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  #1  
Old 07-12-2004, 10:05 AM
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Default How to de-restrict Suzuki LT50 quad

Hey everyone,

A friend of mine is trying to de-restrict a Suzuki LT50 quad for another friend of his. He's found, and removed, the restrictor plate in the intake tract and has gone over the exhaust system looking for restrictions (none were found). We figured it might use an exhaust restrictor like the Yamaha PW50s do.

Prior to the de-restricting surgery, the quad ran fine, if you can call the wheezing almost total lack of power fine, but at least it ran. It had also developed a fuel leak from the carb (probably a stuck float) so that was the original reason for the disassembly. De-restricting it along the way was just an added bonus. The fuel leak didn't keep it from running, it just caused it to go through fuel at a pretty rapid rate (several tankfuls in an afternoon, which is a lot for one of these).

At any rate, he put it back together and now it won't even fire. No changes to the jetting were made or anything else. The pipe was cleaned out and the restrictor plate was removed and that's it. Does anyone know if a jetting change is required? He'd really rather not put the restrictor back into the intake as the quad can't climb the slightest grade with a small child on it in that condition. Living in PA, that rules out most riding areas around here. We're thinking that it might need richened up a good bit, considering that the restrictor plate covered probably 80% of the intake area and it flows a lot more air now with it removed, but we don't have any documentation to go from on it.

Any help would be most appreciated.
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Old 07-12-2004, 11:35 AM
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Default Re: How to de-restrict Suzuki LT50 quad

The carb uses stacked pilot jets, once de-restricted you need to remove the first pilot jet. Also, the main should be a 55 stock, without the restrictors you should start at a 65 and work from there. These motors are extremely low compression, milling 0.020" off the head provides a noticable gain without any drawbacks. In fact, you could go as much as 0.040" without problems or being required to run better-than-pump gas. There is a lot more a person could do, but this is a good start.
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Old 07-12-2004, 12:15 PM
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Default Re: How to de-restrict Suzuki LT50 quad

Faded,

Thanks MUCH for the info! My buddy was thinking exactly what you said on the pilot jet deal, but we didn't try it since we were not sure if that was the way to go or not. I'm not sure if we'll know which of the two is the "first" pilot jet, but we don't have much to change if we guess incorrectly.

The main jet will be a bit more of a challenge as he's only got main jets down to 74 (I think) on hand. He's got a Dynojet dyno in his garage so he's got a fair assortment of jets in the cabinet. So I guess we'll give a 74 a whack and see what happens. Or see if the local dealer has any mains smaller than the 74 he's got right now.

And since I've got a milling machine in my shop, we've always got the head milling bit as a back-up plan. But I don't think we'll do that as a first pass. It IS for kids, after all, we just want to make the thing run well enough to climb the hills in the field where they always ride. When you can walk up the hill faster than ride, it kind of takes the fun out of it.

Looks like some rejetting is in order this week! Thanks again.
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Old 07-12-2004, 12:46 PM
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Default Re: How to de-restrict Suzuki LT50 quad

No problem. The "first" pilot jet is the one that would be removed first, the second one will need to stay where it is. A 74/75 main would be okay to start with, and with a dyno at your disposal you'll make short work of the jetting.

The stock exhaust is also restrictive, you can pull the end cap off and drill extra holes through the exhaust baffle and then replace the end cap.

I think you'll find that even without the restrictors and proper jetting, it won't take to the hills with authority. The head milling will definately help, but to doing it in progressive steps will help you realize the gains.

If it's being ridden on short, tight trails and the rider would be more comfortable with short gearing, it might benefit from being in first gear rather than second. The JR/LT 50 motor is stuck with only finding second gear from the factory, but there is a 1st gear in the box. If you pull the side cover off the left hand side of the bike you can see the shifter mechanism. There should be an oblong cutout that the shifter rides it...if you'll notice, it only allows you to shift up, not down. If you remove this stop plate and remove enough material to allow the shifter to shift down, you'll find 1st gear.
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Old 07-12-2004, 02:20 PM
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Default Re: How to de-restrict Suzuki LT50 quad

It's actually a LTA50, not an LT50. It's one of the new yellow jobbies with suspension and all the rest. Not sure if it changes the transmission picture or not. I didn't see any "shifter" to speak of on the engine when I was looking at it, but we didn't open it up either.

