Help perfect my 89 RM250 Jetting

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5/15/2018 3:59 AM

I've got a 1989 Suzuki RM250, running a PWK Keihin carb from a 1997 (big improvement) and looking to sort out a 1/4-1/2 throttle sputter/burble.

See video below:



As you can hear, when the bikes under load on full throttle it runs clean and pulls well. However at the 1/4-1/2 throttle range just before it really comes on the pipe, it sputters and burbles a little and it definitely bugs me.
Listen at around the 3:50 mark.
Is this a lean or a rich condition? Do I need to alter needle clip or taper?

I know the jetting is close as the plug colour is nice and there is minimal carbon build up on the piston at top end changes. But would love to iron out this rough patch if possible.

40:1 Putoline MX7 - UK Premium Pump 97 octane
Temps typically around 10-25 C
Elevation 0-1000 feet

Carb currently set at:
45 Pilot
A/S 1.5 out
#7 Slide
NEDH Needle on Clip 3
170 Main (have a 168 on order)

Any help would be greatly appreciated!
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@russ_69

5/15/2018 6:44 AM

It sounds, to me, like the needle diameter (H) is too rich. What does it do when you lower, from #3 to #2 , clip position on your current needle ? Your main jet may be a little large as well.

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5/15/2018 7:24 AM

digger wrote:

It sounds, to me, like the needle diameter (H) is too rich. What does it do when you lower, from #3 to #2 , clip position on your current needle ? Your main jet may be a little large as well.

Thanks for the response digger! I was hoping you'd chime in.

Unfortunately I haven't done any testing jetting wise other than with the pilot. I'm reluctant to play around with the jetting on race day in fear of going in the wrong direction.

I have both J and W, NE series needles (cant remember which is the next step leaner diameter from the top of my head)
I'll start with a step learner diameter and then try the clip postion after if required.

Yep main is a little too rich for summer temps, 170 was okay in the winter but will drop to a 168 in now its warmed up.

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@russ_69

5/15/2018 7:52 AM

A W diameter needle is the next step leaner needle in that series , and most series needles, G,H,W,J,K rich to lean starting diameters. The NECW would be 1/2 clip position leaner than the NEDW.

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6/11/2018 3:45 AM

Finally got round to testing jetting. I had some interesting results.....

I changed the 170 main to a 168
The needle I changed to an NECW clip 3, so 1 step leaner needle point taper and 1/2 clip leaner needle clip position.

It made the bike worse, by a lot! Pretty much gutless and flat off the bottom. Even more hesistant, boggy and burbly, with very little throttle response in the first half of the throttle so its obviously too lean?

As usual mid to top pulled very hard and clean, the leaner main felt good and it pulled and sounded better WFO.

So I'm thinking NECH, maybe even NEDH at clip #4 next?

I did get some helmet cam footage of the bike with the NECW #3 needle, will post up when I get chance

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@russ_69