34mm or 36mm round slides

Irish · Full Member Joined: Jun 12, 2007 Posts: 250

I see some guys run the 34mm and others the 36mm, which would be better on a '75 with pods and pipes?

grizld1 · Posted: February 20, 2008, 2:13 am Post subject: Re: 34mm or 36mm round slides

Said this many times, here it is again. Bottom line: a VM36 will perform just fine on a stock motor (which is what you have) but it won't do any more for ya than the VM34 will. Many guys use 34's on 750 racers, and that's not because they want to take their time getting around the track.

ttmaniac · Full Member Joined: Dec 22, 2006 Posts: 468

Do the right thing and get the 36mm flatslides.

A couple bucks more, but a whole lot better.

You can send me the thank you card later.

royfisk · Posted: February 20, 2008, 11:18 am Post subject: Re: 34mm or 36mm round slides

TT could you make up a parts list for the 36mm flatsides? I have a couple pairs of them set up for 2-strokes that I could convert. slide#, needles, jets etc?

ttmaniac · Full Member Joined: Dec 22, 2006 Posts: 468

I think I have the majority of the stuff listed on the official mikuni flatslide thread.

www.650rider.com/index...+flatslide

My pilot is too rich. I'd suggest trying about a 35 pilot for a stock motor.

The other guys say my needles are too rich as well, but it sure runs nice. Still ain't quite road ready till next week though.

Jack, 5twins, and Grizld1 all give their opinions as to a better needle selection and what have you on that thread as well. It's why I started it.

The flatslides are worth the extra twenty bucks or so each that they cost. MAJOR improvement vs. roundslide mikuni's.

I'd say go for it Roy. You won't be disappointed.:)

jimmythetrucker · Full Member Joined: Mar 12, 2008 Posts: 1283

My '79 XS had stock carbs on it. I started tinkering with sprockets -- went from 17 & 34 to 18 & 32. Vibration at highway speed was nearly eliminated. I was pleased with that. But then I found that, with the new sprockets, my old rat ran faster in fourth gear than in fifth because it didn't have enough power to pull fifth gear up to the redline. I could get the redline in fourth OK, but when I shifted to fifth she'd fall on her face and actually lose speed.

So now I've got another XS, an '81 model. I want to go with tall sprockets again, and I will. But I've never used custom carbs and so I'm wondering: Will 34 mm Mikuni round-slides give me enough extra power to pull top end in fifth gear -- or will I run out of steam again like I did with the stock carbs on my old '79? Should I go to 36 mm Miks? Or should I stay with 34's and start tinkering with camshafts and big-bore kits?

jayel · Posted: March 14, 2008, 10:51 pm Post subject: Re: 34mm or 36mm round slides

18/32 figures out to 1.777 try 17/31 it's 1.824 my 74 TX650 pull it ok, I run out of guts before it does in 5th

jimmythetrucker · Full Member Joined: Mar 12, 2008 Posts: 1283

I don't give a hoot about ratios, jayel. I'm wondering if 34 mm mikunis will do the trick or do I need to go to 36mm? And as for as my old '79 -- I guess if it wouldn't pull 18/32 @ 1.777, it wouldn't pull 17/31 @ 1.824, either.

Cooltouch · Posted: March 15, 2008, 1:02 am Post subject: Re: 34mm or 36mm round slides

Jimmythetruckerguy,

Before you spring for a set of RSes, you might want to try XSJohn's needle kits for the BS34s on your bike. I've been hearing great stuff about them. I'll probably be springing for a set myself soon (I have an '81 also).

Best,

Michael

jimmythetrucker · Full Member Joined: Mar 12, 2008 Posts: 1283

Cooltouch -- Problem is that I bought my 81 with two round-slides ON THE BIKE. I ain't buyin' no new needles for stock carbs because the stock carbs are MIA. I've never even seen them and the PO says he don't know where they're at. He bought the machine with no carbs on it and bolted the Mikunis on there himself, thinking that's all that was necessary -- yuppies, you see?

As it is, the bike will idle but that's all. Twist the grip the least bit, it has a stroke and dies. Thud. Both plugs are blacker than a coal miner's ass. I only bought the machine because the compression is so good @ 175/hole.

