JETS AWAY - Jet "kit" question V.2

[can_rizla+.]

OK so i had purchased this jet kit for our bikes off ebay. Only b/c the seller was from Canada...blaw blawblaw not important. I got the jets today (two mains plus two washers) the number on the TOP of the jet reads 133 sooooo....im asking the professionals or anyone who has screwed around with there jet kits if this is a GO or NO install. The seller said he has a gs500 and used them and it worked fine...etc...just want other peoples opinions.

CHEERS

[galahs]

for a stock 2001+ GS that would be fine.... BUT!

It will be too lean if you use a drop in (without restrictor), pods or a lunchbox K&N style airfilter.

plus if you have the airfilter your bike will be VERY VERY lean at smaller throttle openings.

# Pilot or idle jet system. The idle jet controls the idle and on up to quarter throttle, give or take a bit. On some carbs, like Mikuni there is an air jet too. In conjunction with the idle jet there is an idle jet air screw. This screw leans or richens the fuel mixture for a smooth idle and on up to one quarter throttle. From the idle jet, there are little passages cast into the carb that lead to holes just in front of the throttle valve or plate. There can be just one hole or there can be several, depending on the carb design. They effect the mixture as long as the vacuum, in the venturi, is over them. As the throttle opens further, the vacuum moves to the needle jet and jet needle.

# The Throttle Valve. The big slide that opens and closes your throttle has a bevel angle cut in one side of the big round (can be flat, too) slide, toward the air cleaner. This angle comes in several sizes and helps control the fuel mixture from idle to about 35% open throttle.

# Needle Jet. This jet doesn't really even look like a jet, but it is ! It controls the fuel mixture from 15% to 60% open throttle. It sets in the center of the carb, right over the main jet.

# Jet Needle. This is the needle that rides in the throttle slide and goes into the needle jet. This needle controls the fuel mixture from 20% to 80% open throttle. It can come in many different sized tapers. Sometimes, one needle can have several tapers on it. The top end of the needle has grooves cut in it, usually five, and you can move the little clip on the end up or down to lean (down) or richen (up) the mixture. Most late model bikes have needles with only one groove cut in them. This is so you can't richen the mixture, thereby keeping the EPA happy.

# Main Jet. This jet controls the fuel mixture from 60% to 100% open throttle.

Remember our newer carbs have a Pilot, Mid Main and Main Jet

They all kick in at different throttle openings. So your upgraded Main only operates after 60% throttle.

Before that only the Mid Main and Pilot jet are feeding the engine with fuel.

If your going to or have a performance airfilter you should have gone in the range of

20 - 22.5 for the pilot jet

65 - 67 for the mid main jet

and 140 - 150 for the Main jet


I personally have 20 - 65 -145 combo and it works well with my K&N lunch box air filter and Jardine exhaust.

[galahs]

Here's an interesting read from Mikuni (the makers of our carburettor)



1: Main Jet Size: How to Get it Right

Mikuni HSR-series carburetors are remarkably versatile instruments. The standard tuning seldom needs more than small adjustments to accommodate a wide range of engine set-ups. One of the more common required changes is the main jet size.

Aftermarket exhausts have a wide range of flow volumes and the best main jet size is closely associated with exhaust flow. Thus, it is often necessary to replace the standard main jet with a different size to accommodate the wide range of exhaust designs on the market. However, it is easy to get the main jet right for a particular exhaust system using one of the techniques described on this page.

The standard main jet fitted to the HSR42 is a number 160. This size is correct for stock mufflers. Typically, an HSR42 combined with aftermarket exhaust system needs a 165 main jet. The general rule is that HSR42s fitted to engines with loud exhausts usually run best with a 165 main jet.

The HSR45 has a number 175 and the HSR48 a 190. These jets are more suited to modified engines with free flowing exhaust systems.

Keep in mind that the main jet does not affect mixtures until approximately 3/4 throttle. Below that throttle setting, specifically between 1/4 and 3/4 throttle, air/fuel mixtures are controlled by the jet needle and needle jet.

It is relatively easy to get the main jet correct. Follow either of the techniques described below. Both are satisfactory but the Roll-On procedure is more accurate.

NOTE:

The following tuning techniques might result in excessive (illegal) speed and increased risk from the speed and the necessary distraction of doing the test. We recommend that the testing be done on a closed course (track) or on a dynamometer, if one is available.

ROLL-OFF:

The Roll-Off technique is the quickest and is almost as accurate as the Roll-On method. First, one gets the engine warm on the way to a safe roadway. If there is room, use fourth gear as this allows more time to assess the result.

Now, get the engine rpm high enough that it is on the cam and in its power band. This may need to be as high as 4000 rpm with some cam choices. Apply full throttle. Let the engine accelerate for a couple of seconds until it has settled in and is pulling hard. Quickly roll the throttle off to about the 7/8ths position. When you do this, the mixture richens slightly for a second or so.
If the engine gains power as you roll the throttle off, then the main jet is too small and you need to fit a larger one.
If the engine staggers slightly or has a hard hesitation, then the main jet is too large and you need to fit a smaller one.