Flywheel weight?

Compression ratio, ignition timing and fuel mixture make up a balance. When you change one thing, it affects the others. Raising the compression ratio has a similar effect to advancing the ignition or leaning off the mixture. You end up in the same place, when you restore the balance. When you use more fuel, theoretically you should get more power - but it does not really work like that. A squish band head probably gives more power because of the way the mixture behaves within the combustion chamber, Triumph 650 motors do not usually have squish bands in their cylinder heads. The amount of unburned fuel in the exhaust gas would be an indication of combustion efficiency. That is the reason an oxygen sensor is sometimes used - an indirect measurement.

Judging by the comments on this forum, most guys seem to use the recommended jetting and ignition timings for their Commando motors. I do not work like that. For me, recommended settings are only a starting point. I found the taper on the carb needles makes a significant difference, But who would normally try changing them to find out ? With the heavy crank, anything which improves throttle response is good.
I always thought the heavy crank was silly, but now I love it. An up-change with a close box at 7000 RPM is great.

acotrel said:

Judging by the comments on this forum, most guys seem to use the recommended jetting and ignition timings for their Commando motors. I do not work like that. For me, recommended settings are only a starting point. I found the taper on the carb needles makes a significant difference, But who would normally try changing them to find out ? With the heavy crank, anything which improves throttle response is good.
I always thought the heavy crank was silly, but now I love it. An up-change with a close box at 7000 RPM is great.

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The majority of guys on here are getting on a bit and using their bikes on the road. For almost all of us our racing days are gone.

Therefore unless one is at 40 C or an altitude of 3000 metres a nice set of carbs in good condition will tune very well at the factory recomended settings. In my experience even modern fuel tunes fine once you ensure all components are compatible with ethanol.

Similarly ignition. Once you have checked the timing plate is accurate using the ignition timing recomended by your particular ignition manufacturer also works well. Years ago I experimental with timing on Norton engines and provided both cylinders are identical the engine doesn't care much 2 or 3 degrees either way.

If you racing a special motor on methanol (I have done that in a former life) then yes you have to experiment a bit. But not the case for 95 % of us on here these days.

Hopefully, the OP has an answer to the original question of a heavier 850 crankshaft in a 750 engine.
Bad idea. I have both and the 850 is less responsive to more throttle from off idle to WOT. Sure, you can rev it to near redline and dump the clutch and the heavier flywheel will carry you through the gears for a drag strip thrill ride, but you'll be disappointed with response in real riding conditions.
There's enough evidence that 850's are not as quick as 750's and it's not just 19T sprocket vs. 20T.
When I rebuild my 850 it will have get a 750 crankshaft in it.

Deckard said:

Hopefully, the OP has an answer to the original question of a heavier 850 crankshaft in a 750 engine.
Bad idea. I have both and the 850 is less responsive to more throttle from off idle to WOT. Sure, you can rev it to near redline and dump the clutch and the heavier flywheel will carry you through the gears for a drag strip thrill ride, but you'll be disappointed with response in real riding conditions.
There's enough evidence that 850's are not as quick as 750's and it's not just 19T sprocket vs. 20T.
When I rebuild my 850 it will have get a 750 crankshaft in it.

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I suggest an 850 will never be any good while it has the normal wide ratio gearbox. The wide ratios exacerbate the throttle response problem, and it is compounded by a quick taper on the carb needles. When you change up with wide ratios, the drop in revs is large, and you tend to apply more throttle which richens the mixture and the motor makes less power.
With close ratios and slow taper needles, as you accelerate and change up, you lose less revs, so the motor pulls less load, and as the throttles rise, the mixture richens slower - so poor throttle response is not a problem - it disappears.
The slightest bit rich at mid throttle has a big effect in slowing acceleration.
When you compare and 850 with a 750, the overall gearing comes into play I would not expect a 750 to be slower or faster than an 850. The piston weight in the 750 is probably less, then in the 850. They probably rev differently.

Does the 750 crankshaft have the inch diameter hole in the counter weight to get the low balance factor ? I would have thought they are the same crank as the 850 .

Deckard said:

Hopefully, the OP has an answer to the original question of a heavier 850 crankshaft in a 750 engine.
Bad idea. I have both and the 850 is less responsive to more throttle from off idle to WOT. Sure, you can rev it to near redline and dump the clutch and the heavier flywheel will carry you through the gears for a drag strip thrill ride, but you'll be disappointed with response in real riding conditions.
There's enough evidence that 850's are not as quick as 750's and it's not just 19T sprocket vs. 20T.
When I rebuild my 850 it will have get a 750 crankshaft in it.

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I haven't found that to be the case. The slowest of the slow 850s (my mk3) stayed pretty much even with a hotted up 750 up to 90 mph, and that was letting the 750 roll on first.
Then I tried the MK3 against a bone stock 750. The MK3 was quite a bit quicker than that one, however I would like to tackle another stock 750 to make sure.
The hotrodded 750 which the mk3 stayed with to 90 mph is quite a bit quicker than a stock 750. It has a Comstock head with anti reversion D ports, raised compression and a performance cam. The owner has had several Commandos both 750 and 850. He tells me that this 750 is the fastest one he has owned.
The MK3 is still running all original internals from 1975 and its original rh4 head. The restrictive factory Blackcaps have been replaced with the earlier type open fluted Commando silencers. Main jets changed to 260. That's it for performance mods.
Oh and it has old time Analogue Boyer ignition which is supposed to slow down the slow bike even more.

