GPz550 / KZ550 Mikuni Flatslides

Mikuni manufactured TM Series Flatslides for Kawasaki KZ550/GPz550 models in the mid 1980s. Racer Supply in Van Nuys, CA. which is also where I bought my Suzuki GS1000S fairing from was one place which sold the Flatslides. There are not too many GPz550 running these Flatslides. My 81 GPz550 project came equipped with them, I thought they were cool so I kept them and they work just like Mikuni VM29s & RS34s once sorted and vacuum leak free. So I wanted to share what I know. According to Corey Clough who raced GPz550s, most road racing organizations like AFM, AHRMA, VRRA, do not allow flat slide carbs.

Model:
Mikuni TM27-D1 Push & Pull throttle tube
Ad states Fits 83-85 GPz550 79-82 KZ550
Air Screw Setting: 1/2 - 5/8 CCW turn from seated
Pilot Jet Type: VM28/486
Pilot Jet Size (Sample of two I own) 32.5 or 30 (Sea level); Recommend for altitude ( 5000 Ft) Size 27.5
Main Jet type: N100
Main Jet Size (Sample of Two) 107.5 & 115

Installation:
Need Push & Pull Throttle Tube
1. 1981 GPz550 throttle cables P/N 54012-1041 Open & P/N 54012-1042 Closed are one solution.
2. There are no stock carburetor holder which fit & have vacuum nipples for these Flatslides. I suspect the kit came with custom holders.
3. H1/H2 & ZX550 carburetor holders have the correct ID but have no vacuum connection to synchronize the Flatslides.
4. One Carburetor Holder Solution: Purchase a set of Z-1 Lt & Rt carburetor holder. Break out the digital caliper and your Dremmel tool. Using a circular sanding bit in the Dremmel carefully mill out the ID of the Z-1 carb boots. A slow process of milling and measuring because you cannot add material back. Here is the Dremmel set up.


PARTS SUPPORT:
Mikuni Support: None
NOS Parts/Source: None
I am always looking for spare parts ( Floats, needle valve assemblies etc.)

Here is the one link I have found mentioning these Flatslides Carbs in American Motorcyclists circa 1986
books.google.com/books?id=8PkDAAAAMBAJ&p...20flatslides&f=false One of mine

Now that I got a close up look at these carbs they are very different looking from the VM/TM flat slide carbs used on the GSXR motors.
On the VM29/TM33 the bowls and top caps are slanted (because the GSXR head had downdraft runners) and the throttle rod is on the opposite side of the slide.
Your carbs have the throttle rod on the same side as the RS34 series. Your split line on the bodies is totally different from the other carbs, its on the head side of the carbs, the RS34/VM29/TM33 all have the slit line on the filter side.

Thanks for posting this.

One question, do they use a accelerator pump system?

These Flatslides do not have accelerator pump system. I saw a similar set of Mikuni Flatslides recently listed on Ebay for a Kawasaki with an accelerator pump. The listing is gone. Here is another picture.

The floats are plastic. On this set the float lever tangs curled. I guess from sitting

Question re: MIlling: What is the correct profile for the carb holder once its milled out?
There's a small bump inside the Z1 carb that seems to mate to the groove on the carbs. Do you leave that in place, or just go for a flat inside profile?

The correct profile of the Z1 carb boot is is a uniform ID which has a slightly tight interference fit with the Flatslides. The bump or ridge is the first thing to be removed. The artsy part is ensuring the ID is uniform as possible. A digital caliper is necessary and is your friend as you slowly remove rubber material. Measure the ID often till you get the hang of it. You are going to remove quite a bit of material from each carb boot before you achieve a slightly tight interference fit between Flatslides and carb boots.

The Dremel worked well set to the 2-3 speed setting.

PS: Just curious you mentioned the hanging high idle problem. Was that associated with installing the Flatslides in the stock Z1 carb boots?

Ok. Boots are milled out as instructed. I actually found the perfect tool to do it consistently on the drill press. Works out to a "slightly tight interference fit, just a couple hairs smaller than the carb.
Currently waiting for the RTV sealant to cure, I'll give it 24 hours and try to run it again tomorrow.

srsupertrap wrote: PS: Just curious you mentioned the hanging high idle problem. Was that associated with installing the Flatslides in the stock Z1 carb boots?

So I've never actually started this bike with the stock carbs. No idea how they'd behave.

But that's why I was so hesitant to mill these boots out, because no. The guy who sold me these carbs included a set of adapters (pic attached.) He said they were custom made for these, but I *think* they're actually made for the 26mm mikunis, since the bore is a little smaller than the bore on the head. I'd originally bought the Z1 boots mainly to get the carbs synced.

Anyway, the flatslides in these adapters had the exact same behavior as the flatslides in these Z1 boots. The rubber boots on these adapters though have a similar ID and inside bump to the Z1 boots, so it might cause the same issue.

Nope, put the 30 pilot jets back in, and the problem came right back. So maybe these jets are too lean. However, the problem seems to be with the air screws set from 1/4-1 1/4 turns out. When I get to 1 1/2 turns out, it improves, but the bike won't stay running.

I tried spraying WD-40, water, and even propane all around the thing, and can't find any evidence of an air leak.

