GT550 / GT380 throttle slide surprise #1

[yeadon_m]

Folks,

Well I never. I was for reasons I can't recall glancing at the parts lists when I noticed that GT550 LMAB carb throttle slides are not, as I'd expected, all the same. The L and C slides have one part number, the R another. How the dickens can this be? why? whats different, why does it matter and how could I tell, or notice, if I reassembled them incorrectly? anyone know? BTW, the same assymetry is true for GT380 LMAB slides also.

15 13561-34110

..VALVE, THROTTLE MIDDLE / LH
2 CA:2.3

16 13551-34110

..VALVE, THROTTLE RH
1 CA:2.5

Cheers,
Mike

[Suzsmokeyallan]

I haven't seen them as I don't own a 550 or 380 but its safe to assume the cut away is the same, the holes if any are in the same location. So how about the locating grooves position, and/or is there an oil pump adjustment inspection hole on the right carbs slide like the VMs. If there is that slide will have an alignment dimple on it.

[gt-keith]

+1 Oil pump setting dimple.

[yeadon_m]

Gents, of course - you're quite right. I feel a fool.

In a pretence at defence for missing that, the only carb I have in bits right now had three RH slides in it :/) so they all have dimples.

Cheers, Mike

[tz375]

I no longer own a 550 and the one I did have had early type carbs. Yamaha used to put dimples in all 3 as did Suzuki on the J & K models but the Suzuki carb manual says to put the dimple slide in the right carb.

So just for fun, I looked at pictures I had collected from the net and found one where all 3 slides had dimples. Whether Mikuni started supplying them that way or a previous owner had changed them I have no idea, but seems like you are not alone with a three dimple slide set.

[Alan H]

My original 550J had all slides dimpled as that was the way to ensure the carbs were synchronised. Both the low and high top carbs have the brass screws in them for this facility.

[yeadon_m]

TZ, I'll bet you're correct as the chances I popped the only dimpled slide correctly into the right carb when I built the hydrid 550 VM28s under a 380 carb top is just one in three...yet taking off the brass inspection bolt, there's my dimple
Cheers,
Mike

[Alan H]

there's my dimple
Cheers,
Mike

Awww, shucks!!!

[gt-keith]

I appears that all the slides may have a dimple in, so with that in mind could the prefix CA:2.3 for the left & middle and prefix CA:2.5 for the right mean they have different Cut Aways to allow for the fact that the left & middle have a choke plunger and the right does not, hence the different part numbers.
The smaller cut away would mean more vacuum on the engine side of the slide drawing more fuel through the choke plunger on a closed throttle for starting.
Just a thought, but having said that my 380 always needs a bit of throttle to start.

[Alan H]

On the early 550s all the carbs had choke plungers.

[yeadon_m]

Keith,
Yes, I noticed that CA2.3/CA2.5 thing and that was what made me go look for differences. I have a set of three late-380 slides, at least they all came from one battered, dirty set from ebay, and so far I cannot for the life of me see a difference. I will run a vernier over them tomorrow to see if I can find a 0.2mm difference somewhere, if 2.5 / 2.3 is something in mm.

On the late-550 carbs, their slides also have LH and C the same, with RH different. This time, I don't see a CA difference: the C/LH says CA3.0 but there is no remark under the part number for RH.

Once its running off choke, any difference in cutaway between the slides surely makes no sense? and the way the RH and C share rich mixture is accomplished by the C carb having a visibly much bigger air intake than the single cylinder provider of choked mix on the L carb. So I doubt the slide difference is for that purpose.

Are you sure all the slides have dimples? my spare 380 set does, but I've not looked in other sets of 380/550 carbs. That too makes no sense to me because if RH slide is functionally different, adding a dimple to L and C just makes mixing them up more likely!

Alan: according to my parts list, 550J/K also have different part numbers for slides - this time C and RH the same, LH is different. I'm sure you've been diligent in putting the odd one out into the LH carb If you did, what difference in the slide did you see?

Cheers!
Mike

[Alan H]

I'll have to have a look and see, can't remember to be honest!
I've just sent you an email with a pic out of the service manual showing the three carbs with three dots though!
GT550J manual, but as it was 1972, it's the same manual (different carbs mind) as the K, which actually went to the US in '72.

[Alan H]

Thinking about it, IIRC it's the tickover screws. The left and centre carbs were on the left, and the right carb was on the right (outside). So the flat on the slides would be in different places depending where the tickover screw was.

[yeadon_m]

Alan,

That'll explain for the J/K. Good for you!

For your next trick, deduce what the slide part no. difference is for on the LMAB carbs!

Cheers,
Mike

[gt-keith]

Mike, I haven't a clue really I was just throwing an Idea up in the air.

I take lots of photo's when I strip anything down these days (it's a senior thing) And I refurbished a set of 380 carbs a couple of years ago, they're still on the bench in a box.
Of all the photo's I took there's not one of the slides DOH!!! I must have thought even I could remember where those fitted.