How do I figure out what jets to put in my carb?

wingstar

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Fairly certain I read somewhere in this forum that if you get new air filter or exhaust then you have to put different jets in your carb. How exactly do I know what jets to put in?
 
You usually prove out the need through trial and error. Beginning with whether you have the correct ones to begin with, so many people with no logic tend to mess with such things thinking they are 'doing something'. They are all right, messing things up. I for one am firmly convinced that screwing carbs up on bikes is a male puberty rite of passage thing of some sort.

CV carbs like these bikes have tend to let you mod somewhat with no changes at all, they self jet so to speak. The CV quality lets them self adjust jetting for a couple of steps anyway.

Not nearly all mods lead to the absolute need to mod the jetting upwards in the usual thought of way and some mods even lead you toward lowering jet number, telling you your mod likely cost you power. Many cannot accept that, it being drilled into us the jets have to change up.

One other CV trait that can throw one for a loop..........the same ability to self adjust can make an unuseable jetting setup seem to run for a while until the motor carbon coats in fully to the new setup, then and only then will it begin to spit its' guts out. Meaning the new jetting you did that 'runs perfect' that day will likely not be what you suspect is wrong when a month later the engine begins to run into blind staggers at getting on it. Will somebody be able to put 2 and 2 together on that later?

A tip.........most never make the connection, and bike is then doomed to run like crap until next buyer gets it.
 
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You usually prove out the need through trial and error. Beginning with whether you have the correct ones to begin with, so many people with no logic tend to mess with such things thinking they are 'doing something'. They are all right, messing things up. I for one am firmly convinced that screwing carbs up on bikes is a male puberty rite of passage thing of some sort.

CV carbs like these bikes have tend to let you mod somewhat with no changes at all, they self jet so to speak. The CV quality lets them self adjust jetting for a couple of steps anyway.

Not nearly all mods lead to the absolute need to mod the jetting upwards in the usual thought of way and some mods even lead you toward lowering jet number, telling you your mod likely cost you power. Many cannot accept that, it being drilled into us the jets have to change up.

One other CV trait that can throw one for a loop..........the same ability to self adjust can make an unuseable jetting setup seem to run for a while until the motor carbon coats in fully to the new setup, then and only then will it begin to spit its' guts out. Meaning the new jetting you did that 'runs perfect' that day will likely not be what you suspect is wrong when a month later the engine begins to run into blind staggers at getting on it. Will somebody be able to put 2 and 2 together on that later?

A tip.........most never make the connection, and bike is then doomed to run like crap until next buyer gets it.

I don't really plan on doing any mods to this bike engine wise, but I need to replace my headpipes and get new mufflers since the ones I have aren't even for this bike
 
You should know that the OEM mufflers are big because bigger allows more space to kill sound in. Commonly replacement ones are smaller and end up being louder as a result, if not then they are often less efficient than OEM and that alone often kills any jetting increase as unneeded.
 
You should know that the OEM mufflers are big because bigger allows more space to kill sound in. Commonly replacement ones are smaller and end up being louder as a result, if not then they are often less efficient than OEM and that alone often kills any jetting increase as unneeded.
Can I get away with not changing the jets if I replace the exhaust system then?
 
Likely, if it is not like a good breathing true header. They need to be the correct jets for the bike though, if somebody has messed with them that would not apply.
 
You usually prove out the need through trial and error. Beginning with whether you have the correct ones to begin with, so many people with no logic tend to mess with such things thinking they are 'doing something'. They are all right, messing things up. I for one am firmly convinced that screwing carbs up on bikes is a male puberty rite of passage thing of some sort.

This^^^^
Not on a bike, but my "85 5.0 Mustang.
I put on a freer flowing Edelbrock Performer RPM intake, ported and polished the heads, 1 5/8 shorty headers, and thought I would upgrade the old Edelbrock 600 that I had on it to a shiny new Demon that was all that and a box of crayons. I fought with that bigger Demon for six months before I slapped the old Edelbrock on it, adjusted the mixture screws and never missed a beat.
So I didn't mess up with a set of bike carbs, but 20 year old me did learn his lesson on a car. I do believe it is a right of passage...kids today though...do the same thing with a tuning program they think they can manage, end up with melted pistons and some even go as far as to put in a "Cam tune" where they purposely make the car miss on certain cylinders intermittently to make it sound like they have a lumpy cam. Then wonder why there car runs like crap after they get sick of the fake sound and return the car back to a normal tune.
 
When you mod the entire combination must be pieces that will complement and work well with the others.

If that Mustang had come into our shop I would have convinced you to lose the shorty headers (which are not headers at all) and the Demon carb was likely too big.

I had a true AMC AMX two seater that I put a 150 hp. 304 in for a joke and on a dare. 8.5 compression and the combustion chamber mods lowered it likely to 8.0 or so. BIG hydraulic cam @ .540 lift and Edelbrock Torker AMC which was the same as a Tarantula on a 304, limited to top end power, and I modified it on top of that. 1 7/8" Hooker full length headers for a 390-401. The heads were modded a whopping amount by me but still to use the crap 1.78 and 1.40 valves that came in the downstepped valve heads that 290-304 used. I used a modded by me 800 cfm Holley DP. The combo sounded like it chewed rocks when started up with headers open. I capped it back up with the full length turbo muffler exhaust and the car would not even untrack the rear with 5.0 gear and a Ptrac. It was deader than a doornail. WTF??????

