A quick update. I've been fine tuning (without the O2 sensor) and, as suggested by Jim, I went down to a 230 main jet, and, progressively, down to a [HASHTAG]#197[/HASHTAG] air jet. After playing with 45 and 50 pilot jets, I went back to a 40. Hallelujah! the bike has always run well, but it is now really running nicely. The best yet. Starting is easy and idle solid. With the bigger pilot jets you can watch fuel economy plummet and watch it improve when you go leaner. All in all, a really interesting exercise. I should have the O2 sensor plumbed in this weekend, so that should be fun. Also, another dyno run is coming up with my mate's Commando as well. He's had a TriSpark fitted, replacing a Boyer and a bit of fine tuning on his Mikuni. He was having a few starting problems, since cured.
Dyno run (2017)
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Whoops. I meant a #1.7 air jet. Starting point was a #2.0.
SteveA
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Thanks for the clarification, though both sizes seemed pretty large to me compared to my reference literature which suggests around 1.0!
But, a trip to the workshop this afternoon to confirm what sizes are in my various carbs revealed 2.0 in both a pair of 34mm and 36mm!
Didn't go too well, one of them sheered and is still in there!
But, a trip to the workshop this afternoon to confirm what sizes are in my various carbs revealed 2.0 in both a pair of 34mm and 36mm!
Didn't go too well, one of them sheered and is still in there!
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Fuel economy can suffer if carburation is either too rich OR too lean. If it is too lean, running becomes erratic so you tend to use more throttle to get the revs, if it is too rich you also use too much fuel. At the point of best performance, fuel economy is also usually best. I suggest that if you use an oxygen sensor, you might calibrate it against the dyno at the point of carburation where you achieve the best performance. This stuff about stochiometry and the theoretical optimum oxygen value is probably bullshit. It must change to suit the fuel you are using.
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SteveA
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That certainly seems to be the case. Also seems to be the case that people rarely change them!
I have been following this thread with interest. I have an 850 PW3 cam, VM34 rebuilt with 2500 miles so run in. I was given the VM34 which had been used on a 750 then lent out. I have now got it running with a 35 pilot 2.0 air jet 159 P-0 needle jet 6DH3 needle 3 slide. 260 main. Seems to run well but when slowing with closed throttle appears to hunt. If I weeken the pilot it does not pick up cleanly. If I put a smaller Air jet what would be the effect? I have a general dark brown plug colour. Traffic conditions making plug chops difficult, will probable end up taking it to a dyno. I can not find any info of Air jet effects.
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I enjoy reading this stuff because it's real-world experience extrapolated from theory and past practice. People reading should glean that compression ratio does not directly result in compression and a rise in such doesn't mean a rise in bhp. In a stock engine, everything is designed to work in unison and when you change something, you may have to change other things to keep that unity. Just dumping in a bumpier cam or higher-compression pistons may not have any positive effect on their own and certainly will show more results when used with other mods. More BMEP (brake mean effective pressure, a more concise term for the pressure on top of pistons during use) and more rpm will definitely result in increased power. An increase in cam lift, duration and overlap will help those pop-up slugs to not fight themselves until the crank gets spinning. Engines with extreme overlap can be heard to fire straight out the pipe at low rpm where their radical cams cannot capture the charge. Wind it up and it's a different story. As Rob Muzzy once said when asked about improvements to a new race bike he replied, "Higher compression and bumpier cams. What else is new?"
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This flies contrary to everything I have read both in this thread as well as elswhere as well as in practice. Increase in CR will increase BMEP and will increase BHP, limited by knock resistance and heat tolerance. The only exception might be a flat head engine where other trade offs are in play.
Would you care to expound on your statement above little further?
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Raising the compression ratio usually means you use more fuel to maintain combustion conditions in the cylinder head - (richer mixture) and you sometimes need to slightly retard the ignition timing . That is where the extra power comes from. However increasing comp ratio to gain more power might not always produce enough power gain to justify the expense of making the change. The other thing is that when you use petrol as a fuel you don't have unlimited anti-knock as you do with methanol. Methanol runs well in motors at comp. ratios as high as 17 to 1. And if you want to get the most out of it or petrol, you probably have to use high comp. I use 9 to 1 comp. with methanol fuel, so the jets in my carbs are much smaller. In the end you have a situation where you still get the supercharging effect from latent heat of vaporisation of the fuel, but you get better fuel economy and almost as much power. But overall performance is better with methanol, because methanol is much more forgiving of jetting errors. When you use petrol as a fuel, to get best performance, you need to be the super-cool tuner. In the olden days, Australian riders such as Eric Hinton, Jack Ahearn, Bob Brown and Tom Phillis did very well in Europe because they were used to the racing speeds. But they found that the British could get their Manxes going as fast on petrol as we could in Australia when we used methanol. Two friends of mine took a Manx to Goodwood Revival, year before last. They put it back onto petrol to comply with the rules. They did not see which way the British Manxes went - blown to the weeds !
