Author Topic: LT 500 Jetting (Read 8879 times)

Marcman804 · « **on:** October 06, 2015, 11:10:29 am »

I recently bought a Quadzilla and had it rebuilt. I went with the 517 Wiseco kit in the rebuild. Well it had no airbox with the Uni filter directly on the carb and it would break up on the top end so I thought that was the cause and put the stock airbox setup on it. It still breaks up so Im guessing its the jetting at this point. It has the stock Mikuni carb from what I was told by previous owner, FMF exhaust, and I put new Vforce 3 reeds on it. I took the carb apart and the large jet says 380 and the smaller one says 52. (Sorry Im new to carbs, other bikes are fuel injected). Well what do you guys think I should run in the carb with the mods I have? Thanks for the help. The bike is a monster down low and I can only imagine how fast it will be with the correct tune. Posted a pic of the jets and carb.

Glamisrider · « **Reply #1 on:** October 06, 2015, 03:01:27 pm »

Main is too lean.

Try a 420 main and a 35 pilot.

Could also depend on your elevation, what are you at?

And do you know how to tune the pilot circuit?

Mr.Sampson · « **Reply #2 on:** October 06, 2015, 03:02:18 pm »

Mine had a 440 main and a 35 pilot so you could be a bit lean? In the manual it has jetting guidelines ... Good luck

Marcman804 · « **Reply #3 on:** October 07, 2015, 04:12:17 am »

I'm in Virginia and my elevation is 455 ft (139 m). No I don't know how to tune the pilot circuit, would I need to be an expert to do this or can I read it in the manual or online? I see the jets online at www.mfgsupply.com am I safe to order the 420 main and 35 pilot? Can't wait to get this beast running good.

Q2W · « **Reply #4 on:** October 07, 2015, 06:53:00 am »

You may want to think about getting a new slide and do a leak down test. 52 pilot is MASSIVE. Stock pilot is 22.5. Without knowing much else -if you're running an airbox lid, porting, have an airleak, etc - i'd say 380 is fairly close. My guess would be 400 - 380 if your bike has an '88-'90 jug on it. With a pilot of 27.5-30. But than again, your slide looks crazy worn and that will affect the pilot size.

Only buy genuine Mikuni jets. Also, buy a few sizes and start big and work your way down.

You don't have to be an expert, but you will learn. Trust me, very few shops know how to jet well or even work on a 2stroke. Let alone a zilla.

First things first. If it were me, i'd do a leak down test first. If you have air leaks in the motor, you will never be able to tune it.

If you dont know what a leak down test is, here is an example of one.

http://www.suzukiquadracerhq.com/lt250/lt250r-leak-down-test/msg47754/#msg47754

You can literally build one at lowes for less than 20 bucks although you may have a hard time finding a gauge that one reads about 10 - 20 psi. You only need to pump in about 5-7 psi.

Marcman804 · « **Reply #5 on:** October 07, 2015, 07:36:45 am »

Thanks for the info. This is an 88' jug. I am familiar with a leak test, I do this on my turbo car occasionally but we call it a boost leak test. Could it be too rich with the jets that are in it? I just checked the plug and it looks blackened like it may be rich but then again the mechanic who put my engine together told me to run the gas with a little extra oil while its being broken in. Would it break up on the top side if its too rich? I know my Raptor does this when its lean but not sure what happens when its too rich. Sorry for all the questions, again thanks for all the help from you guys. I'll do the leak test as well when I get a chance.

MotorGeek - Jerry Hall · « **Reply #6 on:** October 07, 2015, 08:40:32 am »

Your Raptor will miss fire if it is extremely rich or lean. A two stroke will miss fire when it is extremely rich but will usually burn the piston before it is lean enough to have a lean miss fire.

A rich oil mixture only causes a miss fire when the mode of operation (running the engine at moderate loads) is not generating enough heat to keep the spark plug burned clean.

Marcman804 · « **Reply #7 on:** October 07, 2015, 08:50:34 am »

Thanks man. So what jets should I start with after I do the leak test? Im going to go ahead and order them now so after I do the leak test I'll have a fresh starting point.