Got to thinking about it at home and got mad at all the wasted work and went out to car and began to cut the tailpipes off it. The exhaust setup had been supposedly a 'trick' one but I put direct header mufflers on it alone and no tailpipes and barely able to put it on the street like that. The car thumped so hard at idle it almost shook front wheels off the pavement, VERY radical. That exhaust cutting was a monster game changer, suddenly the engine began to rev like it was made to, before only hitting a brick wall at 5000+ rpm, now it went to 7500 with ease. With a 4 speed Borg Warner T-10 and the gear the car would literally haze the tires until you got into maybe halfway through 3rd gear. Uncapped 1/4 mile with short 28.5 tall X 10.5" wide slicks and shifting at 7500 the car instantly began busting off low 12's and then 12.0's, then getting serious had it running in the high 11's at close to 120 same night. The biggest problem on the leave was the wheelie the car would yank, it was spending too much time going up instead of out and an AMX specific problem because the cars are so short (96" wheelbase). It dropped into the 11's by killing bite with more air in the slicks and tightening up the front shocks a bit.

That full length tailpipe killed at least 100 hp. off that particular engine because it was so radically tuned for free-flowing breathing at top rpm. If I had put big valves in it and bumped the compression up I would have had an 11.50 car easy. The motor came out of a Rambler station wagon, that was part of the joke. My guess is that it made an honest 375-400 hp.

The more radical you get the more an engine will puke its' guts out if even one part in the combo is seriously mismatched. I've seen it done since then so many times I cannot count.
 
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Just saw the date on the original post... oops. Well I've already written everything so I'll leave it here.

The main thing is doing only ONE CHANGE AT A TIME. I can't stress enough how important this is. Fortunately we're dealing with a twin and not a four-cylinder... that starts getting old fast! Make sure you're starting with the factory spec for your bike; typically it's 135 for the Main and 42.5 for the Pilot.

A few ideas:

1) If you know what size jets you need, go ahead and buy the next few sizes as well. You'll probably need them in the long run and you'll save on shipping and waiting time as well. You may even be able to swap some sizes with a member here.

2) Get a spare set of carb gaskets. Sometimes when you pull the bowls, the gasket is swollen or you tear it or... having a spare gasket set on hand means you can keep working. It's cheap insurance, then you have extras for the next time you need them. Gasket 3F7-14984-00-00

As far as your exhaust/pipes/filters/jets go... it's just going to take some time to get it to a place you like... but that's time well spent. You'll also know your carbs really well when done.
 
I've found that it is extremely unusual to ever need to mess with the pilots even on highly modded engines, yet I see people do it all day long. The pilots if right will be too rich at mixture screw open too far and too lean at cut off, modern EPA settings can make you slightly lean of course. If you by the odd chance need more pilot, one size up bigger and STOP. If you are convinced you need more you have something else WRONG.

One thing that is a solid mistake is using the pilots and mixture screws trying to set fuel higher in the powerband, seen a lot of it mentioned here lately. If you have a smooth idle and no balk or bog at just off idle then leave them alone! Why? Because it can mess you up higher when you open them too much trying to get engine richer up high, then the artificially too rich bottom end soaks the plugs to wet good and black and then you are lost, so many of you cannot recognize that at all. You do NOT run the bike at too hot all the time and the cruise then kills you to wet the plugs as cruise is mostly off pilots and low needle. So, the plugs wet and do not burn clean enough to stop missing and hard starting as well as loss of cylinder(s) and cold pipes.
 
If the carb gaskets are truly such I've been making my own for years out of 1/16" thick bulk rolled up gasket material from the car parts stores.A leather hole punch like found at a craft store will help scads too as well as a Xacto hobby razor knife. I even made complicated inter carb body and TB baseplate gaskets for like 4 barrel carbs and they worked perfect and saved the cost of a $40-$50 carb kit. You can buy one new gasket and there is your one to copy off of.
 
No reason not to be though, the skills there are not hard except for maybe the patience. So many though have that as the major stumbling block. I've saved thousands simply looking at a part a couple of minutes to say 'I can do that too' and rebuild the part often better than OEM and for sure cheaper and often at rates so high per hour I never would get paid that in the real world. Like using thirty cent washers to bring slipping automatic transmissions back to life for 20 more years. After the usual 'you need a new transmission' advice from a shop, and what they all say now although commonly not needed.

Nobody wants to do the common 'fix only the broken part' any more, they all think the shops will go broke if they don't sell the entire motor or trans or whatever. There's a whopping space in there you can put yourself in to make literally hundreds of thousands of dollars in, I've done it for 50 years and you don't have to be the smartest guy to do most of it either.

FYI, on bikes like Honda that use a shaped big rubber o-ring instead of gasket you can actually use a created paper gasket OVER the old rubber o-ring left in place as a 'groove stuffer' of sorts for support and works fine for years, BTDT. Some of those rubber o-ring bowl gasket prices will KILL you, they are almost unobtanium. You just have to pay attention to paper gasket fine detritus or whiskers hanging off the holes you cut or punch, they coming loose will clog jets. I even use paper on some car o-ring locations as well again leaving the original o-ring in place even old and hard, even better.
 
Agree. Sometimes cutting your own gasket makes much more sense. And yes... having a spare set of gaskets probably applies moreso to rubber gasket bikes (like Honda). My first bike was a 4 cylinder Honda; I remember pulling the carbs and replacing jets, only to find upon reassembly that the gaskets were far too swollen/large to be reused right away. Live and learn.
 
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