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Ian WilsonSun Nov-13-05 01:31 AM

"Gearing....and a bit of jetting"



Hello chaps, a bit of an opinion please. My Mk3 Commando is standard except for a Boyer (analogue) and a single 34mm Mikuni, that I'm very pleased with. When I got my bike it had a 19t gearbox sprocket, which was OK but impractical for high speed work. So I fitted a 21t, that seems to be much better. But it still seemed too low for long distance 85mph cruising. However, next year I'm doing two long motorway jaunts, in Germany and Italy, where 85mph is slow! So when I did the layshaft bearing repair I fitted a 23t sprocket. Acceleration is not a criterion, but relaxed long distances at high speed is. At the same time the engine was running a bit rich, so I replaced the 260 main jet with a 230. It's all running very well now, and cruises at 85 with under 4,500 on the tacho. Objective achieved? NO. 85mph is about all it will do flat out. It just won't pull through it and go faster, though on a downhill it runs up freely. So, the question. Is 23t just too tall for a standard Mk3 motor? Do I need to put the 260 main jet back, or try the 240 which is recommended for general use? Should I have fitted the 22t? A combination (which?) of these? Do I have to accept that I have to go a little slower after all, though on an autoroute or an autostrada this is more dangerous that going very fast? Anybody else tried to 'high speed cruise' a Commando? All suggestions welcome, thanks in advance. Ian Wilson


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andersonjmaMon Nov-14-05 02:28 PM

#1. "RE: Gearing....and a bit of jetting"
In response to Reply # 0



I have no personnal experience tuning a Mikuni for a 850 but I recomend this booklet. I have the Concentric version and it is the most comprehensive single source of info on Concentrics I have seen. I expect it would be just as good for Mikunis. Again I have not run this sort of gearing on an 850 but what happens is you drop down to third. Does it accelerate up to the red line ? Going from 19 to 23 on the g/box sprocket means you have gone up about 21%. This is close to the 22 % step between third and four on a standard Commando gearbox so using third should give you a reasonable test of original top speed. Does the bike go over 100 mph in third ? I would say that 85 mph on an 850 almost no matter what gearing sounds like there is a problem somewhere.


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Ian WilsonTue Nov-15-05 10:22 AM

#2. "RE: Gearing....and a bit of jetting"
In response to Reply # 1



Thanks for the thoughts. Actually, the engine is just coming to pieces at the moment. You see, I was out on Sunday afternoon with a Kawasaki 650 and a CBR600. The Commando was running very well and on a long straight I opened her up. The speedo never went past 85 and the tacho never passed 5,000, but there was suddenly some clattering and I slowed down. The bike seemed to be running OK so I took it easy and rode the rattly engine gently home. Upon examination, the LH exhaust cam seems to have gone AWOL, it is nearly worn away and that valve was hardly opening. The head and all other parts were fine, though there was some scuffing on the LH piston that will probably mean I'll have to change it but the bores still showed the honing marks from eight years ago! So now I have to strip it and fit a new camshaft. I also have a new dilemma. Which camshaft? Any suggestions? Oh, and the CBR600 rider was behind me when this happened. He said that he usually follows me showing 75mph or so, but when the engine let go he was showing 105.... Clearly the speedo and tacho lie. Oh well, that's something I've learnt, if nothing else!


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dynodaveyTue Nov-15-05 11:53 AM

#3. "RE: Gearing....and a bit of jetting"
In response to Reply # 2



On the cam business... I can offer that the NEW Norton Andover cams are NOT of the same quality as approx NOS 70-71 cams. The current NA cams have this marking "NA" in the casting and upon measuring, I have found they are not as hard as the old cams(IIRC about 35RC). They are not nearly as bad as the cams that originally came in the MKIII, but I have serious doubts about them. One new cam split in the AA mounting hole exactically in line with the casting part line. It was returned for warranty consideration, but have not heard good news yet.... The 1/4-bsf AA mount bolt breaks off at 14ft/lbs, and 14 ft/lbs does NOT break/split open an old OEM cam(70-71 era). So the assertion that it was over torqued will fall on deaf ears IF that is their response. We reluctantly used a megacycle NR cam to replace the defective norton cam.


