270 Question

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norsa1

norsa1

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The 850 motor is built in the offset crank 270 degree configuration. It runs, it idles, it makes power just like it should.
The compression reads 125 lbs per cylinder, nothing excessive, yet the engine is a beast to kickstart.posting.php?mode=post&f=1#

I am running an RGM beltdrive which gears up the ratio a small amount and I suspect it may be part of the problem

We are talking a 170 lb man leaping off a second storey building trying to get up enough speed to depress the kickstarter. It is that difficult.

Any theories why it is so hard to start?
 
What cam profile is it, and what is the CR (just carefully measure the oil required to fill the combustion chamber to find out). If you have a mild cam, with little opening duration, this accentuates the compressions ratio.
One othe point with a 270 crank is to ensure you are over compression on both cylinders so you get the crank spinning and the inertia will help overcome the compression and start it.
 
norsa1 said:
The 850 motor is built in the offset crank 270 degree configuration. It runs, it idles, it makes power just like it should.
The compression reads 125 lbs per cylinder, nothing excessive, yet the engine is a beast to kickstart.posting.php?mode=post&f=1#

I am running an RGM beltdrive which gears up the ratio a small amount and I suspect it may be part of the problem

We are talking a 170 lb man leaping off a second storey building trying to get up enough speed to depress the kickstarter. It is that difficult.

Any theories why it is so hard to start?
Click to expand...

Total seal rings?
 
With a 270 degree crank you must have a big space then little space happening between compression strokes, since the standard 360 crank is gives perfectly even spacing. My suggestion is to utilize this spacing to make things easier. Using the big space is the only way I can get my 1360 Vincent to kick over.
Try squeezing it just thru compression on the appropriate cylinder, leaving the big space there for flywheel momentum to roll it thru on the next compression stroke. So you will always be starting it on the same cylinder.
If when you squeeze it thru it rolls too far, put the bike in gear an roll it backwards a few inches until it comes hard up against compression. Now you should have a full swing at getting it thru on the next compression stroke.

Glen
 
norsa1 said:
I am running an RGM beltdrive which gears up the ratio a small amount and I suspect it may be part of the problem
Click to expand...

I use that RGM beltdrive also. It puts a larger sprocket on the motor so it should make it easier to kick over. Jim
 
Supposed to run smoother on rigid mounted frames. Won't run smoother on an iso frame unless shimmed for zero side clearance, as the rocking couple produced will not be taken by the iso's well.
 
I have not found them to be particularly smoother -just different. Jim
 
It's primarily done to emulate the sound and feel of a 90 degree V twin, but there would need to be a fair bit of legwork to figure out what balance factor suits the frame best, likely through removable mallory weights on the crank and multiple tries. It's also not the ideal angle, whereas 294 is, but again, this is all guesswork until you put a motor in a chassis and test it. It gets rid of the nasty rocking couple a 180 has and evens out the firing order a bit, and somewhat reduces the massive crankcase pressure change and balance issues a 360 twin has at the cost of the even firing order.

I have a pretty negative view on all of this, as I generally feel that if you find the sound and feel of a particular bike not to your taste, it's best to pick another bike. Going with a crank like this means major rejetting work (must run dual carbs), custom/modified camshaft, custom/modified crankshaft, which is probably not worth it when you could just sell the bike and pocket the change and pick up a bevel drive twin.
 
Dave wrote "sounds like a lot of effort just for a little smoothness"

Maybe, but I don't have a 270 degree crank in anything as of yet. I will say that as I age and ride more and more miles each year, smoothness is becoming more important. If a 270 degree crank helps, then go for it.
The Norton 961 uses a 270 crank and that might have been Kenny Dreers idea.
The 961 sounds much like a Ducati because of the common crank angle.
To my ears, it is not a great sounding engine, whereas the British parallel twins are. Ducati and 961 owners might not see it that way.
Also, almost every road test of the 961 has included mention of a considerable amount of vibration, so if the 270 crank is smoother, it is still not smooth.
I suppose if you want a smooth new British bike, the Triumph Triple seems to be it. And those sound like a Maserati V12 going down a straightaway. The great gobs of horsepower and torque are a side benefit.

Glen
 
the 270 needs a balance shaft to actually be smooth, like the TRX 850 which is actually one of the nicest designed production bikes I have seen in a long time.
 
worntorn said:
Supposed to run a little smoother

Glen
Click to expand...

It's also supposed to increase mid range torque, or at least, improve the way in which the torque generated reaches the back wheel, according to Phil Irving (as I recall).

I think his basic idea was that with a 90 degree cank, one piston is at, or close to, maximum velocity when one is (technically) stopped at either TDC, or BDC. Thus cancelling out vibration and preventing momentary power loss and deceleration / acceleration.

However, I speak merely from memory of what I've read. I've never run one.
 
Fast Eddie said:
worntorn said:
Supposed to run a little smoother

Glen
Click to expand...

It's also supposed to increase mid range torque, or at least, improve the way in which the torque generated reaches the back wheel, according to Phil Irving (as I recall).

I think his basic idea was that with a 90 degree cank, one piston is at, or close to, maximum velocity when one is (technically) stopped at either TDC, or BDC. Thus cancelling out vibration and preventing momentary power loss and deceleration / acceleration.

However, I speak merely from memory of what I've read. I've never run one.
Click to expand...

I tried several different offsets on my isolastic racebike. The dyno readings never changed. I ended up back at 360 degrees because that was the smoothest at high RPM. Jim
 
worntorn said:
Maybe, but I don't have a 270 degree crank in anything as of yet.
Click to expand...

You do have a Vinnie (or 2), which is sorta half way there, and almost as good. !

:mrgreen: :mrgreen: :mrgreen:
 
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