I don't think we'll be dyno testing it, since his isn't set up to run ATVs. "Dyno testing" will consist of backyard laps to evaluate the changes. That should get us in the ballpark though.

I'm not sure how much performance they're actually looking for out of this thing. The big one right now is it will barely climb many of the hills on the farm where the kids frequently ride. Put an adult on it and forget it, it's downhill or flatland only. Yeah, I know, it's a kid ATV but my ATC70 is a hoot to ride around the yard. Looping wheelies on it don't hurt like they do on my CR250. Or at the very least, I don't intentionally loop the CR like I do the ATC.

Is the "first" jet the smaller of the two or something? Or is there some other way to determine which is "first" to remove? OK, I think I'm getting it, the two pilot jets aren't side by side, but one on top of the other. You remove one, the smaller of the two, and there's a larger jet behind it. So the smaller jet meters the fuel and the second does nothing. Then you remove the first one and the larger, second pilot jet now does the metering? Is that right? I didn't see the carb innards myself, I only heard about the "two pilot jets," so forgive my ignorance. But from the way you worded it, it sounds like it's obvious which is which so I'm guessing that I'm correct.

If the 74 main is too big, I think the exhaust drilling will help on that front, so we might have no choice but to open up the stock pipe like you said. Sure, we could get a smaller main jet, but what fun would that be?
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Old 07-12-2004, 03:47 PM
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Default Re: How to de-restrict Suzuki LT50 quad

Quote:
It's actually a LTA50, not an LT50. It's one of the new yellow jobbies with suspension and all the rest. Not sure if it changes the transmission picture or not.

I'm not sure either, I know they were all the same from the mid-80s up till just recently...not sure on the new ones though.


Quote:
Is the "first" jet the smaller of the two or something? Or is there some other way to determine which is "first" to remove? OK, I think I'm getting it, the two pilot jets aren't side by side, but one on top of the other. You remove one, the smaller of the two, and there's a larger jet behind it. So the smaller jet meters the fuel and the second does nothing. Then you remove the first one and the larger, second pilot jet now does the metering? Is that right? I didn't see the carb innards myself, I only heard about the "two pilot jets," so forgive my ignorance. But from the way you worded it, it sounds like it's obvious which is which so I'm guessing that I'm correct.
You're getting it...they're stacked on top of each other, in order to get the 2nd one out you'll need to remove the 1st.

Sounds like you have it under control, have fun! :
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Old 07-12-2004, 05:34 PM
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Default Re: How to de-restrict Suzuki LT50 quad

Yep, I just talked to my buddy and he filled me in on the stacked pilot jet so we're good to go on that front.

He's also got Dynojet jets, not Mikuni ones, so the 74 might be OK after all since I think they run smaller than the stock numbers. Either way, we'll see what happens.

Thanks again for the help.
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Old 07-12-2004, 05:41 PM
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Default Re: How to de-restrict Suzuki LT50 quad

M,

Be sure to post back and let me know how it goes...it's always good to hear the outcome of these things, ya know?!
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Old 07-12-2004, 05:50 PM
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Default Re: How to de-restrict Suzuki LT50 quad

Will do. It will probably take a few days as he's got to order an exhaust gasket for it. The pipe is now leaking at the head so that's got to be dealt with first. But once we get it running up to snuff, I'll post the details on what it took to do it.
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Old 07-12-2004, 05:53 PM
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Default Re: How to de-restrict Suzuki LT50 quad

Cool deal :, can't wait to hear what you think of it when you're done.
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Old 07-18-2004, 02:17 AM
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Default Re: How to de-restrict Suzuki LT50 quad

Hey, what's this about a first gear? I have an 85 LT50. Are you saying it has a lower gear? There is no shifter on the outside. It is just gas and go. It would be awesome if there is a way to shift it down.
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Old 07-18-2004, 08:24 PM
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Default Re: How to de-restrict Suzuki LT50 quad

Quote:
Originally posted by 91CR500
There is no shifter on the outside.
Might want to look for one, maybe yours broke off? : There were only minor changes from '84 untill '02...
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Old 07-19-2004, 10:29 PM
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Default Re: How to de-restrict Suzuki LT50 quad

I took a good look at mine. There is no place on the outside of the case for a shifter at all. Are you talking about the LT50 four wheeler? That is what I have and I see no place for a shifter. I have not pulled the side cover off yet. Might have to try that.
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Old 07-31-2004, 11:25 AM
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Default Re: How to de-restrict Suzuki LT50 quad

