keychange Posted October 31, 2010 Report Share Posted October 31, 2010 Hi - I have the Amal blues, just can't get good all round performance after spending dough on new jets, needles etc - I have been fooling around with a 2nd hand Mikuni VM26 from a DT175. It is in good clean condition and I have made sure all the jets are clean and operating. I have rigged up a suitable adapter to connect to the original air box. When I kick her over it fires then dies - after a few kicks I remove the plug and it's always wet. I did get her running roughly only at high revs and loads of smoke ( engine must have been loaded up) so I removed plug and left it overnight to dry out ( fuel off). Next day I removed the air filter - double checked seat needle and float (no idea how to measure with this set up) but there is no flooding through overflow (which isn't blocked) . But no improvement it just won't run - sort of thinks about it then dies. I would have thought I would get it to go even with wrong jets and then gradually tune from there but it just won't run. Does this suggest that the jetting is hopelessly wrong or should I be looking for something more obvious. Quote Link to comment Share on other sites More sharing options...
mrb505 Posted October 31, 2010 Report Share Posted October 31, 2010 Hi Andrew How are you, I have the jet sizes for the mikuni that you tried of mine, if it worked okay you could compare the sizes to the one your trying now. Main 150 Pilot 30 Needle jet 193N8 Needle 4EJ4 Slide 20 cutaway With the float I think you measure the bottom of the float to where the gasket sits while holding it upside down, as in the valve being shut by gravity from memory its something like 22-24mm Hope this helps Ross Quote Link to comment Share on other sites More sharing options...
keychange Posted October 31, 2010 Author Report Share Posted October 31, 2010 Gidday Ross good to hear from you, Was yours a VM26? I have been having so much fun on the TLR I have been ignoring the Cota which apart from improving the lean, hot running by returning everything back to how I got it - also means I have massive dead spot in the high end. Better than the erratic behavior I had before but still not an enjoyable ride. I know the cutaway on my VM26 is only a 10 (I will check the other jets etc later) would the cutaway make that much difference. The Cota is much more reliable with the old carbie stuff installed but it's frustrating to know that it can be so much better - if only it was reliable. You should come over for a ride one weekend ... I have improved a little Quote Link to comment Share on other sites More sharing options...
mrb505 Posted October 31, 2010 Report Share Posted October 31, 2010 Hi Andrew, This mikuni is a little smaller measures 24.8 and is marked as a 248 A2 wouldn't have a clue what it is off originally but seems to work fine on the 325 and 250 sherpa's, That is a very small cutaway I thought this one was small at 2.0 Yeah I'd love to come over for a ride one day Quote Link to comment Share on other sites More sharing options...
keychange Posted November 1, 2010 Author Report Share Posted November 1, 2010 (edited) I have gone back and rechecked and it has the number 286 then 03 on inlet side -I hadn't noticed before) I can't find much info this - it has the independent floats as per VM26 ??? Edited November 1, 2010 by keychange Quote Link to comment Share on other sites More sharing options...
keychange Posted November 3, 2010 Author Report Share Posted November 3, 2010 I am hopeless with details !! The cutaway is marked 1.5 The main jet has no identifying number I measured it as close as I can at 2.4mm or .095" does that sound right? - idle jet is 25 Quote Link to comment Share on other sites More sharing options...
keychange Posted November 3, 2010 Author Report Share Posted November 3, 2010 (edited) Now appears the carbie is from a something like a DT1E 1971 and that what I thought was a strange shaped main jet is a "setter needle jet" - the main jet is actually down in the float bowl screwed into the Banjo bolt (drainer) it is marked R - 90 How does a main jet at the bottom of the float bowl work??? Edited November 3, 2010 by keychange Quote Link to comment Share on other sites More sharing options...
mrb505 Posted November 3, 2010 Report Share Posted November 3, 2010 Long as the seal (o'ring) between the two halves is there and working its all the same just one is lying in the bottom of the bath the other is like a straw sipping from the bath. The good thing about the one lying in the bottom is that its easy to get it out and clean it, the downside is that it picks up any crap a lot easier . You can always borrow my mikuni again and give it a good test see if it works okay over a longer term then maybe set up your mikuni with similar jetting it might save a lot of guesswork. Quote Link to comment Share on other sites More sharing options...
keychange Posted November 3, 2010 Author Report Share Posted November 3, 2010 (edited) aha - there is no o-ring - well that explains everything .... I'll see what I have in my o-ring box. Thanks again Ross for the offer and I may take you up - I just couldn't figure out how this carb could be so far off as to not even run badly and now I know why. I fitted an o-ring and away she goes. It dies if I put the air filter on so my o-ring may not be sealing correctly - No amount of air screw adjustment affects it with air filter fitted but it works a treat without it. Anyway I have ordered some larger main jets - I guess that having the main jet on the feed side of the system means it affects the fuel flow for all stages but idle. I guess I also need the larger idle jet as well. Edited November 3, 2010 by keychange Quote Link to comment Share on other sites More sharing options...
hekto Posted November 4, 2010 Report Share Posted November 4, 2010 aha - there is no o-ring - well that explains everything .... I'll see what I have in my o-ring box. Thanks again Ross for the offer and I may take you up - I just couldn't figure out how this carb could be so far off as to not even run badly and now I know why. I fitted an o-ring and away she goes. It dies if I put the air filter on so my o-ring may not be sealing correctly - No amount of air screw adjustment affects it with air filter fitted but it works a treat without it. Anyway I have ordered some larger main jets - I guess that having the main jet on the feed side of the system means it affects the fuel flow for all stages but idle. I guess I also need the larger idle jet as well. About the main jet your assumptions are wrong. The main jet only affects the mixture when you are running full throttle.The needle and the needle jet covers most of the throttle positions. But the idle jet (pilot) also affects the size and position. More info: http://www.dansmc.com/carbs2.htm Quote Link to comment Share on other sites More sharing options...
