Dr. Blair on Dyno Hill- Place your bets!

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My 750 hit a wall at about 83mph with 240 main jets. I had both 260's and 220's in the parts bin, so I swapped the 220's into the bike and went out with the jet key and 260's in my pocket.... With the 220's (stock recommended size) my bike zoomed right past the 83mph wall that was there with the 240's...

The spark plugs looked very clean with the 220's in place. I probably should have tried 230's also to see if I could get the same result with slightly richer main jet. Also, my bike has dunstall decibel mufflers with a stock cam and valves so the other variables are noted.
 
I understand what the needle does all too well. Every part in the carburetor matters and changing the clip position can help build more power before on the main. 250's would probably help with your tuning style. I'll keep my mouth shut. Good luck
 
o0norton0o said:
My 750 hit a wall at about 83mph with 240 main jets. I had both 260's and 220's in the parts bin, so I swapped the 220's into the bike and went out with the jet key and 260's in my pocket.... With the 220's (stock recommended size) my bike zoomed right past the 83mph wall that was there with the 240's...

The spark plugs looked very clean with the 220's in place. I probably should have tried 230's also to see if I could get the same result with slightly richer main jet. Also, my bike has dunstall decibel mufflers with a stock cam and valves so the other variables are noted.
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If your jetting was too rich and you raised the comp. ratio or increased the ignition advance, you would probably get the same effect, as you would by leaning-off. For any given octane rated fuel, the best combustion conditions are always a balance between jetting, comp. ratio and ignition advance.
You will notice that the manufacturer of the motorcycle usually specifies all four factors. Most Commandos have probably moved well away from standard. The exhaust system in particular can change the balance.. If you fit a megaphone to the exhaust of a well-tuned motorcycle, without re-tuning - you will often burn a piston.
 
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Schwany said:
I understand what the needle does all too well. Every part in the carburetor matters and changing the clip position can help build more power before on the main. 250's would probably help with your tuning style. I'll keep my mouth shut. Good luck
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I would definitely experiment with clip position if tuning this setup for permanent use on this bike.
I could be wrong, but going by past experience and the Carb Tuning guide, this engine should be on the mainjet for this pull. There's been no hesitation on transition so far, opening the throttle feels like turning on a switch.
250s are going in right after the 270s.

Glen
 
FWIW, many years ago when I did a lot of dirt riding, I had a Bultaco 350 Alpina (air-cooled, two stroke) that ran best with (main) jetting leaned to one size larger than engine seizing size! That was several sizes leaner than factory but it produced more power on the dyno, with the power increasing with every size leaner. I went down one more size and the engine seized after 10-15 seconds at WOT! Disassembled/cleaned everything up and put it back together with the "pre-seize" jetting and rode it for years...

SO...best power and dangerously lean can be one jet size apart!
 
MexicoMike said:
FWIW, many years ago when I did a lot of dirt riding, I had a Bultaco 350 Alpina (air-cooled, two stroke) that ran best with (main) jetting leaned to one size larger than engine seizing size! That was several sizes leaner than factory but it produced more power on the dyno, with the power increasing with every size leaner. I went down one more size and the engine seized after 10-15 seconds at WOT! Disassembled/cleaned everything up and put it back together with the "pre-seize" jetting and rode it for years...

SO...best power and dangerously lean can be one jet size apart!
Click to expand...
I think you just captured 2 stroke tuning! Life on a knife edge!
 
MexicoMike said:
FWIW, many years ago when I did a lot of dirt riding, I had a Bultaco 350 Alpina (air-cooled, two stroke) that ran best with (main) jetting leaned to one size larger than engine seizing size! That was several sizes leaner than factory but it produced more power on the dyno, with the power increasing with every size leaner. I went down one more size and the engine seized after 10-15 seconds at WOT! Disassembled/cleaned everything up and put it back together with the "pre-seize" jetting and rode it for years...

SO...best power and dangerously lean can be one jet size apart!
Click to expand...

One notch difference on the needle is quite a big step when you are using petrol. But you need to be careful of the taper. I run my 850 lean, right down the needles. But it means I am forced to deed the throttle on, as you would do with any two-stroke with a large carb. I can understand guys being freaky about leaning-off the jetting, because you can damage the motor if you go too far and continue to ride the bike. But once you are used to setting the mixture,- like anything - it becomes easy.
With my bike, as soon as I ride it, I know straight away if it is too lean. What is more difficult, is to know when it is too rich.

