XR 1000 flat slide carburettors

[hayleyl]

Hell all - wondering if anyone can help with a fix for sticking carb slides on an XR1000. I have a 1984 XR1000 (black and orange one) with a stage 2 factory kit that was fitted when the machine was new - carbs, pistons, cams and valves etc. Carbs are synchronised really well but they are beasts when the slides stick through air pressure - slides get sucked to the body of the carb. Not so bad if you're having a bit of fun up in the rev range - but if you're in traffic and the buggers stick then snap open - watch out! - it can be super dangerous.
I was thinking of replacing carbs with 'roller' slide carbs but I think they may look too modern. Is there a trick / mod that can be performed to eliminate the slide sticking problem on these flat slides? (I have the stock delortos but they are not functional with the factory stage 2 kit)
Cheers Hayley.

[EKHKHK56]

That sounds scary! A couple ideas come to mind. I understand these carbs are touchy about cable routing at the carb end. Most have a router on top of carb.
Certainly want to eliminate any question of the cables binding also from wear or dirt. The carb may have a worn or gummy slide. Cleaning and micropolish would help. Also on fiberglass tanks I've heard of different fuels eating any linings that have been applied and gumming up carbs. Any way to put on a push pull cable set up? These are a couple thoughts, I haven't actually ran into this problem/solution yet myself. Erik K

[55panman]

Hi, have to ask why the Delortos aren't functional with the Stage 2 kit. And what exactly did the stage 2 kit consist of. Always ran 38 round slide Mikunis on my
XR750 which I'm sure was quite a bit hotter, more cam, better porting, more compression, than that XR1000. They always worked great came off the corner excellent and were easy to tune. Ron

[DaveC]

The go to guy on the 39.5mm Mikunis for the XR1000 is Bob E. Sounds like you may need some silicone spray on the slide or stronger return springs. I run the 41mm Dellortos with the E cams, Branch heads and manifolds, JE Pistons and the Supertrapp XR750 mufflers. The Mikunis offer more cut away options on the slides (7 I believe) and the Dellorto only has 3 cutaways, so you have more options to address and attempt to solve the exhaust reversion problem between 3200 and 4200 RPM. However, the Dellortos are much easier to change needles and jets that the Mikunis.

[55panman]

Often reversion problems come from way too much throttle slide cut away. If you buy a carb kit from Sudco they come with too much cutaway. When you open the throttle up more, especially if you wack it, the intake goes to lean because it gets more air and the fuel flow can't keep up. If the cutaway is smaller it doesn't get as much air and the mixture remains right. I can wack my throttle wide open from an idle on my flattrackers and not get a bog. This is with the old round slide Mikunis and they don't have or need an accelerator pump if you run as low a cutaway as you can get. Ron

[sportsterpaul]

I would revert to the Del-Ortos. I tried the 40mm flat-slide Mikuni's on my 1980 show bike with Thunderheads. I tried three different carbs, all used ones off eBay. All of them stuck open just off idle. I consider them death machines. Note the larger bore Mikuni carbs have little rollers on the slides. You would not think that not idling down is dangerous, but trust me, it is.

I was quite mystified by the sticking. When I took the carbs apart, I could see there was all this goofy linkage inside to try and press the slide down directly so play and wear would be compensated for. It did not matter. The linkages were a bit different in the 3 carbs, they all stuck. I consider this to be their "end stage" behavior, like a cranky old lady in a nursing home.

I sold them all on eBay, warning that they stick, and good riddance. I will never use a Mikuni anything, since they have not recalled and crushed every single one of these monstrosities. I put an on old butterfly Keihin on my show bike, like I have on my '77, '79, and '62. My XR-1000 has Del-Ortos, I would not have bought it if it had Mikunis on it.

I do hear the round-slide 38mm Mikunis will work great, and are as smooth as a Tillotson, which had the best off-idle performance of any carb, when it was not catching fire. Kiehin is Honda, and they know a thing or two about automotive engineering. If I keep having trouble with my old butterfly Keihins, I will put the late-model CV Kehins on my bikes.

[hayleyl]

Thanks everyone for the advice - looks like I need to revisit the Delortos and throw the flat slides in the bin. Many thanks Hayley.

