jaan Posted April 3, 2008 Report Share Posted April 3, 2008 Which Carb and what size is worth to try for Montesa Cota 348 - instead of Amal? Quote Link to comment Share on other sites More sharing options...
02-apr Posted April 3, 2008 Report Share Posted April 3, 2008 Can't remember the details but Sammy Miller did a Mikuni for them at the time which made it a bit smoother - may still be listed? Quote Link to comment Share on other sites More sharing options...
motovintage Posted April 3, 2008 Report Share Posted April 3, 2008 (edited) go with the same size as your amal, (probably a 28mm) I like the mikuni (PM or PJ ?)round slide, it's an easy carb to work with and get parts & jets for, I have a kiehn on my 123 and like it, either one is way better carb than the Amal in my oppinion, after getting the kiehn on and getting the jetting right it produces quite a bit more performance, better bottom power and a lot better top end power. Edited April 3, 2008 by MotoVintage Quote Link to comment Share on other sites More sharing options...
hrc1 Posted April 3, 2008 Report Share Posted April 3, 2008 My 348 came with a Mikuni fitted. Haven't had time to give it a good run yet, still fettling & last month was taken up with a non stop run of "must ride" trials! I could have a look & get pics & jetting if you like. One day I'd like to try a modern carb on this - Keihin or Delorto VHST. Both I've used on other modern bikes with good results. Quote Link to comment Share on other sites More sharing options...
peterb Posted April 4, 2008 Report Share Posted April 4, 2008 Hello Jaan, I have successfully used a standard DellOrto PHBL 26 carb, jetted for a Gas Gas PRO 250/280/300 but fitted a 95 main jet for many older bikes. Bultaco 325, 250's and an Ossa MAR. They are easy to work on and easy to get a hold of jets, also not too dear. Performance is good. Bye, PeterB. Quote Link to comment Share on other sites More sharing options...
michaelmoore Posted April 4, 2008 Report Share Posted April 4, 2008 For use on an AHRMA machine here in the USA the first issue would be "what carb is legal to use on this bike in vintage competition?" The trials group may not be as "sniffy" about that as the roadracers can be, but it always pays to look at the rule book first. IIRC, the Bing was considered to be an improvement over the AMAL Concentric on the 325 Bultacos. My 1974 model had the AMAL. It seemed to work OK but they start wearing out as soon as you take them out of the box. In some of them you can fit a nice chromed brass slide out of a Mikuni VM and solve the body wear issues. But the needles/needle jets seem to wear too and may need to be replaced every season or two depending on how much you ride and how sensitive you are to minor changes in carburetion. A friend was recently buying a lot of jets for some big VM Mikunis that he was fitting to an XS650 Yamaha and he was running into "sorry, we've discontinued those jets/slides" replies from Sudco on some of his orders. So I'd suggest you make sure what you are allowed to run and that you can get parts before you start looking at "which carb actually works the best." cheers, Michael Quote Link to comment Share on other sites More sharing options...
Guest majestyman340 Posted April 4, 2008 Report Share Posted April 4, 2008 Which Carb and what size is worth to try for Montesa Cota 348 - instead of Amal? Pretty much any Jap carb is going to be a major improvement over the Amals, which are very poorly made as well as expensive. Currently an Ebay seller has VM26 and 28 Mikuni carbs listed for around Quote Link to comment Share on other sites More sharing options...
jaan Posted April 4, 2008 Author Report Share Posted April 4, 2008 (edited) I have now on the bike an Amal 2627 (Made in Spain). The slide was worn out but I have ordered a new one. The bottom rubber seal of the choke plunger was a little bit worn so I ordered it too. The first owner of the bike said that he has had problems with cold starting. I have too a new Keihin 28 mm and an older Mikuni 28 mm. Thus I could try which of them works best. However I have no ideas about the good jetting of Keihin and Mikuni. I'd really appreciate, hrc1, if you could look at the jetting of your Mikuni. Edited April 4, 2008 by Jaan Quote Link to comment Share on other sites More sharing options...
jaan Posted April 4, 2008 Author Report Share Posted April 4, 2008 Pretty much any Jap carb is going to be a major improvement over the Amals, ---- How about Bing Carbs? They are obviously used on some spanish trial bikes. Are they German or Spain made? Quote Link to comment Share on other sites More sharing options...
