The hex head jets your carb uses are numbered in terms of "bulk flow", at home I think I have a list somewhere explaining it all clearly, when I get home tonight I will see if I can explain clearly.
Forum whore
The hex head jets your carb uses are numbered in terms of "bulk flow", at home I think I have a list somewhere explaining it all clearly, when I get home tonight I will see if I can explain clearly.
Forum whore
I was going to scan the sheet I have, then thought I would just type in a few lines of the text to see if it was on line, low and behold it was..... some wording has been changed to show the guy did the tests himself, but the basics of the information is what I have here........ (Don't worry that it is written by a two stroker.....the concepts regarding the flow rates of the jets in comparison to their number are applicable to whatever engine you like, 2 or 4 stroke
http://rd500lc.free.fr/technics/jet_size_fr.htm
Richban, after reading the article, you may well see how and why I believe your current main jet is too big.....look at it as a %age increase over the original...........
I also believe the following attachment will aid you in your quest......
When I am having trouble setting up a carb, I like to mark out the slide position on the throttle tube (1/4 1/2 3/4 1) so I can better see where I am with my slide position in relation to RPM.
Also, while it is far from scientific, when I am setting up an engine with a carb that has the hex jet system, and I am unsure where to start, I simply use a comparison to a carb on a similar engine, that uses the mm numbering system.
Basically, I mean, just consider the numbering system of the hex system as DOUBLE the .mm system
Nowm, I know this is in no way scientific, but it is a good starting point.
For example,
I know that (xxx engine, with blah blah exhaust, and a Dellorto carb uses a 145 mainjet.
If I simply change carbs, to, say a Mikuni that has the bulk flow system, I will start the dyno using a 290 hex jet (double the .mm size)
This is a good "ball park" figure.
sure, I will more often than not have to go slightly leaner, but it is normally quite close.
Using this "system", your 540 would equal a 270 (2.70mm) .mm system main jet.
I'll put it this way, a 42 PS Honda RS125, running a well designed pressurised airbox, achieving over 200km/h runs about a 210 mainjet (2.10mm), (but more often than that less....180, or maybe 190 (1.8, or 1.9mm), I really don't think your FXR150 is going to need that much fuel!
Experience tells me on an engine like yours, I would start at around a 120 main (using the "rule of half" equals 240 in the hex system, which, ironically is where you started!)
Plenty of us have fooled ourselves into thinking we have created an engine than nothing short of fuel supplies rivaling the atlantic ocean would suffice, only to find that, in the end, only 2 sizes bigger mains where required.......... or even not at all.....
By the way, a few years ago, I too was fooled into thinking I needed to go to 200 mains in a 450cc engine, turned out the needle tapers where wrong....even fooled the lambda sensor! in the end, after getting that right, the mains ended up at 128, up from 125
Forum whore
Hi Rich
From Wickipedia
"The carburetor works on Bernoulli's principle: the faster air moves, the lower its static pressure, and the higher its dynamic pressure. The throttle (accelerator) linkage does not directly control the flow of liquid fuel. Instead, it actuates carburetor mechanisms which meter the flow of air being pulled into the engine. The speed of this flow, and therefore its pressure, determines the amount of fuel drawn into the airstream." From http://en.wikipedia.org/wiki/Carburetor
Having thought about it, it could be your 520 jet is now the right size. A carb works by having a narrower section (venture) in the throttle bore where the main jet is. Your carb should be slightly smaller in the middle than the ends.
As pressure times area of the main jet determines how much fuel is discharged, in having "D" slotted the carb you could have reduced the venture effect to the point where you need the area of a really big main jet. Its all about hydraulics.
The problem with a reduced signal at the jets, is you get a wishy washy response when you open the throttle and the engine won’t respond well to a big handful.
If this is the problem you could try making the engine side of the carb bigger. You could rework the carb so the round part of the bore tapers out on the engine side, to recover some of the lost venture effect.
Hardcore Biker
Buy a Loncin Rich!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!
It's not what you ride but how you ride it!!
