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Jetting older Bikes

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I have been fighting jetting of my 96 RM125 for over a year now. Have spend countless hours reading threads, trying to make some headway with this bike. Whatever I did, it would not idle, but would foul plugs. Moved the needle to full Lean(position 1) went down to 35 slow jet(stock 50), Main Jet 162, stock 170. Still fouling plugs and low end sputter. Since most of my riding is less than 1/2 throttle, the mainjet changes were in desperation, more than need.

Mechanical Condition

Compression 155psi

New Crank Seals

Boyesen 2 stage reeds Stock cage

Stock pipe and silencer(new Packing)

Clean Air filter

Altitude 600ft

Temp 50-90F


I actually have made some headway with my RM125, figured I would share it with this forum. I have increased my needle diameter 1 size and then another size. So I am at at R1471NS which is 2.715mm diameter. Noticed the tuning airscrew left it out less than 3/4 turn with a 40 SJ (the airscrew actually now has an effect) so went up one size to 45 SJ(closer to stock). It idles great now, and has a lot less sputter and 4-stroking at just off idle throttle. The bike actuall pulls the front wheel off the ground in second. There is still room for improvement. It still cold fouls plugs. So I purchased the next higher size, which is the biggest I could find. This is a DGN, which has the same L and taper as stock, with a 2.725 diameter. I am hoping this is the magic bullet. If not looks like Slide filing is in order.

I really think these older bike run like crap because the jet needle wears oval over time screwing up the off idle response. The difference in size from adjacent diameters is less than 0.0004 inches(which is 0.0008" larger for my change). There is no way this can be detected or seen. Unfortunately with Keihen PWK this is not serviceable item. New carb would fix it also, but that would be no fun.

I welcome any comments

Edited by mark3066
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You make no mention of what ratio oil/fuel you run and what the bike had been jetted for ran. The lower the ratio (more oil to fuel) the richer the jetting must be.

Yes, the needle jet emulsion tubes wear, last about 100 hours tops, needles about twice that unless the bike is soley used for plonking in which case both last about the same.

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Thanks for the comments I am running 32:1 and sticking with it. At least that seems to be the most common recommendation when jetting.

I have thought about a hotter plug, but I am concern I would be masking the jetting problem Currently running BR8ES until I sort this thing out, then I will go to the exensive R6918B-8 plug. Since I am switching out plugs every 3-4 hours, I figured this is the less expensive way to go.

Glad to hear about emulsion tube wear, because that is the theory I'm riding on. I am surprised Keihin doesn't feel it is should be a replacement item.

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I have almost the exact same problem with my 1993 RMX 250, foul 2 or 3 sparks per ride.

Change the main jet to stock 150

change slow jet to stock 60

but never really check on the needle jet. assuming it was stock.

Did change the clip position one up. But nothing seems to work.

When you took out the plugs where they all black or just wet?

I take mine and they dont look that black oily nasty burn.

A little over tan, but wet as hell.

And i am really tired of changing things and getting no results.

Will dig on the needle thing. :thumbsup:

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My plug is actually black and dry, but I may be ridding slower than you. If you crack the throttle just a little, does the bike sputter and smoke. That is where I think I have my problems, which the needle diameter should address.

Another thing, if you don't get enough time on the sparkplug it may be brown on its way to black. I pulled mine after about 10 minutes and thought I nailed the jetting problem, nice and brown, but it was just on its way to black:banghead:.

As another note, I have don't have a 250, but a 60 sj seem a little large. My stock SJ is 50, went all the way down to 35 with no change in low speed operation, that is what led me to the needle diameter.

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