brownie001 Posted November 28, 2016 Report Share Posted November 28, 2016 Hi, I fitted new carbs to my bikes and did some testing on Sunday morning. Also tried my action cam out for the first time. The 320 is sorted but the 125 still soots up the plug after a short while. I'm scratchin my head and a few other places besides. The carb is brand new, original spec. I have fresh 102 octane with no methanol mixed at 50:1 full synthetic oil. The exhaust has been cleaned and the middle box repacked. The timing is spot on and a brand new NGK BP5ES plug. The filter is clean, there are no leaks, new crank seals. To be continued........... 2 Quote Link to comment Share on other sites More sharing options...
iconic558 Posted November 28, 2016 Report Share Posted November 28, 2016 Are those later 125 case reed induction? Quote Link to comment Share on other sites More sharing options...
pschrauber Posted November 28, 2016 Report Share Posted November 28, 2016 Mmmh in tho video you rode mostly on idle and quarter throttle. Just for reference let the engine run free a bit on road or lane with longer tracks between half and full throttle. May bee it's just sooted a bit from inside, this will too burn up carbon in the mid section of the exhaust. In my personal experience especially after local club trials on limited ground the engine feels a bit clogged and I then do some km trails (not trials) riding and voilá the engine sounds and feels better (more awaken) again. 1 Quote Link to comment Share on other sites More sharing options...
brownie001 Posted November 28, 2016 Author Report Share Posted November 28, 2016 Hi, first up Iconic558. Both bikes are vintage 81, no reeds involved. Now Pschrauber, the video was filmed on the 320. There is no problem with that anymore. The 125 is a different matter, it has to be revved, it hasn't got the low down grunt of the 280 cc engine. I do tend to open the throttle a lot more on that. Even after getting the thing really nice and warm and running up and down the access road a couple of time, the plug stays black and sooty. I'll get there in the end but it's prooving to be a difficult thing to solve up to now. Quote Link to comment Share on other sites More sharing options...
turbohead Posted November 28, 2016 Report Share Posted November 28, 2016 Your 280 sound and run like it should, just as you say. Regarding your 125, it is most probably a case of too much fuel down low in the register or even too much fuel all the way up. Try smaller main, drop the needle to get it running almost right and then start tuning step by step. If you are running a standard (Bosch) points ignition, check once again points, timing and so on (don't forget the condenser, change if suspect). A small selection of jets and needles and some therapy in the woods is all you need to get it perfect. Quote Link to comment Share on other sites More sharing options...
brownie001 Posted November 29, 2016 Author Report Share Posted November 29, 2016 Hi Turbohead, I picked and fussed around with the original carbs and settings on both bikes. The 320 engine is completely rebuilt and the 125 is very low mileage but the carb was from the original high use engine. The 280 is now spot on. As for the 125, no points involved, CDI black box. Have a new spare which made no difference. After a long mail exchange with Martin Mathews, he told me that in his experience, the motors run best with the standard carb spec's which is what I have for both new ones and of course the old ones too but there the bodies are original of course. So there you go, the search goes on....I'll get there one day ;o] Quote Link to comment Share on other sites More sharing options...
iconic558 Posted November 29, 2016 Report Share Posted November 29, 2016 Just a "been there before scenario" check the needle is correct.....difficult to see may need a magnifying glass....well I do but my eyes are shot and that the float isn't punctured. Had a problem once on a Mikuni ( yes different) thay ended up being fuel leaking past the float needle valve due to bad O ring...just sayin'. Good luck Quote Link to comment Share on other sites More sharing options...
brownie001 Posted November 29, 2016 Author Report Share Posted November 29, 2016 Hi Iconic558, bear in mind the carb is brand spankin new supplied to spec as per the original from 81. All was checked over before installing it. Everything is spot on with the carb. That's what's so frustrating really. It could be lots of things, most of them have been checked and double checked or renewed. I've had loads of good advice and more than enough 'I once had...' Believe me, I've done everything which has been suggested. All I need now is for someone to tell me that I have to sprinkle horse hair over a toad while its sitting on the bike and I need to be drinking a warm brown ale stood on one leg under a full moon and I'll try that as well ;o] Like I said earlier, I'll get there in the end.... Quote Link to comment Share on other sites More sharing options...
