black111r Posted September 16, 2011 Report Share Posted September 16, 2011 Hi PASTED IN FROM CLASSIC SECTION Well I have got the new Mikuni VM26 set up with a 150 main and 30 pilot. Starts pretty good, and runs cleanly but it is now missing when it gets hot. I am running it on 33-1 and with the air screw by 1.5 turns out. Rubber inlet hose in good condition with no leaks. Looking at the plug I have noted that the NGK BP-7ES look pretty hot (ie no hint of soot, and a copper hue). Could this be the problem. Any thoughts of the next step to get the bike running cleanly. Is it worth changing the plug (to what type?), revert back to 25-1, or backing out the airscrew. Or all of the above! Thanks in advance Chris Posted 13 September 2011 - 06:31 PM Hi I am wondering if anybody could help me. I have just finished restoring an OSSA 250mar Mk 2(1975) and have bought a new Mikuni VM26 for it as the bike was running badly with an old worn Amal. It was hard to start and lost its tune very easily and constantly oiled the plugs However after fitting the new Mikuni VM26 it starts easy but hiccups and wont rev out when even slightly warm. The new carb has been set up with a 120 main and a 2.5 pilot but when running it appears starved of fuel. Any thoughts on what the correct jetting and settings should be? Any thoughts most welcomed Chris White Report MultiQuote Reply Edit #2 tony283 Advanced Member Group:Members Posts:337 Joined:08-March 07 Bike:Yamaha/Triumph/RE Club:AHRMA/ITSA/BARHAM Posted 13 September 2011 - 08:43 PM Use a 150-160 Main and a 35 pilot with airscrew at around 1.5 - 1.75 turns out. Tony Tony Down Tony's Blog "A Black Cat is Lucky!" Report MultiQuote Reply #3 black111r New Member Group:Members Posts:2 Joined:13-September 11 Location:Brisbane, Australia Bike:Ossa 250mar 1975 Club:WDTC Posted Today, 05:31 PM Hi Well I have got the new Mikuni VM26 set up with a 150 main and 30 pilot. Starts pretty good, and runs cleanly but it is now missing when it gets hot. I am running it on 33-1 and with the air screw by 1.5 turns out. Rubber inlet hose in good condition with no leaks. Looking at the plug I have noted that the NGK BP-7ES look pretty hot (ie no hint of soot, and a copper hue). Could this be the problem. Any thoughts of the next step to get the bike running cleanly. Is it worth changing the plug (to what type?), revert back to 25-1, or backing out the airscrew. Or all of the above! Thanks in advance Chris Quote Link to comment Share on other sites More sharing options...
keychange Posted September 16, 2011 Report Share Posted September 16, 2011 Read this then double check what mixture you should be running http://www.smokstak.com/articles/oils.html Quote Link to comment Share on other sites More sharing options...
black111r Posted September 17, 2011 Author Report Share Posted September 17, 2011 Many thanks, what a great series of thoughts on the link between oil/petrol mix, jet sizes, plugs and the the temp that engines run at. Next step is back to the manufacturers recommended 25-1 and see how that helps. Interestingly enough I have only recently tried leaning out the mixture due to every other person that I talk to at trials recommending that I do so. i.e "with modern oils you can go to 50-1, 100-1" etc etc. One question though. If an engine is running hot and misfiring because of it what would be the correct plug type to change to. It currently has the manufactures recommended NGK BP-7E. rgds Chris Quote Link to comment Share on other sites More sharing options...
keychange Posted September 17, 2011 Report Share Posted September 17, 2011 (edited) I'd stick to the recommended plug and try and fix the problem rather than treat the symptoms. What can cause the symptoms: timing, jetting and mixture setting, air leaking around inlet or in carbie, crank seals... it could be a combination and so you just have to start (with easy first) and work your way through. By the time it's over you will really know your engine much better Edited September 17, 2011 by keychange Quote Link to comment Share on other sites More sharing options...
