is the only useable combination; it's just the one that is known to work without any fussing.

 

Oh grasshopper, if you seek the answer to your question instead of having the answer fed to you like a duck pre-foie gras you shall be twice enlightened.

 

Sorry I gave you a link to the info you need..... Plus a whole lot more.

So if you want a yes or no answer---

Yes or no... Depends

Here's another---- do your own homework

http://xjbikes.com/forums/threads/xj550-cam-swap-aftermarket-options.40742/

 

by the discription above you will want the 11 not the 1. the 11 will be the latest version. like windows 3.1 vs windows 3.11
both will work. you probaly can only get 11 being the latest production part.

 

My understanding is that Mikuni revisions aren't necessarily denotations of improvements, but that they are really meant to decribe changes that are made for a specific application (starting a taper a few mm lower on the needle for example).

 

Hey k-moe..... That was not a yes or no answer-

 

Thanks K-Moe. I understand the taper, and reason for it. Most just shim them. I don't know if this is a good practice. I have seca cams and seca idle, main and pilot air jets. Just wanted to know if i need to use seca needles.Im going to order the seals., butterfly screws and fuel O rings from XJ soon. trying to get everything in one order.Thanks for the comic relief Hog. funny. thanks Dirty

 

You have a wife. You know the routine

 

Lol

 

I get it, A bith i aint. Thanks. Later dirty