Suzuki Central Forum banner
1 - 4 of 4 Posts

·
Registered
Joined
·
42 Posts
Discussion Starter · #1 ·
If someone out there could explain to me how to put in a new main jet. i would be very happy if you could help me. I just put on my HMF slip-on, uni airfilter, and i am taking off the lid. HMF sent me a 170 main. Do you think that is too big? Any help would be appreciated.
 

·
Registered
Joined
·
1,258 Posts
in the March issue of Dirtwheels, they said they put a 170 main for their shootout, so it's probably right.

Little Fat
 
G

·
Re: Jetting for a uni and a HMF!!!!

QUOTE
If someone out there could explain to me how to put in a new main jet. i would be very happy if you could help me. I just put on my HMF slip-on, uni airfilter, and i am taking off the lid. HMF sent me a 170 main. Do you think that is too big? Any help would be appreciated.[/b]
Well, I can get you started. As for the jet size you will have to experiment to find out what's best for you bike. Many have only gone to a 165 main jet. Most say not to do a needle change and to leave the "air screw" alone. They say leave the pilot jet alone unlees you are having cold weather start problems. Anyway, read the following info, look at the pics. The actual change of the jet is easy, except the first time and the getting the float bowl screws out so you can get to the jets is a headache! Read on...


Rejetted my KFX400 today and geeeeeeeez, a major headache getting off the float bowl screws as others have mentioned here. I had to use a needle nose vice grip plier (NEEDLE NOSE VICE GRIP) to grab the screw heads and loosen the screws. (Some people say an "impact screwdriver will do the job of getting the screws loose) The stock phillips slots quit even though I used a new, properly sized phillips head driver with lots of pressure. The last screw was so tight I could not even budge it with the vice grips so I drilled the head of it off and then it came out easily. I replaced the stock screws with plain steel cap screw socket (allen head) screws "4mm x 10mm "purchased at Home Depot. Be sure not to go longer then 10mm or risk damage to carb. Use plenty of anti-seize and be sure to work the screws in and out so the anti-seize coats all well. Anyway, jet change was easy BUT nightmare this first time because of soft metal stock bowl screws. I might mention Kawasaki uses those screws as other metal screws will react with the aluminum and kind of weld themselves. MUST use anti-seize on replacement regular steel screws or risk problems.


Look below now for pic of the carb. The pic is labeled. The main jet is the the one that looks like it is sitting in a brass/gold colored bowl. You remove that main jet with a flat tip screwdriver and replace it.. easy to do. By the way, did not remove the carb to do the jet change. Loosened clamps and turned the carb. Had to unbolt temporarily the small black box that resides on the other side of the carb. Once the carb is turned toward you a bit you can easily get to the float bowl screws. Again, they are a pain to get off. After changing the main jet reverse the above procedure.. turn the carb back in position, retighten clamps. Note, if you can't get to the float bowl screws to do what you need to do to get them off, slip the carb out of the rubber carb boot and lower it an inch or 2 and turn it toward you til you can get to it. Gently, don't pull, force, etc.. Go easy turning it, pulling on it as it has cables going into the top of it, etc.. SO, that is pretty much it. Quite easy to do except for the float bowl screws as I and others have said. IF you are not familar with tools you might want to get someone who is to help you get the float bowl screws out. Be sure to go to Lowes or Home Depot, etc. and get new allen head screws as described above before you start the jet change procedure!





 
G

·
QUOTE
If someone out there could explain to me how to put in a new main jet. i would be very happy if you could help me. I just put on my HMF slip-on, uni airfilter, and i am taking off the lid. HMF sent me a 170 main. Do you think that is too big? Any help would be appreciated.[/b]
AND then you need to do Plug Chops:


Many of you will get by with a MAIN JET change and not really need to do anything more. You won't need to touch the pilot jet or fool with the air screw or change the needle. When doing your jetting it is always best to start rich and work down! Best to do the plug chops and such as mentioned in the following paragraphs.

The way I learned is that you do jetting in two stages.


First, you need to get the -idle and low speed- jetting correct and then work on the -high speed or main jetting-. One could be right on and the other off. Treat as a 2 stage test and fix. As I said, Get the -idle and low speed- jetting correct and then work on the -high speed or main jetting-.


