Jetting for Low(er) Speed running – I am stumped!

Jetting for Low(er) Speed running

I’ve started this new thread as the original one ( was “smoother idle” but what I’m trying to fix (now) is actually above idle.

So, standard needle is GDDUS, running with a #45 pilot.

I borrowed the DRZ needle OCEMN which has a thinner straight portion (N-P-R-S is 4 sizes = 4/100ths mm measured), plus a fractionally thinner tapered section.

The EMN needle gives superbly smooth low speed running but more testing reveals its too rich at light load in the 1/8th to ¼ throttle range eg cruising on road 45-55mph.

The symptoms is surging, too rich - if I go back to the stock, leaner, needle there is no problem! I was using clip 3, but tried it right down to the 1st clip, ie needle in lowest position, which helped the surge but didn’t cure it enough to be acceptable.

I tried another needle GDDUQ, which is identical to stock but two sizes thinner on the straight part (2/100ths mm)

Now, this needle was slightly better than stock but not as smooth as the OCEMN at low revs, but was rich enough to introduce the surging again at 1/8th to 1/4th - frustrating!

Continuing the test: Stock needle and size larger pilot #48. No surging, but the low speed running only slightly better than the #45.

Then try the #50 pilot (again), with stock needle, and yes, this gives the best combination out of all. No surging, but … the low speed running OK, but not as good as using a needle with thinner root ie OCEMN.

Raised the stock needle (5th notch), various pilots - surging again – doh!

So….. I can’t get the smooth low speed running without introducing the too-rich surge at 1/8 to ¼ light load.

Just thought, I’m using a #165 main, sea level, would a larger size have a significant affect at low throttle?

Any ideas??!!!!



Below is a day’s worth of testing – a lot!

Needle/Clip: EMN #3

Pilot: #45

Fuel screw turns out: right in

Result: Too rich snatchy idle, good running low revs, surging ¼ th

Needle/Clip: EMN #2

Pilot: #45

Fuel screw turns out: right in

Result: good running low revs, but still too rich idle, surging ¼ th

Needle/Clip: EMN #2

Pilot: #45

Fuel screw turns out: (?)

Pilot air jet: removed! (to lean pilot)

Result: Better idle, good running low revs, surging ¼ th

Needle/Clip: EMN #1

Pilot: #45

Fuel screw turns out: (?)

Pilot air jet: removed!

Result: Idle uneven, good running low revs, surging at ¼ th better but still annoying, popping on over-run

Needle/Clip: EMN #1

Pilot: #45

Fuel screw turns out: (?)

Pilot air jet: Back in!

Result: Idle better, good running low revs, surging as per previous, popping gone

Needle/Clip: UDQ #4

Pilot: #45

Fuel screw turns out: (?)

Result: Good idle, quite good low revs, surging as per previous

Needle/Clip: UDQ #5

Pilot: #45

Fuel screw turns out: (?)

Result: Good idle, low revs same as previous, surging ¼ th back

Needle/Clip: UDQ #4

Pilot: #48

Fuel screw turns out: (?)

Result: Idle worse than previous, quite good low revs, surging at ¼ th better but still annoying

Needle/Clip: UDS #4 (stock) **reference point**

Pilot: #45 (stock)

Fuel screw turns out: (3-3 1/2)

Result: very lean and snatchy at low revs until engine properly hot 10-15 mins), then quite smooth, no surging at ¼.

Needle/Clip: UDS #5 (notch richer)

Pilot: #45 (stock)

Fuel screw turns out: (3-3 1/2)

Result: lean and snatchy at low revs until engine properly hot. Quite smooth low revs, but surging ¼ th.

Needle/Clip: UDS #4

Pilot: #48

Fuel screw turns out: 1/4 out

Result: Idle a little glitchy, quite smooth low revs, a little surging at very light load, fine-tuning fuel screw helped

Needle/Clip: UDS #5

Pilot: #48

Fuel screw turns out: right in

Result: Glitchy running around idle (too rich), some ¼ th surging showing again

Needle/Clip: UDS #4

Pilot: #50

Fuel screw turns out: right in best

Result: A little glitchy at idle but otherwise good low speed running (but not as good as EMN needle). No surging. The best overall setup, and remained good after several hours riding in 20 deg C. Question mark over fuel screw setting – ie no room to go any leaner for hotter days


You are confusing yourself.

Select the pilt, set the fuel screw for idle. Then LEAVE IT ALONE.