pschrauber Posted November 29, 2016 Report Share Posted November 29, 2016 Timing of the ignition is very right? You don't have cracks on the intake rubber part that is inside the engine? You have just slightly oiled the filter foam which means everywhere is just a film of oil and no spots with soaked oil? Quote Link to comment Share on other sites More sharing options...
iconic558 Posted November 29, 2016 Report Share Posted November 29, 2016 Maybe time to crack a beer open and take a step back and go back to basics. Has it ever run properly. If not and air/fuel/timing are ok then ya looking at mechanical silliness...like piston back back to front....rotary valve timing off....crank seal fallen out, believe it not it can give symptons of running rich due to such poor running. Saw that on an 1986 Honda CR125 a few weeks back just when I thought I'd seen it all! Ya got the right attitude, always remember there aint no witchcraft involved although sometimes it doesnt seem that way. PS chime back if them toads work! Quote Link to comment Share on other sites More sharing options...
brownie001 Posted November 29, 2016 Author Report Share Posted November 29, 2016 Hi Pschrauber, not much you can do wrong with the 125 timing, the inake rubber is new, the foam filter has been renewed and oiled as needed with a very light film. Nevertheless, I'm now taking nothing for granted and will go through the A to Z at the weekend and check everything again until I find what's wrong, maybe ? Quote Link to comment Share on other sites More sharing options...
motoswm Posted November 29, 2016 Report Share Posted November 29, 2016 Did you spot on the original carb settings for the 125 that the mixture screw is 2.5 turns out ?? Quote Link to comment Share on other sites More sharing options...
brownie001 Posted December 3, 2016 Author Report Share Posted December 3, 2016 Hi Martin, yep sure did. I have the specsheet from the Registro Storico web site. The carb was also built to the spec found there too. Quote Link to comment Share on other sites More sharing options...
brownie001 Posted December 11, 2016 Author Report Share Posted December 11, 2016 Great day out yesterday at the Kerzenheim Nikolaus trial. Took the 125 along and got it warmed up nicely to be able to check the compression. I saw 100 PSI on the guage. I was expecting around 150 PSI as the motor is supposed to have a 9,5:1 compression ratio. Can The 100 PSI be interpreted as a very low compression or are there other factors to be taken into account ? Thanks in advance...... 1 Quote Link to comment Share on other sites More sharing options...
turbohead Posted December 11, 2016 Report Share Posted December 11, 2016 (edited) This is an old and sometimes much disputed issue, I'm afraid. Compression testing is standard procedure on any car engine, but on a small 2-stroke engine things get a bit more complicated. Your reading at 100 psi is what a chain saw or lawnmower normally give, but are you sure you have done a reliable test..? Most compression test gauges are built for large-displacement automotive use, instead of small displacement 2-stroke motorcycle use. For example, the internal hose volume of a standard gauge is over 10cc and the entire combustion chamber volume in a 125 cc 2-stroke is around 10cc. There are special gauges for motorbikes where the hose has a 2-3cc inner volume. The much smaller hose volume of such a gauge mean the engine will need to spin a lot fewer times to get a reliable and repeatable reading. A few tricks when compression testing a small 2-stroke engine: Engines that have sat for some time may have oil leaked in from somewhere. The added ring sealing offered by the oil can greatly increase the pressure reading. Cold Engine only. Hot engines will yield lower numbers, and varying temperature will yield varying readings. Throttle wide open. To allow the same air volume entering the engine at all test runs. Spark plug in cap and grounded to motor. To make sure there is no damage to the ignition system Push the bike in 2nd gear. Push the bike at walking speed for about 10-20 metres. This make sure the engine always spin at the same speed. Repeat the push test a couple of times. Your kickstarter will not give the same engine rotation speed from time to time causing variations in the reading. However, compression tests are not the ideal or most reliable way to test a 2-stroke for trouble. A two stroke engine need compression above and below the piston rings, a two stroke engine must have a sealed bottom end that will hold both compression and vacuum. To test this, you have to place the piston at BDC, plug the intake and exhaust, then pressurize the crankcase through a fitting in the spark plug hole, and check if pressure remain for five or six minutes. The vacuum test is done the same way, with a vacuum pump and the piston in the same position. Edited December 11, 2016 by turbohead Quote Link to comment Share on other sites More sharing options...
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