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October 16th, 2012, 12:18 AM   #1
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Keihin FCR Vacuum Release Plate & Seal

The Keihin FCR-MX Carbs are notorious for having this plate wear / crack / rattle etc....

To buy the plate and the seal is an excercise in frustration when it comes to the pricing that gets quoted around the place. The way to go is this: Buy the Honda 'kit' which comprises the seal and the plate for around $64.00 + shipping. The Honda part number is 16037-MEB-671 . To buy the parts here in Oz (as a genuine Honda part) will set you back around $214.00 retail so make sure you go to a US Honda parts supplier.

Before you start the job (giving the carb a birthday) it's worth having a read of this: http://www.dirtrider.com/features/pr..._carb_rebuild/

Cheers,
Bushpig
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October 16th, 2012, 01:12 AM   #2
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Re: Keihin FCR Vacuum Release Plate & Seal

Hi and welcome

There is another problem, even if you have a new plate and seal you can also have grooves on body were the wheels work, so even with a new plate and seal you can have too much gap between the plate and the body, so you will have one intake of air uncontroled, that is leaning your mixture.

Here one example from me:
[attachment=1:29h5418z]FCR_Body_Grooves.jpg[/attachment:29h5418z]

I have repaired that grooves but probably did not done a good job, because I still have more gap than I wich, then I tryed to make a different seal but could not done it, its very dificult, I had to make one mould and its too expensive to do it for one piece.
Now I'm playing with gaskets, here is a pict:
[attachment=0:29h5418z]Plate_gasket.jpg[/attachment:29h5418z]
This is a gasket with metal inside to not be compress by the presure, it happens.

This is one ongoing job, will try to develop the idea.


ZAGA
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Keihin FCR Vacuum Release Plate & Seal-fcr_body_grooves.jpg   Keihin FCR Vacuum Release Plate & Seal-plate_gasket.jpg  
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October 16th, 2012, 01:40 AM   #3
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Re: Keihin FCR Vacuum Release Plate & Seal

Correct. There's only one (of two) wear related problems being addressed by replacing the plate & seal, but they do need replacing.

Your photo shows the grooving really well, and unless they've seen it for themselves many people just don't believe it's an issue. I've heard of people filling them (the grooves) with Devcon and all sorts of other things but there has to be a better way but I'm just not sure what it is. No doubt someone will come up with a fix, and when they do......

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Bushpig
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October 16th, 2012, 03:12 AM   #4
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Re: Keihin FCR Vacuum Release Plate & Seal

Hi

I don't know a better way, that is how I've done it:
[attachment=2:3c5t876y]FCR_body_repair_1.jpg[/attachment:3c5t876y]
[attachment=1:3c5t876y]FCR_body_repair_2.jpg[/attachment:3c5t876y]

Then I used a file and many hours
I've used DEVCON clear epoxi and the plastic steel, the plastic steel its easier to work but the clear epoxi its lasting more (I think)

[attachment=0:3c5t876y]Devcon_2 Ton Epoxy_1.jpg[/attachment:3c5t876y]

If you find other way, let me know


ZAGA
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Keihin FCR Vacuum Release Plate & Seal-fcr_body_repair_1.jpg   Keihin FCR Vacuum Release Plate & Seal-fcr_body_repair_2.jpg   Keihin FCR Vacuum Release Plate & Seal-devcon_2-ton-epoxy_1.jpg  
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October 16th, 2012, 11:01 AM   #5
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Re: Keihin FCR Vacuum Release Plate & Seal

Gentlemen,
I have yet to experience this joy.

Thinking laterally, what would happen if the wheels were of slightly larger diameter to compensate for the wear? It's possible the grooves will be too uneven for this to work, but if it did it would be a far easier job to make some oversize wheels.

Cheers... Paul
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October 17th, 2012, 01:07 AM   #6
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Re: Keihin FCR Vacuum Release Plate & Seal

Hi Paul

That was one of my first thoughts, but unfortunatly no, that is not the solution.

The thing is, you want the slide and most important the plate to go to the front of the carb (engine side) so it will be agaist be body carb. By doing this you will seal all other intakes of air from the top of the carb. The only air you want in, is the one you control, wich is the one that goes underneath the slide, by normal intake process.

In a new carb you have a very little gap, and that gap is compensated by the seal that act as a spring, keeping the plate allways against the carb body.