I'm going to try going up a jet size.

Edit 2: Went up 2 jet sizes (because that's what I have) and it gets better. Still not great. But it's also definitely running rich, cause now the exhaust is smoking a little.

Edit: Next jet size up is *better* but idle still hangs up. Best behavior seems to be with air screws 2 turns out, which is about the limit.
Going to have to order the next size up and try those.

Side question: Any chance this is the spark advancer? I put a timing light on it, and it seems to behave correctly, not sure if the springs might be worn or something.

Sorry to read the flatslides have the same exact behavior in the milled out Z1 boots, e.g. hanging idle. That cured mine but lets move on. Looking back this is an interesting photo. I replaced these O-rings when I cleaned and rebuilt my set. Find the tube extending into the carb bowl, then move up and you will see the o-ring. I know you have beat the vacuum leak theory to a pulp but its probably worth shooting the joint where the o-ring resides with WD-40 or propane.

Note the picture shows my air screws position notes written on the greasy newspaper when disassembled 3/4 turns out

srsupertrap wrote: Sorry to read the flatslides have the same exact behavior in the milled out Z1 boots, e.g. hanging idle. That cured mine but lets move on.

Nothing's ever easy. Appreciate the help.

I replaced these O-rings when I cleaned and rebuilt my set. Find the tube extending into the carb bowl, then move up and you will see the o-ring.

I've been avoiding taking that apart, specifically cause that gasket is NLA. It's still got the factory tamper-resistant screws in there. Since that o-ring seals the choke, I'd have expected that it would leak gas if it was bad. But you're probably right, I'm at the try-everything point. Hell for all I know, these NLA gaskets could be the leak.

I'm going to go back to my smallest jets (27.5) just as a baseline. Before milling the boots, those gave me the smoothest idle. After running those and actually writing down some notes, I'll get the front plate broken down today and hope I'm not cutting new gaskets by hand. Do you know what size those o-rings are by chance?

I'm also going to re-check the valves now that the bike's been run up to temp a few times.

Also: Needle position shouldn't matter for this specific problem, right? I have these set to the middle.

Edit:
Went ahead and replaced those o-rings. None were bad or even hardened. I'm starting to think that this problem isn't these carbs. Maybe valves, timing advance, dunno anything else that could affect it.

Still need to check the valves again.
Did some color-tuning on this run.
27.5 pilot jets + 1.5 turns out: Idles smoothly, and the idle *slowly* returns if the throttle is very slowly opened.
From 1/2 to 1 turns out, the idle hangs, but with a colortune, the color of the flame is a perfectly "bunsen burner blue"

It turns orange as expected when the throttle opens, but then when the throttle is closed and the RPM is hanging, the colortune shows the same blue from the ~3-5k hang down to normal ~1100 idle. Never turns white, so it's not lean at idle.

The O-ring size in the picture is ID 3.5mm, OD 7.2mm & Width 1.8mm. I got those O-rings from McMaster Carr P/N 9464K12. The NLA gasket shown in the picture is paper thin. I do not think it seals anything.

I sounds like you replaced all the seals (Boots & O-rings) that could contribute to a hanging idle. What do you have sealing the carb boot vacuum ports? I have connected the carb vacuum port between cylinder 1 & 2 to the vacuum petcock. The needle position shouldn't be contributing to the symptom. I am running a 27.5 pilot @ 5000ft

How long does your engine need choke? I start right away on half choke and then push in the choke after 30 seconds and it idles. I checked my valves about 100 miles after rebuild when my idle was hanging and found the valve lash had not moved. That does not mean anything one way or another.

Sorry not more help but I have plenty of O-ring if you need any for a full tear down. Curious did the seller state these carbs were working 1. perfect, 2. Ok 3. Hadn;t run in years or 4. No idea

srsupertrap wrote: I sounds like you replaced all the seals (Boots & O-rings) that could contribute to a hanging idle. What do you have sealing the carb boot vacuum ports? I have connected the carb vacuum port between cylinder 1 & 2 to the vacuum petcock. The needle position shouldn't be contributing to the symptom. I am running a 27.5 pilot @ 5000ft

Right now it's just capped with a rubber cap and a fuel clip. I plan on running it to the petcock once the bike is on the road.

How long does your engine need choke?

Full choke and a throttle blip till it fires. After about 10 seconds, it'll run on half choke for about 20-30 seconds, and then idle normally, with the idle rising to normal speed as the engine warms up.

Sorry not more help but I have plenty of O-ring if you need any for a full tear down. Curious did the seller state these carbs were working 1. perfect, 2. Ok 3. Hadn;t run in years or 4. No idea

I appreciate all the time you've taken on this. I'm starting to wonder if there's something wrong with these carbs. Nothing is drilled or sanded, I don't see any cracks. Seller said he ran them with a 615 kit, but no details.
I've got a couple more pilot jet sizes to try and the valve check, but if those don't work, I'm just gonna call it and go back to stock carbs and airbox. (And tbh, it wouldn't surprise me if this problem follows to those carbs as well.)

Edit: Valves are all in spec. Head bolts are all torqued to spec. Exhaust gaskets are tight, no leaks found.