PO said he tried to get it running but doesn't know anything about carbs and never got farther than where it's at now. I looked at it and saw at once that the carbs were never installed properly but were instead just kinda smeared on there.

What's wrong? Start with the fact that the carbs aren't even a set. They are two left carbs rather than a left and a right. Clearance between the carb tops and the tank is so small that the throttle cables are kinked where they enter the carbs. The guy never knew about 90-degree cable guides. As you might imagine, there are other hardware problems as well. Had the poor slob got it running, he could never have ridden it far.

The upshot is that the right-side carb, at least, will have to be replaced with a REAL right-hand carb. So I thought now would be a good time to ask: Should I buy one new, right-hand 34mm or a pair (left & right) of 36 mm's? So I asked the question about the problem I had when I tried gearing up my '79.

I've never messed with Mikunis, either, and the only data I have to go on is the problem I had when I geared up my '79. I don't want to spend $300 for a set of new Miks and then find out I should have bought a set the next size bigger.

So if somebody would address the question I asked in the first place: Will two 34 mm's suck enough air into the engine for good top-end power with tall gearing or will I have to go to 36 mm's or will I have to soup it up even further by adding a cam or a big-bore kit or something? Or, if you prefer the short version: Do I buy one 34 mm Mik or two 36 mm Miks?

Answer please.

Jimmy

Cooltouch · Posted: March 15, 2008, 11:34 am Post subject: Re: 34mm or 36mm round slides

Jimmy,

There's lots of folks who race XS650s and use the 34mm roundslides. Your bike will operate with 36s but there's no need for them. If I were in your shoes, I'd get a singe right carb -- Sudco has good prices -- and then start going through the ritual of dialing them in. There's a number of folks here with extensive RS experience, and who can probably give you a quite accurate initial jetting scenario to get your bike running more or less properly. Sure sounds like yours are set way too rich.

About your high compression numbers -- those are higher than spec by about 25 psi, as I recall. I would not be surprised if your bike has a significant carbon build-up inside the combustion chambers. Might want to run some gas treatment in your bike for a few tankfuls after you get it running.

Best,

Michael

jimmythetrucker · Full Member Joined: Mar 12, 2008 Posts: 1283

Thank you, Michael. That's exactly the kind of info I needed. I will get a single right carb with proper jetting pre-installed. I will tear down one of the lefties and clean and rebuild it. As for the jetting on the lefties -- there's no tellin' how they're set up until I get 'em off the bike and open 'em up. The install was so badly botched it's impossible to get any idea how they're jetted at this point. But thanks to you I know where to start from and how to proceed from here.

Thanks again.

Jimmy

grizld1 · Posted: March 15, 2008, 3:54 pm Post subject: Re: 34mm or 36mm round slides

No need for a new carb body! Jet lists for the VM34 have been posted repeatedly, here and elsewhere, tuning procedures as well. Awhile ago 5twins (IIRC) found extemely good prices on Mikuni brass at www.crafttrading.com and you might check 'em out. Just do a little homework, buy the brass, and have at it. You won't solve problems just throwing money at 'em.

Cooltouch, "RS" in Mikuni carbs does not mean "round slide." It designates a banked flat-slide design with accelerator pump.

xsjohn · Last edited by xsjohn on March 16, 2008, 10:11 am; edited 1 time in total

jimmythetrucker.....I'm pulling 18/29 gearing with the BS34 carbs.......my hgeavy friend rides it too with no problems..........

xsjohn

Cooltouch · Posted: March 15, 2008, 7:38 pm Post subject: Re: 34mm or 36mm round slides

grizld1 wrote:

Cooltouch, "RS" in Mikuni carbs does not mean "round slide." It designates a banked flat-slide design with accelerator pump.

My apologies if I misled anyone. Sometimes I get lazy and abbreviate.

jimmythetrucker · Full Member Joined: Mar 12, 2008 Posts: 1283

grizld1 -- What do you mean there's no need for a new carb body? Can I turn a left Mikuni carb into a right Mikuni carb with a screwdriver and some small tools?

