Need jetting change with new header?

You never need to change jetting with exhaust upgrades. Anyone who says different does not know how a carb works.

Opening up your air box or a less restrictive air intake is another story and is only affected under wide open throttle and high rpm's

Idle circuits are unaffected

Altitude changes are the only occasions jetting of both will need to be changed and if your jetting is spot on for your altitude it likely won't need to be changed unless your altitude changes by 5,000 feet

Otherwise you'll be "close enough" unless you race and are looking for optimal performance


Way to many variable for this statement to be true.

You are assuming that the vacuum affecting the jets is in direct relationship with the change in exahust scavenging, which is not always the case.

Way to many variable for this statement to be true.

You are assuming that the vacuum affecting the jets is in direct relationship with the change in exahust scavenging, which is not always the case.

i disagree, it doesn't make any difference if you have increased exhaust flow by scavenging or by less back pressure. the fact is the air fuel mixture is only affected by the resistance to air flow through the filter.

furthermore a bike at idle places less demand for air flow through the filter verses WOT

so if you have resistance in the intake tract it will only be seen at the highest demand for airflow at WOT and high rpm which you ride at maybe 1% of the time if ever!


even if you connected a shop vac to the engine side of a carb you would still get the same air fuel ratio because the ratio will be determined based on the jetting at any given throttle position.

unless you change the density of the air (which changes with altitude and air temperature) or have restriction in the airbox your ratio will not change.


if your filter clogs your mixture will get richer because now you've changed the pressure inside the throat of the carb and is now less than the barometric pressure acting on the fuel in the float bowl. therefore your going to get more fuel.

on the exhaust side of things, the carb sees no difference only that more air is flowing through it which does not change the mixture at all.


people who live with the idea that every change they make requires a jet change are only kidding themselves. its really just a placebo effect. fact is you can be anywhere near 14.7:1 + or - 2-3 parts before your going to notice any engine performance change.


when i had my honda CRF 450X i jetted it with the JD jetting kit and it started and ran perfect at my altitude 6500 feet as well as on the top of taylor pass at 12,000 feet and also ran perfect in Glamis sand dunes at close to 0 feet!

never changed a damn thing!

so if anyone thinks a jet change is needed because you opened your exhaust and made a 1 hp improvement by freeing up exhaust back pressure and increased scavenging, you really had your jetting way off to begin with or may just need your head examined.

not to mention the fact that exhaust modifications will only be noticed at WOT anything less than that is a placebo effect because you spent 400 bucks on it, you want to believe that money made you some power.

i bought my Ti exhaust because i wanted a spark arrester, and i wanted to save a few pounds in weight. not because i thought it would give me any power gains because they are negligible. 

Ok, new question. This is a problem that has seemed to arise since I did the top end rebuild, and changed jets in the carb the first time. Now that it's back to what it was before I thought it might be fine, but here's my issue.

On a cold start with the choke out, when the bike fires up, it won't have a very high idle until I blip the throttle, then it will idle like the choke is on. Why? I believe this is what makes it take a couple kicks when cold to start

Bass, are you saying bore/stroke, cams, headwork don't change fueling at all either? How about 2 strokes, does porting/pipe not change it at all?

Bass, are you saying bore/stroke, cams, headwork don't change fueling at all either? How about 2 strokes, does porting/pipe not change it at all?


The core of his original point is that by and large, changing the exhaust on most bikes like a YZF won't require a jet change for exactly the reason he's laid out, which is that the crab delivers fuel based on the air that flows through it, and a moderate increase or decrease requires no correction. That's true.  In general.  It's also true that if the jetting on the typical 4-stroke is optimized for any altitude, the bike will generally be fine at altitudes as much as 5000 feet higher, or 3-4000 feet lower. 


What isn't true is that this remains axiomatic across an infinite range of air flow, and eventually the carb will start to drift either rich or lean depending on a number of things, including the main and pilot AIR jets.  What also is not true is that all custom exhausts change the volume of air throughput.  Compared to stock systems on most YZF's, aftermarket exhausts make only minor improvements in a part of the rev range, if they do that much. 


Please understand that when you start in changing cams and such that you are introducing factors into the mix that actually change the dynamic airflow behavior in the intake, which as one effect at part throttle and low speed, and another at full throttle and high speeds, and that does usually call for jetting changes.

Bass, are you saying bore/stroke, cams, headwork don't change fueling at all either? How about 2 strokes, does porting/pipe not change it at all?

Correct, on an FI bike you would have to increase the FI duration because the injector timing is programmed based on the amount of air expected to be in the engine and since you have total control of the fuel duration IE fuel ratio you would have to program the ECM to deliver more or less fuel across the entire RPM range and throttle positions to compliment the additional air needed to fill the cylinder. the AIT and MAP sensors will give the ECM the info it needs to calculate air density of the air and with that info the ECM dictates at a given rpm and manifold pressure how much fuel is needed. since it is programmed for a given engine size and with a given injector size and given fuel pressure the injector "on time" can be calculated for that engine. if you went up in core and stroke you would either need a larger injector and / or increase the injection pulse duration to accommodate the larger displacement.


on a carb, fuel is delivered as a function of air drawn through the venturi but limited based on the jetting. the needle of the carb is a valve essentially that opens and closes based on the position of the needle and the taper of the needle to regulate the fuel flow at any throttle setting.

