OKO carb for ETZ251

[alexxx]

Hi Folks,

I am considering to fit an OKO carb (Taiwan made)
Seems to be a popular replacement for a lot of bikes, has anyone here tried one?
I contacted the company and they suggested a 28mm carb for the ETZ251 but I am concerned that it would not fit over the carb mounting spigot, is that 30mm OD (or just under) to match the current BVF?

Here is the US website
http://www.usa-oko.com/

[dirtsurfer]

I haven't tried one. Is the price similar to Mikuni?

[Puffs]

Hi Alex, ta for that link.

I have no experience with OKO, but I very much like the idea of a clear bowl - allows you to see the level. That can be quite useful.

Make sure you can mount it properly. These carbs seem to be made to fit inside a rubber muff, as is common elsewhere, but the standard alu ETZ251 inlet manifold requires a precise fit. The OD of the spigot of my original carb is approximately 39.7mm. That means the 32mm K104 might fit, with some persuation; for the smaller carbs I cannot find the spigot OD. But that's likely smaller. But of course you can replace/adapt the inlet manifold.

Just out of interest, why replace the original BVF 30 N 3-1?

[Blurredman]

Interestingly,

It does seem apparent that in previous original articles about the MZ's, it looks like they used a "packing" as the original documentation says, in the inlet. They don't mention what type of 'packing' and why it should be installed. I've never used this item and i've got on fine..

[Puffs]

packing = gasket?

[dirtsurfer]

Many years ago , some bikes had a thin fibre bush for packing.

[Puffs]

I can imagine one would use something like this:

plastic bush.png (28.51 KiB) Viewed 78 times

or thinner, but on mine it is (and as far as I can see, has always been with this manifold) just metal on metal, with a good fit.

The original manifold has an M6 bolt on either side to clamp the carb's spigot, but the thick alu will not allow for much movement. So the fit will have to be good & hard, if you want to use that original manifold. But of course you don't have to, see below as an example for an alternative I drove a couple of years with (+ shortened inlet skirt).

RV ETZ, top.jpg (24.68 KiB) Viewed 78 times

RV ETZ, carb side.jpg (39.35 KiB) Viewed 78 times

[dirtsurfer]

.......see below as an example for an alternative I drove a couple of years with (+ shortened inlet skirt).

RV ETZ, top.jpg

RV ETZ, carb side.jpg

That rubber packing really looks like a piece of re-purposed radiator hose and is that a reed valve in the second picture? and Intriguingly use of 8 screws to hold in place!

[Andy_C]

I can imagine one would use something like this:

plastic bush.png

or thinner, but on mine it is (and as far as I can see, has always been with this manifold) just metal on metal, with a good fit.

The original manifold has an M6 bolt on either side to clamp the carb's spigot, but the thick alu will not allow for much movement. So the fit will have to be good & hard, if you want to use that original manifold. But of course you don't have to, see below as an example for an alternative I drove a couple of years with (+ shortened inlet skirt).

RV ETZ, top.jpg

RV ETZ, carb side.jpg

Puffs.

So you grafted a reed valve onto the bike - yes?

Can you give us some details - what did the reed valve come from, what changes were made to the porting etc?

Just curious.

Thanks.

[Puffs]

dirtsurfer wrote:
That rubber packing really looks like a piece of re-purposed radiator hose and is that a reed valve in the second picture? and Intriguingly use of 8 screws to hold in place!


Yeah, so? Radiator hose comes in exactly the right size, is resistant to petrol, reinforced, and very strong. Pray tell: what would you use for the flush spigot of the stock BVF?

Andy, it's a reed valve that exactly replaces the original inlet manifold. I have changed very little to the porting inside the cylinder, but doubtlessly I rounded the top of the exhaust a bit & some widening of the ports, but nothing special. I took a couple of mm off the inlet skirt, & lightened & optimised the piston a bit, but otherwise a standard 3-ring piston.

Riding-wise it gave more power low down: the gains were particularly in the lower rev regions, so that the bike became less sensitive to shifting in order to keep it on top of the original narrow power band. It wasn't really any faster (around 85mph), but that wasn't my intention either. The bike was a commuter.

The reeds didn't come from any other bike, I made them for this reed valve, from aramide & epoxy. I made the entire thing, except for the re-purposed radiator hose and the 8 screws, those were made by someone else. I used it for some 4 years.

Of course if you want to really increase the power of the engine you'd have to reduce the dead volume between the reed & the piston, so then you have to shorten the inlet path & drop the reed box into it. In this one, the reeds stop a few mm short of the end of the box.

It all depends on what you're after (and that's also why I asked: Why replace the BVF?). If you want to go fast, you better use a different bike.
+++++++++++++++++++
Edit: the piston I used with the RV is the one on the right, it's a Megu. The one in the middle is a new Almot, the one on the left is a RAM with about 2000 miles. The inside of the raised skirt is shown below.

RAM, Almot & Megu with lifted skirt.jpg (16.83 KiB) Viewed 76 times

Megu with lifted skirt, inside.jpg (30.03 KiB) Viewed 76 times

Porting is indeed pretty much standard, see download/file.php?id=4989&mode=view

[Andy_C]

Puffs,

Thanks for that - I was just curious.

[dirtsurfer]

Yeah, so? Radiator hose comes in exactly the right size, is resistant to petrol, reinforced, and very strong. Pray tell: what would you use for the flush spigot of the stock BVF?...

I would use a high quality product like this one from my local bmw supplier

The reeds didn't come from any other bike, I made them for this reed valve, from aramide & epoxy. I made the entire thing, except for the re-purposed radiator hose and the 8 screws, those were made by someone else. I used it for some 4 years.

well done

[Puffs]

Well, of course I used BMW radiator hose, hand-crafted in Bayern.

@alexxx, from your other thread that recently surfaced I get the impression that your carb is just poorly adjusted (too lean). Or clogged, or that you have an air leak somewhere, or that your fuel tap doesn't give the required flow. These ETZ251's can run fine with the original BVF, but of course if you feel it's time for a new OKO/Mikuni/Bing/Keihin/Dell'Orto/... just go for it.

[alexxx]

Thanks folks for all the advice and help.
I think I will try one of these carbs as a cheap alternative to a Mikuni.
I have been looking for a used but good Bing to try but no luck
I am basically trying to get rid of the terrible surging on the over run and erratic tick over.
The surging is so bad that on long downhills I have to pull in the clutch.

[Andy_C]

Alexxx - before spalshing out on a new carb are ypu 100% sure that the existing (BVF) carb is free from any obstructions, and that your timiing is set correctly ?

When I first had my ETZ is used to buck and stutter and was very unpleasant on the overrun, it is pretty good these days. I spent a lot of time fiddling with carb settings - mine has a BVF and it is happiest with 2.5 turns out on the airscrew, and 4 turns out on the bypass screw.

Finally, do you have any leaks in the exhaust system ? Mine used to have a small leak where the silencer joined the downpipe and that had a huge effect on surging - a small leak and it would surge like mad.

Just a few thoughts, whatever you decide to do, good luck.