guy53 Posted February 22, 2018 Report Share Posted February 22, 2018 I have a B model 250 with the original Mikuni that I've been trying to setup for a few years. The motor as a .005 inch head gasket, is on 3rd os, the exhaust port is '' lifted'' .020 inch, the rest of the ports are not modified, not really polish but clean. There is no modified reed valve. I run Shell super gaz with a good oil at 50:1. I fitted a Uni Filter and the motor is equipped with a Wes exhaust. I played with the timing without any real improvement.I have been fiddling with the needle eight, low speed and main jet, air screw etc. but the motor is always running hot at close throttle to 1/4 open . I fitted a VM 26mm played with it all of one summer and went back to the original Mikuni. I am thinking maybe an OKO carb might cure the problem, any experience ? Guy Quote Link to comment Share on other sites More sharing options...
section swept Posted February 23, 2018 Report Share Posted February 23, 2018 (edited) Stay with the Mikuni but first return it to the original specification/settings. You don’t mention float height and as this is important because it is easy to bend without realising it must be checked accurately. The ignition timing needs to be accurately set to Yamaha’s specs. The spark plug also needs to be as standard but an iridium to the same spec is ok. Ensure all breather/vent pipes are correctly routed, not kinked or blocked including the carb fittings. With a Wes exhaust you would possibly need to go up a couple of sizes on the main jet and try the engine performance; plug chop is a good way to establish main jet and needle type. But as I say start from standard settings first to establish a base line, note every adjustment and make small adjustments. Check the main bearing crankcase seals as you could be drawing air from unwanted places. For the engine to be running hot at low throttle openings are you allowing for the lower air flow around the engine? What method are you using to measure engine temp. A richer mixture may help with cooler running at slow speeds but may also cause 4 stroking and bogging down/stalling issues. Have included a picture of my E model this has standard jetting but synthetic reed valve and modified stop plates, also standard exhaust as Yamaha knows best.??The idle mixture screw has two settings, one for work and one for off-road that’s why you can see the original nylon cap in place. This cap gives approx. quarter turn in or out but the cap must be removed to perform the initial set-up. I’ve not ignored you’re stated 0.020 thou lifted exhaust port but I doubt that will have had much if any effect to the engine performance temperature wise. Not advertising but I found Yam Bits to be very helpful with parts and I dare say a certain Mr. A might offer some useful advice?? Edited February 23, 2018 by section swept Added material Quote Link to comment Share on other sites More sharing options...
feetupfun Posted February 23, 2018 Report Share Posted February 23, 2018 The OKO is certainly good. I just fitted a new one to my TY250A to replace the original TK and it works very well. Not a fair comparison though because the TK had started to wear on the needle and needle jet. It is just as good as a standard Yamaha TY250D carby that is in very good condition that I have on a TY250D motor. The problem you describe is one I have lived through on a couple of bikes, but not any Yamahas. I had exactly the same issue with my Cota 348. The original AMAL was terrible so I soon replaced that with a KT250 carby that ran perfectly except for when the motor was fully warmed up and it would then act very hot off-idle. I also had the same issue with a Bultaco Alpina 350. The original AMAL MK1 was OK but suffered from an attitude problem in that when the front of the bike was low and wouldn't idle. First off I fitted a new 26mm VM Mikuni sold to me as jetted correctly for the bike, but I spent a few hundred dollars and about 25 hours of test and tune time trying to get it to go as well as it did on the AMAL, with symptoms that sound just like yours. As well as trying different needles, needle jets, pilot jets, idle air jets and different slides, I also modified the downstream edge of the slide with a groove intended to increase the fuel just off idle. Trouble was that even with that serious sort of modification, it would run rich off idle until it was warm, run well for a brief period then run hot and what seemed like too lean just off idle as it reached full operating temperature. On this bike and the Cota 348, I then fitted new OKO carbies and was impressed with the improvement, but the problem was still there to a lesser degree. I then consulted a two stroke engine builder/tuning expert who told me that it was a common problem that is due to the combustion properties of modern fuel being different to the 1970s fuel. He also told me that it was a common practice in vintage MX circles to modify the combustion chamber shape to get rid of the problem. The idea is to replicate a modern design shape and squish zone in the old engines and that it cures the problem very well. He also suggested that running a slightly advanced spark would reduce the problem and I have done this in the two bikes I mentioned and it did provide a benefit. I went about 0.6mm more before TDC on both motors. 2 Quote Link to comment Share on other sites More sharing options...
guy53 Posted February 23, 2018 Author Report Share Posted February 23, 2018 Section Swept, I have 2 Yamaha Mikuni that I fitted on the bike, the one that originally was on is the one that use to make the test. Quite a few times after testing a new setup I change the carb for the '' other '' original to compare. Also I've been telling fellow worker to make change one step at a time and keep notes of the change and the result on different machines for more than 40 years, so I practice what I teach. I know many will change low speed jet not be satisfied play with the needle, no go change float setup....and than .. what was the original setup. I think the Wes exhaust to be an improvement. I did not mentioned that I ride more than I do sections so the motor don't get hot than cooler than hot again. David, your theory on the combustion chamber is interesting, if you have link on the info on that I would be grateful if you would pass it on. I have a couple head on the shelf, I would use one for test. I am shocked at the timing you set your bike, mine is setup at around 3mm BTDC. We had a couple of days with the temperature on the plus side, I think the left cover is going to be off the motor soon. Guy 1 Quote Link to comment Share on other sites More sharing options...
