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View Full Version : Stupid RF900 Mikuni Downdraught carbs - require expert advice



riffer
22nd February 2007, 21:32
Okay, the bloody RF's shat another jet needle. Same carbie, number 1, as last time. Only this time I can't find the bottom of the needle that broke off.

I know. I pulled the float bowl cover off (and lost the bloody float needle valve taking the float out - damn things fall out and bounce on the gargre floor and are never seen again) and stripped the top of the carb and there's only the top of it left where it mounts to the diaphragm.

So, I have a spare jet needle, or I can buy another one. I've pulled out the needle jet (or the emulsion tube, if you prefer) and the whole assembly and I'm looking at it now inside the house.

I'm damned if I can work out why this sucker is snapping jet needles. I've checked the plastic holder for the slide and there's no more movement in no.1 carb than there is in any of the other ones. The needle jet doesn't appear to be ovalised (although it looks pretty worn to me - I'll get one of the experts at Welly Motorcycles to look at it tomorrow when I order a new one I reckon).

Carbs are pretty simple (once you disregard the fact that they're full of tiny little bits) but this ones got me buggered.

What's causing this problem? Is it the needle jet? Or is there enough wear
in the throttle slide to make this rattle or what?

Bloody Mikuni Downdraught carbs are a liability... this month's disposable was supposed to be going towards a new helmet dammit.

Ixion
22nd February 2007, 21:37
Did you replace them all when the first one broke? Or was this another "old" one. It certainly shouldn't have broken a NEW needle in that time

riffer
22nd February 2007, 21:41
Four brand new genuine Suzuki parts.

At ridiculous cost - about $15 each.

and a long, long wait from Japan for them to arrive too.

Which is why I'm scratching my head.

It's not lasted long. I replaced all of them as three were worn last time but it was No. 1 that broke last time. (http://www.kiwibiker.co.nz/forums/showthread.php?t=39307&page=2&highlight=emulsion+tube)

Didn't do the needle jets though as they're $30-40 EACH.

riffer
23rd February 2007, 08:33
Okay - a few pics here.

For the unitiated the black bit is the the throttle slide (what the jet needle sits in and what goes up and down the carbie when you pull back the throttle) and the white bit is the throttle slide holder, a bit of plastic that fits into the cab body and allows the throttle slide to go up and down.

There seems to be some wear at the final level of travel of the throttle slide holder which has caused it to flare out and this I believe may be introducing an element of yaw into the movement of the throttle slide, particularly under conditions of extreme acceleration (like trying to lose Fat Jim in the Welly traffic). I have rotated the throttle slide slightly to sit it hard up against one side to accentuate this wear level. Pictures 2 and 3 really show the level of flaring evident.

Is this amount of wear within tolerance? What the hell are the tolerances anyway?

And yes, you can't get these parts anymore so we're going to have to have a completely different conversation about how to fix this slide holder...

Motu
23rd February 2007, 08:51
That's a carburettor?!!!! Plastic this and plastic that....I think this could be half the problem.I'm so happy cars don't have carbs anymore - when a customer comes in with a carb problem I just say we don't do those anymore,go somewhere else sucker.

Crisis management
23rd February 2007, 09:06
I would think that breaking a needle is fairly unusual, typical "out of spec" is due to wear. For the needle to break it must be side loaded at some point which assumes there is a mis alignment somewhere.
Typically needles float horizontally, only being restrained vertically to allow for a bit of slop in the slider mechanisim....so either there is way too much slop or the needle is being restrained horizontally.
I suggest you reassemble that carb with another needle and try manually working the throttle slide to detect where the interference occurs. Coat the needle with something like engineers blue and look for where the first point of contact occurs on the needle.

If that fails, send it to me, I'll put it on my desk for two weeks, look at it occassionally and then send it back......:innocent:

Good luck

riffer
23rd February 2007, 09:16
I took it in to Wellington Motorcycles and had a good long talk to Bruce the workshop manager.

According to him, there's too much wear in the throttle slide holder and throttle slide, as both are made of plastic and the needles are just going to break if I don't replace these parts.

You can get a new throttle slide & diaphragm $119 each
You can't get a new throttle slide holder.

I also need new needle jets $45 each


Plus the float needle valve only comes as an entire assembly at $78

So that's $750 already...

I'm thinking its going to be cheaper to get another set of carbs from a wrecker, and use bits off both the get a good set.

Oh - and some fuckwit loctited the float bowl retaining screws on the carbs too so I rooted them with the impact driver getting them off...

not a happy bunny.

Crisis management
23rd February 2007, 09:24
Bugger, thats 6% of the price of a new SV1000!! (Guess what I've been looking at)

Second hand maybe ok if a low mileage example is available but you may be better off getting new and being sure that the parts are worth paying for.

