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Lord of Combustion
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Discussion Starter #1
there are lots and lots of carburetors out there....lots of differences...so i will condense the majority of the thread to info pertinent to my all time favorite 4-stroke carb... the KEIHIN FCR carb.... keihin is a subsidiary of Honda Motor Corp... and their 2-stroke and 4-stroke carbs are state of the art in performance....

they were the innovators that introduced a vacuum breaking plate slide design and also put the slide on rollers.... all this allowed for lower and lower spring pressure to reliably close a throttle on a 4-stroke.... big 4-strokes were notorious for having enough vacuum to draw and hold the slide stuck to the front of the slides bore.... against all but the heaviest of workable springs.... the more powerful the engine, the more the stuck wide open problem.... FCR changed all that....

also... for any aftermarket stuff i talk about here... you can hit up site sponsors :

KB Motorsports or Woodwards Specialties to get a killer deal on anything you decide you need....

it's going to take a while to complete.... but starting with something is the first step.... so here we go...

REMEMBER THAT STOCK IS WAY OFF when you start to mod your machine.... stock jetting is made for a restrictive closed off airbox and a terrible exhaust system... plus the factories tend to go lean anyways to help with emissions.... when you start modifying your machine by new exhaust and pulling your air box lid.... your jetting is going to be WAY OFF.... we will fix all of that by the time the thread is complete


gotta start at the beginning... so on with carb theory first... i'm going to use the word vaccuum here with some extra literary license.... pressure differential is really what i mean in a lot of cases....higher pressure in one area/ lower pressure in another causes a flow from high to low.... like the intake stroke of an engine opens a low pressure area and the higher pressure of the atmosphere outside rushes in to fill the cylinder in order to have equal pressures everywhere ...inside and out....

the carburetor is a decvice that works on pressure differentials in order to introduce an atomized / burnable amount of fuel into the air flow heading into your engine.... the finer the droplets can be "atomized" , the better the combustion will be.... big droplets only burn off the "skin" of the drop...the area that can easily react with available oxygen....

a carburetor has several different "fuel circuits".... meaning that there is a different circuit of the carb that is responsible for fuel flow depending on the amount of throttle opening....

these circuits all get fuel from the "float bowl" .... bottom of the carb....
called a float bowl because there are sealed air chamber "floats" that are responsible for keeping the bowl at steady level of fill.... they do this by pushing a valve closed when the gas lifts/floats them to the correct level... much like the back of a toilet could be called a float bowl...water rises and lifts a float until it actuates a valve to stop flow at the correct level....

now...once the gas is at the correct level in the bowl.... we can talk about the different percentages of throttle opening... and which jet circuit is primarily responsible for the fuel mixing at that opening...

FLOAT BOWL PIC
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FLOAT ASSY AND NEEDLE VALVE THAT CONTROL FUEL FILL IN FLOAT BOWL
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CLOSED THROTTLE:

throttle closed is when your fuel is controlled by the idle fuel screw... located at the bottom middle front of the carb.... affecting air fuel mixture at closed throttle / engine idling....

the fuel opening it feeds through is a very small one that you can see at the bottom of the bore on the engine side of the closed throttle slide... its straight up into the bore from the screw itself....

note :careful when adjusting it... dont ever close it too tight or you will damage the needle end or the seat...

all the way turned in is no fuel flow...and as you open the screw counter clockwise, you increase fuel flow...

on lots of machines the screw is an absolute beotch to get to... which makes it challenging to adjust while running.....

there are aftermarket fuel screw replacements that have heads you can use fingers to adjust (instead of screwdrivers) and numbers to read for reference positioning.... but all the ones i have seen have had the larger/ finger friendly head on them make them more prone to moving by vibration... and they like to vibrate loose and change adjustment... and in worst cases, they fall out...

but on my last go around, i decided to use the easiest to adjust ... as well as cant fall out style of idle fuel setup.... the RD Flex Jet... easiest way to adjust your idel fuel mix to perfection

if only all headaches could be fixed so easily...

