UPDATE: 2009 SX50 HELP! (scroll down)

Related:
Create New Tag

8/16/2019 11:19 AM
Edited Date/Time: 8/17/2019 3:21 PM

EDIT: I'll check the advice below to make sure the piston clears the exhaust port. Right now I'm having bigger issues, though: the darn thing won't start. I have the new top end in it, the spark plug is new, I've cleaned the carburetor (it didn't need it, but I cleaned it anyway,) the reeds look good, there is no blockage in the exhaust nor in the airbox... no go. It doesn't help that the kickstarter on this thing only has about a 45-degree angle of operation when you kick it. I don't feel like I can get a good bit of momentum into the crank.
Anyway, is there a secret trick to getting an SX50 to start?

|

Braaapin' aint easy.

8/16/2019 7:01 PM

Also, the gasket kit came with three base gaskets; two thick and one thin. There was only one (thin) in the bike upon teardown, but I don't know if that's correct. Is this an option for lower compression?

|

Braaapin' aint easy.

8/16/2019 8:36 PM

Falcon wrote:

Also, the gasket kit came with three base gaskets; two thick and one thin. There was only one (thin) in the bike upon teardown, but I don't know if that's correct. Is this an option for lower compression?

I don’t have any torque specs for you, but the different base gaskets are to help you set the correct squish...

|

8/17/2019 8:12 AM

I see. Is there a recommendation for stock?

|

Braaapin' aint easy.

8/17/2019 8:54 AM
Edited Date/Time: 8/17/2019 8:55 AM

Falcon wrote:

I see. Is there a recommendation for stock?

I think the 50s are set somewhere around .040”... It Would probably work better around .028”-.030”... just make sure when changing base gaskets that the piston doesn’t partially block the exhaust port at the bottom of the stroke...

|

8/17/2019 3:22 PM

I'm posting again at the bottom to help reduce confusion.
Trying to start a KTM SX50 with a brand-new top end. It has fresh gas, a new spark plug, reeds and carb are clean and in good condition, but the thing won't fire up. No rag in the airbox, no blockage in the pipe. Help!

|

Braaapin' aint easy.

8/17/2019 5:46 PM

Is it sparking when you kick it over? Possible kink in wiring loom?

|

8/17/2019 6:50 PM

Bright blue spark

|

Braaapin' aint easy.

8/17/2019 7:49 PM

Is the plug wet after trying to kick it a few times ? Should be if it is pulling fuel through the choke circuit.

|

8/17/2019 8:02 PM

I don't think it's wet. I haven't looked too closely at that. I gave the carb a quick clean today, including the pilot but I didn't disassemble the choke or anything. maybe I'll jump in there tomorrow.

I actually tore it all back down today to make sure I didn't do anything wrong in the top end and I noticed some excess oil on the crank and in the bottom part of the cylinder. It could just be premix but I'm wondering if there's a main seal problem. Would that keep the thing from starting though?? unsure

|

Braaapin' aint easy.

8/18/2019 4:49 PM

OK, I checked the plug after kicking and it doesn't seem wet. I went through the carburetor again and removed the jets, the idle adjuster, the air screw and the choke knob & cleaned all the circuits out. There did not appear to be any obstructions or gunk. I'm pretty confident it's not a fuel problem. The reeds are perfect and there's good compression. Still won't run though.

I'm pretty much confident in the spark too, but I understand the stators on these things go out regularly and sometimes the voltage isn't enough to overcome the pressure created by compression when the cylinders are all buttoned up. I took a look at the ignition side and there's some fairly significant rust. What do you think? Bad stator? Is there a way to test this one? I don't think I know anybody with a part I can swap onto this bike to try it out. I'd hate to spend $100 and have it not be the issue.
Help! I'm getting tired of kicking this thing! I think I put a tenth or two on the hour meter just by kicking!


|

Braaapin' aint easy.

8/18/2019 5:46 PM

If you did the top end make sure to deck the cylinder. It could require a couple different thickness of gaskets. Search YouTube in decking a KTM 50SX. WMR sells the tool for a few bucks. Works wonders.

I have a 09 SX50 and it was a nightmare to start until I found this out. Now it starts within two kicks.

|

8/18/2019 5:49 PM



here is the link
|

8/18/2019 5:58 PM

I understand the deck height thing, but this bike won't even begin to start. It hasn't even shown one iota of combustion at all. This is not a "runs poorly" or "takes a long time to start" or "won't quite start" situation; it's a "now has a half hour of break-in on the cylinder from the kickstarter but not one single detonation of fuel/air" situation.

I've even tried starting fluid down the carb throat and nothing. Nada, zilch. It's like trying to kickstart a toaster; there's nothing at all happening.

|

Braaapin' aint easy.

8/19/2019 3:04 PM
Edited Date/Time: 8/19/2019 3:23 PM

Have you ever heard this bike run ? Looking at the stator, may want to check the crank seal behind it. If the plug is not wet after kicking, it is not drawing fuel through the intake, into the crank case. Poor or low vacuum due to a leak somewhere ,possibly.

Pull the carb, shoot the starter fluid directly into the intake. It should at least pop when kicked over in this manner.

Is it making at least 100psi cranking pressure on a gauge ? Low compression will also hinder the amount of "pull" of the fuel charge .

What was the clearance on the piston to bore ? It could still be "loose" ,even with a new piston /ring. Low cranking pressure makes for no or hard starting. I bought a complete KTM cylinder, piston kit for about $210 for an old JR I rescued way back.

