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.

 

hold on though, whats the scoop on your setup? if your not running pods, and all you did is modify the mufflers by putting on straight pipes, then you should be fine, from wha I've heard, and understand, the collectors provide most of the back pressure, and taking the pipes off wont hurt, it sure didnt on my maxim, ran like a champ

 

no pods. Just straight pipes. Ran fine before, loses power now at medium to high RPM. Still runs and I can ride it, but it is definitely running weaker.

Idles fine. Accelerates a little weaker than before, and cuts out as I get into the higher RPM range.

Could this be caused buy a leak in one of the boots on the engine side of the carbs? The reason I was thinking it was the mufflers is because this happened right after the mufflers were removed. Prior to that the bike ran strong all the way through.

What are those boots called? Not the ones going into the airbox, but the ones on the other side of the carbs. How hard are they to replace, and do they require gaskets or anything else like that?

 

Mike, send chacal a PM and ask for a quote on those boots/sleeves, he'll guide you in the right direction. Are yours hard or soft?

 

they seem hard, harder than the ones that go into the airbox.

 

u listening to the wrong bark of the wrong dog bro. go with your instincts. it could NOT be the manifold intake boots as it didnt haeppen BEFORE u took off the baffles and collector. Start with things that are CHEAP before you run to spend money. Try the chore boy idea and secure them with some cheap clamps and steel screens so they dont blow out. if this helps the the detective is on the right trail. Listen to Rick and some o the other pro's before u listen to an EXHAUST GUY. They KNOW these bikes. TRUST THEM>>>!!!! Timetoride has helped me on this issue as i wanted to go with pods but it appears this is a DEMON IN HEAD idea. Rickcomatic is the tech guy here,take his advice. Dont go hunting the shark in the woods. there is only grizzly bears there.

 

Yeah, I am gonna go spend like $4 at home depot and see if it changes anything.

I am throwing mufflers back on friday either way. The novelty wore off rather quickly, I am paranoid about getting pulled over as it is, I don't need anything drawing more attention by the cops anyway.

Probably replace the old boots anyways if it isn't super difficult.

 

as u wish but those boots are super thick and will last almost forever unless u damage one removin em. they are a hard ruber composite and am unsure if they have metal interiors but i think they do they are kinda heavy for such a small object. maybe they are wrapped in a composite rubber coating ijust dont know 4 sure.

 

thanks. I will lift the tank and get a better look and see if there is any sign damage. maybe they are good.

 

mike how good are you at drilling steel bolts out of aluminum heads??????
I ask because thats what you will be doing if you try to remove the intake boots. If they do all come out go buy a lottery ticket one for you and one for me :lol: . The boots can be repaired with a coating of high temp black RTV gasket maker. Do you even know for sure that they leak. Try the propane vacuum leak test. And yes they do have metal in the flange area.
Good Luck,
James

 

Why does everyone always say things like "Your bolts WILL BREAK OFF"? I removed all four of my carb to manifold boots and not a single bolt broke. Matter of fact, I only even had to use penetrating lube on one of them.

I've been telling everyone on all my forums recently, keep an eye out (VERY CLOSELY) for frozen/stuck bolts, as drilling them out isnt too much fun, but unless you're attacking the engine willy-nilly with an air-wrench to take it apart, you should be able to feel a stuck bolt before you break it off.

On-topic, if you do major exhaust/intake mods, and don't rejet, you COULD burn a valve, melt a hole in a piston, or any one of a plethora of bad things. The best part? The bike will probably run stronger than you've ever felt it while it's doing it. Engines LOVE to run lean .... it's just horrible for them.

 

hi switch I know what you're saying about the bolts and all, I fell that if you tell someone that they'er going to break them off they will be more carefull and hopefully won't break them off. I'd rather see a person write back "HA GOTEM ALL OUT" than "dam what do I do now?" (ya know what i mean)
See Ya,
James

 

Lowlife, I can understand that, it still irks me though. I'd rather see people saying things along the lines of 'go nuts, but be warned X, Y, and Z could and probably will happen, so you might have alot more work ahead of you' etc, etc etc.

I'm a member on several boards, so I apologize if anyone took personal offense with my comments, it was not directed at anyone in particular, just that this is a very common trend on any motorcycle community I'm a part of. It gets rather old.

 

No offence taken Switch, good lookin out
James

 

I have a K&N air filter in the stock air box, 4-1 exhaust, shaved head, I run 8mm wires with new Dyna coils and I have neber opened my carbs. I have had fuel/air mixture tests done and also used a Colortune plug and it has all turned out the way they say it should. Changes made on the intake or exhaust of a bike may not be what causes the problem, sometimes it just amplifies it and you notice it more. Maybe I just got lucky but I would never reccomend changing jets just because someone says you will probably have to. Make your changes, do the tests, come up with your solution. Every bike is different. Make sure your bike is in solid running condition before you go changing everything around.