Marcman804 · « **Reply #8 on:** October 20, 2015, 10:41:57 am »

Ok I installed the new slide, did the leak test and there were no leaks. I have V3 reeds, FMF exhaust, Uni open filter on the carb. I tried the 400 main and 40 pilot but it was still rich when I did the plug chop. So then I went to the 380 main and the 35 pilot and its still rich. What other changes should I make, what are other ways to fine tune the carb? I read on here about setting the needle but I'm not familiar with this, could someone elaborate? It idles perfect and the bottom end is strong but breaks up on the top side when wot. Thanks for all the help so far guys.

MotorGeek - Jerry Hall · « **Reply #9 on:** October 20, 2015, 02:41:03 pm »

What size is the uni that you have clamped onto the carb?

Marcman804 · « **Reply #10 on:** October 21, 2015, 06:42:35 am »

Its a 3" filter.

MotorGeek - Jerry Hall · « **Reply #11 on:** October 21, 2015, 11:54:56 am »

Quote from: Marcman804 on October 21, 2015, 06:42:35 am

Its a 3" filter.

That is a small air filter and may be the reason you are needing to run small main jets. The jetting specs that most guys are giving your are based upon having the stock rubber boot between the carb and a much larger air filter. A LT 500 with anyones big or small filter clamped on the back of the carb, will not usually produce as much power as one with the properly designed intake system.

I would suggest that you get an air box or use the stock carb to air filter boot then maybe some of these jetting recommendations will work

Mr.Sampson · « **Reply #12 on:** October 25, 2015, 06:00:14 am »

I also need jetting help ...
87' big reed
87.50 bore
Ported professionally
Clamp on K&N with tube
41.5 mm pj carb
Good ole FMF pipe
Just need a starting point switched from a mikuni tm38ss stock carb

MotorGeek - Jerry Hall · « **Reply #13 on:** October 25, 2015, 01:54:41 pm »

Quote from: Mr.Sampson on October 25, 2015, 06:00:14 am

I also need jetting help ...
87' big reed
87.50 bore
Ported professionally
Clamp on K&N with tube
41.5 mm pj carb
Good ole FMF pipe
Just need a starting point switched from a mikuni tm38ss stock carb

Do you have the 1987 TM 38 carb body or the 1988 to 1990 TM 38 carb body?

How long is the tube between the carb and K&N?

Marcman804 · « **Reply #14 on:** October 30, 2015, 03:28:44 am »

I have the stock airbox setup but it seemed to run better with the open filter. I got a bigger 6" Uni filter and it did help a little but its still rich. I guess I'll keep playing around with the jetting. Since I have a decent idle, should I change the main or the pilot jet? And I also, I have been reading other's posts where they are mentioning the air screw, I thought this was for how fast the bike idles down after letting off the throttle but it seems people are stating as it assists in jetting.

MotorGeek - Jerry Hall · « **Reply #15 on:** October 30, 2015, 07:01:31 am »

The air screw is a fine tuning circuit for the pilot jet. The air screw changes the amount of suction placed on the idle circuit and the transition hole that is feed by the pilot jet.

The slide stop screw controls the idle speed.

Marcman804 · « **Reply #16 on:** October 30, 2015, 07:19:58 am »

So once I get my jetting much closer I should use that to fine tune it. I have an idea of where the screw is but want to be sure exactly where its located. And as far as my jetting, the plug is still really black, should I go down on main to 370 or 360 or go smaller on the pilot?

Q2W · « **Reply #17 on:** October 30, 2015, 07:38:39 am »

The important thing is that you have a tube coming off the carb to an air filter. Lots of ppl eliminate the air box, as did i, open filter is def the best so long as you arent mud riding and clean it regularly. Most still use the stock tube we just make a bracket to support the air filter at the airbox location. SB Designs still makes a bracket i think. You can find in his store on this site.

It's great you got the bike idling. But there are a lot stuff you need to do.

First thing i'd do is set the slide height.
Using the thumb screw (large screw with the spring on it) turn it counter clockwise until the slide is at the very bottom of the carb - as far as it will go-
Make sure your throttle cable is adjusted out so that the slide can go all the way down.
Set your slide height to 3mm-ish by turning the thumb screw clockwise. You will see the slide move. (take off the air filter, etc, so you can see the slide in the carb)
Once you have that set NEVER touch the thumb screw again.

All future idle adjustments will be done with Air/fuel mixture screw.

Second, Adjust the throttle cable.
Set the throttle cable so that it's not in a bind or has any slack. The adjustments are at the handle bars. Careful not to get it too tight so that it raises the slide. Make sure the slide is able to rest at the previous adjusted location.

