What happens if you don't re-jet carbs?

If you run straight pipes and don't re-jet, how does the engine perform?


If you put pod filters on and don't re-jet, what happens?


Will it run? Run like crap? Cut out, idle rough?

 

All of the above possibly. Pods are the DEVIL on our bikes. The whole rejetting process is uber tedious I've been told. I've known four or five people to get it right out of the hundreds that have them
A good rule of thumb I've been told to follow, is to go two jet sizes up. Seeing as you'll be getting more air faster through your engine, you'll want the right amount of gas too.
Throw your bike in your sig and post up your location bro
-Chris

 

what about replacing stock mufflers with shortys? Do you think that would that require rejetting?

 

Thought about that too. Check it.
When you change your exahust you are changing the pressure on the "Intake" side. So, shorter mufflers would cause a larger pull through your air filter.
SO!
Getting an airfilter that lets less air in would not require re-jetting. Feel meh?
Like, the UNI filters let more air in than the stock paper filters. So I had to colortune and sync to compensate. Get what I'm sayin?
I'm gonna do a new exhust in a year or so but its gonna kinda be like straight pipes. 4-4
-Chris

 

I've thought about this a lot and it really doesn't make much sense. Your intake and exhaust valves are never open at the same time. All the exhaust does is move the exhaust gasses away from the motor so it doesn't choke it out.

Suction is caused by the vacuum as the piston drops. If the exhaust valve was open, it would suck air/exhaust gasses through the exhaust port and choke out the engine.

If anything, you'd think that SMALLER pipes would cause a problem as the engine has to work harder to push the exhaust gasses away.

I could be wrong, but really, it doesn't make sense to have to adjust the intake side when it's a totally separate system from the exhaust.

 

Also, check this out

http://en.wikipedia.org/wiki/Back_pressure

Sort of explains what I mean a little better

 

Generally, no.

 

You end up trying to explain to 90% of the people that your bike runs great without rejetting, and getting angry when they don't believe you.

 

the collectors provide alotof the back pressure, I had my mufflers off for a little bit as i was working on my sportsters, and bike ran just fin

 

so then tell me what is the REAL purpose of jet kits for then? if we cannot add pods and increase fuel to the compression chamber to correct air flow issues whats the point? increased HP's? More massive pull off the line? i am drawing a blank and confused ever so much more

 

The "REAL" purpose of Jet Kits is to make money for the company that supplies them.

There is a raging debate about the effectiveness of Jet Kits in solving the problem of incorrect Air~Fuel Mixtures after abandoning the stock Airbox design on our XJ-Bikes.

Some guys who bought a Jet Kit back in the early 90's, when there was a buzz about Jetting, after a article in CycleWorld about a Chrome plated XJ-Bike made to go faster, teased guys into yanking-out the Airbox and: "Throwing-in a Jet Ket"!

Some guys managed to get it right.
Most are still trying!

 

jhicks13, here ya go, read about "valve overlap"
http://www.btc-bci.com/~billben/camlore.htm

 

mufflers create back pressure that affects the amount of exhaust that passes threw them and in turn affects how much air that the intake pulls into the piston

 

Yeah, but isn't opening the exhaust usually not enough to need rejeting? I heard that if you open up the exhaust you can usually just adjust up your fuel misture screw on the carbs and should be good. Unless you do pods. But between the naysayers and the go for it guys on the pods issue, I think it all depends on how good of a job you do. I don't think it's imposibble to get the carbs running right after switching to pods either.

 

I have a stock air box with a 4 into 1 exhaust and had to go up two main jet sizes and shim the needles before the bike would quit running lean at 5k rpms. i never noticed a performance issue, but my plugs told another tale. i think i need to add a bigger main air jet also, as now the bike stumbles a little bit when transitioning from running off the pilot circuit to running on the main circuit.

don't change your air box. don't change your exhaust.

 

When you make a Mod to the Stock Exhaust. If you Mod reduces Back pressure, ... you usually need to Re-Jet.

On street Machines, and Bikes run at lower rpm's, the Bike needs some back pressure. Having the pressure is good. Exhaust back pressure keeps the Air~Fuel Mixture to the cylinder from being pulled out during valve overlap. On Inline-4-cylinder engines, the exhaust pulse from one cylinder can affect the Air/Fuel Mixture at an accompanying cylinder.

The backpressure from the manifolds, pipes, collector and mufflers helps to even out the exhaust flow, for better low and mid range power.

Before you decide to abandon your Stock Airbox because someone said they think you can get better performance along with better looks on the Bike.

Tell them you've read that adding Pods on the Bike is like trying to solve a Rubic's Cube.
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>, ... BLINDFOLDED!

 

rejet can depend on engine condition, altitude, and air filter

think of the engine as a pump if the supply is restristed or the delivery then that is the amount that the pump will move

 

Ok here is the situation;

I took off my stock mufflers, after the collector thing that all 4 headers go into, so basically I have 24" straight pipes welded on where the mufflers WERE at.

After this procedure, the bike seems to be running ok at low/mid RPM's but at high RPM's it seems to kind of sputter out like the engine skips a beat and I feel a loss of power.

The guy who welded the pipes on for me says that he thinks that there is a small crack in one of the boots on the engine side of the carbs, but since the bike was running fine before the straight pipes, and weak at high RPM's after the straight pipes, I suspect that the lack of mufflers has adversely affected my bikes performance.

Unfortunately, I have no way of just popping my mufflers back on I can't test this theory.

I was thinking of some slip on shorty mufflers if and see if that solves the problem, but if it is a leak on the boot, I don't want to be barking up the wrong tree.

So I guess my question is;

What is most likely the culprit of my loss of power at high RPM, a leak in one of the boots on the engine side of the carbs OR the lack of back pressure that the stock mufflers was providing?

 

No Back Pressure.
Exhaust Flow robs some of the Intake Mixture in the Overlap ... causing a Lean and a Less Fuel Mixture.

 

To fatten-up the top end of fuel delivery only without messing up the rest of your performance range, that you are happy with?

Without changing the main jets, you re-shape and thin-out the last inch of the needles on the sliders, and after buying an extra set of control springs, you clip a coil or 2 which makes the sliders slide farther.

You would be blindly experimenting to tune your carbs to work with your other mods. Nobody can tell you how much thinning and clipping to do.
You would proceed in small steps and take notes.

You can test your back-pressure theory with aluminum screening and hose clamps. (tacky, but temporary) If that works, you can secure some pot scrubbers in your straight pipes.