The final verdict on the LTA50 was to remove the first pilot jet and leave the rest of the jetting stock. He tried a larger main jet, but the airbox lid had to be removed to make it work. It was going to need some more tuning, as well as jets he didn't have, to make it work so he put the stock main back in and left it that way. It still runs better than it did with the restrictor plate in the intake. There's more to be had in that engine, but tuning the crap out of it really wasn't the idea so that's how it was left.
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Old 08-02-2004, 08:10 AM
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Default Re: How to de-restrict Suzuki LT50 quad

An update:

The LTA50 got a workout yesterday from it's pint-sized rider and it seemed to run pretty well for him. I told his grandfather that we could still make it faster for him but he said it's quite fast enough as it stands. At least until we can break him of the habit of not looking where he's going on it. He tends to look at us and gas it, rather than where the quad is pointing, so faster probably isn't better just quite yet. But removing the restrictor made a big improvement as the quad will now climb hills for the boy at will.
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Old 08-04-2004, 03:18 PM
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Default make us fast!

Faded, you seem to know alot about these machines so I am going to pick your brain. We have an 2002 LT50 and my son likes it but it is so darn slow that it is no fun for him. He is 10 years old and has raced in the fair for since he was 4. (we had an 87 model prior to the 2002 model). What can we do to make this as fast as possible. He is only allowed to race one more year and our fair is the first week of September. He has more than enough talent to win but has never had the fastest bike. I would like to put him in the winners circle this year!! I have tried to seek help everywhere and the only answer I get is ..."those little machines aren't made to go fast." Well, I think if I took my chainsaw engine out and put it in there it would go faster!!
I am not very mechanically inclined though so if you are very technical I probably won't understand. I know people though that will understand so go right ahead.
QUESTION: If I was to give you my 2002 LT50 and told you to make it FAST and RACE WORTHY, what would you do to it?
My son and I would appreciate any help.
Thank you in advance. Mark
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Old 08-05-2004, 05:22 PM
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Default Re: make us fast!

Not sure how much time/money youíre planning on putting into your LTA50 modification, but Iíll try and give you an idea on a few things you can do that should help. I canít really list them in order from what would be easiest to the things that will require more time/money/fabrication/work, so I just start listing things and let you pick out what you think you can handle.

Airbox: The stock airbox is very small and restrictive. If youíre only looking to go out and win one race then I would consider a clamp-on foam (like a UNI Pod filter) or K&N filter for all out performance, otherwise you can ditch the airbox lid and move on. The clamp-on filters will allow more airflow and should out perform the stock airbox minus the lid. Size accordingly to your carb (as discussed below).

Intake/Carb: Again, for all out performance the stock carb is on the small side @ 12mm. I havenít confirmed anything yet, but you should be able to benefit from the XR50 rage and source either a stock or slightly larger XR50 carb for cheap and adapt it to work. Iíve seen the 16mm units on e-bay sell for around $20. I havenít check fitment of the carb or found out if the JR throttle cable will work, so if you go this route youíre on your own. This will require a new intake manifold to be fabricated as well, but wouldnít be all that hard to bend a small piece of pipe and machining a couple of flanges. Also, the jetting will have to be sorted. . . If you choose to stick with the stock carb/intake there are a couple things you can do. Start by match porting (using a gasket) the intake manifold (actually itís an elbow) to the cylinder. The idea here is to create the smoothest path for the air to flow; sharp edges from ports not lining up will hurt performance. Also, donít do your finish sanding/filing for a smooth finish. Ending with a 220 grit or comparable should provide a good texture that will help promote tumbling. With the carb off look through it with the throttle open and make sure the slide moves up far enough to clear the bore of the carb. If the bottom part of the slide hangs down in the bore then remove the slide and gently file the top (very important here, donít file the bottom) of the slide until itís enough that the slide will clear the bore and not impede air flow.

