GT550 LMAB carb confusion (again)

[yeadon_m]

Folks,

I've noticed numerous posts about especially the later / ganged VM28 carbs on this bike, and I'm adding another.

The parts lists confidently tell us the needle jet is 172 O-8 (for the L) and P-0 for the MAB; and the jet needle as 5DH21-4 for LMAB.

Well, thats simply not what they shipped! at least to UK and Italy. Here's the evidence, one piece stronger than the other.

My UK 550B came with 172 P-4 NJs and 5DN1-3 JNs. No biggie...perhaps someone changed the internals between 1977 and me getting the bike. Though I'd bet not.

Recently, I stumbled on a brand new in factory wrappings set of GT550 ganged carbs, from a guy who 20 years ago bought a container load of NOS parts from Suzuki Italia. As they were keenly priced - and when is the right time to buy rare parts? Ans: when you see them I grabbed them, and started dismantling today.

I found to my surprise that the NJs are O-9 !! I've not yet got to the JNs as the lower throttle boots are black goo and I'll be taking the slides out for cleaning / replacing soon.

I also found that - in contrast to the parts lists that show (for any given carb) the throttle slide has the same part number LMAB - the slides in these NOS carbs are categorically not the same as the slides in my UK 550B carbs. The 'Italian' carb slides all have a notch in their outlet side (see pic).

If anyone (Tz?) can offer a view on what these different JNs and NJs (as well as 'notched' slides) are likely to do to fuel/air mixtures at different throttle positions, I'm all ears

Also, if anyone should have insights as to why the parts list vs the found reality should differ as I've found, that'd be good to learn about. For me, lesson learned is don't treat the parts listings even from Suzuki as gospel, just a good guide.

In due course I'll have these carbs set up and tested on my bike, for the sheer fun of finding out if new carbs make it nicer, different in any way.

Cheers,
Mike

[gt-keith]

Mike, looking at your photo's may I offer a suggestion.

Given that in an earlier thread regarding your hybrid GT380/GT550 carbs we assumed that the carb bodies were the same apart from the venturi dia and throttle slide, if the drillings in the body are the size and the Suzuki carb manual says they are bypass 1.4mm and pilot outlet 0.8mm it's fairly safe to assume they are also in the same position.
So going back to the photo, the larger throttle slide looks like it "masks" the bypass hole compared to the position of the bypass hole and throttle slide on my GT380 photo.
So maybe the cut out on the 550 throttle slide is to remove this "masking" effect.
As the pilot jet and pilot air screw only control idle to slow speed it's unlikely to have much effect through the rest of the throttle opening.
The two photo's also show clearly the 'extra' metal in the GT380 venturi compared to the GT550.

[yeadon_m]

Keith,

Thanks - your suggestion sounds plausible for a 550 / 380 slide difference except for one thing: on the other 3 sets of GT550 ganged carbs I've had all the slides look like yours, ie no notch at the bottom. So these slides are different / a variant on 550 slides I've seen.

Since I posted this, I now have the following data:

UK GT550B VM28 carb needle jets are 172 P-4 and jet needles 5DN1-3 (pilots #30, mains #105 / #102.5 / #105)

The Italian GT550 (? L?) VM28 carb needle jets are 172 O-9 and jet needles are 5DH21-4 (pilots #30, mains #100 all cylinders)

Someone may know which of these combinations is richer / leaner at different throttle positions and I'd welcome any comments.

Cheers,
Mike

[Suzukidave]

Mike , do the different looking set of slides have the same cut away . Looking around i have seen reference to 2.5 and 3.0 cut aways ?

[yeadon_m]

Dave,

Yes, on my 550 carbs, both those with a without notches are marked #2.5 underneath.

I am now speculating that the purpose of the notch is to ensure a richer mixture when rolling off (to reduce bucking / roughness - a smaller version of that often experienced with the BS40 GT750 carbs). But its purely a guess from looking at them. I have not yet fitted them to my 550!

Throttle notches aside, what you make of the different needle jet / jet needle combos on the two sets of 550 carbs? will one automatically be richer or leaner than the other? if you don't know, do you know how I could research and find out?

Cheers,
Mike

[tz375]

As far as I can work out, the notch is to allow air into the motor on a closed throttle probably to reduce surging.

On the needle jets, higher letters and higher numbers indicate larger diameters.

O-9 is 2.645mm
P-0 is 2.650mm
P-4 is 2.670mm

letters go up in 0.05mm increments and numbers go up in 0.005mm increments

On needles the second letter indicates that the needle has two taper angles. Larger letters are steeper angles, so a 5DN will be richer at the top end than a DH all other things being equal, but the 21 or other number on a needle indicate that the taper starts in a different place than a needle with the same letters - in this example DH - but different number and there's no code for that.