scot taco Posted February 23, 2018 Report Share Posted February 23, 2018 Guy, you could try reaching out to these guys at Mid Atlantic vintage trials.They have been selling and setting up OKO carbs on all sorts of different bikes for quite a while and might be able to help you out. http://www.mid-atlantictrials.com/Menu.html 1 Quote Link to comment Share on other sites More sharing options...
brewtus Posted February 23, 2018 Report Share Posted February 23, 2018 1 hour ago, scot taco said: Guy, you could try reaching out to these guys at Mid Atlantic vintage trials.They have been selling and setting up OKO carbs on all sorts of different bikes for quite a while and might be able to help you out. http://www.mid-atlantictrials.com/Menu.html Awesome link! Methinks I'll try an OKO on my newly arrived 304 Montesa. Quote Link to comment Share on other sites More sharing options...
guy53 Posted February 23, 2018 Author Report Share Posted February 23, 2018 I had check their site before posting and if I go the OKO way, they'll be the one I contact. Guy Quote Link to comment Share on other sites More sharing options...
feetupfun Posted February 24, 2018 Report Share Posted February 24, 2018 (edited) 18 hours ago, guy53 said: Section Swept, I have 2 Yamaha Mikuni that I fitted on the bike, the one that originally was on is the one that use to make the test. Quite a few times after testing a new setup I change the carb for the '' other '' original to compare. Also I've been telling fellow worker to make change one step at a time and keep notes of the change and the result on different machines for more than 40 years, so I practice what I teach. I know many will change low speed jet not be satisfied play with the needle, no go change float setup....and than .. what was the original setup. I think the Wes exhaust to be an improvement. I did not mentioned that I ride more than I do sections so the motor don't get hot than cooler than hot again. David, your theory on the combustion chamber is interesting, if you have link on the info on that I would be grateful if you would pass it on. I have a couple head on the shelf, I would use one for test. I am shocked at the timing you set your bike, mine is setup at around 3mm BTDC. We had a couple of days with the temperature on the plus side, I think the left cover is going to be off the motor soon. Guy Guy the 0.6mm is not the timing. It is the amount further advanced I set the timing compared with standard setting. As an example, if standard for the bike is 3 mm, I use 3mm + 0.6mm = 3.6mm I don't have a hyperlink to info about combustion chamber design. I spoke to the tuning expert on the phone. Edited February 24, 2018 by feetupfun Quote Link to comment Share on other sites More sharing options...
b40rt Posted February 24, 2018 Report Share Posted February 24, 2018 I'm surprised no one has tried a Dellorto, almost standard fit on Bultaco's these days. Quote Link to comment Share on other sites More sharing options...
guy53 Posted February 25, 2018 Author Report Share Posted February 25, 2018 Thank's for the pointer David, I checked my notes this morning and I'm running at 3.3mm so not that far. I will pay a visit to a friend of mine about the combustion chamber when I get a chance. B40rt, I haven't seen a Dellorto on a TY here, I've always heard they worked well but where more difficult to tune. Buy Quote Link to comment Share on other sites More sharing options...
b40rt Posted February 25, 2018 Report Share Posted February 25, 2018 17 hours ago, guy53 said: Thank's for the pointer David, I checked my notes this morning and I'm running at 3.3mm so not that far. I will pay a visit to a friend of mine about the combustion chamber when I get a chance. B40rt, I haven't seen a Dellorto on a TY here, I've always heard they worked well but where more difficult to tune. Buy No experience of TY's, but have used the same settings on 240 & 280 rotax, so doesn't seen to critical !!! Quote Link to comment Share on other sites More sharing options...
zuma Posted March 7, 2018 Report Share Posted March 7, 2018 On 24/02/2018 at 9:13 AM, b40rt said: I'm surprised no one has tried a Dellorto, almost standard fit on Bultaco's these days. Hi, I've got one : PHBH26 from a cota 315 (re-machined to 28) with a homemade airbox plus wes exhaust system, and it's perfect ! 2 Quote Link to comment Share on other sites More sharing options...
b40rt Posted March 7, 2018 Report Share Posted March 7, 2018 @zuma good man, would be even better if it was a petrol screw one ! Quote Link to comment Share on other sites More sharing options...
feetupfun Posted March 8, 2018 Report Share Posted March 8, 2018 @zuma that's a fabulous looking airbox Quote Link to comment Share on other sites More sharing options...
feetupfun Posted March 8, 2018 Report Share Posted March 8, 2018 These carbies go very well too. I recently fitted this OKO to my A model when the original TK carby started to suffer from needle and needle jet wear 2 Quote Link to comment Share on other sites More sharing options...
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