Second hand is not neccessarily value for money on high wear components.

Paul in NZ
23rd February 2007, 09:31
Fuck me (thats just an expression)

I'll stick with the old AMAL on the Triumph thanks... I could buy 3 whole carbs NEW for that.

Motu
23rd February 2007, 10:26
I'd put a couple of DCOE Webers on it.....

pete376403
23rd February 2007, 10:52
can you get small (I'm guessing 36 or thereabouts) downdraught DCOEs? It would be an interesting exercise, and DCOEs look nice

Fatjim
23rd February 2007, 10:54
What's causing this problem?.

Trying to drag the Fatjim off at the Melling lights! Thats twice now.

Fatjim
23rd February 2007, 10:56
Fuck me (thats just an expression)

I'll stick with the old AMAL on the Triumph thanks... I could buy 3 whole carbs NEW for that.

Yeah, and whats the chance of it ever running smoothly again?

riffer
23rd February 2007, 10:59
Trying to drag the Fatjim off at the Melling lights! Thats twice now.

I hate you James. :dodge:

Bike killer.

F5 Dave
23rd February 2007, 11:05
Crumbs, the slide has worn as well! Hadn't considered that they were plastic too. Could have sworn mine were ally coated black & there was some ally exposed on the load bearing surfaces. Maybe I dreamt that.

I was devising a plan to mill the top 5mm off the guide & glue a new section of plastic on the top & milled to suit. Wonder what type of plastic to use? You could adjust in a bit of taper to fit the worn slides.

riffer
23rd February 2007, 11:21
Crumbs, the slide has worn as well! Hadn't considered that they were plastic too. Could have sworn mine were ally coated black & there was some ally exposed on the load bearing surfaces. Maybe I dreamt that.


You're dreaming mate. It's some kind of really hard plastic, almost bekelite-ish.

wots it do mister
23rd February 2007, 12:18
If you wanna go plastic try uhmwpe (ultra high molecular weight polyethylene) pronounced umpfe has high abrasion resistance & impact strength deisgned for sliding applications but you may have to find a
end user to see if they had any off cuts & if that dosn't work its also food
safe so you could allways use it to scoop your yoghart.

riffer
23rd February 2007, 12:22
Found some replacement carbs.

Just have to wait till payday to buy them.

Eurodave
23rd February 2007, 12:24
I'd put a couple of DCOE Webers on it.....

If theyre downdraughts hes after Id use a pair of Weber DCNF's ala Ducklattery Paso etc

F5 Dave
23rd February 2007, 12:28
Not a bad first post, thanks for recommendation, cool, if a little unwieldy handle.

Eurodave that's just a gross Icon

Eurodave
23rd February 2007, 12:34
Eurodave that's just a gross Icon

Yes,& thankyou for noticing,but it is quite fitting for this forum & the amount of excretia posted

Fatjim
23rd February 2007, 13:20
I hate you James. :dodge:

Bike killer.

Now who's bike is quite!


Get well soon.

F5 Dave
23rd February 2007, 13:54
Quite what?

riffer
23rd February 2007, 14:01
Nah - he meant quiet.

We had a chat the other day at the Melling lights about how his bike was too quiet, and I told him if he gave it a bit of herbs I'd listen from behind and give him an opinion of the sound.

Of course though, he's so old, fat and slow it got boring and I had to leave him behind after a while to do some proper fanging through the traffic. :dodge:

Fatjim
23rd February 2007, 14:12
yeah I meant quiet. I still do.

vagrant
24th February 2007, 16:07
Just for interests sake, how many K's has your RF done?

crazybigal
24th February 2007, 16:53
this may be of some help to you
http://www.factorypro.com/jets,needles,emulsion_tubes,pilot_jets.htm

riffer
24th February 2007, 17:00
Just for interests sake, how many K's has your RF done?

More than any other RF I've ever seen. 101,000.


this may be of some help to you
http://www.factorypro.com/jets,needles,emulsion_tubes,pilot_jets.htm
Yep. Spent a bit of time on factorypro's website. But the thing is, I can spend up to $1200 rebuilding the carbs, and I still haven't fixed the main problem - the case of the throttle slide holders, which you can't buy new. Now I could go down the rapid prototyping route, but its going to cost a lot more than $500 to get four new ones designed and CNC'ed.

I've got a pair of carbs coming, which have less than 30,000kms on them, for less than $500.

crazybigal
24th February 2007, 17:14
and if you wanted to try another carb, Keihin fit. their 39mm one should work.
http://www.trademe.co.nz/Trade-Me-Motors/Motorbikes/Parts-accessories/Performance/auction-88974587.htm

crazybigal
24th February 2007, 17:20
yeah but you will prob run into the same problems! looks to be a design fault.
mind you they prob never intended to get 100k out of them!