Let me know, please.

Jimmy

grizld1 · Posted: March 15, 2008, 11:17 pm Post subject: Re: 34mm or 36mm round slides

Doesn't matter! The VM36's aren't even available with left/right versions. If you don't like reaching in for the right carb choke lever, install a pull-up control on the right--lots cheaper than a new carb body. I use a cable-actuated setup on my VM36's--kinda pricy new, but I got the pieces used from the salvage carb stash at a local independent shop and soldered up the cables.

jimmythetrucker · Full Member Joined: Mar 12, 2008 Posts: 1283

Hmmmm! I didn't know about the choke cables, and I confess the idea never occurred to me. What I was thinking about was easy access to the choke and to the tuning screws. It would be nice to access the screws on the right carb from the right side rather than poking a screwdriver across from the left side and fumbling around in the dim light and deep shadows that are always present between carbs -- most especially when you're out on the driveway at night, pissed off and in a hurry.

I hear you saying I can get a "pull-up choke kit" to gain easy access to the choke on the existing carb. OK.

Does the "right" carb then NOT give right-side access to the tuning screws?

And to what do I hook the manometers when I want to balance my rehabbed Mikunis? The PO's install has the vacuum petcock (there's only one petcock on the '81 model) hooked to the vacuum port on the left-side manifold. The right-side vacuum port is plugged with a rubber cap. Do I hook the manometers to the manifold vacuum ports or to ports on the carbs themselves? I ask because there are a couple of un-plugged brass tubes on the carb bodies, which I'm not sure should be allowed because I don't yet know what those tubes are supposed to be for.

Also, I wonder -- since we're gettin' kinda far afield here -- should we start a new thread on this, fellas?

jimmythetrucker · Full Member Joined: Mar 12, 2008 Posts: 1283

XSJohn -- With that 18/29 gearing, what kind of top-end speed can you make? Does the Air Force shoot sidewinders at you when you break the law or what?

xsjohn · Posted: March 16, 2008, 10:35 am Post subject: Re: 34mm or 36mm round slides

To be honest I have never been much over 85 but it' still pulling strong.......when I worked for Yamaha I had to test the roadracers and I guess I got tired of trying to kill myself.......

xsjohn

grizld1 · Posted: March 16, 2008, 11:05 am Post subject: Re: 34mm or 36mm round slides

Jimmie, either way you go you're gonna be reaching in with a long screwdriver to get at something, since the mixture screws and the throttle stop screws are on opposite sides of the carbies.

xsjohn · Posted: March 16, 2008, 11:26 am Post subject: Re: 34mm or 36mm round slides

Maybe this guy can help.........xsjohn

jimmythetrucker · Full Member Joined: Mar 12, 2008 Posts: 1283

xsjohn -- My old '79 used to pull 90 in fourth (not yet redline but that was all she had). When I shifted to fifth she'd drop off to 85. It used to cruise real easy at 65, though. It had all the power I wanted at that speed, and it got great mileage.

I once did the arithmetic (speedo @ halfway to tach redline, times 2) and concluded that, with those 18/32 sprockets, it SHOULD do 120. I never knew if the problem was carb type/size or jet size. At the time (1996), I just thought I'd over-geared to old sow and decided to live with it.

It really was sweet, as long as I didn't ask too much of it. It was a rat when I got it and I rode it the rest of the way to death. It was at the end of the third set of swing-arm bushes (that I KNEW of) when I sold it to some kid for $400. The sprockets looked like two rings of fishhooks, but he never noticed. The tank was red and the head-pipes were chrome and the mufflers were loud, which I think was all he really knew or cared about. I'm sure the engine had a lot left in it, though. It didn't burn oil and it was smooth as silk.

Best.

Jimmy

xsjohn · Posted: March 16, 2008, 6:58 pm Post subject: Re: 34mm or 36mm round slides

Jim.....I gear mine high so I can do the express without reving it out so much..........not any other major reason.......still have the hardened 30 that I used for years......mikes higher 5th and the 18/30 would be better for me I bet........that would put the express even higher and make the lowers better too.........

xsjohn

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