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Ian WilsonSat Nov-19-05 07:01 AM

#7. "RE: Gearing....and a bit of jetting"
In response to Reply # 3



Thanks to all for this. I'll keep you posted. Dynodavey, you say you reluctantly use the Megacycle cam. Why 'reluctantly'? I have been recommended this and the only question is that it will need flat bottomed followers. What are they and in what way do they differ? Where can I get them from? Can I not use the std followers? I've looked at your cam profile and I am not a good enough engine man to know why. Many thanks, Ian


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andersonjmaTue Nov-15-05 06:38 PM

#4. "RE: Gearing....and a bit of jetting"
In response to Reply # 2



The tacho and speedo both lie?. Sounds a bit unlikely. Its hard to see that you were really doing 105 mph if you had almost no left hand exhaust cam left. That would be pretty good. I will leave it to Dave to advise on reliable cams however I do recomend you inspect the followers very very carefully with a magnifying glass for cracks and partially loose stelite pads. They should be refaced or replaced when fitting a new cam. Also watch for any sticking of the follower and clearance should you go for a different cam profile. There seems to be a difference of opinion on whether to use special cam protection running in paste when assembling the motor. Overall most people recommend it. I always pour some oil down the push rod tunnels immediatly before starting for the first time. See Les Emerys running in procedures on his Norvil Fair spares website. Having said that his recomended running in procedure seems good I have had problems with his cams. I bought one from him a couple of years ago ago and when I measured it up the base circle and quietening rams were inaccurately machined. I understand others have seem the same thing Look in Dave's cam section on this site. Good luck John


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Ian WilsonWed Nov-16-05 10:01 AM

#5. "RE: Gearing....and a bit of jetting"
In response to Reply # 4



Well, I should own up and say that the tacho has never read beyond 5,000 and I forget the last time I took the old girl past 80. Anyway, it was faster than I usually go! The rattling, and therefore (I think) the inlet LH cam failure occurred while at high speed. I would think that once through the case hardening the cam would wear rapidly. It was the noise that made me ease off. I'm guessing that a little lubrication problem might have shown itself. Also odd was the scoring to the inlet side of the LH piston but no damage at all to the bore. The 2-yr old honing marks are still visible. The LH inlet cam follower is wrecked but the other three all look OK. Yes, while it is apart I'll check the oil pump. So, some advice please. I'm going to do two long motorway trips next year, so I have geared it tall. What cam does the accumulated knowledge suggest? Given the use I am going to give it, I have had the Combat cam and the PW3 cam suggested. Also, do I replace all the cam followers, just the damaged one or just the LH pair? All suggestions gratefully reeived! Thanks again, Ian


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Rik PWed Nov-16-05 02:03 PM

#6. "RE: Gearing....and a bit of jetting"
In response to Reply # 5



The standard tacho usually seems fairly accurate if it hasn't gone into self destruct mode. It usually is the D/S Inlet lobe that goes which makes me wonder if lubrication is a factor. I don't imagine that the cam is softer at one end. The mark III completely lacks the oil bath which 500 Dommies had and which gradually disappeared over the years. I have used a PW3 in a tweaked 750 It is nice. quite useable low down and certainly keeps on pulling when it's on song. Don't know how happy it is with a single carb. though. If you fit a PW3, you will also need to ease the base of the cam follower tunnels and make sure the followers can rise sufficiently. Piston to Valve clearances will also need checking. After my umpteenth failure in my Mk III, I am now running a cold-chilled cast iron current Andover Norton cam. Up to now, no problems and the same goes for a couple of other people I have spoken to. The iron cam has been around for 4 or 5 years now. Shiny particles on the magnetic drain plug remain something I live in fear of. You will certainly need to fit a pair of followers and, although they are not cheap, I would not fit used followers on a new cam. I'm sure it would invalidate any warranty. Don't avoid dismantling the crank and cleaning the sludge trap either. Those nasty stellite particles will have found their way in there.


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jan guestFri Dec-09-05 12:00 PM

#8. "RE: Gearing....and a bit of jetting"
In response to Reply # 0



the gearing you can pull depends a lot on your riding position and weight you are carrying . I think for a STD Commando with upright riding posision 23T is too long ( tried it once ) I also do often long distance trips on the German autobahns , the autoroutes de France and Italian autopistas . My overall gearin is 4 to 1 . Also fairly long but I have an 18" rear wheel and my bike is very light and low . It cruises easily at 85 - 90 mph ( a must in Germany and France ) and I can pull it over 120 if I want to . But on long distances I keep it below 5000 rpm .


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