ausy300pro Posted July 2, 2009 Report Share Posted July 2, 2009 has anyone tried a bigger carb on their 175 ? I have just finished rebuild of engine for my Majesty, it's now 185 cc with electronic ignition a DT head and gasket for more compression plus ports have been cleaned up and matched to liner etc, also has boyesen reeds, I am also going to skim flywheel a little to make it pick up a little quicker, thing is the standard carb is a tiny 22mm and I think it must be undersized with the mods that have been done, has anyone fitted a bigger carb ? I was going to go to 24mm and stick to a mikuni of similar spec to the original, is this worthwhile and would it be considered legal and fair for twinshock trials ? as usual any opinions/ideas much appreciated (also thought of going to 26mm as even a 125 gasser has a 26mm carb jetted to suit) Quote Link to comment Share on other sites More sharing options...
andy.t Posted July 3, 2009 Report Share Posted July 3, 2009 ausy300pro, I ain't no expert on the subject but its my understanding that the small carbs provide higher air speeds which are more suitable to the low speed of a trials bike, not sure if going big is the answer, I think I would try bigger pilot jets on the original carb for a start. must admit I did think of trying one of those flat slide OKO carbs with a 24mm bore, I don't think anyone would complain about mods like this in the trials that I have done. My personal opinion on mods like sticking two shocks on TY mono is just pushing things too far. Quote Link to comment Share on other sites More sharing options...
pat Posted July 3, 2009 Report Share Posted July 3, 2009 (edited) ausy, It would be nice if someone could be convinced to cast a batch of these engine parts ... http://www.mijossi.com/3ty/dossier/ty239.html flatslide 26/175cc or 28/180-239cc would probably work perfect and an adequate reedcage on top of the improvement. Cheers, Edited July 3, 2009 by Pat Quote Link to comment Share on other sites More sharing options...
ausy300pro Posted July 3, 2009 Author Report Share Posted July 3, 2009 I see where you are coming from with the small carb high gas speed for nice running at low rpm (andy t) but the modern 125.s all run with 26mm carbs so don't think that would be the case if a smaller carb was better, I was not wanting to go to a flatslide oko or keihin as that seems to be going towards the mono parts on a twin shock type thing, as you say I would consider putting twin shocks on a mono bike against the spirit and is actually against the rules of most twin shock classes (the wording is usually as originally fitted), I will probably try my rebuilt engine with the original carb and play about with jetting to see how it performs, I am more bothered about bottom and mid range than outright power, I am not going to be attempting to splat up an 8 foot step on my little TY ! I have seen some good mikuni copies, thay look exactly like the original carb and take all the mikuni parts, jets needles etc so may get one of those to try. can get a 26mm copy carb for Quote Link to comment Share on other sites More sharing options...
feetupfun Posted July 3, 2009 Report Share Posted July 3, 2009 Note the TY239 is actually 206cc and they have used the same idea to achieve the 206cc as on my TY175/205 (ie a 72.5mm ID sleeve) except they have started with a late model DT175 barrell - probably to allow the larger reed cage to be used. There is more machining work required to use the later model DT175 barrell due to having to modify the TY175 bottom end to fit the different pattern barrel retaining studs. The exhaust pipe has been rerouted to clear the head finning too. A carby bigger than the standard 22mm does help the top end, even on a 175cc motor, but will not help the bottom and mid range, and from my experiments, it is hard to get a 26mm round slide Mikuni to work as well as the 22mm carby in trials use. Trail riding is fine, but for precise control, the 22mm carby is better. I still use the standard carby on my 206cc motor and it goes great. As far as the 26mm carbies on modern 125 trials bikes, it is a bit hard to compare because 125s are not ridden much at low RPM, but their high RPM performance is important. I notice that the Rayer bike pictured appears to have had the swingarm lengthened. Quote Link to comment Share on other sites More sharing options...
worldtrialchamp Posted July 3, 2009 Report Share Posted July 3, 2009 have you seen this? Quote Link to comment Share on other sites More sharing options...
pat Posted July 4, 2009 Report Share Posted July 4, 2009 (edited) I do respect that engien character for trials could be a matter of taste. Replacing the TK on my TYR with a flatslide made major improvement (toghter with a proper reedcage design), offers more adeqate jetting, much better responce, more adeqate flow, mix air/fuel, idle adj, more torq and a WIDER power band not moved. Eg, with same gearing I almost can use the 2 Edited July 4, 2009 by Pat Quote Link to comment Share on other sites More sharing options...
ausy300pro Posted July 4, 2009 Author Report Share Posted July 4, 2009 what was the new reed cage you fitted ? I am going to try bike with original carb for now, may go bigger carb of same type in future but want it to be a legitimate twin shock (32 year old one) bike, you can go on for ever fitting modern parts and end up with a bike that is more modern parts than original ones (which I don't want to do as I might as well buy a new bike !). Feetupfun I know what you mean about the 125's and a good point that, to be honest I had forgotten as it's three years since I last rode my son's 125 gasser in a trial, you are right they don't have much bottom end it's all done with lot's of clutch slip and high revs so probably not a worthwhile comparison to the carburettion on a TY175. Quote Link to comment Share on other sites More sharing options...
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