Stupid RF900 Mikuni Downdraught carbs - require expert advice

[riffer]

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]

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]

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.

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

[riffer]

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]

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]

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......

Good luck

[riffer]

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]

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]

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]

I'd put a couple of DCOE Webers on it.....

[pete376403]

can you get small (I'm guessing 36 or thereabouts) downdraught DCOEs? It would be an interesting exercise, and DCOEs look nice

[Fatjim]

What's causing this problem?.

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

[Fatjim]

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]

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

I hate you James.

Bike killer.

[F5 Dave]

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.