tup Posted February 21, 2006 Report Share Posted February 21, 2006 I am restoring a pair of Montesas and need some help with the jetting. I have new Mikuni VM26 carbs for each and have been given jetting for both from a couple of sources here in the US (expat Yorkshireman), but the 348 which is finished would not pull the skin of a rice pudding. Any one have any experience of putting Mikunis on the 348/349. Thank You. Quote Link to comment Share on other sites More sharing options...
feetupfun Posted February 22, 2006 Report Share Posted February 22, 2006 I am restoring a pair of Montesas and need some help with the jetting. I have new Mikuni VM26 carbs for each and have been given jetting for both from a couple of sources here in the US (expat Yorkshireman), but the 348 which is finished would not pull the skin of a rice pudding. Any one have any experience of putting Mikunis on the 348/349. Thank You. <{POST_SNAPBACK}> If the 348 runs reasonably smoothly at most throttle positions and RPMs with your new VM26 carby, the problem you have with your rice pudding is probably something other than the carby jetting. If the jetting is wrong, it will run either rich (4 stroking and lots of exhaust smoke) or lean (flat spots and pinging) or won't run at all. The rice pudding description is not enough info to go on. Is it a standard modern off the shelf VM26 carby bought from Mikuni? Is it an OEM Mikuni VM26 ie made for a 1970s Yamaha, Suzuki, Kawasaki? Is it a modern carby jetted for a Beta reed valve trials bike? They are all VM26 Mikunis. You can look up the standard jetting for a VM26 from the Mikuni website. If you want more help, please give more info about the symptoms and the type of VM26. Quote Link to comment Share on other sites More sharing options...
tup Posted February 22, 2006 Author Report Share Posted February 22, 2006 Hello and thank you for the rapid reply. The Mikuni is new from Sudco the US importer and was fitted with the slide, needle and jets as quoted to me by a US Montesa fellah. The engine has new points, condenser and spark plug all set as per Montesa specs. New crankcase oil seals, and shows 125 pounds secondary compression. I am running 80:1 with synthetic oil. It seems to be running rich as I do get a fair bit of blue smoke and a wet plug. It may be that I am expecting something that I will not get with the 348 with its extra drive side flywheel etc. I use a TY 250 at the moment and the performance of that, particularly off closed throttle, is my point of reference. The 348 will not pick up the front wheel and the exhaust note sounds a bit flat. Basicaly I am being lazy and hoping that someone can give me a closer carb spec to save me some time. We all live in hope. Mike. Quote Link to comment Share on other sites More sharing options...
feetupfun Posted February 23, 2006 Report Share Posted February 23, 2006 I'm still not convinced from what you have written that there is something wrong with the jetting and I don't want you to go on a wild goose chase. The four local completely standard 348 Cotas in my area all perform very similarly and all have lots more low and midrange torque than a TY250 twinshock (434 and 493). If your 348 is actually capable of being ridden and continuous running yet won't even lift the front wheel, there is probably something else wrong other than the carby jetting. Because you have told us that you have the ignition set right, replaced the crank seals and the compression sounds OK, I'm assuming you have not checked the following other possibilities for problem sources: Blocked or otherwise restricted exhaust pipe/exhaust port Restrictive air inlet/non-standard airbox/incorrect air filter medium Leaking air hose between carby and barrel Incorrect port timing ie a ported barrel or a barrel from an MX or enduro Montesa or the piston skirt shortened at the back. PS Are you sure you have not set the timing 1.8mm after TDC rather than before TDC? PPS 80:1 synthetic oil ratio is quite extreme for an air cooled bike. I would suggest somewhere between 30:1 and 45:1 David Quote Link to comment Share on other sites More sharing options...
tup Posted February 23, 2006 Author Report Share Posted February 23, 2006 Hello David, Thank you for the time and effort that you are putting into helping me work my way through this. I have the standard airbox and a new, oiled filter. A new rubber connector for the carb, and as far as I am aware no restrictions in the exhaust (though I will whip it off and have another look). Ignition timing is set BTDC, though I am a little intrigued by the fact that I do not seem to be able to over advance to the point of getting it to kick back. Over the thirty years that I have worked on bikes I have sometimes used this ploy as a starting point when I did not know the timing spec, just another little mystery. I cannot vouch for anything inside the motor though as far as I am aware everything is standard 348. As to the oil ratio, everyone I know here in the NE USA uses at a minimum 50:1, and friend who is an aknowledged Ossa expert, plus a Montesa dealer of many years tell me 80:1 for the 348. I have run the TY on 50:1 for the last six years without any problems or signs of wear on its annual strip down. Anyway, thanks again for all of the help, it is much appreciated. Mike Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.