[55panman]

Sportster Paul, I ran 38 round slides on my 1977 XR750 flattracker and they always worked flawlessly. I hardly ever had to change the jetting, maybe just up or down 1 or 2 on the main jet and they came off the corner so nice. They're cheap at swap meets and have a ton of needles, needle jets mains, slow jets and slides.
All parts are available and Bill's Custom Cycle has the cables for them using a Barnett throttle. You can look up the jetting in the XR750 info or I could find it.
As I said I always ran the lowest slide cut away. I used a 1 1/2 cut away. The nice thing about them is the parts are so plentiful. Ron

[sportsterpaul]

A flat-tracker-- wait wait-- does this mean you push-started a bike with no accelerator pump in the carb? I am totally impressed.

I have met people that said they had a 38mm Mikuni on an Iron Sportsters and loved them, including a buddy that said his was a magneto bike. They told me the starting procedure was a bit different, and that you had to dial them in, but once dialed, it would start fine. All my bikes have a coil, so I would think they would be easy to start as well.

I have a buddy that owned a automotive machine shop and used to help his uncle build Offenhausers. He said an acceleration pump is a kludge and will talk about the "signal" of the manifold pressure and such. He loves CV carbs like the Mikuni. I like the 38mm round-slide because it is about as simple as the butterfly Keihin. And that big round slide has enough area so the manifold pressure pulls it back down, even if your return spring broke.

I put the flat-slide Mikunis in the same bin with that Lectron I had, a curiosity good for drag racers where there is only on and off, and no middle.

The CV Keihin solves the bog problem by letting the manifold pressure control the slide, and a butterfly controls the throttle opening. Much more complicated, but perhaps the best of both worlds. I do like a smaller venturi like the 38m since I think most guys over-carb their bikes the way we over-carbed our cars. I remember the rich kid that put some 1200 CFM Holley on his Chevy, only to find it un-drivable. I think the smaller venturi is why Tillotsons feel so good coming off idle.

I'll start reading up on the 38mm. I used to have one from a flea market, but it would be nice to have a new one, that can take alcohol in the gas.

[55panman]

Yes I bumped started my 1977 (ex-Ronnie Jones #16 rookie bike) XR750. Back it up on compression 4 or 5 steps, up on the seat, and let the clutch out. You had to leave the throttle closed until the motor caught and then just EASE!! the throttle out or it would kill the engine. I could bump it MYSELF! with leathers, helmet and a steel shoe on my HighPoint motocross boots!! But then I am Big for a dirt tracker and can be awfully determined. Since you were sidesaddle and no front brake, it could get rather exciting getting it stopped in a SMALL, crowded pit!! It took a few extra seconds to get your left foot on the peg and throw your leg over the seat to get to the brake, especially if you were dogging other bikes and people in a crowded pit. I would use a 36mm round slides Mikunis on TT tracks.
You are right about people over carbing. One of the best lines I ever heard ( and I always think of it still) was Bill "Grumpy" Jenkins, who said "YOU HAVE TO BE ABLE TO DRIVE THE CAR". Grumpy was a DRAG racer!! The original Pro Stocker and the man who made the Chevy "409" and "427" what they were. Meaning that
huge 1200 cfm carb was no good if, as you got off the start line and the secondaries tipped in the car bogged or the tires went up in smoke you couldn't pedal it because it didn't respond to the throttle. Big carbs are for Miles and Bonneville. Down on main street, or out of a slow corner, I love to be able to wack the throttle and have the bike respond immediately. The Mikuni won't be affected by alcohol. You can order new Mikunis from Sudco. If you need info and parts, it's them, and there's a guy in Wisconsin, can't think of his Shop now but I can find it, who will get you any parts in 2 days. Used a 42mm round slide on my 1955 Panhead Bonneville record setter 136 mph, actually hit 141 then slowed because I couldn't get it to feed enough fuel for 3 miles wide open. Fuel feed can be a problem at Bonneville. The internal fuel feed passage isn't big enough for that. Unfortunately have gone to a S&S even though I really worked to make the Mikuni work because I really like them. Ron