michaelmoore Posted April 4, 2008 Report Share Posted April 4, 2008 Jaan, if the slide in your AMAL is worn then it is very likely the body is too. Putting in a new slide is not going to take care of all the wear. That's why there are people who bore the bodies and sleeve the slides or substitute a Mikuni slide. The AMAL slide and body seem to be made from the same metal and that causes them to wear quickly. Once you get different materials (like an aluminum body and a chromed or brass slide) the wear goes down quite a bit. Also keep in mind that the AMAL is prone to distort if the flange clamp nuts are tightened too heavily. Generally, I think that if you can get away from the AMAL you'll be happier in the long run. cheers, Michael Quote Link to comment Share on other sites More sharing options...
jaan Posted April 4, 2008 Author Report Share Posted April 4, 2008 The AMAL slide and body seem to be made from the same metal and that causes them to wear quickly. Once you get different materials (like an aluminum body and a chromed or brass slide) the wear goes down quite a bit.Generally, I think that if you can get away from the AMAL you'll be happier in the long run. cheers, Michael Thanks, MichaelMoore, I'll do as you suggest. (Maybe some guy after me wants to restore "ex-Jaan bike", then he has an Amal. ) It seems to me that the body of my Amal (Made in Spain) is aluminum and the slide zinc. However I'm not any metallurgist. Quote Link to comment Share on other sites More sharing options...
hrc1 Posted April 4, 2008 Report Share Posted April 4, 2008 (edited) The mikuni is jetted: Main 95, pilot 20. No markings on needle jet. Thats how it came - I can't comment on how this runs as its being fettled before I give it a bit of a test.... The carb has an oval bore, in the slide area and ends up a 30mm going into engine. I have some pics - I'll add later. As pub beckons right now! Edited April 7, 2008 by hrc1 Quote Link to comment Share on other sites More sharing options...
michaelmoore Posted April 4, 2008 Report Share Posted April 4, 2008 Whatever the AMAL MK1 Concentric is made out of, it doesn't last very long. I've seen mention that both bodies and slides were zinc (possibly the Mazak alloy?) and it wouldn't surprise me if some had one part in one metal and the other in a different one. The MKII is supposed to be a lot nicer. It appears that running metals that are too similar in hardness against each other can cause some problems. I sent the 932 Concentrics from my B50MXs out to have the bodies bored and chromed Mikuni slides fitted. The reports I've seen is that modification makes the wear, well, about like that in a Mikuni. The AMAL can meter well (I've heard the MKII Concentrics can be quite good) but the MK1 Concentric won't do it for long because the wear is so rapid. Once the air starts going around the slide instead of under it I doubt you'll have a very happy time with it on a trials bike. http://www.jba.bc.ca/Bushmans%20Carb%20Tuning.html http://www.britbike.com/ubb/cgi-bin/ultima...11;t=001129;p=1 http://www.freewebs.com/bsacontent/pdf/ama...tricstuning.pdf http://www.ntnoa.org/Carbs.htm The material on that sites may be of some interest to you if you do persevere with the AMAL. cheers, Michael Quote Link to comment Share on other sites More sharing options...
jaan Posted April 5, 2008 Author Report Share Posted April 5, 2008 Thanks MichaelMoore and hr1. I'll use Mikuni or Keihin on my 348 and will leave my Amal Mk2 for the historians. Quote Link to comment Share on other sites More sharing options...
jaan Posted April 5, 2008 Author Report Share Posted April 5, 2008 For use on an AHRMA machine here in the USA the first issue would be "what carb is legal to use on this bike in vintage competition?"Michael Obviously flatslide carbs are not legal to use for example on Cota 348 there in the USA? If they however are legal in some country, is it worth to try them? Does flatslide carb work better than roundslide carb on twinshock trial bike? I've heard that they do work fine with full throttle (motocross, classic TT) but not so fine with low rpm. axulsuv is very pleased with OKO flatslide carb on his twinshock Fantic (Fantic > "Carb! Swap from delorto to OKO"). Quote Link to comment Share on other sites More sharing options...
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