More than any other RF I've ever seen. 101,000.



I've got a pair of carbs coming, which have less than 30,000kms on them, for less than $500.

Fatjim
24th February 2007, 19:02
See if they do an equivalent slider for a Honda. It's more likely to be engineered to last the distance.

These Suzuki things just aren't built for the Melling lights.

riffer
24th February 2007, 19:28
and if you wanted to try another carb, Keihin fit. their 39mm one should work.
http://www.trademe.co.nz/Trade-Me-Motors/Motorbikes/Parts-accessories/Performance/auction-88974587.htm

It looks on the surface to be a good idea, but I'm just not sure of what problems you'd face throwing sidedraughts in a system that's designed for downdraughts. Fitting the airbox would probably be the least of the problems.


yeah but you will prob run into the same problems! looks to be a design fault.
mind you they prob never intended to get 100k out of them!

Yep - the way I see it, if the old ones lasted for 100,000 kms then I should expect at least 50,000 out of the next ones. And at that stage there'll be a few other things to worry about as well, so it will probably be time to move on to the next one.

And Fatjim - the next time we meet at Melling lights - your bike is toast. I'll have the carbs from an RF900RV Manta Ray Special on there. Might just advance the ignition a tad too while I'm at it. Better bring the Bayliss and not the hairdressers bike... :dodge:

crashe
24th February 2007, 19:31
Hey there is someone on Trade Me in the motoring section having problems with these Mikuni carbs as well.

If anyone can help this dude out........ post in his thread.

http://www.trademe.co.nz/Community/MessageBoard/Messages.aspx?id=15615315&threadid=15615315

Mikuni Bike Carb Jet Sizes?
Does any one know if the jet sizes in Mikuni carbs relate to their bore size? ie 130 jet size = 1.30 mm. Or does it relate to fuel flow rate? I believe that Weber jet sizes relate to bore size. glyn4 5:49 pm, 24 Feb

crazybigal
24th February 2007, 20:41
for sure, prb a bit much dicking around with it. i see the suppliers of Keihin list a carb to retro fit the rf900 model 016-504, mind you cost you more than your bikes worth!


It looks on the surface to be a good idea, but I'm just not sure of what problems you'd face throwing sidedraughts in a system that's designed for downdraughts. Fitting the airbox would probably be the least of the problems.



Yep - the way I see it, if the old ones lasted for 100,000 kms then I should expect at least 50,000 out of the next ones. And at that stage there'll be a few other things to worry about as well, so it will probably be time to move on to the next one.

And Fatjim - the next time we meet at Melling lights - your bike is toast. I'll have the carbs from an RF900RV Manta Ray Special on there. Might just advance the ignition a tad too while I'm at it. Better bring the Bayliss and not the hairdressers bike... :dodge:

Fatjim
25th February 2007, 09:40
The Bayliss is the hairdresser's bike mate.


BTW, the Yawn needs the carbies balancing. you must be an expert by now, fancy giving me a hand?

F5 Dave
26th February 2007, 10:01
Hey there is someone on Trade Me in the motoring section having problems with these Mikuni carbs as well.

If anyone can help this dude out........ post in his thread.

http://www.trademe.co.nz/Community/MessageBoard/Messages.aspx?id=15615315&threadid=15615315

These Mikuni carbs? That's like saying These Dunlop tyres. There are probably more mikuni carbs than any other single make & half squillion different types. Your Vertigo probably has Miks (or maybe Hitachi). This guys issue is just of jet measurement (which can't be answered as mikuni run both flow & drill size jets & as usual he has assumed that everyone is a mindreader).

crashe
26th February 2007, 10:10
These Mikuni carbs? That's like saying These Dunlop tyres. There are probably more mikuni carbs than any other single make & half squillion different types. Your Vertigo probably has Miks (or maybe Hitachi). This guys issue is just of jet measurement (which can't be answered as mikuni run both flow & drill size jets & as usual he has assumed that everyone is a mindreader).

Oh ok..... I just spotted the thread... saw it was the same brand as Riffer's...

Cos I know jack-shit about carbs....

So thought ahha...... someone on here could help him out....

Guitana
26th February 2007, 11:55
Bound to be a good set of carbs at a wreckers for a good price!!
You may have to sell your guitar!!!!!

riffer
26th February 2007, 12:02
Fuck off Rick!

The Gibson's a keeper!

Got the carbs sorted thanks. Just requires some juggling of the next month's finances a bit.

riffer
26th February 2007, 12:03
BTW, the Yawn needs the carbies balancing. you must be an expert by now, fancy giving me a hand?

Believe it or not Jim, I've never actually balanced carbs in my life. I will be needing to though when I get the new ones, as they will be set up totally different.

Perhaps someone with some skills could run a clinic for a few of us guys.