yz timing and restrictive exhaust

Robin your exhaust is very restrictive with both the quiet core and only 6 disks. This is a new jetting area for me as I run the exhaust with much less restriction. The bike with YZ timing really needs RPM and flow to work porperly.

That being said with your current pipe and YZ timing I would try


Fuel Screw = 1.5

DVP #2

MJ= 168 no more than a 175

second choice everthing the same but a 48Pj with the fuel screw at 3/4 turn out max.

Good luck


I just verified my setup and I was i tooth off on the intake valves (I did 20 km in this way)!!! Now being correct toothely speaking, I tried 48,0.75 turn, 172 and third clip on the dvp needle. Close to be correct. The bike hesitate and is hard to control on 0-1/4 throttle so I'm going to try the 45 pilot and 1.5 as you recommended. If it has no effect I'll go for the second clip.

In any case, I can already see the benefit of the yz timing over the wr at least on road. It kicks sooner and rev easier and I'll have to watch the speedo and the cops now.

Concerning the main, with wr timing and 168, i was hearing a metal sound so it was way too lean. I raised to 175 and the sound diseapeared. A while after I downsized to 172 adn it pull harder. May be I'll try the 170 one day but it is close to the way too lean 168.

I just switched to yz timing. Bike is canadian 1999, megamax 2 with fmf ceramic coated exhaust (sold by clark, excellent!!), air box top removed.

The tricky thing is that I use just 6 plates WITH the quiet core. Yes, it is almost as quiet as the stock bike but with little bit more bang than stock (not difficult to beat!!). I'm running on road 80% and 20% trail.

I switched to dvp needle and yz timing 2 days ago. I started to jet the bike but it seems harder than expected. First attemps was at 172,48, 3rd clip but the bike stalled with fast throttle open and seems low on power compared to my old setup (172,48,3rd clip, wr timing, DXM). I raised the clip and the problem was less present. Now I'm at the first clip and the jetting is still far than perfect!!

May be next try will be a setting often seen on the discussion group, something like 175,48, 4-5th clip but this is a blind trialand I'd like to understand what happening.

Big question: How using a more restrictive exhaust affect the jetting of a yzedded wr compared to a yxedded bike with a non-restrictive exhaust (like most of you use)



i've slipped in a little article on my exploits last weekend called "teeth & pins"- have a look.

over here in britain the pipes i believe aren't as restrictive as yours & they are about 92db as standard.

the YZ timing has been a huge improvement & as you can see from my notes i was lucky first time with the jetting first time out.

in my experience the jetting goes up initially when you de-restrict a bike at either the pipe or inlet. after this all tuning brings the jetting down.

once when i had my ducati singing i got down to the smallest main jet that dell o'rto made!!!

I changed the pilot for a 45 and it helped but not that much, I was not able to find a pilot screw turn that start easy, does not backfire a lot and rev freely. I then changed the needle for the dxm (the original wr canadian needle). This cured the 0-1/4 hole completely and now the bike is close to perfect to me. My only complaint is that starting that is harder than before. However I'll take the time to figure out exactly what happening before doing another move on the jetting.

I did a 10 km on road and it boost the mid a lot. I can now raise the front wheel just with gaz. Remember that I am geared 15/45 and this is not and open muffler! My second gear is almost the third gear of the stock wr of my brother!!!

I won't go back to the wr timing for sure, the bike surge is perfect and controllable for me, I love that power!!! I should post the next try to jet it better. Maybe the next stage is the octopus surgery...


Please enter you exact jetting configuration I think you said


Fuel Screw = ??

Needle = DXM but what clip position ??

Main Jet = ??



The current setting is: complete megamax pipe with insert and 6 disk. mj=172, pj=45, fuel screw around 1.25. Yz timing, top airbox removed. geared 16/45 with dunlop 606 rear and mt21 front.

the bike is more poweful that ever but I can't say if this is the optimum. The only symptom is that the bike is harder to start when cold. It just have a little constant backfire on closed throttle deceleration.

Oups, I forgot the dxm needle at third clip!!


For starting I would try the fuel screw at 2.5 and see if it helps. You may get a little better response out of the DXM needle by raising in 1 clip position; might want to give it a try some day.


Just two quick question.

1-Do the needle straight portion diameter influence starting or just off idle response?

2-The bike just have a small hesitation when opening the throttle quickly. Is it a good test to see if the clip is at the good position? I would try the clip at second position from that point of view but I was wondering if the bike could be too lean elsewhere.

If I put the fuel screw at 2.5 turn, the popping on deceleration diseapear completely. I heard that little backfire poping at deceleration is normal. Does that mean that no poping is not normal and a symptom of another jet misjetted (like the needle or the pilot?)

We have a big ride this week-end. I'll gear downer if possible, 16/45 is pretty fast in a tight trail!!!

I tried the second clip tonight and it looked low on power from 0-3/8. I then tried the fourth clip and it was better but it is hard to say which (the third clip in first and now the fourth) one have the best setting, I have to test and take time to feel the bike.

For the same combustion circuit (inbox and exhaust), Should the main jet be the same when changing timing from wr to yz? I mean, my mains was perfect at 172 before and I just changed the timing on the bike (jetting too). Should I expect to have to raise it by switching to yz timing?

Robin with the DXM needle when I said raise the needle one clip position I ment go from #3 to #4 from the top it would probably work a little better for you.

The starting is primarily controlled by the pilot jet, the pilot air jet and the fuel screw setting when starting without the choke. With the Choke then the start jet is in play. The needle straight section diameter has a very slight effect on starting but will work in conjunction with the Pilot, and fuel screw for between approximalty 0 to 1/4 throttle.

Snapping the throttle is not a good test for needle clip position. A minor bog sometimes cannot be eliminated. A major bog when snapping open the throttle can be eliminated with jetting and proper accelerator pump discharge set up.

A little poping on decelleration is ok but so is no poping its up to you. This minor poping on decelleration is caused by a slight lean condition when the thorttle is rapidly closed. When you open the fuel screw you are slighlty richening the fuel mixtre from 0 to approx 1/8 throttle opening so when the throttle is closed rapidly the richer mixture may be enough to not cause any poping. The same effect can be had by reducing the needle straight section diameter but other changes would have to be made. Through jetting you are trying to get the most consistent response top to bottom for your bike setup riding conditions.

In general the YZ cam timing wants to be a little richer coming onto the main jet. This can be controlled by needle taper and main jet size. With the restrictive exhaust setting you are running the main jet selection will probably not vary too much between WR and YZ timing.

With everything I seen in this discussion so far sounds like the following jetting would be vary close for your situation

PJ = 45

Fuel Screw = 1.5 to 2.5 (testing required)

Needle DXM #4 oR #5 (testing required)

Main Jet 170 to 175 (testing required)

With your big ride coming up I would make set the needle at #4, go with the 172 main and set the fuel screw a 2.0. Ride it this way for awhile and get a good feel for it and then make only one change at a time to dial (fine tune) in the remaining final jet settings.

I thought I answered most of these question last night but must have screwed up the posting process.


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