What I've tryed to do was a rubber seal that was a spring with more range to compensate the wear, but its realy hard to do, only if I made a mould to do it, for now I'm using a gasket to limit the travel of plate, because I don't want to do the epoxi thing again, I just don't have the patience to go all over again


ZAGA
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October 17th, 2012, 07:36 AM   #7
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Re: Keihin FCR Vacuum Release Plate & Seal

hi zaga

good to see you having more understanding about my new mod, maybe i should try to explain as i have used this mod for many hours on my bike.
first of all i was having problems with the fcr it had an unpredictable idle, sometimes when you shut off the idle would stay high then sometimes it would idle perfect then other times it would drop to nothing and stall.
when the bike was off idle it was great but i ride motox and i ride hard so this was a problem as its near impossible to hit the same lines on every lap with a wild idle, so i started researching and found the problem with the slide wheels wearing away the carb body so i stripped the carb down to asses the damage .
after looking at the grooves that the wheels had made i decided that filling them with some kind of epoxy was not for me! what if the epoxy came loose and jammed the slide open ?? i would end up with some serious injuries.

i also own a mk1 fcr so i stripped this down to make some comparisons, i found hardly any wear at all so i took all the wheels off both slides so i could get a more accurate reading of slide to body clearance, i found my problematic mk2 had some 0.5mm more clearance than my mk1.

i tried swapping the rubber seals from under the slide plate with no improvement , so i measured how much the plate sat away from the slide with the rubber seal in place it was 0.3mm , i then made a new seal i had no rubber so i used good quality gasket paper i used 0.8mm as this was 0.5mm thicker .
i fitted the plate back on the slide then back in the carb it looked perfect not tight at all but not too much clearance either. i fitted my fcr back on my bike and she runs like a dream drops to the same idle every time with out fail .

but after many outings and many hard hours of riding my crazy idle started coming back , so i stripped the carb again to find that the gasket i had made had been compressed by the induction force, so i then came up with another idea i thought what if i used a thicker gasket but pre compressed it in the press before fitting??

unfortunatly i dont have the right thickness gasket paper here to make a new 1 and i have had no time to pick some up but i am very confident that when i do it will work just as amazingly for a lot longer or forever ( i wish lol ).

so for the price of a piece of gasket paper smaller than a credit card its well worth a try

cheers kris
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October 17th, 2012, 09:47 PM   #8
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Re: Keihin FCR Vacuum Release Plate & Seal

kris,
why not use a piece of brass or steel shim of the required thickness? Easy to get, available in very small thickness increments, cheap and will not compress.
Just a thought.

Thanks for the excellent description of your solution by the way.

Cheers... Paul
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October 18th, 2012, 01:11 AM   #9
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Re: Keihin FCR Vacuum Release Plate & Seal

hi paul

the original rubber seal behind the plate is a stretch fit as in it has to sit under a lip and if i had used a piece of metal of some kind i would never have got it to fit as metal will stretch but not spring back like gasket paper.

the 0.8mm gasket i made lasted for many rides the crazy idle did come back as the gasket compressed but it still was not as bad as without it .
so i got 15-20 riding hours before it compressed , now i am definatly thinking that if i use 1.2/1.3mm gasket paper and pre compress to my desired 0.8 that the 15-20 hours could well turn into 150-200 hours.
i bought my fcr cheap with this idle issue i payyed 120 for it but it was fully kitted out with all of taffys best bits which alone would cost around what payyed for the whole package and after stumbling upon this very easy cheap mod my carb has gone from ready for the bin to ready for racing , the jetting kit and super needle is working very nice and the carb conversion has made my 650 the ultimate off roader .

cheers kris
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October 18th, 2012, 01:42 AM   #10
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Re: Keihin FCR Vacuum Release Plate & Seal

Hi all

None of this solutions are excelent ones, the excelent was to have a new carb but as kris pointed this is a very expensive piece, and like him I bought mine used and cheap and spent the same also with Taffys bits.

The thing is, I would not advice the gasket alone like a fix, because I don't think the wear grooves are even, but I'm not shure, and when the seal is enough to fill the gap it can compesate the uneven wear because it act as a sring all the way arround the plate. I think the grooves are deeper on the bottom of the carb because its there that it spends more time and is when it is exposed to more pressure (don't know if I explained well ??).

Anyway if we don't have the coins for a new one, we have to find some kind of fix Now I'm thinking of a teflon shim that you can cut with a lathe and might be a more durable fix, that won't compress.

Kris I will send you one of my gaskets, it has metal inside and it won't compress as easy, for you to try.


ZAGA
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