IDLE FUEL SCREW PIC
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next up:

PILOT JET

the pilot jet is an short term intermediate circuit that takes over for a short time from completely closed throttle to about 1/8th to 1/4 throttle where the needle starts to dominate

you can think of it as responsible for the mix "just off idle".... and also a big part of fuel delivery when you have a "chopped throttle condition" higher rpms followed by a sharply closed throttle....

if you get a lot of popping at chopped throttle... your pilot is too small... you get one pulse going through being too lean for spark ignition... and the next pulse richens it up so it can light...but then that lights the lean mix that made it to the exhaust without the spark lighting it... pop..pop..pop

if your HEADPIPE GLOWS at idle, it will be either the idle fuel screw too lean...or the pilot too lean... or a combination of both.... 48~50 pilot is a common sweet spot for pilots jets...and 1.5 to 2.5 turns out from closed on the idle fuel screw is a common sweet spot

PILOT JET PIC... between main jet and idle fuel screw "down in the hole"
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PILOT JET IS TOP LEFT IN THIS PIC
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next up

NEEDLE JET

the needle has primary control over the fuel flow from approx 1/4th to 3/4ths throttle opening.... but it starts to come in at about 1/8th throttle...

the needle has different sections... first up is the base diameter...straight section of the needle that first gets exposed to the bore and vacuum when the throttle is just beginning to open.... the thichness the needle starts with is important for fuel flow at low opening and also for initiating flow ramp up when it hits the tapered section of the needle....

there are "clip positions" or grooves cut in the top of the needle that allows for the circlip that holds it in place to be moved up or down to change the part of throttle opening amount where the needle transitions from straight section to tapered section in the bore....

the tapered section of the needle being pulled farther and farther out of the main tube...allows for more and more fuel flow.....

the stock needle is made for the restrictive closed airbox and restrictive stock exhaust....

the first needle to go to when you start modding your yfz is the needle from its yz450f dirt bike brother... the NCVQ needle

I HAVE TO GET A BETTER PIC OF NEEDLE REMOVED FROM CARB... BUT THE FLAT BLACK ABOVE IT IS THE SLIDE/THROTTLE....NEEDLE IS THE THIN VERTICAL THING IN THE MIDDLE
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GOT SOME BETTER PICS .... the center ring / venturi of the carb and a needle sitting in the position it travels through the center ring in.... needle taper is evident in that it gets thinner as it is pulled out farther (more fuel flow) ...and clip positions at top of needle to change the starting height of the needle.... clip near top is leaner ...clip near bottom is richer... not an fcr needle, but you get the idea :)

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another pic of the center ring with the red pointer showing the orifice that the pilot jet feeds through into the carb bore.... when the slide is closed, there is significant amount of vacuum seen everywhere on the engine side of the slide.... in front of the idle jet orifice , in the carb body , is another tiny hole that the idle fuel screw feeds fuel to the carb bore though..

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MAIN JET

the big daddy of the fuel meterers...lol... the bottom of the main tube that the needle rides is where the main jet lives.... once the tapered section of the needle is pulled out enough.... the bottom line of fuel delivery is the main jet at the bottom of the tube....

main jet is primarily responsible for fuel flow from 3/4 to W.O.T. (wide open throttle)

NOTE.... 4-strokes can be off by pretty big amount on jetting and still seem to run fine.... lean makes excess heat and can damage an engine ...rich dampens the burn AND washes oil off the cylinder walls... neither condition is good....
in dyno tuning i have seen rich conditions that seemed to run fine while riding, but have actually been keeping the machine down by as mch as 5 full hp at peak...... you can lose 5 hp and not know it by the way it runs... above that point in rich running you will notice a "false rev limiter" where the engine cuts out as though its on rev limiter... butits actually from being too rich....

MAIN JET POSITION.... THE OFF WHITE PLASTIC THING COVERING THE JETS IS A SPLASH GUARD MEANT TO KEEP FUEL CONTAINED BY THE JETS OVER ROUGH TERRAIN
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MAIN JET IS BOTTOM OF PIC...BELOW THE MAIN JET TUBE (NEEDLE PULLS IN AND OUT OF TUBE DURING OPERATION, WHEN NEEDLE IS PULLED ALL THE WAY UP, MAIN JET IS IN CONTROL OF FUEL)...ALSO SHOWN ARE PILOT JET AND JET SPLASH GUARD
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MAIN JET CAP/ BOTTOM OF FLOAT BOWL / ACCESS TO MAIN JET.... IDLE FUEL SCREW AND MAIN JET ARE THE 2 YOU CAN SEMI EASILY WORK WITH WHILE THE CARB IS INSTALLED
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Lord of Combustion
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Discussion Starter #2 (Edited)
ACCELERATOR PUMP

great big HUGE deal on the fcr carbs....and another reason for them being the greatest carb for 4-stroke performance ... the accelerator pump...

as carbs get bigger and bigger for more top end performance... you will have a problem with throttle lag and bogging...even as extreme as the engine shutting off .... from a lean condition during large / fast throttle position opening changes....