I used to tune a 360 Bultaco scrambler for a mate of mine. I rode it a few times - it was amazing the way it made other bikes go backwards. The biggest exponent of the 360 Bultaco was Bert Flood. He had Ginger Molloy riding his road racer in Europe against the twin cylinder GP two strokes.
 
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SteveA said:
I think you just captured 2 stroke tuning! Life on a knife edge!
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Try doing it using methanol as a fuel. The results can be really spectacular. If you ever try it in a two stroke - if the bike suddenly starts going extremely fast, you probably have fuel starvation and it is about to seize. I did it with a T250 Suzuki and beat a new TZ350 G on an airstrip race circuit. It can be very nerve wracking, when you use methanol in a two stroke.
When you race a two-stroke, you learn very well how to tune your bike. That is how I learned to tune my Commando 850 engine.
There is a very big difference between fast and slow, but not much difference in the jetting for each.
 
I was reading something in a magazine the other day, about checking the size of needle jets by machining a plug to fit the jet and measuring that. It was silly stuff. The difference in internal diameter between fast and slow jetting when you use petrol is probably about a quarter of a thou of an inch. That is less than the difference between a good and a bad fit.
Some of those guys who write magazine articles, have probably never raced. Think about the taper on the needle and how much the clearance between it and the jet changes, when you move it one notch.

I never worry about having a slow motor, because most slow motors can easily be made to go faster than you might think they can.
 
worntorn said:
The bolt on horsepower does seem pretty elusive at the moment!
I've never found any change in wot performance with needle clip position change. The chart shows the range for effectiveness of clip position and other factors .
I will try various mainjets.
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Which manufacturer supplied that chart?
 
Went with the 270 MJs late today, got the same result as with 280s, 106 kmh.
I have 250s here but the roads are salted now.
Will wait for warmer temps and some rain to wash the salt away.

Glen
 
Most Commandos seem to have solid mounted carbs. With a 54% balance factor, the vibes are still there at high revs regardless of the rubber mounted motor. If the carbs get bounced, you can get fuel frothing or even bounce the float needle off it's seat - both can raise the fuel level in the bowls and richen the mixture. If that is happening, changing the jets won't have much effect. When I bought my Mk2 Amals, they were alcohol kitted and had rubber mounts. After a relatively short period, the rubber in the mounts went hard and cracked, so I replaced the rubber. They are not Japanese mounts - pity !. When you buy British, you get that sort of thing.
 
Changing the jets from 260 to 280 brought the speed down. 270s didn't bring the speed back.
So this exhaust likes the 260s. It might like the 250s a little more.

Glen
 
acotrel said:
Most Commandos seem to have solid mounted carbs. With a 54% balance factor, the vibes are still there at high revs regardless of the rubber mounted motor. If the carbs get bounced, you can get fuel frothing or even bounce the float needle off it's seat - both can raise the fuel level in the bowls and richen the mixture. If that is happening, changing the jets won't have much effect. When I bought my Mk2 Amals, they were alcohol kitted and had rubber mounts. After a relatively short period, the rubber in the mounts went hard and cracked, so I replaced the rubber. They are not Japanese mounts - pity !. When you buy British, you get that sort of thing.
Click to expand...
Al
Maybe go back to the beginning and read what this test is about!
 
Hi Bruce
Yes, I'm not getting anything out of this setup so far. In fact the very best result on that hill has been with the standard balance pipes plugged to simulate separate pipes. That showed 113kmh at the top, every time.
The silencers were open peashooters for that combo.

So it seems most Commando owners are doing alright with their early type standard twin exhausts, non balanced, with free flowing peashooters, provided it's a stock bike.
It would be interesting to test some more exhaust types. If I get the opportunity, I will.

Glen
 
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An improvement in torque usually affects acceleration more that it does top speed, if you don't change the gearing. If you improve the torque and raise the overall gearing, you will be faster at the end of a long road, but your gain in acceleration often won't be there. With top speed, in the end you have two things working against one another - horsepower and resistance. With a 2 into 1 which is not restrictive, you should not lose any top end, but you should get there faster. If you raise the overall gearing, you convert that gain in drive into speed. However, if you get out of a corner faster, your top speed at the end of the next straight will be higher
What you find, when you race with a commando engine, you need to use acceleration around corners more than you need to worry about being fast at the ends of the straights.. If you are going to get passed, it will usually be towards the end of a long straight. And when the guys do that, they need to get around the next corner after braking down from a higher speed than yourself.
 
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