920 lightweight

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worntorn said:
So did I until encountering this when tuning the 1360. An old time engine builder and racer told me that the overbored Triumphs and Nortons generally used smaller mainjets than the standard bore bikes, all other things being equal. I found his info to be correct for the 1360.
Later on I realized this is part of the Bernoulli Principle on Venturi effect.
The flow through a Venturi ( mainjet) increases at the square of velocity.
The larger engine with the same inlet tract and carbs has to have higher flow and velocity at a given rpm as it has a larger cylinder to fill in the same amount of time.
The larger engine needs a proportionately larger amount of fuel and air. It gets that with the air but gets more than that with the fuel because of the exponential relationship between velocity and Venturi flow.
That was how it went with the 1360. It required mainjets that were smaller than used on the 1200 cc version of that motor.

For some reason this motor liked the 270s. I will try 280 s today.
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Have you tried a colder plug? Just curious if the compression change was enough to require it.
 
Not yet, although it's still in the break-in phase, short runs, no high revs, etc ,haven't allowed it to get very hot.

Glen
 
worntorn said:
Out riding yesterday I realized its smoother than the 850 below 3000 rpm. Both are good above that point but the 850 has annoying vibration around 2500 rpms. The 850 also shakes it's front wheel at idle, made worse if the idle is set high. The 920 doesn't seem to shake at idle even with the current 1200 rpm setting.
Both bikes have the same standard weight pistons and rods but the 920 has much higher compression, which should add to vibration, if anything.
The 920 crank was dynamically balanced, that might account for the difference in vibration levels.

Glen
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Sounds right to me. Significant difference over whatever the factory did after my crank was dynamically balanced on a solid mounted Norton motor. Mine still vibrates at idle some. Underway it's nice for a solid mounted twin cylinder engine with two pistons going up and down at the same time. I thought my crank was balanced to 86%, but it could be 68% based on my poor memory of the conversation I had with Raber about doing the work. Receipt does not mention anything but balanced crank for the work done.

I had a Commando that vibrated so much at idle it would spin around in a circle on a flat surface when on the center stand. I've heard that is nothing new though.
 
It makes a difference if you approach a hill with a lot of revs because then you are not relying on throttle response. It is similar to turning into a very strong head wind. Also your gearbox internal ratios affect performance. Close ratios keep the revs high, so as you open the throttle the motor responds quicker. If you approach the hill flat out in top gear, that might give you an indication as to whether you have the right main jets, But it has been my experience that getting the needle jets right is more important. You have a heavy crank and the rate at which the throttle opens affects the way it spins up. If the needle jets are very lean, the motor will respond better. If it is spinning up from already high revs, it has less distance to go until it reaches maximum power.
 
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To keep things simple, these tests are always in 4th (top) gear. For the Nortons, the runs are always at the same approach speed, 100 kmh.
On the 920 the test is from 3800 rpm. It manages to accelerate to about 5100 rpm at the top of the hill.

The 850 has the same overall ratio, so it also is at 3800 rpm in 4th gear at the start of the hill. It accelerates from 3800 to 4300 rpm at the top of hill. It is carrying about 150 lbs more weight and has to do it with less power.

Some more numbers, in Imperial-
The average acceleration for the 850 on this hill tested as above is 1.2 feet per second per second.

The average acceleration for the 920 , same hill, is 3.2 feet per second per second.

That is a big boost when you are against a headwind and needing to pass a line of Semis or holiday traffic on an uphill passing lane.








Glen
 
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You know, all the big 4s use visibly "stepped" slide needles; so that at certain throttle points, the "jetting" changes significantly.

I wonder, do the FCRs have these multi-tapered needles, and/or has anyone experimented with custom needles designed to increase fuel flow at various throttle points?
 
worntorn said:
To keep things simple, these tests are always in 4th (top) gear. For the Nortons, the runs are always at the same approach speed, 100 kmh.
On the 920 the test is from 3800 rpm. It manages to accelerate to about 5100 rpm at the top of the hill.

The 850 has the same overall ratio, so it also is at 3800 rpm in 4th gear at the start of the hill. It accelerates from 3800 to 4300 rpm at the top of hill. It is carrying about 150 lbs more weight and has to do it with less power.

Some more numbers, in Imperial-
The average acceleration for the 850 on this hill tested as above is 1.2 feet per second per second.

The average acceleration for the 920 , same hill, is 3.2 feet per second per second.

That is a big boost when you are against a headwind and needing to pass a line of Semis or holiday traffic on an uphill passing lane.








Glen
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150lbs, the weight of a pillion passenger...
 
The Triumph Thruxton Dual seat will click right on. I don't want to purchase one as they are about $500 and look like a warm turd, especially if finished tuck and roll brown.
It would be useful to have one for about ten minutes.

Glen
 
What that photo tells is me is just how clever the marketing guys are…

They actually succeed in getting people to voluntarily buy those… AND feel good about it…!
 
Fast Eddie said:
What that photo tells is me is just how clever the marketing guys are…

They actually succeed in getting people to voluntarily buy those… AND feel good about it…!
Click to expand...
Wot??? Don't you like brown seats Nigel?;)
 
I was real close to buying a used T140 back in '79. The poo-brown seat was a buzzkill.
920 lightweight
 
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