the deal is that for each throttle position, you need a certain rpm range that creates the proper amount of airflow to create vacuum necessary to pull enough fuel through the desired circuit...

fast throttle openings means that the throttle is at a more open position before the engine rpm/vacuum has risen enough to pull fuel through it.... so a lean condition .... and bog can / will occur.... the bigger the carb to the size of the engine, the worse this problem would be....

the solution was made long ago on performance car carbs... like holley carbs.... single pumper and double pumper are common terms from the hot rodding community.... pumper means accelerator pump.... and what that is is a pump device on the carb that doesnt need vacuum to deliver fuel through it during big throttle stabbings....

the accelerator pump is like a small syringe setup that mechanically pumps a spray bottle like shot of fuel into the carb bore.... it is short term shot that provides the necessary fuel to cover the throttle position change until the engine rpm/ vacuum rises to the point it draws the appropriate amount of fuel through the appropriate carb circuit....

BOTTOM OF ACCELERATOR PUMP ASSY
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ACCELERATOR PUMP NOZZLE / SHOOTS/DIRECTS PUMP SHOT THROUGH CARB BORE/ VENTURI.... STRAIGHT INTO INTAKE PORT
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which brings us to another jet

LEAK JET

now that you know what the accelerator pump is... there is a jet associated with it... leak let... with different engine mods and setups, a different amount of accelerator pump shot will be needed to have sharp throttle response during fast/large throttle openings....

think of the accelerator pump like the syringe...and instead of the syringe having a needle on it , there is a small hose that goes from your syringe to the bore of the carburetor... the leak jet would be a changeable size jet that acts like a hole in the side of the hose...it determines how much of your pump shot will "leak out" of the hose , back into the float bowl, before it make it out the end of the hose into your carb bore....

pulling the airbox lid... more mods... bigger carb bore...bigger engine... all will need a smaller and smaller leak jet size to ensure less leak out and more of the shot is actually delivered.... in lots of cases for modified machines... a #35 to #40 leak jet end up being close to correct....

gotta pull the float bowl each tine to change this jet in the bottom of the bowl... when its right it right... but it can still be a pain to deal with...

RD and Boyesen both make accelerator pump caps (bottoms) that have an adjustable leak jet as part of them... i will cover the Boyesen QuickShot 3 in this article.... not necessary if you have the right leak jet... but what a pleasure to tune with.... sort of like "leak jets of the rich and famous" lol

LEAK JET LOCATED IN BOTTOM OF FLOAT BOWL / SMALLER JET = BIGGER PUMP SHOT
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anyways guys..... i have SOOOOOOOO very much more to cover ...TONS of pictures.... and more tuning info than you want to think about...

but i will have to come back in and fill in all those blanks....
at least i have started the 2-finger typing job from ****... baby steps :)
 

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Lord of Combustion
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Discussion Starter #3
ATHENA DUAL SPRAY ACCELERATOR PUMP NOZZLE KIT...... atomization is key in fast burn.... sredum modifies the pump nozzle of the oem part so it can deliver more fuel more quickly... this setup from athena has 2 smaller delivery holes that shoot a V pattern into the carb bore as opposed to the single stream of the oem nozzle...

the kit comes with tools... custom FCR gasket/o-rings .... smaller leak jets.... the dual spray nozzle.... and even a piece of wire and instructions for the Zip Tie / Wire Tie mod for a more direct pressure on the pump diaphragm (i use o-rings for the mod so there is much less chance of diaphragm rupture, will detail later)

the smaller holes of the V spray nozzle would be more prone to clogging from particulates in the fuel.... so i chose to run a good in-line fuel filter in the fuel line for added insurance...

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BOYESEN QUICK SHOT 3..... custom accelerator pump cover / main chamber.... the cover is the chamber that the accelerator pump diaphragm actuates on to push the pump shot from... this one has custom bleed holes to keep air out of the fuel chamber... and it takes control of the leak jet function by giving you a solid plug to put where the leak jet would go....and then the bleed off function of the leak jet is handled by the adjustable screw that is now part of the cover..... awesome way to have instant acces to changing / fine tuning your pump shot.... and not have to remove the float bowl to swap leak jets.... if the leak jet size is good... you are good... not really a need for the QS3 cover... but the cool factor and convenience factor is pretty high on this piece :)

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R&D FLEX JET.... another item that you dont really need... but the cool and convenience factor is off the charts for the price of this item.... the idle and engine idle quality is adjusted by the idle fuel screw at the bottom front of the carb.... difficult to get a screwdriver on while the engine is running (or not running) .... and the billet ones that have a finger turnable head have always vibrated and fell out on the ones i have tested.... PROBLEM SOLVED with the R&D Flex Jet.... cable operated and easy access for fingertip tuning... plus it doesnt vibrate loose ... just awesome...