keychange Posted November 4, 2010 Author Report Share Posted November 4, 2010 Yes Hekto but in this set up doesn't the fuel have to negotiate the main jet in order to get to the needle jet... I am only guessing - to be honest I originally thought the setter was the main jet. With the o-ring around the setter how else does the needle jet get fuel - but through the main jet. Although I agree this wouldn't appear to address the air filter issue. I also admit I am confused that the air filter is causing the engine to die (its clean) to me it means the mixture is still very rich - so I have to assume my o-ring isn't providing a suitable seal on the setter. This would allow the engine to suck fuel direct from the float bowl through the setter. Eveything I have read about Mikunis on Montesa mentions main jets much bigger than the #90 I have at the moment and so I have tried to order a range of larger - foolishly tried some crowd in New Zealand ( SME ) cheap but hopeless communications. I will try and pick up an o-ring tomorrow in Dorrigo - but I am not holding my breath Quote Link to comment Share on other sites More sharing options...
hekto Posted November 5, 2010 Report Share Posted November 5, 2010 Yes Hekto but in this set up doesn't the fuel have to negotiate the main jet in order to get to the needle jet... I am only guessing - to be honest I originally thought the setter was the main jet. With the o-ring around the setter how else does the needle jet get fuel - but through the main jet. Although I agree this wouldn't appear to address the air filter issue. I also admit I am confused that the air filter is causing the engine to die (its clean) to me it means the mixture is still very rich - so I have to assume my o-ring isn't providing a suitable seal on the setter. This would allow the engine to suck fuel direct from the float bowl through the setter. Eveything I have read about Mikunis on Montesa mentions main jets much bigger than the #90 I have at the moment and so I have tried to order a range of larger - foolishly tried some crowd in New Zealand ( SME ) cheap but hopeless communications. I will try and pick up an o-ring tomorrow in Dorrigo - but I am not holding my breath I'm most familiar with Dell Orto - Italians freak u' know! I think that Mikunis works very close to how Dell Orto's do. If you don't know where you are with the mixture it can be a good idea to start with the standard jetting and take it from there. Then make a 'tape tuner' for your gas grip, with marks for 25%, 50% 75%, 100%. There can be a great difference in what you feel is the throttle position and what it actually is. It take some while to get it optimal. This manual is very detailed and gives a lot of excellent explanations: http://www.guzzitek.org/documents/carburateur/DellOrtoMAJ01.pdf Quote Link to comment Share on other sites More sharing options...
keychange Posted November 5, 2010 Author Report Share Posted November 5, 2010 (edited) I'm most familiar with Dell Orto - Italians freak u' know! I think that Mikunis works very close to how Dell Orto's do. If you don't know where you are with the mixture it can be a good idea to start with the standard jetting and take it from there. Then make a 'tape tuner' for your gas grip, with marks for 25%, 50% 75%, 100%. There can be a great difference in what you feel is the throttle position and what it actually is. It take some while to get it optimal. This manual is very detailed and gives a lot of excellent explanations: http://www.guzzitek.org/documents/carburateur/DellOrtoMAJ01.pdf Dell Orto's cop a lot of flack but I have always found them reliable and straight forward - I bought my Guzzi 750 Nevada in 2002 and sold it a year ago - it was really nice little bike and so easy to tune and jet - thanks for advice, my problem has been with carb first identifying what it is then finding out what standard means... not easy with Mikunis Edited November 5, 2010 by keychange Quote Link to comment Share on other sites More sharing options...
keychange Posted November 6, 2010 Author Report Share Posted November 6, 2010 (edited) I got a fuel grade o-ring from town that fitted very well. I decided to enlarge the main jet OK I know you aren't supposed to but I figured I had nothing to lose and I've done it many times in the past (including my Moto Guzzi) so I took it up to the same size hole as an Amal 150 main jet. Fitted the carb and she started first kick and soon idling nicely. I fitted the air filter and still all good so I went to take it for a run and as soon as I opened the throttle it bogged down and never recovered absolutely loaded up. After 20 minutes checking everything discovered that the slide retainer pin had slid out of position and so the slide was rotating in the barrel. However when I had cleaned and reassembled I was back to the original fuel blowing back out the air inlet and loading up. I have to conclude that the o-ring has failed - I had double checked it and it appeared to be a good snug fit - is the Mikuni o-ring a special design or am I barking up the wrong tree. Is it a single use o-ring ? I did get a spare should I try it? I did replace the o-ring and no better - I have also lowered float level with no improvement... so I think it must be the o-ring Edited November 6, 2010 by keychange Quote Link to comment Share on other sites More sharing options...
hekto Posted November 7, 2010 Report Share Posted November 7, 2010 A 2-stroke engine where the piston controls the intake will always 'spit' out some fuel/gas of the intake. It depends on the timing and will normally be at low RPM's and during starting. This is due to the fact that the intake timing always will be symmetrical around TDP. Different arrangements with membranes and discs eliminates this. This is in general and I'm familiar with the actual design of your bike. How about the choke-system? Normally it's an independent air channel opened with some kind of a piston. It supplies fuel/air in a channel that surpasses the normal air intake. You write the fuel send backwards is building up? Is this during tick over or when you drives on it? It couldn't be oil that builds up in the crank? I can't see that the o-ring should do these problems. Quote Link to comment Share on other sites More sharing options...
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