99 RM250 Fouling Plugs

Hi,

I have had issues with fouling plugs since i have owned this bike and had messed with jetting until it only fouled them every 2 to 3 hrs.

I ended up with 42 pilot, 152 main and needle in 4th clip position with 32:1, which I thought was a tad lean?

So after reading heaps of posts I put in a new top end, replaced the right hand side crank seal, added a FMF shorty pipe and boyesen power reeds in an attempt to rectify the problem.

It certainly gave it a power boost, but fouled 2 plugs (one was an NKG Iridium IX at $20 AUD :thumbsup: ) within 1.5 hrs. I did change the mix to 40:1 on the advice of a bike shop owner... would this make that much difference?

Carb jetting is currently 45 pilot, 152 main needle clip middle position with 40:1.

What are your suggestions for mix, jetting and needle clip position?

Thanks.

You should do a compression test.. even though you put a new top end in the cylinder walls could be worn causing low compression and it takes heat and compression to get combustion so if your not getting the compression your not getting a good burn.

Yea, a 152 jet shouldn't be fouling plugs and a word to the wise if you are fouling plugs, don't try and solve it with more expensive plugs!

What pipe do you have as it almost appears that you have exhaust lagging in the pipe and backing into the cylinder.

You didn't say how it ran on 32:1. Going richer to 40:1 definitely contributed to the problem as 'rich' is why plugs foul in the first place.

Okay I will do a compression test, but i only have access to one of the older style testers not the screw in type.

Should the compression be around 150psi?

It still has factory pipe (a couple of small dents), I only put a new silencer on. It ran okay at 32:1, it just had oil dripping down the silencer and bogged/hesitated a bit.

The bike definitely runs better now with more power and less bogging, but I guess the fouling problem wasn't the right hand crank seal like I thought/hoped.

I only got the expensive plug because that was all the bike shop had left for my bike, but I was hoping it would help...

Okay I will do a compression test, but i only have access to one of the older style testers not the screw in type.

Should the compression be around 150psi?

It still has factory pipe (a couple of small dents), I only put a new silencer on. It ran okay at 32:1, it just had oil dripping down the silencer and bogged/hesitated a bit.

The bike definitely runs better now with more power and less bogging, but I guess the fouling problem wasn't the right hand crank seal like I thought/hoped.

I only got the expensive plug because that was all the bike shop had left for my bike, but I was hoping it would help...

At 150psi you want to think rebuild. Good compression is in the 170-200psi range. Your problem is jetting not the amount of oil you run. However, it is an MX bike and really needs to be revved to keep the combustion temps up. Think about how you ride. Are putting around the trail and loading the bike up with raw fuel or are you on the MX track running wide open throttle? either way look to your jetting and start their once you confirmed that you have good compression.

Cheers!

I was actually revving the engine pretty hard on both occassions the plug died, so I guess that backs up the low compression theory.

I did the break in procedure as follows:

Start the bike blip the throttle until too hot to touch cylinder and ride it gently to running temperature let cool down for an hour+... then did that again.

Rode it under half throttle for 15 to 20 mins then let it cool for an hour.

Then i gave it hell, and after about 20 mins the first plug died...

The other was thrashing it around a slippery paddock.

Because you foul plugs doesn't mean that you have low compression. You may have a weak spark or the jetting is really piss poor.

here is a jetting chart.

1999RM250Jetting.jpg

Heres a link on how to jet properly. http://www.thumpertalk.com/forum/showthread.php?t=749204

Okay, I will check the compression first up... as for jetting... according to that chart I should prob have 152 main, needle clip in 2nd clip and 42 pilot with the bike stock...

I currently have 152 main, needle 3rd clip and 45 pilot.

So due to the boyesen reeds, maybe a 150 main needle 2nd clip and 45 pilot might be the go?? And also go back to 32:1...

The bike definitely didn't smoke and spooge as much as before the mods, so they helped somewhat.

Thanks for the help so far :thumbsup:

The boyesen reeds shouldn't necessitate a change in jetting and only if you were doing a v-force. The turbine core longer silencers tell you to drop jetting which you usually don't need to do either. I think you are now below stock with the lean jetting and you are going to create more problems...........ummm, what oil are you using as some spooge and foul plugs more than others!

I'm using Motul 800, but have also tried a shell one in the past and had the same problems

I can't find the compression tester and will have to order one, so prob wont be able to check that until next week...

You said the shorty helped, take the silencer off, too, and then run it and while loud, the neighbors can live with it for a bit for a diagnosis. Oh, and please don't say you are running some high octane fuel.......................!!!!!!

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