moment88 Posted February 9, 2008 Report Share Posted February 9, 2008 (edited) I have a '74 Montesa Cota 247. I have owned it for approximately 28 years. After many years of mediocre service, my Amal carb began to give me serious problems at my last trials event. I am running with the stock Amal carb (rebuilt, in great condition). At the previously mentioned event, the throttle response deteriorated rapidly as the sections clicked by. It took longer and longer for the revs to fall as the day went on. It was not a hot day. By the third loop, I could barely maintain "3's" and control the bike, as the lingering revs would propel me MUCH farther into a turn than I anticipated. I am aware of the consequences of over tightening the carb flange mounting bolts resullting in a "warping" of the slide chamber. I did not do that. My throttle cable was not kinked or binding. The mounting flange has been "surfaced" to reduced the possibility of any air leakage. It has new gaskets and o-rings. I am now giving serious consideration to defecting to a Mikuni carburettor. Does anyone have an opinion on this or any experience with making such a switch? How about jetting? How do I know where to begin? Finally, will I be able to procure a non-spigot mount Mikuni the correct size for this machine? Suppliers or sources? Any thoughts or advice on this matter would be greatly appreciated. OR, does anyone have any ideas on how to make the stock Amal a worthy component on my machine? Thanks, Matthew Edited February 9, 2008 by Moment88 Quote Link to comment Share on other sites More sharing options...
dfwilson Posted February 9, 2008 Report Share Posted February 9, 2008 I have a Cota 247 with a Mikuni and it ran great (it is apart right now to fix the frame). The conversion required an approximately 1" thick spacer. I made the spacer out of Delrin plastic. DFW Quote Link to comment Share on other sites More sharing options...
feetupfun Posted February 9, 2008 Report Share Posted February 9, 2008 (edited) Matthew Yes it sounds like unwanted air is entering your 247 motor. Don't assume it has to be the fault of the AMAL (but it may be). Because you have been diligent with your meintenance of the carby and have taken the trouble to avoid the usual pitfalls relating to air leakage and flange mounted carburettors, and that you know the history of the bike, you would know if the crankshaft seals have been replaced recently. If your bike has the original seals, they would be quite hard by now and susceptible to leakage when combined with any wear in the crank bearings (main bearings). Because you didn't mention heavy smoking as the performance deteriorated, it may only be the magneto side crank seal leaking at this stage. If on the other hand, you have replaced these seals in the past few years, the leakage may be the result of another issue. Here are some other possibilities: Cylinder base gasket leaking - would be visible as an oily deposit somewhere on the gasket joint. Body wear of the AMAL. You mentioned rebuilding the carby but didn't say if it had been resleeved. The slides wear very fast and you may have replaced the slide many times. The slide contact surface in the body also wears but at a lower rate and can cause leakage. Yes it is fine to fit a Mikuni carby. Mounting flange adaptors are available from Mikuni suppliers so you can fit a spigot mount carby to your 247 barrel. Suggest Hugh's Bultaco, Craryville, New York state as your Mikuni supplier. If you want your 247 to continue to look original, you can still buy new standard AMAL carbies. David Edited February 9, 2008 by feetupfun Quote Link to comment Share on other sites More sharing options...
moment88 Posted February 9, 2008 Author Report Share Posted February 9, 2008 David: I think your suggestion of the inspection of the crank seals is a very worthwhile endevour. You know how time passes, and maybe I haven't checked (or replaced) them as recently as I think I have. A few years back, I had a guy in Arizona (3C carbs?) re-sleeve my Amal body and slide with brass to reduce the inevidable "Amal" wear, but evidendtly, he went out of business, which leads me to believe that maybe that wasn't such a good idea (or, just not profitable). It was supposed to correct that problem of excess air leakage around the slides of Amal carbs. It is ironic that you recommended Hugh's Bultaco only because his establishment happens to be in the same state that I live in. Small World huh? Matthew Quote Link to comment Share on other sites More sharing options...
swooshdave Posted February 10, 2008 Report Share Posted February 10, 2008 There are at least three places in North America still resleeving the Amals. I hadn't heard about the place in Arizona, are you talking about AMR? Because they still seem to be in business. There is Lund in Washington and Triton Machining in Canada (they no longer have a website so you have to call). I used Triton and was very happy. Of course now with the weak dollar I may have to try somewhere else in the States. As for which way to go I prefer the simplicity of the Amal. Once you have a sleeved Amal, most of the problems go away. Even if I bought a new Amal I'd get it sleeved. Quote Link to comment Share on other sites More sharing options...
jaan Posted March 4, 2008 Report Share Posted March 4, 2008 Hugh's Bultaco sells 28 mm Flat Slide Mikunis too, have you tried them? Are they much better than round slide Mikunis? Quote Link to comment Share on other sites More sharing options...
swooshdave Posted March 4, 2008 Report Share Posted March 4, 2008 Hugh's Bultaco sells 28 mm Flat Slide Mikunis too, have you tried them? Are they much better than round slide Mikunis? If it matters, some competitive organizations don't allow the flat slides (AHRMA for example). Quote Link to comment Share on other sites More sharing options...
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