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JD, TAFFY. How does the FLATCR work?

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I drive a minibike but come read posts here too. I posted the same questions on 250F board but got no responses. I read Jetting Q's cover to cover but need background first before specifics of which jets worked best.

1. The air jets (PAJ and MAJ) have almost no effect on mixture (according to manual). Why replace them?

2. Are the air jets mainly to supply air to the mixture or is their primary purpose to provide air to an emulsifier tube (put bubbles in the fuel stream). Does it even have emulsifier tubes?

3. How do the air and fuel screws relate the the pilot jet and pilot air jet, if at all?

4. Is there a hard cutoff between any of the circuits (like a stop rod) or are the lower throttle position circuits simply overwhelmed by the higher ones?

Any other background information about how these carbs work would be appreciated. I'm sorry but I grew up at a sawmill, our equipment was much different. I know less now than I did 25 years ago.



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the keihin guide is totally wrong about the needle straight. it says it affects the bike at 1/8-1/4 or something. no i haven't looked at it this time! but i think it's still there.


they're really tough q's. you know how to pick them!


1) the MAJ bleeds into the ET and therefore leans the mixture off at the top end. the larger the MAJ the more it bleeds in thus leaning the mixture. the idea is that the engine needs a weaker mixture past maximum torque/power. the higher the revs the more it bleeds (leans).

the PAJ has it's own circuit and it might not (in your opinion not mine)change the ratio within the PC but within the bigger circuit it does! put it to you another way; if i have a 4 lane motorway with 4 cars per second going by and then i have a slip road onto it. i have 1 car per second joining in the first of two lanes. they only use the first lane of the two because the other lane is always empty to keep the ratio right (air/fuel).

i can speed up the amount of cars joining but they still only use the one lane to join. now that means the ratio of the pilot circuit is the same but it's effect on rejoining the main circuit is different!

2) it has an ET. as explained the MAJ bleeds the fuel to lean it on the main circuit whilst the PC has it's own circuit therefore the more air you can send through the more fuel it will try to pick up.

3) examples of ratios

38PJ/45PAJ is rich

38PJ/60PAJ is about right

38PJ/75PAJ is lean

one can push one up or down or the other up or down. Jetting Q's offers about the right ratio-near THE END.

they effect a little of the starting and a little of the 1/2 throttle, but they mainly effect 1/8 to 1/4 throttle. if you have the lean mixture (38/75) or any ratio that's lean it will show with the leanness getting greater at the top edge of it's range with a light vibration and shudder. if it's a rich ratio (38/45)it will be wooly at the very bottom and and not too bad higher up.

the more you open the pilot screw from closed the richer the whole slow speed circuit will be. period. the correct PS turns is 1.25 to 1.75 turns from closed.

so if i have a lean combo of 38/75 i will probably open my PS to 2-2.5 turns, likewise if i have a rich combo i might even have it turned all the way in! this can happen!

so the PS can be used as a rain check on your PC ratio.

4) i don't quite follow you on the last q. the main jet is totally in control of the needle. it even has an effect on the PC. but it shouldn't really effect the starting mixture.

working the other way the needle straight effects the starting and about the first 1-2 yards as you pull off. after that the rest i've said already.

the only thing i'm not 100% happy about in the above answers is the ratio of the MC and the PC at low revs. my motorway analogy had one car joining four which i know isn't true. it's more like two motorways of four lanes converging but i can't be sure.

marc you know a lot more than you're letting on. your q's hit the button every time.

whatever you're doing i hope you'll let me know. you definately ain't no "good'ol boy".


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If I got you right on Q 4 you want to know if something physically closes each circuit off as another picks up.

If so the answer is no.

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taffy, thank you. After reading your response and rereading the keihin FCR tuning guide a few more times, things are finally sorting out. I wish there was a tech white paper on the thing.

tbronco, Yeah, I didn't really think there was, just wanted to be sure. Thanks.


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