Cylinder/Head: A problem with these motors is that the piston doesnít uncover the transfer ports 100% when the piston is at BDC. If youíre looking to build a full race motor then to fix this youíll need to have a spacer plate machined to fit between the cylinder and the cases effectively raising the cylinder and uncovering the transfers. Iím not sure exactly what thickness is needed, but I think it is around 1.5-2mm. Youíll have to disassemble the engine and check to see where youíll need to be. If you do this you might as well take the time to port match the crankcase, spacer plate and cylinder. By using the spacer plate you will alter port timing, add to the squish clearance and lower the compression ratio even lower than it already is (5.6:1). This will need to be corrected with porting changes and machining to both the jug and the head to get the compression ratio and squish back to where you want it. It is possible to add additional transfer ports as well, but I havenít really talked to anyone that has done this so Iíll leave it at that. These are pretty big steps that will require more effort than most are willing to commit to, so if you decide to skip these steps you can at least have the head milled for increased compression and tighter squish clearance. Starting with 0.020Ē will give you a noticeable increase and you can go as much as 0.040Ē and still run pump gas. Also, make sure that the reach on the spark plug is such that the top of the threaded portion sits flush with the dome of the head. If not, have your machine shop turn down the spark plug mating surface until itís flush.

Piston: Wiseco offers a 1mm overbore piston. While youíre having your head machined take the cylinder with you and have them bore it for the oversized piston. This will bump you up to a whopping 52ccs of screaming yellow fury. Before you fully assembly your motor install the piston on the rod and then slide the cylinder over the piston. Rotate the crank so that the piston is at BDC and check to make sure that the piston skirt doesnít block the intake port. If it does modify the piston skirt so that the piston skirtís intake window is large enough that it doesnít cover the intake port when the piston is in the basement. When youíre ready to do your final assembly set your ring-end gap to 0.010-0.012Ē

Exhaust: There are a handful of people that make aftermarket exhaust for your quad. I canít comment on their usefulness but Iíve noticed most donít follow conventional two-stroke pipe design (divergent cone, body, convergent cone, silencer). Most are spendy as well @ about $300 no matter who you go with. Iím sure they all help some, but the $-per-hp ratio probably isnít that great. If you keep the stock exhaust ditch the spark arrestor (for closed competition purposes only of course) and drill a few more holes in the interior baffle. Also make sure that you donít have any unwanted carbon deposits or build-up in the header section of the pipe.

Oil Injection: If it were mine I would remove it and run premix only. You should be able to pull the banjo bolt, line and top cover off of the oil pump and remove the plastic gear thatís inside. Pull the banjo bolt and the line out of the top cover and replace them with a 6.0mm x 1.0 bolt.

Jetting: Set the float level before you do anything. Then you can pull out the stacked pilot jets (remove the first one, leave the second). Youíll probably need the next larger pilot jet, so you will want to source it ahead of time. Your local Suzuki dealer can get it for you. I would start with a main around 75 when using the stock carb and run the needle in the 4th position and work your way leaner from there.

Spark Plug: The stock BPR-4H plug is too hot unless you run with the restrictors and/or your just putting around the yard. Youíll want to run the cooler plug, NGK BPR-5H.

Last edited by Shifting; 08-05-2004 at 05:29 PM.
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Old 08-24-2004, 10:35 AM
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Default Re: How to de-restrict Suzuki LT50 quad

Just an update:

Last weekend was the first time the quad got an extensive test since we de-restricted it and it passed with flying colors. It ran well and had no trouble pulling the ankle-biters up any hill they tried to ride. While I'm sure that there's more to be had from it with further mods, it's definitely a lot better than it was in stock form.

Thanks again for all the help.
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Old 08-27-2007, 12:12 PM
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Default Re: How to de-restrict Suzuki LT50 quad

Hi. new here. I think the 85 lt50 I have has been derestricted already because it will haul my 200+lbs up the hill and wheelie like crazy.
It does stall often when backing off the throttle so I suspect jetting issues. Anyone have good photos or diagrams of the carb? I will check plug and look for restrictions tonight. Also, the screw ahead of the idle screw, is it air or fuel, ie, does screwing it in make you lean or richer? Thanks for any info.
Any links to downloaded/scanned owners manuals or shop manuals around?

Mike, Canada
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Old 09-09-2007, 02:26 PM
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Default Re: How to de-restrict Suzuki LT50 quad

As far as I can see, there is no restrictor in either the exhaust or intake. The main was 57 and pilot stacked. I have tried the 72.5 main I have and removed the first pilot (the one to come out first). It still stalls occasionally when he gets off the throttle or makes hard turns (to the right?!) but less. What is the float supposed to be set to on this carb? Also, someone talked about needle clip in fourth position, is this the top or bottom position?
I am going to skim the head by 040,try a bpr 5 or 6 plug and report back.
Thanks. Mike
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