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Tuning bikes on petrol is a British thing - like bloody cricket ! You have to have that bloody-minded nit-picking mentality. Fitting a performance cam to a standard motor probably gives more performance gain than you would ever get by raising comp. ratio when using petrol.
Apart from the Fullauto head, same tune as me. Quite a bit different jetting to me though. I could make a few observations, but the only problem is that I'm an idiot! Jim Comstock pointed me in the right direction but I still had to try " a few things first". Hence, I'm an idiot! But at least my understanding is better, which is what counts. Three sizes bigger main jet than mine seems to be a good place to start. Maybe a size or two down!
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Just that engines are designed as a system and changing one thing does not necessarily make a positive change without other changes. I should have been more specific that full benefit from a rise in compression will only be realized when accomplished with other changes, better breathing, more airflow, more radical cam timing and lift. If it were an automatic increase in performance, why wasn't it done at the factory? Because to make it worthwhile, other changes would have to go along with it. Then you run into the hurdles of more money, more noise and (these days) greater emissions. As private owners, usually money is the limiting factor. If your compression increase bumps the knock threshold, you have to retard the timing and there goes some power gain. And no matter what you do, if you don't increase flow and rpm, you're not gonna get much of a power increase. Just bunpimg compression may increase low-end torque, but may hold the engne back at higher rpm. Possibly the engineers at Norton set the parameters as such that the engine could take advantage of a slight increase in compression., say to 9,5-10:1 without other changes. Combat engines were what, 10.5:1? And they were given higher-lift cams and bigger carbs to go along with it. How much diofference would the cha nge have made if it were compression alone? Only dyno testing would tell for sure but I dont think anyone around here, racers or street riders would just bump compression and leave everything else alone. Would be interesting to see dyno charts for a stock engine, and compare them with charts for the same engine with 9.5:1 pistons and then 10:1 pistons with no other changes.
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That qualification is exactly what I was so subtly alluding to. Yes, rarely will you hear of anyone just bumping compression ratio up or down and you can say the same about many other engine modifications including cam profiles etc.... Earlier in this thread, I posted an URL on the Otto cycle and efficiencies where you can calculate (or look up table) the net changes in efficiencies which pretty much equate to improvement in torque. Yes, the proof (characteristics of performance improvement) is in the pudding as “funny” things sometimes happen to the overall torque curve when changing compression ratios but will see an overall general increase in torque with a “reasonable” (and manageable to your circumstances) increase in compression ratio.
Your question about if it were an automatic increase in performance, why didn’t they do it at the factory is a fair question. Please correct me if I am mistaken here but I seem to recall the earlier big twins did in fact start at lower compression ratios and eventually increased. Regardless of whether they did or did not, some practical reasons for not increasing CR would include availability of adequate octane rated fuel across the various markets they were selling to, ease of starting and heat. Can you imagine an E-Start coming out of the factory with a 10.5:1 compression ratio?
In a much broader sense, take a look at what the factories are cranking out today; whether it is a car or motorcycle. I think they have more or less eclipsed the rather agrarian compression ratios of our Commandos. Some of this is due to water cooling and some of this is due to such things as combustion chambers amenable to supporting higher compression as well as better fuels, variable timed cams, knock sensing ignition, and a whole bunch of other black magic.
This bit about increasing compression ratio was brought up by someone else on this forum years ago who asserted that the least costly and easiest engine enhancements one could make on a Commando were increasing compression ratio, improving the valve seat profiles (performance valve job), and maybe cleaning up and improving the port and bowl profile a bit? I tend to agree with this as there’s no splitting of the cases involved.
The reality is Commando owners as a whole, generally tend to want to screw around with their rides a bit and it becomes a slippery slope. That afternoon of doing a plug chop becomes an evening tear down, ordering cams, pistons and custom exhaust because “something was not right” in the zoo!
Your question about if it were an automatic increase in performance, why didn’t they do it at the factory is a fair question. Please correct me if I am mistaken here but I seem to recall the earlier big twins did in fact start at lower compression ratios and eventually increased. Regardless of whether they did or did not, some practical reasons for not increasing CR would include availability of adequate octane rated fuel across the various markets they were selling to, ease of starting and heat. Can you imagine an E-Start coming out of the factory with a 10.5:1 compression ratio?
In a much broader sense, take a look at what the factories are cranking out today; whether it is a car or motorcycle. I think they have more or less eclipsed the rather agrarian compression ratios of our Commandos. Some of this is due to water cooling and some of this is due to such things as combustion chambers amenable to supporting higher compression as well as better fuels, variable timed cams, knock sensing ignition, and a whole bunch of other black magic.
This bit about increasing compression ratio was brought up by someone else on this forum years ago who asserted that the least costly and easiest engine enhancements one could make on a Commando were increasing compression ratio, improving the valve seat profiles (performance valve job), and maybe cleaning up and improving the port and bowl profile a bit? I tend to agree with this as there’s no splitting of the cases involved.
The reality is Commando owners as a whole, generally tend to want to screw around with their rides a bit and it becomes a slippery slope. That afternoon of doing a plug chop becomes an evening tear down, ordering cams, pistons and custom exhaust because “something was not right” in the zoo!
