Carb replacement, again!

[pfg49]

I've just bought a Rickman built from a donor 78 SE. The bloke I bought it from, built it and never rode it, in fact he did 2, and literally never rode either! He likes building bikes, but not riding them. Each to his own I suppose. The engine is bog standard, except for the pipes which have removable baffles, and I'd imagine the baffles will be restrictive.
Having gone round it and done a spanner check to tighten up the loose nuts and bolts, I gave it a blast. It ran very well for about 5 miles, then 1 cylinder started to misfire. Nursed it home, did a few checks and thought it might be the condenser. I was going to buy an electronic system anyway, so I bought a Boyer and fitted it. NO change!! I've changed everything on the ignition, so it must be fuel. Off with the carbs and I found this.

Lots of crap in the float chamber and the carb body on the side of the soldered float. Took everything to bits, gave all the passageways in the carbs a good clean and a blast of air, bent the tang to try and compensate for the extra weight on the float, and put it all back together. Did 15 miles, no problem. Went out with my son on his Suzuki 550GT 2 stroke, got 5 miles and the misfiring is back.
Now the carbs aren't bad, but I will have to buy a new float, and several other bits and pieces including the diaphragms which don't look to stunning, and are a bit hard, so sod the expense, give the cat a goldfish, I'll buy some new carbs. Now my choices are, Chinese Mikuni or Keihin rip-offs, genuine VMs, or something more exotic. For me it's the rip-offs or the genuine VMs.
I had some Chinese Keihins on my 71 BSA Lightning, and my 74 Triumph T150 triple, they worked well, neither set was spot on, but fairly close, but I only got rid of them because I couldn't find anyone to rolling road them. I've read on here about the trials and tribulations connected with the Keihins, and I didn't even know you could get rip-off Mikunis. My questions to the wise and experienced carb swappers on here is, has the build and quality of the Keihins got anybetter since mrriggs marathon struggle, has anyone used the Chinese Mikunis, and finally, as I have a 78 SE engine, do I really need 38s or should I use 34's? I always worry about too big a carburettor on an engine.

 

[RustiePyles]

If you’re going to put aftermarket carbs on save yourself the headache and get a set of VM34’s from Gary Hoos. They’ll be jetted near perfect when you get em and he’s the cheapest around. I’ve never had any luck with Chinese knock-off’s, XS’s are finicky about carb tuning already I wouldn’t compound that with cheap carbs.

 

[grizld1]

Hmmmm. You have a high dollar Rickman there, but you're thinking about bargain basement knockoff carbies? For goodness' sake, man, give that fine ride something decent!

Re. venturi size, a 38 mm. diaphragm slide carbie has about the same unobstructed venturi area as a 34 mm. mechanical slide pot. The OE BS34 works well, and Euro versions were particularly good. If you're thinking about new carbies, the VM34 is the usual choice, but the smaller VM32 would yield more effective venturi that the OE BS34 and be less demanding to tune.

But if you want something as classy as that grey Rickman (which BTW has me turning green with envy), check out the TM34 4-stroke flat slide pumper kit from www.mikuni-topham.de. There's a lot of work in the custom throttle shafts and brackets in that kit, and the price is a bargain for what you get. I've used 36 and 33 mm. Mikuni 4-stroke flat slide pumpers on my XS650, and in terms of ease of tuning and performance gain they're a world away from the old VM and OE carbies.

 

[Kevin Werner]

Rickman frame, the stuff of wet dreams.

 

[arcticXS]

I have those TM/RS 34 flatslides from Topham on my XS, and they are great. I believe they may also be available from Allens Performance in the UK.
Another alternative is the GPZ500 carbs, that got a lot of attention here on the forum some years ago.

 

[grizld1]

BTW "RS" in Mikuni talk means "Radial Smoothbore," not "Round Slide." The carb that arctic and I referred to is the TM34-B120, which is an RS design labeled as a TM because it is sold by Mikuni as a single carburetor rather than in a bank of 4 like the RS series. The TM34-120 has a bell crank slide and an accelerator pump. The conventional TM34 has cable top slides and no AP and requires very different jetting. Unfortunately Mikuni America does not import the TM34-B120. Like many parts for carburetors that Mikuni America does sell, they have to be ordered from Europe. I'm certainly not implying that Mikuni America delivers poor product support for some of their most expensive motorcycle carbs--I'm stating it up front, loud and clear.

 

[gggGary]

Other than having a set on the shelf (don't say it GBear!) I haven't run the GPZ carbs but if you insist on penny pinching they should be a top choice.
that exhaust is going to fuggle your tuning either way....
2 more go buy a new float, use the 38's, they work well. Have they got airboxes in front of them?

 

[grizld1]

Yeah I'm gonna say it, ya lazy rascal! Those TM36/68 pumpers are all ready to hang in a pair of OE BS34 boots, and I'll betcha dollars to donuts you could hang 'em on Madness and go without messing with a thing except maybe--just maybe--a step or two up on the mains. That is if Madness has gotten the attention the poor beast deserves. Grrr.... But right, better to invest time and money in OE carbs than waste both on junk knockoff carbies.

 

[Smuggy]

Hi @arcticXS,

What throttle cable did you use with your set of TM/RS 34s. Did stock work fine?

 

[pfg49]

Smuggy,
I didn't build the bike, I just bought it, and I am changing it to how I want it. As I was working with the carbs on Saturday,I do think that the throttle cable is standard though, and it works fine.

 

[arcticXS]

Hi @arcticXS,

What throttle cable did you use with your set of TM/RS 34s. Did stock work fine?

The carbs came with throttle cables (push/pull setup with 2 cables, and a new throttle grip.

 

[pfg49]

I started with what you see at the beginning of the thread, and I've now got to here. I much prefer the spoked wheels, more in keeping with the Rickman style, and the next thing is to get a "Desert" tank at 12 litres, because the one on the bike is only 5 litres!

 

[toglhot]

I looked at the Mikuni knock offs and was all set to buy them, then I was informed the threads on the jets were different that genuine Mikunis, so I bought some VMs from Mikunioz. I did have problems with interference from the petrol taps, but spun up some manifolds, so that fixed that. Looks like there is plenty of room on your bike though. Hoos Racing has has rubber manifolds to suit, longer manifolds like the ones I made, or the two piece manifolds. Looks like the short rubber manifolds would fit your bike though. The VMs are great carbs, instant response, cheap parts, simple, give a slight performance boost, super easy to jet and tune and best of all, you get to bin those stooopid BSs. Being an off roader, the longer manifolds could also suit, the longer manifolds, on paper at least, improve low end performance, in reality you'd never notice though. The inlet tracts would need to be in the vicinity of 200mm long to realise any performance boost, but again you'd hardly notice. Performance mods like longer inlet tracts,, porting, etc, are only worthwhile in concert with other mods like lumpier cams and so on.