Before you start, have if possible, 3 preferably new, unused (new not a must ) spark plugs gapped and ready to use. You will need at least three plugs for this test.


-Idle and low speed- jetting:


After installing a new plug, you want to ride around at part throttle gently accelerating but keeping below ½ throttle. You want to do low speed as much as possible. After about one mile, pull in the clutch and kill the engine with your kill switch. Coast to the side of the trail. . Remove the spark plug. Look at the insulator from the tip down as far as you can see. Look at the base ring also ... which is the bottom circular end of the plug. Use a flashlight and magnifying glass if necessary. If your having a hard time reading the spark plug, after the jet pass you can put the plug in a vice and hacksaw around the plug at the washer. Break the threads off with vise-grips, and the porcelain will be easy to read. You are also concerned with the grounding tip or the base of the plug.

**(Note: Some people do jet mainly by reading the tip and the base ring/ bottom of plug and disregard the porcelain.) **


This pic shows how a rich plug's base ring looks.


Colors on base ring you might/will see:


White or no color - LEAN (Bad News) - increase jet size
Dark Black - real rich; drop jet size
Medium brown - rich, drop jet size
Medium brown with some light spots - rich, drop jet size
Kinda turning from dark to light tan - closer, drop jet size
Mostly light tan - drop one size, you're pretty close.. see to how it feels
95% light tan - the way you want it!


Anyway, as far as the porcelain, you are looking for an off white, eggshell to very, very light tan on the porcelain. If there was not sufficient time to thoroughly color the whole plug, the insulator may still be white, just be sure to read the base ring and be sure there is a visible dark ring or at best 95% light tan ring at the base of the so you know you are not running lean.


With Idle and low speed- jetting, if you are reading to rich, you may need to lean out the pilot jet or close the air screw slightly. If you are reading very white or no color at all (BAD NEWS) you will need to open the idle mixture more or go up one pilot jet size. If the plug reading is good, you are set to do the main jetting.


Now the MAIN jetting....


--To do the MAIN JET you need a place where you can accelerate at wide-open throttle through at least third gear, preferably fourth gear SAFELY. The procedure is simple. After riding to the location and with a warm engine, change to a fresh plug. Start the engine and immediately accelerate at wide-open throttle as described above. Do not idle or use part throttle. After accelerating, pull in the clutch; hit the kill switch and coast to a stop. Remove the plug and examine as before.


Colors on base ring you might/will see - You saw this color list above:


White or no color - LEAN (Bad News) - increase jet size
Dark Black - very, very rich; drop jet size
Medium brown - rich, drop jet size
Medium brown with some light spots - rich, drop jet size
Kinda turning from dark to light tan - closer, drop jet size
Mostly light tan - drop one size, you're pretty close.. see to how it feels
95% light tan - the way you want it! With your MAIN jetting correct, your bike should rev out and feel strong all the way up through the rev range. Remember over rev and the rev limiter kicks in and you will wrongly think that there is a jetting problem when there is not. Do your plug checks so you know!


If the plug is reading very white of no indication at all, you are lean (BAD NEWS) and need to go up one size on your main jet. If you are reading a tan or darker, you will need to go down at least one size depending on how dark the plug reads. You will need to repeat this procedure until the plug readings are correct.


Fine tuning: When you are getting close on the main jet you can fine tune with the needle if yours has a clip raising the clip lowers the needle and leans out the mixture. Lowering the clip richens the mixture. If your carburetor is not equipped with a clip, thin washers can be used to do fine tuning if needed. A Dyno jet needle has places where you can move the clip to. After getting your plugs reading right, your jetting is on and you will be producing the most power.. Anyway, that is pretty much it.


Once again, many of you will get by with a MAIN JET change and not really need to do anything more. You won't need to touch the pilot jet or fool with the air screw or change the needle. Best to do the plug chops as mentioned. Remember you would be probably be jetted abit differently in the cold weather then Summer riding. Some guys jet when each season comes around. Also, Another thing I learned is it is better to run a little rich at the sacrifice of a tad bit of power then to chance running lean and possibly causing engine damage!
When doing your jetting it is always best to start rich and work down!
 
1 - 4 of 4 Posts
Top