If the cylinder has a lot of wear, they used to offer A-D pistons. When I bought my kit, my dealer said they only offered C pistons , I believe, which is also what the kit came with. The small bikes are very sensitive to the vacuum signal, or amount of "pull" of the fuel charge.

|

8/19/2019 3:29 PM

Understand, most can not wait to unload the 50cc race bikes. They will have you chasing your ass a lot of the time.



|

8/19/2019 3:31 PM

Check the impeller on the water pump. Sure way to cook a KTM 50 top end when they fail.

|

8/20/2019 8:54 AM

If you have good blue spark as stated, then try starter fluid to see if it will at least fire up for a second or 2. If so, then it is a fuel supply issue. If not then you need to recheck spark.
I would also clean the flywheel of rust.

Paw Paw

|

8/20/2019 10:44 AM

Thanks for the continued help, guys. I'm going to test the stator for resistance today; I just found out the spec for that, so we'll see.

Project Racer, I'll try the "no carb" starter fluid idea if the stator turns out to be good. I just changed the top end myself; it's a "B" piston in an "A" cylinder and it seems to fit properly with no rocking at all. You may be on to something with the air leak idea, though. I did notice some oil on the cases below the piston when I re-inspected it. I figured that wouldn't cause it not to start, but if it's a big leak, maybe you're right about the vacuum pressure on the upstroke.
I have seen this bike run before, so I know it's just one small thing.

PawPaw, I'll clean that flywheel off and see what happens.

|

Braaapin' aint easy.

8/20/2019 3:39 PM

UPDATE: No go on the starter fluid down the intake throat. Not even a pop.
The stator tests within normal range for resistance (510 Ohms.) Like I said, it's giving me a solid spark outside the cylinder with the plug grounded to the studs. I cleaned off the rust just to be sure, but I'm pretty sure we've eliminated that as a culprit.
The plug looks dry to me, but I don't know how long I should be kicking it to see. Is 10 kicks enough? 100? I've completely gone through the carb, the floats are set correctly, the fuel shutoff valve is operating, and there is plenty of fuel present in the bowl when I remove the carb. The reeds seem to open and close easily.

So, to recap, I have compression, spark, and fuel (I THINK,) and still no combustion. What the actual F? This thing is going to kill me.

|

Braaapin' aint easy.

8/20/2019 6:24 PM

If the plug is dry, can not smell fuel on it. No Bueno. How long since it last ran ? Have access to any pressure testing equipment ?

If it is making at least 100-110 pounds of cranking pressure, and the crank seals are holding pressure it should run. 100-110 would be considered low IMO, but it should a least give a hit.

|

8/21/2019 7:55 AM

I can get a compression gauge for the plug hole but I don't know how I'm going to test the underside.

I'll do that and if the compression looks good we'll be that much closer.

|

Braaapin' aint easy.

8/21/2019 11:17 PM

I had a similar issue. I took off the pipe and kicked it over and it started up. I did end up replacing the stator about a month later though. I was also advised by someone to start them with the rear wheel off the ground.
I pulled my hair out fixing 50sx!! As soon as my daughter was tall enough I put her on a 65 and had no issues since. Good luck

|

8/22/2019 3:16 PM

OK, so I was starting to lean back toward project racer's idea about a poor crank seal. I think it's the clutch side, not the ignition side. Here's my reasoning:
-Stator is dry as a bone, with no sign of any oil or exhaust residue
-Dry plug
-Heavy oil on the crank (too much to be combustion residue, I feel.) Same color as the the trans oil.
-Brand-new top end rules out compression (mostly - still have to put it on a gauge, but I'm confident this is not it)
-Good spark and stator tests within range

All these point toward fuel as the limiting factor. However, I tried the old "saran wrap on the carb throat" trick to see if the reeds were operating when the kickstarter was engaged and sure enough I'm getting some suction. Arrrrrrgh! I may just replace the clutch-side seal anyway to see if that's it. After all, saran wrap isn't hard to move and maybe there's just not sufficient vacuum going on under the piston.

Can that seal be removed and replaced without splitting the cases?

|

Braaapin' aint easy.

8/23/2019 3:36 PM

OK, I'm onto something. The clutch-side crank seal must be bad. We're getting way too much oil in the crankcase just by kicking. Check out the pictures and let me know if you concur - and remember, this bike has not yet started. That's dirty old oil from the gearbox coming into the combustion chamber.



|

Braaapin' aint easy.

8/23/2019 5:51 PM

It could also be a bad center case gasket.

Paw Paw

|

8/23/2019 6:17 PM

Leakdown test would’ve told you instantly where your leak was... Personally I’d replace the stator and clutch side crank seals... they’re easy to access and replace with the engine in the frame... plus they’re cheap... if it still pulls oil into the case after that then I would assume bad center case gasket; when was the last time this bike ran?

|

8/27/2019 2:20 PM

It ran about a month ago. From what I understand, it was slowly getting harder to start and losing power, so I thought = top end.

I'm going to do the clutch side crank seal first and see if that's the issue. If not, I'll drop in the stator side too and see. After that, I'm done with this one! grin

|

Braaapin' aint easy.

8/28/2019 12:05 PM

is that a crack in case? in line with the hose clamp

|

8/28/2019 2:47 PM
Edited Date/Time: 8/28/2019 2:50 PM

bf884 wrote:

is that a crack in case? in line with the hose clamp

Looks questionable.

|