[Vintageman]

One can see that Suzuki spent much time tuning the GT550 as it progressed.

I have seen Yam, Suz, Kaw and Honda jetting not agree with parts manual and swear I was the first to touch the carbs.

I think engineers improve tuning for various reasons and don't recommend updating what was all ready sold... only if safety or warranty issues. A little less/diffferent performance oh well. So I always consider the latest jetting to be the best performance or possible fuel economy/emissions... other things change too but still worth a look

The early GT550 72/72 I feel has a definite jetting flaw in one spot: as you just come off closed throttle. It is too lean. And, if the engine is revving high (pings and seen couple bikes with holed piston from coasting too long) . That is you were at higher RPMs and slow down and close throttle versus just idling for example. If you close all the way OK but if you hold open just a little ping ping ping.

This notch is very interesting. Here is what I think is going on (here is one reference http://3cyl.com/mraxl/manuals/kawcarb/" onclick="window.open(this.href);return false; there is second but can't find now). It has to do with the Pilot Outlet and Pilot Bypass function. The Pilot Bypass changes its role based on slide position. When the slide is way down it adds more air in parallel to the pilot air passage coming from bell mouth and both supply air to the Pilot Outlet. When the slide starts to open it transitions to supplying fuel versus air. I think this notch reduces the bypass' dual role, I think it would cause the air pressure between the two outlet to be the same and thus the bypass is more a mix fuel supplier versus air boost for pilot.

This is good trick/option versus say changing Pilot passage hole sizes.

Yeadon_m. If you ever pull that slide out with notch can you measure its width and height.

When you streat ride and you have problems at a given throttle position you now have another tuning option at your disposal me thinks!

[yeadon_m]

Guys,

TZ, Vintageman, very interesting info! thank you Vintageman for the manual, an easier to read version of what I've seen before.

I agree that there's little doubt that the notch will modify the transition from idle to off-idle. If I should take these carbs apart (which seems HIGHLY likely at some point as I jigger it all better I will measure the notch dimensions and report back.

Of all the GT triples, the 550 has to be the one most mucked-about with where it comes to carb settings. Maybe this means nothing other than that the development team was more diligent, rather than the 550 gave them more problems than 380/750? we will probably never know. I quite like mysteries, more yet to find out!

Cheers,
Mike

[Vintageman]

It should be noted that the Suzuki Factory manual ->
http://www.oldjapanesebikes.com/mraxl_G ... rb/22.html" onclick="window.open(this.href);return false;

and the Kawasaki factory manual -> http://3cyl.com/mraxl/manuals/kawcarb/" onclick="window.open(this.href);return false;

swap"Pilot Outlet" and "Pilot Bypass"

And Mikuni agrees with Kawasaki -> http://www.mikuni.com/pdf/vmmanual.pdf" onclick="window.open(this.href);return false;.

Well kind of does look at page 2 figure 3 is updated

So I would say the hole sizes for each (i.e.gt-keith) use Suzuki manual, but be careful just incase there is a typo. I am going to verify/cleanup my 1973 GT550 two pilot outlet sizes

What's the other larger size Bypass in the table (Suzuki carb manual for later ganged carbs).

[yeadon_m]

I always assumed the bypass referred to bypassing the slide hence its the one on the engine side....

On my GT550A ganged carbs, the bypass was slightly bigger than 0.5mm as a soft wire of this diameter would just slide through it. Pilot outlets were >1mm but <1.5mm, those being the reference probes I had to hand.

Cheers,
Mike

[Vintageman]

yeadon_m,

What do you call the signal hole version?

What I did not realize, until now is that the 72/73 gt550 has a smaller hole 0.5mm (closer to engine) versus 0.8mm. Explains better to me why they went from 27.5 to 25 pilot

Anyone know what that the other 5.15mm Bypass is refereeing to on the later GT550 in Suz cab manual?

[tz375]

That 5.15mm relates to the starter circuit and is probably the outlet orifice.

The pilot jet outlet is 0.8mm and the pilot jet bypass is 1.4mm.

Mike, which was the larger outlet on your carb, under the slide or close to the engine?

[yeadon_m]

TZ, the smaller (~0.5mm) of the holes I posted approx size of is the one closest to the motor, the 'pilot bypass', and the larger I measured as >>1mm and <1.5mm (so your 1.4mm reference fits perfectly), the 'pilot outlet'.

Vintageman - thats so interesting that pilot jet reduced when outlet sizes increased.

Cheers,
Mike

[Alan H]

Can't sleep again Mike?

[yeadon_m]

Alan,
If I make it to 6am thats a good night's sleep. Kentish folk are up catching worms by then. I know the sun comes up later in God's Own Country.
Cheers,
Mike