black111r Posted September 18, 2011 Author Report Share Posted September 18, 2011 I'd stick to the recommended plug and try and fix the problem rather than treat the symptoms. What can cause the symptoms: timing, jetting and mixture setting, air leaking around inlet or in carbie, crank seals... it could be a combination and so you just have to start (with easy first) and work your way through. By the time it's over you will really know your engine much better Firstly I will try the mix back at the recommended 25-1, using the recommended NGK BP-7E. Lets see how that goes thanks for all the help and I will revert. Chris Quote Link to comment Share on other sites More sharing options...
black111r Posted September 19, 2011 Author Report Share Posted September 19, 2011 Okay what should I try next? Mixture back to 25-1, Correct plug- NGK-BP7E gapped to 0.20 (0.51mm). Inlet hose perfect with no air leaks, clean filter. New carb (Mikuni VM26, 150 main 30 pilot) Starts easily with the choke. Choke off, then bike runs cleanly. After say 3-5mins of just riding around bike starts to misfire v badly and will not run with an open throttle, will still idle though. The plug comes out clean, copper colour with no hint of soot or white powder. This type of problematic running is new. With the amal is was starting that was hard but it is hard to say whether this new problem was developing. Is it time to try a new Motoplat coil, the existing looks original so is the best part of 35 yrs old. The coil does not feel hot to the touch when the misfiring starts and I do know that the coil is well grounded to the frame. Any better alternatives? Any thoughts Chris Quote Link to comment Share on other sites More sharing options...
keychange Posted September 19, 2011 Report Share Posted September 19, 2011 (edited) I would replace the condenser before the coil- if like the Montesa it is mounted under the flywheel - this is a good opportunity to remount it under the tank near the coil. However I would also reinstall the Amal just to see what happens - as that will prove if the carbie is the problem. With Amals don't just tickle the fuel - make sure the bike is fully upright or even leaning to the right as you tickle it and start with throttle full open... once you get used to them they are usually pretty easy to start. When you say "inlet hose fine" that doesn't cover all issues. The carbie flange (especially on the Amal) becomes distorted over the years and once the engine warms up things expand and air gets sucked in there. The mikuni should be better but it must have the correct o-ring or else you will have problems too. In either case do not over tighten the carbie flange or your throttle will stick wide open Don't despair just take a long breath and realise that it may take time but it will be worth it in the end. Keep the updates coming.. Edited September 19, 2011 by keychange Quote Link to comment Share on other sites More sharing options...
black111r Posted September 19, 2011 Author Report Share Posted September 19, 2011 Followings Andrews advice i took a deep breath and have put the old Amal back onto the bike. Just to spite me the bike started first kick! However after running for 3-5 mins (as before) it started to misfire in exactly the same manner as before. I took the chance to inspect the rubber inlet spigot hose- it was perfect with no splits or deformity in the spigots. Pulling the plug this time, however, did show a lot more oil so it would suggest that the new Mikuni carb is helping the fuel to be burnt more efficiently. But back to the misfire. Should I replace the Motoplat with a new coil or just replace the condenser. Which is the most likely to be causing the problem? The HT lead does show signs of being burnt against the exhaust however I have shielded these areas with hose so I do not think that it is shorting. in fact the lead has always been like this so i do not think that this is the cause of this misfire. It is also sealed into the Motoplat so replacing it will mean a new Motoplat coil. Any thoughts Chris Quote Link to comment Share on other sites More sharing options...
keychange Posted September 19, 2011 Report Share Posted September 19, 2011 Definitely the condensor first - they break down much more often, especially under heat and they are much cheaper than a coil to replace. But also check the points although they normally would be bad from scratch - you might have short in the low tension wire that only shows when hot ie: metals expand when hot and so can short out hot and not when cold. However condensor is a very common cause of misfire when hot - they only cost a few dollars Quote Link to comment Share on other sites More sharing options...
black111r Posted September 19, 2011 Author Report Share Posted September 19, 2011 Definitely the condensor first - they break down much more often, especially under heat and they are much cheaper than a coil to replace. But also check the points although they normally would be bad from scratch - you might have short in the low tension wire that only shows when hot ie: metals expand when hot and so can short out hot and not when cold. However condensor is a very common cause of misfire when hot - they only cost a few dollars Any ideas on a local source for a new condenser I can order one in from the UK (In motion 8.00GBP) but this will mean another week of waiting. Also I believe i will need a flywheel puller? Chris Quote Link to comment Share on other sites More sharing options...