Third, take note of how many turns your air/fuel screw is set too.
The air/fuel screw is a small flat headed screw on the opposite side as the thumb screw.
Turn it clockwise until it's tight.
Turn it back counter clockwise 1 1/2 turns. This is a common starting point.

Fourth, Start your bike.
Let your bike warm up blipping the throttle to keep it running and for lubrication.
from here, you need to do some fine adjustments to the air screw to set the idle.
The thresh-hold i like to use is 3/4 turns out - 2 1/2 turns out.
Turning the air screw out (counter clockwise) allows more air into the pilot system and leans it.
Turning the air screw in (clockwise) allows less air into the pilot system and richens it.
If your bike is idling high, it needs less air. if it's idling low or wont stay running, it needs more air.
Slowly turn the air screw which ever way you need to get your idle right. TAKE NOTE OF THE AMOUNT OF TURNS!
I like to adjust it 1/4 turn at a time until i get close and finely adjust it from there.
If you cant get your motor to react to the air screw adjustments or you have to go below 3/4 turns out or over 2 1/2 turns out, your pilot jet needs to go up or down in size.

It seems intimidating, and it is at first, but you should catch on really quick once you dive into it.

We'll worry about setting your needle height and main later.

Good luck!

Marcman804 · « **Reply #18 on:** October 30, 2015, 07:46:53 am »

Thanks for that info man, the bike idles perfect!!! I have the stock airbox in place for if Im going to be in mud and I also have the air filter directly on the carb. Im looking now to see if there is a bigger filter I could try to see if that will lean it out some until the new jets come. Just wondering if I should go down any on the main or the pilot. The bottom end is amazing but it breaks up horribly still on the top side. My brother has an extra Keihin carb from his Banshee, would I be better off using that and if so what would be the best starting point on the jets. I wanna get this thing right asap, my buddy with his honda 450 is talking trash lol.

Q2W · « **Reply #19 on:** October 30, 2015, 08:04:34 am »

dont touch the pilot if it idles. It's likely the needle position or main.

Marcman804 · « **Reply #20 on:** October 30, 2015, 10:10:07 am »

Im going to go down on the mains, just ordered 4 different sizes but how do I just the needle position if its not the jetting? And again man, I appreciate all of the help. Once I get it running right, I'll let you know how bad I smoked him.

Q2W · « **Reply #21 on:** October 30, 2015, 01:11:40 pm »

take the carb off.

Unscrew the top screws on the carb and pull the slide out.

pull up the spring back so you can see down inside the slide. there are 2 screws down there. Take them out and you should be able to get the needle out.

There is a c-clip on the needle. If you raise the c-clip to the next position it drops the needle and will lean out the bike at mid-ish throttle. Moving the c-clip down the needle with raise it making it richer.

If it feels super rich at mid throttle, raise the clip 1 position at a time until it feels nice and crisp.

Good luck spanking that shee!

Marcman804 · « **Reply #22 on:** November 01, 2015, 02:10:19 pm »

Thanks man. If it wasn't raining I'd give that a try right now. I'll do it tomorrow hopefully when the weather let's up. But I also tried to see if the powervalve was working by leaving the bike in neutral and giving it some good revs, I didn't see any movement inside... could this add to the problem of it being rich?

Q2W · « **Reply #23 on:** November 02, 2015, 07:58:56 am »

Quote from: Marcman804 on November 01, 2015, 02:10:19 pm

Thanks man. If it wasn't raining I'd give that a try right now. I'll do it tomorrow hopefully when the weather let's up. But I also tried to see if the powervalve was working by leaving the bike in neutral and giving it some good revs, I didn't see any movement inside... could this add to the problem of it being rich?

I don't think so but I honestly don't know.

If the PV is stuck open (it's supposed to closed at higher RPM) it will hurt your performance on the top end.

Marcman804 · « **Reply #24 on:** November 05, 2015, 05:35:49 am »

I finally got around to taking the cap off of the power valve adjustment and the spring was a mangled piece of s##t (I attached a pic). Is there anywhere I can get just the spring or is there anything I can do as an alternative? And also, if the spring is not working would that cause the power valve to remain open or would it remain closed.

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Author Topic: LT 500 Jetting (Read 8879 times)

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