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The last (gasp) real factory performance mod-wise fo the Dominator Twin was the short stroke. What do we have these days? REALLY short-stroke, big-bore, high-revving, liquid-cooled, OHC engines wih more more electronic nanny-control every year.
How you set one up to ride depends on the purpose. Nothing wrong with bumping compression over stock a bit on the street (along wih that valve job etc. mentioned. Likely that's the highest design spec wihout a cam, but everyone knows the results of increased power spec and all the race-born and aftermarket fixes for what blows. I have to mix fuel for the SS clone already. A day ride might involve carrying some small bottles of snake oil for fillups with the tiny tank.
How you set one up to ride depends on the purpose. Nothing wrong with bumping compression over stock a bit on the street (along wih that valve job etc. mentioned. Likely that's the highest design spec wihout a cam, but everyone knows the results of increased power spec and all the race-born and aftermarket fixes for what blows. I have to mix fuel for the SS clone already. A day ride might involve carrying some small bottles of snake oil for fillups with the tiny tank.
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Comnoz gave Fullauto the advice to try a different air jet.
This is interesting because as far as I can see, most people who use Mikunis don't even know there is one, let alone change it from the size that most 32 to 36 Mikunis seem to be supplied with. What I have been able to read so far says that the air jet has an effect in combination with the main jet at WOT, and that incorrect sizing can cause weakening or enrichment at the revs built toward maximum rpm.
But Fullauto is generally saying he is not habitually a WOT rider. I am, because mine is a race bike. The bike runs fine on 34mms with 2.0 air jets. Having fitted 36mms on pretty much the same jetting the symptoms I am trying to resolve are that the motor makes good power on corner exit and onto WOT, but as we get to top gear in the higher rev range, there is some misfiring not apparent at other throttle settings, gear and rpm. Through the gears to motor actually has a few hundred more rpm that it had with 34mms. So I am happy with the direction to larger carbs.
The only reference I have on advised air jet size is Victory Library's VM manual, which suggests a 1.0 on 34mms and a 0.7 on 36mms. I am within close range of the manual's base recommendations, except considerably weeker on the main jet, driven by an early dyno run that proved more power with smaller mains. Dyno time is a good suggestion if you know of one in the wilds of Normandy, but no point going to a dyno without a good range of jets for potential changes. I have no alternative air jets and am trying to establish what I should purchase for testing.
At €10 a pair plus silly shipping costs, I am not going to buy 15 pairs, but it will be equally frustrating to buy 1.0 and say 1.5 and then only to establish, it is better with a smaller size, but not near enough with what I chose.
This is interesting because as far as I can see, most people who use Mikunis don't even know there is one, let alone change it from the size that most 32 to 36 Mikunis seem to be supplied with. What I have been able to read so far says that the air jet has an effect in combination with the main jet at WOT, and that incorrect sizing can cause weakening or enrichment at the revs built toward maximum rpm.
But Fullauto is generally saying he is not habitually a WOT rider. I am, because mine is a race bike. The bike runs fine on 34mms with 2.0 air jets. Having fitted 36mms on pretty much the same jetting the symptoms I am trying to resolve are that the motor makes good power on corner exit and onto WOT, but as we get to top gear in the higher rev range, there is some misfiring not apparent at other throttle settings, gear and rpm. Through the gears to motor actually has a few hundred more rpm that it had with 34mms. So I am happy with the direction to larger carbs.
The only reference I have on advised air jet size is Victory Library's VM manual, which suggests a 1.0 on 34mms and a 0.7 on 36mms. I am within close range of the manual's base recommendations, except considerably weeker on the main jet, driven by an early dyno run that proved more power with smaller mains. Dyno time is a good suggestion if you know of one in the wilds of Normandy, but no point going to a dyno without a good range of jets for potential changes. I have no alternative air jets and am trying to establish what I should purchase for testing.
At €10 a pair plus silly shipping costs, I am not going to buy 15 pairs, but it will be equally frustrating to buy 1.0 and say 1.5 and then only to establish, it is better with a smaller size, but not near enough with what I chose.
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When you increase the comp. ratio of a four cylinder 1000cc motor to 11 to 1, you only have to kick over 250cc at high comp. With an 850 Norton twin, it is 425cc. When you jet to compensate for increased comp. ratio, common practice is to increase the main jets to a size which you know is too rich, then adjust the needle jets and needles. Then use plug chops to get the main jets right. I don't usually even bother to do the last step. When I ride, I feed the throttle on as I would with a two-stroke. If the mains are a bit too rich, it doesn't really matter, in racing you are not on them long enough.
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I can't make it through all these ramblings but I will say my Howard Gem Rotivator with a Fairbanks Morse Magneto will fire first rotation when primed with a snort of fuel after sitting for very long periods. It is a twin cylinder Wisconsin engine. I have eyed that magneto as it is a wasted spark set up I believe. I have another one on a single cylinder Wisconsin. So there.
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