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OEM IDLE FUEL SCREW AND THE FLEX JET SIDE BY SIDE

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MAGNETIC FLOAT BOWL DRAIN / MAIN JET COVER..... necessary? maybe not... but what the heck.... looks trick...and if it catches ANYTHING that could plug the pump nozzle... then it's worth $20 bucks to me.... problem was that the oem cap had radial openings to allow for faster fuel feed to the main jet area... the magnetic cap did not.... couple minutes with a drill and i was happy with the result... it may have fed fine on WOT without the holes... but i didnt want to downgrade any function from the part it was replacing....

OEM LEFT... MAGNETIC ON RIGHT.... notice that lack of open radial area between body and threads on magnetic cap

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PIC WHERE YOU CAN SEE THE MAGNET

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AND WITH THE RADIAL FUEL FEED HOLES IN THE MAGNETIC CAP

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PIC OF THE COOL STUFF YOU REALLY DONT NEED ...INSTALLED.....LOL

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Lord of Combustion
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Discussion Starter #4 (Edited)
i'm going to add in some about A/FR.... Air Fuel Ratio..... henceforth typed as AFR to save time... (Wide Open Throttle will be WOT too)

a lot of what we do with tuning the FCR carb.... or any carb with an accelerator pump system.... is going to have a lot of tuning not only dependant on the jetting itself, but during changes in throttle openings.... it will have a lot to do with what the accelerator pump adds...

remember... without accelerator pumps, you would not be able to run as large a carb as we do on todays performance 4-strokes... without having stumble and bog/stall problems going from low to WOT throttle.... without accelerator pumps , the carbs used to be sized smaller in order to keep enough vacuum through them to pull fuel better from the changing fuel circuits.... same theory as inhaling through a straw VS inhaling through a 2" diameter pipe... opne is more limiting in flow, but keeps vacuum high.... accelerator pump carbs , when set up correctly, dont need to be small to keep vacuum high... the accelerator pump tosses in enough fuel to cover the fuel needs during large throttle position changes, until engine rpm and vacuum rises to the level to draw proper fuel amounts through the metering circuits of the carb (each different jet or needle , anything that is responsible for vacuum operated fuel delivery, can be referred to as a "metering circuit")

ok... so anyways.... the accelerator pump and the metering circuits of the carb have overlapping functions during throttle openings....

Richer mixtures means more fuel in the mix... leaner mixtures means less fuel and more air in the mix...

"Stoich" or stoichiometric (will check spelling later) .... is the amount of oxygen needed to mix with fuel (diff for diff fuels) ...in order to completely burn the fuel... you can only put as much fuel in the engine as their is oxygen in the air to burn with it.... putting more fuel in than that will just dampen the burn.... (that's where oxygenated fuels add power... they are not limited to only the amount of air coming in the intake tract, they bring oxygen in the fuel , so more fuel can be put in the engine and will be completely combusted... more fuel burned= more power... BIG topic...more later)

in the early days of performance... richer mixtures were used to dampen the burn on purpose... that was one of the methods used to control unwanted styles of combustion such as detonation or pre-ignition.... combustion chambers were radically differnt too... and no TPS system to vary your spark timing based on throttle opening and rpm.... lots of reasons why 10:1 used to be considered fairly high compression....12:1 compression was definately not streetable and was race and race fuel application.... and 14:1 was scary high and dangerous compression level for gasoline engines...

now, 11~12:1 compression is stock/ normal / pump gas range.... this is because of a few things put together...like better engine controls...better/ faster burn combustion chambers... bore to stroke ratios...

anyways... it also means that we dont need to have unnecessarily rich air/fuel ratios to dampen undesirable sorts of combustion.... we can now go lean and hot and make more power safely.....

the yfz is particularly happy with leaner AFR's.... they make their best power at 14:1 AFR and slightly above, like 14.5:1 (fuel dependant)... those levels would scare the **** out of race tuners from the 70's... talk 12:1 compression ratio and 14:1 AFR in those days and you would be a candidate for the looney bin.... but now it is commonplace...