a different needle can make it rich at the wide open setting for example but leaner at the midrange. the same needle moved up or down a position can change the entire mixture but keep the profile the same.

a carb will to a degree compensate for air density because the more air that is flowing across the venturi will in turn deliver additional fuel since the higher the volume of air the more vacuum is created to draw the air out of the jets.


in any case since the fuel delivery takes place as a result of air being drawn across the venturi, fuel will flow to fill a larger cylinder or a smaller all the same. if you bore and stroke a bike it will draw more air past the venturi for each intake stroke. since the mixture is a function of the jetting, fuel will flow exactly the same whether your intake stroke is a short pulse or a constant suction such as from a vacuum hose. the fuel ratio remains the same!

why would it change? since the air draws the fuel in, anytime you have airflow, you have fuel delivery! the more air you flow, the more fuel you get it's all in proportion.

of course a larger carb can flow more air and fuel with less restriction than a smaller carb can. make the carb too big and you have less ability to control fuel flow because the venturi becomes less effective. you have to have a restriction in a carb as it passes the venturi for the Bernoulli theory to take place. the air has to speed up as it passes through for the Bernoulli effect to take place.


the only reason an accelerator pump exists on a carb is because when you whack the throttle open, and air being less dense than fuel, "meaning it can move faster than fuel" you'd end up with a temporary lean condition until the fuel starts to flow. the air will rush in way faster than the fuel can be supplied through the regular jets since it has to be drawn through a tiny hole and has more mass than the air. it just simply cannot react that fast so additional fuel has to be sprayed (in an correct quantity) to supply the additional fuel to stay consistent with the additional in-rush of air. if you open the throttle slowly you'll notice that the AP pump delivers little to no additional fuel, this is why the leak jet exists, the additional fuel that would have been shot into the throat of the carb gets leaked back into the float bowl rather than enriching the mixture going into the engine.


on a 2 stroke there is 1 additional variable to take into consideration, if you mix oil in the fuel verses an auto oil injection bike you could make the bike leaner if you mixed too much oil in the fuel. but on a 2 stroke a fair amount of fuel is also wasted out the exhaust pipe and never burnt! I have not played with 2 strokes a lot but I can imagine that they may be a little more finicky with regard to fuel mix and due to the fact they have oil in the fuel they probably don't run at STOICH 14.7:1 the same as a 4 stroke does because the oil itself is a factor in the combustion process.

personally I would love to see a 2 stroke become commonplace that is direct injected into the cylinder! this would make for some wicked torque and a much more fuel efficient engine since all the fuel is burnt because it has no way to escape through the exhaust port.


all a cam timing and lobe separation does is create better airflow and for a carb this is no issue as I explained earlier, if the air flows, the fuel flows simple as that. however on a automobile they incorporate a MAF sensor in the intake. this sensor can detect the amount of air flowing in grams per second, and it can tell the ecm how much air is flowing with a high degree of accuracy and therefore fuel can be added as needed, (to a degree) a good tuner will of course optimize the fuel for a given cam operation since the RPM if the engine will cause the scavenging effect to be optimal at some rpm's and not optimal at others. also the mixture can be verified correct and adjusted by monitoring the exhaust gasses at the o2 sensor (closed loop operation) since a bike has no MAF sensor it's fuel delivery has to be programmed and may not always be "perfect" but since no emission's are needed it will be "close enough"


keep in mind, few people who tune bikes or cars have every put an o2 sensor in the exhaust to verify the adjustments they made were necessary or correct. unfortunately in this industry you have a lot of people who tune things themselves and usually tune or jet un necessary because of something they read on an online form or heard from a friend who supposedly knows how to tune.

you can talk the talk all day long that you know how to tune but unless you've verified your jetting changes were needed and your adjustments verified with en O2 sensor you may have just made the bike richer or leaner without knowing any different. it will still run fine because to a large degree it does not have to be at exactly 14:7-1 and will still run just fine anywhere from 12-16:1 without issue.

without anything external to tell you where you are mixture wise how do you know if you were spot on before you changed your jetting or too rich or too lean?


when I fly airplanes we monitor fuel mixture based on cylinder head engine temperature, since we don't have an o2 sensor and the engine runs at a constant speed we have to change mixture based on our altitude. we fine tune that mixture based on cylinder head temperature. if its too hot, we richen it up, if it's too rich, it will be colder and likely sputter and we lean it out. sometimes we run it lean until the temperature gets to its peak and lean it out 50 degrees cooler, this is essentially a very lean but fuel efficient way of running your engine we call it "lean of peak" for endurance reasons.


sorry for the long rant, but maybe you can find some value in this post to help you later on.

happy trails.

Sorry, but changes other than air filter can require changes to jetting on a carb'd bike. It'll typically still run ok if the changes aren't huge, but AFRs will change. Carbs are about the only mechanical calculators still in existence and while they're pretty good, they aren't perfect. 


Quite a few people run widebands for tuning, it's hardly uncommon. I borrowed one from a friend to work out jetting on my truck. 

Maybe on the YZ a main jet change is not required.


On my WR,  change from a YZ pipe  to a Dr D full stainless with Baja header required a change from 160 main to a 170 main, or there was a near WOT flat spot even at top rpms.

A 180 main made it gurgle at early wide open throttle.

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