keychange Posted September 19, 2011 Report Share Posted September 19, 2011 I am assuming the Ossa has some similarities to the Montesa - I just found my post on the Australian trials forum subject "Cota 247 Running Lean and Overheating" a post that ran from June 5th to Sept 7th last year In that we discussed and I replaced the condensor with no improvement NOTE: my Cota was not misfiring - you originally said missing and then that evolved to misfiring ~~ the condensor will certainly do that. The advice I got at the time was "You can use just about any condensor that is ment for pionts ignition. I have used VW beetle units and mounted them up under the tank next to the coil. Mounting the condensor up there keeps it a lot cooler and much easyer to change as well." I got mine from a local GM dealer - very old stock in a box of old parts probably from 6 cylinder - it worked ok and still does. Have a look on eBay for flywheel puller - if it's like the Montesa and Bultaco then you have to have one if you plan to keep the bike - there is no way of getting by without it. Quote Link to comment Share on other sites More sharing options...
charliechitlins Posted September 19, 2011 Report Share Posted September 19, 2011 Looking at the intake spigot is no good...you have to test it. You can spray water on it while the bike is running to see if the idle changes. You can also point a propane torch at it (don't light it!!) and the idle will change if it's sucking propane. It's probably electrical, though. Points and condenser are cheap and easy to replace. Often the first sign of a coil going belly up is it works for a few minutes, then starts to miss. Quote Link to comment Share on other sites More sharing options...
black111r Posted September 19, 2011 Author Report Share Posted September 19, 2011 Looking at the intake spigot is no good...you have to test it. You can spray water on it while the bike is running to see if the idle changes. You can also point a propane torch at it (don't light it!!) and the idle will change if it's sucking propane. It's probably electrical, though. Points and condenser are cheap and easy to replace. Often the first sign of a coil going belly up is it works for a few minutes, then starts to miss. Thanks to all As part of yesterdays work I took the carb off along with the rubber intake spigot. It is one that I have recently made from new 35mm internal dia automotive hose. No splits but once the bike is running again I will try the tests you suggest to see if air is leaking in Back to the bike today for another strip down! Chris Quote Link to comment Share on other sites More sharing options...
black111r Posted September 20, 2011 Author Report Share Posted September 20, 2011 The bike has been stripped down and the first thing I notice is that the HT lead is slightly loose where it enters the Motoplat HT coil. Quite possibly it could have been worked loose with the constant plug changing that I have done. It is also pretty burnt where it has been in contact with the exhaust. though I cannot see metal through the flattened plastic sheathing. I has Araldiited it into place and will get the bike running tomorrow to see if that issue is the cause of the mis-fire when warm. the burnt areas can be shielded from the frame using a slipover hose and i can cable tie it all tight. The second point that I have noticed is that the LT wiring loom exiting from under the Magneto is pretty flattened as suggested by David. I have tried to check the internal condition of the four wires within the loom but will need to remove the flywheel to do so as it is all pretty tight. I have take a hammer to the aluminum bash plate as you could see rubbing marks on where the wires exit from under the magneto. I will try and give the area for the loom a little space and get the bike running as noted above before attempting Davids rewiring tip. Will keep you posted and thanks to all for their tips and advice. Chris PS If the coil proves to be defective does anyone know a source for an Motoplat HT coil (it has the letters FN on the metal bar), Or a spare knocking around? Speed and Sport in the US has a universal HT, does anyone know whether this will do the job? http://www.speedandsport.com/index.php/parts/ignition/ignition-coil/ Quote Link to comment Share on other sites More sharing options...
keychange Posted September 20, 2011 Report Share Posted September 20, 2011 it is getting complicated with two posts on the same subject - As usual David's advice is good, I mentioned earlier a short in the LT - I had to replace mine also removing the lighting wires at the same time to reduce the clutter and pressure on the now single wire in the loom exiting the engine. I am not sure idf was the right thing to do but it works for me and I have no intention of refitting the lights. Quote Link to comment Share on other sites More sharing options...
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