Loose spark plug symptoms!?

Hi I've been having problems with my bike fouling the plug, not getting on the powerband, making a lot of smoke, cutting out, not starting and generally being very sucky!

The last tank of fuel I mixed exactly 50:1 so I figured it wasn't the mixture.

I've been swapping plugs out really often and noticed oil spatter out the top of the head, on the carb and underneath the tank. I assumed it was coming from the exhaust expansion chamber to the muffler but actually seemed to be from the top of the head!

I normally finger tighten the spark plug then "nip it up" just with the wrench, but I figured this time I would put some torque on it and hey presto, the bike came back to life!

I was suspecting the power valve or the ovaled exhaust expansion chamber flange but it looks like these were all symptoms of simply a loose spark plug!

This is my first two stroke and it seems they need more attention paid to the spark plug torque than on a four stroke.

I figured I would ask if those symptoms sound consistent with a loose plug anyhow, since it seems to have solved my problem.

Edited by flynn_


A loose plug is either too lose and 'blows out or it leaks a little spooge, making a mess. A real loose plug might make starting more difficult. Proper installation with a NEW plug is bottomed out and 1/4 turn. The gasket on a plug is a crush gasket and once used, is crushed and reuse, does not get tightened as much. It is important for the threads on the plug and the head to be clean as well.

sounds like you are jetted a bit rich aswell

Hmm, well I got all excited there, not sure what the problem is then.

I don't know what ratio my bike is jetted for, I'm using 50:1 but the jetting might be setup for 60:1 for all I know.

Apart from tightening the spark plug more I put 2 liters more of gas into the tank with about 1/2 a tank of 50:1 already in.

I really thought it was just a plug that wasn't tight enough, maybe there is still another problem.

The exhaust flange isn't seating great but that hasn't changed and yet it seems to be running back normally now.



Here's a pic of my jetting from the service notes I got when I bought the bike.

It was setup way back in 2004!

What ratio does it look like it has been jetted for?


Edited by flynn_

You've not yet said what bike. Your elevation?

Hello buddy.

It's a KTM 250 EXC, 2000 plate.

I live 1/2 from the sea so literally sea level.

I was out on it again today, it bogs down and doesn't get into the power, then after a couple seconds of spluttering, goes crazy and tries to put me into the car in front of me!

Only over about 1/3rd throttle though.


The engine doesn't sound happy, which could be something as simple as the mixture or pistons/rings/reed valve/power valve. It has a sort of muffled popping sound to it and the plug is always wet when I pull it, never dry.


I am gonna get my Street bike back working tomorrow then I can tear into the KTM cause I figure that is what it needs.

Edited by flynn_

I don't think you understand how jetting works.  Bikes are not "jetted for an oil ratio".  The oil ratio stays the same and jetting is changed to ensure the air-fuel mixture is correct.  

I meant in the sense that changing the oil/fuel ratio changes the air/fuel mixture.

Anyway the behaviour of the bike at the moment is really odd.

When I got the bike it ran crisply and perfectly.

The only things I've done to it are put the fuel in the tank and while riding some knarly terrain, the expansion chamber has taken a knock and ovalled the flange that sits over the cylinder head flange.

I do not have a car so I simply pour the gas into the tank and then put the oil on top so it can sink through the gas.

From what I've read with modern 2 stroke oils that should be enough to get a good mixture.

At the moment I'm literally getting around the corner and the motor gas died down and fouled the plug up.

I'm gonna run the tank dry and put the exact mixture back in tomorrow but I really think there is something else wrong.

It's hard to tell in such a situation because there can be numerous false positives and red herrings going on all over the place!

You are running at 50:1, so that's not too crazy one end of the spec(so stick with it) , looks like the jetting installed is slightly leaner than stock.  Looks like a 15 year old bike, with carb issues, ring issues, reed issue  ect


But oil mixture is not your real issue, probably best just to take it to mechanic, so they can check out.

Depending on how extremely you've "ovaled" your pipe, that could easily be your problem. If it's actually crushed enough to restrict exhaust flow, it will run really poorly. Also, running a loose enough spark plug for any period of time can cause serious damage to your top end (piston/cylinder and related components). This is from allowing too much air into the combustion chamber and really leaning out the mixture. Let's hope that isn't the case, but you probably better pull the jug and make sure.

Have you replaced the piston ever? If not, what is your compression? How many hours or KM are on the bike?



Hi not taking away from other suggestions or maybe other causes, but is that the jetting on the list that you have in the bike just now? You need to check your jetting regularly (we keep copious jetting notes for race and practice tracks for mx in the UK) and broadly speaking you jet leaner in the summer (Less oxygen in the atmosphere) and richer in the winter (More oxygen). There are a lot of other factors like how you ride, track surface, altitude etc. Anyway for now, if it is as Baltic in England as it is in Scotland that jetting is too lean, particularly if you have a NOZ H which affects 1/3 to 2/3 throttle opening. KTM recommend base settings for 6degrees C to 15 degrees C, under 1000 feet as a/s 1.5 out, 45 pilot, NOZG on 3 and a 180 main. The NOZ have an overlap between needles but you cannot achieve this with your H as KTM recommends clip 3 for the G. If you are in mud or sand continually you should up the main to a 185. Hope this helps. Bob.

PS - stop mixing your fuel like you do - put proper measures in a plastic can and give it a good shake. If neat fuel goes into your combustion chamber you WILL take the back (intake side) of the piston crown out.

Edited by 7Tktm

Flynn, 7Tktm covered it like most the guys... You problem is jetting for sure, I run 60:1 when doing single track typically but will go to 50:1 when dealing with desert riding or racing for that extra lube in extreme conditions. In case you do not have an owners manual to look up jetting specs here you go 250-380 KTM Owners manual


Edited by Diamond-Dave

Good post. His 2000 carb in the UK should have NOZ needles and if he has an H installed it is too lean. Always best before asking to declare what jetting you are running. makes things easier to diagnose.

On 11/4/2017 at 5:15 PM, flynn_ said:

I do not have a car so I simply pour the gas into the tank and then put the oil on top so it can sink through the gas.

Stop doing that.

Get a gas can, put the right amount of oil in the gas gan, add gas/petrol, shake a lot. When you want to fill the bike's tank, shake it again and pour it in.

21 hours ago, pat22043 said:

Stop doing that.

Get a gas can, put the right amount of oil in the gas gan, add gas/petrol, shake a lot. When you want to fill the bike's tank, shake it again and pour it in.

Its ok to mix at the pump. Anyone that dual sports 2 strokes has to do it. 


 Id just bounce mine around to mix or add the oil first.  I usually closed the petcock before dumping the oil in. 

Just now, seer said:

I'd just bounce mine around to mix or add the oil first.  I usually closed the petcock before dumping the oil in. 

Be sure to close the petcock, then shake the bike. I'm not sure I would go with "OK" to do, but if you have to.... I carry an emergency metal flash of premix when I think I'll need more fuel than my bike carries.

I wonder if gas/petrol stations in the old Communist eastern European areas sold premix for their Trabants and other two stroke cars. I had a buddy with a two-stroke SAAB back in the early 70s

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