in my experience.... if i can make it happen on yfz's jetting ... i prefer to go with 13.5:1 ~14:1 AFR up to the torque peak... and 14:1 ~ 14.5:1 AFR from torque peak to the rev limiter.... that has been what has worked best for power.....

the TRX engines do best at about a half point richer than the yfz's .... actually most of the machines like 13.5:1 to 14:1 afr.... the 5 valve combustion chamber of the yfz just happens to like things a bit leaner than most....

and at idle , i like for them to sit on the dyno and purr at a 12:1ish AFR.... start good... idle happy... no glowing headpipes....

i will have to add in more as i can today....stay tuned
 

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i'm going to add in some about A/FR.... Air Fuel Ratio..... henceforth typed as AFR to save time... (Wide Open Throttle will be WOT too)


the yfz is particularly happy with leaner AFR's.... they make their best power at 14:1 AFR and slightly above, like 14.5:1 (fuel dependant)... those levels would scare the **** out of race tuners from the 70's... talk 12:1 compression ratio and 14:1 AFR in those days and you would be a candidate for the looney bin.... but now it is commonplace...

in my experience.... if i can make it happen on yfz's jetting ... i prefer to go with 13.5:1 ~14:1 AFR up to the torque peak... and 14:1 ~ 14.5:1 AFR from torque peak to the rev limiter.... that has been what has worked best for power.....

and at idle , i like for them to sit on the dyno and purr at a 12:1ish AFR.... start good... idle happy... no glowing headpipes....

i will have to add in more as i can today....stay tuned
so you would tune the TRX same as a YZF? I don't think the engines have similar combustion characteristics..EX head+ valves+ combustion chamber design..
 

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Good right up mixxer, thanks for taking the time to put it all together. One part I have always wondered about is the R&D Power Bowl 2. Looks like a nice piece, I'm sure it works the same way the QuickShot 3. But a little more bling. Haha
 

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x2 on the R&D Power Bowl 2
 

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I got one and love it..
 

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Lord of Combustion
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Discussion Starter #9
so you would tune the TRX same as a YZF? I don't think the engines have similar combustion characteristics..EX head+ valves+ combustion chamber design..
I added a bit to the original post.... the yfz likes things just a tad leaner than most others...

the trx , and most others, make best power 13.5:1 to 14:1
 

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Lord of Combustion
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Discussion Starter #12
very glad to help out
 

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very glad to help out
John . to your comments on the YFZ carb . you once told me good luck on getting that to work on a LTZ . is there something inherently different about that carb ?
 

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Lord of Combustion
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Discussion Starter #14
John . to your comments on the YFZ carb . you once told me good luck on getting that to work on a LTZ . is there something inherently different about that carb ?
nope... just jetting to work out
 

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Lord of Combustion
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Discussion Starter #16
yes
 

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Lord of Combustion
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Discussion Starter #18
cant tell you... all depends on what type of engine and what type of racing you are interested in....

this wasn't meant to be a thread for me to tell people what jets to use and what carb to use for what....

I am just using it as a way to explain carbs in general so people will know what they are doing when they want to dial in carbs... sort of a guide as to what they are looking at and looking for....

I will ask you what dyno work have you done and what have your experiences been with different size engines with different size carbs so far??

my area of work is in track based endurance racing engines... motocross / hare scrambles / woods / flattrack....

drag racing is not at all where my interests lie.... so you will get better drag race info from shops that specialize in drag racing
 

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I have been battling this carb now for awhile and I am hoping I almost got it. Thanks for all the input everyone has given me so far. I am going to be an expert in the FCR carb by the time I'm done.

I've been having trouble fine tuning my 42.5 FCR Sparks Carb. I started with with a 200 main, 45 pilot and NCVQ needle 4th slot from the bottom. It was hard to start hot and at WOT it seemed to be "held back" like too much fuel was coming in. I was most concerned with the hard starting hot and checked my valves multiple times, not it. Spoke to ghott because he had a similar issue; his pilot circuit was too rich because needle was too high and main was too big. I dropped (moved clip up) one slot and dropped my main and seems to start much better now hot (thanks ghott). Now, from idle to 3/4 throttle runs great, but WOT falls on its face with a 192 main, and major popping on decel. I'm pretty sure I can solve it by going up to 195 (I think), but my question is with the pilot. I have a 40 in there now and AFR at idle is in the 15s and screwing the a/f screw in or out doesn't do much. When it is all the way in it idles higher and AFR drops a little. And after running it up and down the street a couple of times my 1/8 throttle becomes choppy, but idles fine. All this tells me I need to go back up on my pilot, right?
 
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