Select teh main jet. Then LEAVE IT ALONE.

Now.... work with the needle. Being the taper and length of the ENM seems good and only the section at small throttle openings is a problem (bettween idle and 1/4 thottle) which is the straight diameter, you need to try a EMP or R (my guess would be the R), on clip #3.

Do not try to change the pilot/fuel screw to compensate for a too lean or rich needle. You do not add more salt to your steak if you put on too much pepper.

OK William, fair points raised. However in this case (or this bike!) things are not quite as straightforward as we would like.

The WR450 seems to be very sensitive to jetting changes, unlike the DRZ which will run happily and smoothly on a wide variation. Also, where I agree that setting main and pilot as separate items is the “ideal” way, on this bike the pilot and needle have such a dramatic affect on each other that I had no option but to try combinations.

Your suggestion to try a EMP or R needle is unquestionably right in theory, but my point is that even using a slightly richer needle, a “Q” did not make it rich enough for my required low speed running, but that was enough to give the rich-surge at ¼ throttle. So an R needle may not show much surge but won’t actually achieve any other improvement.

Hope that makes sense?


I am no expert, but it sounds like a fly wheel weight might solve all your issues.

I have a 08 YZ450 and its really crappy to ride slowly with the little kids. but i would not trade it for anything when the pace picks up.

You need to keep in mind also, the effect and amout of fuel passing through the pilot circuit. It is quite small and increses significantly once off idle. It would take a few changes in pilot jet to equate to the difference between a P and R needle for equivilent fuel flow. I am sure you are familiar with these:



You may find this FCR-Tuning Guide a usefull tool also.

I'm so glad I don't have to mess with this anymore. The YZ needle, new leak jet, quickshot2 and adjustable fuel screw worked well for me. I have a very faint surge that I only notice at steady throttle on the road(This bike is not designed for steady road throttle). I never ever notice it off the road. I believe the TPS was once considered for the surge problem. Wouldn't hurt to check it.

Thanks William, yeah I have studied those diagrams to death!!

TPS? - I tried disconnecting that a while back and it had a very noticeable drop in power at lower revs - it is there for a reason!

I have a possible breakthough to announce, more once I've done another test to confirm. Unfortunately the bike is currently out of action in the sick-bay - more in a new post later!



Y’ know sometimes you just have to try things out as the theoretical stuff just doesn’t always work.

So, I had tried different needles and different pilot jets cos that’s what the theory says right for slower speed running?

Well, that failed miserably so tried a big fat main jet drilled to approx #175 (stock is #165)

Instant fix!!!!

Only slightly surprising as I had discovered years ago that the main jet can influence mixture quite significantly down to ¼ throttle, or even less.

The main is now replaced with a proper #170, which is the recommended - same great running at all throttle positions, fantastic.

I also bought a #168 which I will try out at some point – I am using a 94db pipe and not sure if the recommended #170 is meant for a straight-through?

The #48 pilot is pretty good, but a #50 seems better to me – I run the fuel screw at ½ turn out which gives very smooth (lean) on-off throttle transition, whereas the #48 gave some lean surge set the same way (about 1.5 turns out for this one)

Very, very pleased. :worthy: Just a pity it took so long to figure this out! :lol:


Just a quick update - there's a mini-heatwave here in the UK and its 30deg C which is pretty much as high as it normally gets (though its 32 today!).

Anyway, went out for a short ride yesterday and the jetting with the #50 pilot was spot-on despite the high temp.

Excellent, very happy with the bike :worthy:


Which needle/clip position did you settle on? Also, I'm curious what size leak jet you are running and if you have played with the AP circuit at all? Thanks!

I’m using the standard needle GDDUS, 4th clip (it’s a UK bike)

What is worth highlighting is that this jetting comb #170, GDDUS needle, #50 pilot gives very clean carburetion, no surging, bog, blah.

I think that those that are looking at the TPS to cure their surging need to rejet! And I would suspect that aftermarket needles are the culprit, running a bit rich in the search for that extra 0.5hp…

SXP – I haven’t touched the leak jet – its sock as per the manual. I have the AP backed out a little from stock to delay it (screw turned in), but even before I touched it there was no bog, I was just experimenting.

From my experience the AP actually CAUSES a bog if it comes in too early as the engine cant cope with the burst of fuel at low throttle. Adjusting it so it at least misses the slide is the starting point.


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