Forum whore
Quite.Originally Posted by TZ350
Erm, I think the best way to explain it Richban, is, ah, (don't take this the wrong way), but, you may well have upset your carb with your mods, that, as Teezee has pointed out, has given you the symptom of a reduced signal at the jets.
I subscribe to this theory, as, well, there ain't no way in hell you need a main jet that big, and the increasing of the mainjet size, is simply masking the problem,
Basically, I don't have a chart that goes up that big....it really is methanol sized..... so I can't tell you how much fuel per minute that baby flows, (you can work it ouot if need be) but, er, let's just say it's likely that if your carb was working correctly, and the mainjet was flowing to it's full capacity, then, er, on a full tank, you will get about 15 laps of Manfield!
Basically, the carb is not working correctly to be able to draw enough fuel out of the main.......... because if it was, you would be using about a liter a minute!
OK, maybe a little less!
Forum whore
ok ok yes yes no no. Well I know where you are coming from. The thing is, it is all so tunable. Change one thing and it responds correctly ie raise the needle, richer at 3/4. It does seem a big hole and I have taken the main out as suggested and it did not work over about 1/2 throttle. Could it just be the difference between a bleed type needle jet and a primary? Will one setup flow more than the other? Mr Miknui says the carb will work nice for the hp range its in so thats not an issue. Now it has a completely new carb on it, I will test each part of the carbs operation as much as pos to see if there are any unpredictable results. I have seen this carb offered as a upgrade for CRF150's I will try find the spec.
I always mark the throttle when tuning so we will see.
I would think more Niagara falls that an ocean.
And shut up Fish, you're just scared. Andrew must have told you how crazy fast the bike is, he is quaking in his boots. And we all know I would have won the GP if he didn't push me off. Yes thats right Adlam, pushed! That is why you were so desperate to find the video evidence after the race and destroy it. And the look of concern when I got back from the hospital. Concern of a friend, or the guilty look of a saboteur. ha. Number one plate indeed.
Hardcore Biker
bring it on big boy
It's not what you ride but how you ride it!!
Hardcore Biker
go fi5hy on the loncin !!!
Blowin up bikes since ages ago
Who's Fi5hy?
"Some people are like clouds, once they fuck off, it's a great day!"
Forum whore
wats this loncin you speak of?, sounds slow....
cbr's are taking over.....eveyone else forget about it
Forum whore
Thats not actually correct. A 540 hex head main has a 1.93mm hole.
Forum whore
You are actually to to far off. 10 laps of manfield used about half a tank. And thats pretty much full throttle none stop.
I had a quick run up the road with the new carb for the first time. It went great but the lean problem off full throttle that I fixed with with raising the needle was back. Then I remembered the 3mm bell-mouth extension on the carb. I ripped it off and back to normal yee ha. More volume in the carb now so intake length may need to be shorter. I have a 4 different needles to test so after the weekend I should have it really sorted. As far as main jet size goes I will see what happens after the normal setup process. I did run one of these carbs on a 190cc Et4 vespa and it ran a 460 main, setup on the dyno buy Craig at motormart he knows his shit. I am not to worried as the performance can't be faulted. (Yet) But will keep an eye on things.
Forum whore
Like I say, it's not perfect and very "ball park"
But, I am interested to see where you end up!
Think of it this way, you have (after modifying your carb throat), increased you mainjet size, by almost 100%, does that mean your engine, on full throttle needs 100% more fuel (and, is actually getting it)
I believe that is not true, and the bigger mainjet is masking the fault, albeit successfully from the sound of it!
I don't believe that it is possible for this big jet to be providing all the fuel that it can!
I'm curious, did it make noticebly more power after the mods?
Forum whore
latest carb mods no not yet. Will let you know after some proper testing this weekend. Plug has gone from full turn of colour on the base ring to 3/4 turn a little leaner full, throttle chop.
All mods put together about 30% rear wheel hp.
Forum whore
And now we know that Dad does more than test other peoples ideas he also applies what he reads to develop his own ideas, ideas that have stumped other people before, like making a working plenum chamber. He also posts all the good things he finds and shares his knowledge.
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