The period roadtests of the day found the 73 and early 74 850s to be very quick. With the advent of the Mk2a and Mk3 models the very quiet and restrictive Blackcaps went on to meet California noise laws. That killed performance, but the fix is easy.


Here is what Motorcycle Mechanics had to say about the new 850 model at the dragstrip in 73 -

" In a straight race against a 750 the 850 was a lot quicker than the 750 over the first 1/8" mile and was still pulling away slightly on the last part of the strip"

Cycle World 750 dragstrip results
SEPT 68. 4.84 (19T) TOP RATIO -
13.47 SEC., 96.35 MPH

Cycle World 750 dragstrip results Mar 71. 4.84 (19T)TOP RATIO-
13.11 SEC., 101 MPH

Cycle World 850 dragstrip results May 72 (1973 850) 4.60 (20T) TOP RATIO-
12.95 SEC, 102.38 MPH

Same rider and dragstrip for all three Cycle World tests.


Glen

acotrel said:

I suggest an 850 will never be any good while it has the normal wide ratio gearbox. The wide ratios exacerbate the throttle response problem, and it is compounded by a quick taper on the carb needles. When you change up with wide ratios, the drop in revs is large, and you tend to apply more throttle which richens the mixture and the motor makes less power.
With close ratios and slow taper needles, as you accelerate and change up, you lose less revs, so the motor pulls less load, and as the throttles rise, the mixture richens slower - so poor throttle response is not a problem - it disappears.
The slightest bit rich at mid throttle has a big effect in slowing acceleration.
When you compare and 850 with a 750, the overall gearing comes into play I would not expect a 750 to be slower or faster than an 850. The piston weight in the 750 is probably less, then in the 850. They probably rev differently.

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Good for what?

Close ratio gearboxes are useless on the road.

And pretty hopeless for shorter circuit sprint races where most classic races are held in Oz and NZ . You can use a Daytona first gear and close up 3 and top a little but with a full close ratio box by the time you get moving in first gear most of the field will be around the first corner. A six speed box is obviously different.

Deckard said:

When I rebuild my 850 it will have a 750 crankshaft in it.

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IMO, for a road bike, that would be a mistake.

When I built my Seeley 850, I thought the motor was horrible and I never believed in it. After a bit of development of what goes with it, I now believe the 850 motor is excellent. With mine, the changes are minimal. It runs on methanol at low comp. - that hides up the tuning errors. But the close ratio gearbox cured its sluggishness. Even with methanol, carb jetting is critical - it works together with the gearing and the exhaust system. Using petrol would be a pain.

With the heavy crank, wide ratios and a race cam and big ports - if the revs drop below the power band, how would you get the motor to start pulling again ? If you slip the clutch or change down, you lose the plot. How much lighter is a 750 crank ?

I hear all this talk of red lining and wonder why my 850 mk2a with close ratio box, pea shooters and PW3 cam fitted by Norman White struggles to get past 6000rpm ,this is with clip ons and rearsets...........?

Colin, what are the carbs and mainjet size?

Glen

Glen........
mk1 premiers 32mm , new on five years ago, 260 mains , i had tried 220 up to 300 no change ,,,, when Norman did the cam his exact words were "you know this is an 850 ", I have had this conversation on here before about performance difference between 750/850,,,, one main issue is that the previous owner bored out the inlets to 32mm (RH10 head) saying it was a standard mod.....
Norman said no no no , i have seen sleeves available from USA to take it back to 30mm but not so sure about it
i m still toying with this idea.......

Does make you wonder about the effectiveness of that porting job.
The stock MK3 850 (-blackcaps) with 32 mm rh4 head will romp thru 6500 rpm in the first 3 gears.
I shift mine at 6200 +- when trying for maximum acceleration.
What did Norman mean by " you know this is an 850?"
That is is untuneable as compared to a 750?

I recall that Jim Comstock fitted a PW3 to an otherwise stock 850, can't recall the model year/ head type. It made a couple of extra bhp at 6500 rpm but had a big drop in midrange bhp (6?)vs same bike with stock cam.
The owner really liked the high rpm powerband and didn't mind the midrange loss as that accentuated the " on the cam" part of the rev range. Kind of like a 2 stroke, I guess. They can be fun.

It worked well at the top and so should yours!

Glen

ludwig said:

IMO, for a road bike, that would be a mistake.

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Hmm, anyone tried that and regretted it?
VR880's had lightened cranks and performed well. IIRC, steel rods would have made them more reliable.
I will be upping the CR at least with Carrillo rods anyway.

colin davie said:

I hear all this talk of red lining and wonder why my 850 mk2a with close ratio box, pea shooters and PW3 cam fitted by Norman White struggles to get past 6000rpm ,this is with clip ons and rearsets...........?

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What you don't say:

Is this in every gear or just top?

What gearbox sprocket do you have fitted.

And......the simple solution......fit an STS Fullauto! I'm sure AN will have them back in stock soon!

Simple isn't cheap!

colin davie said:

I hear all this talk of red lining and wonder why my 850 mk2a with close ratio box, pea shooters and PW3 cam fitted by Norman White struggles to get past 6000rpm ,this is with clip ons and rearsets...........?

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My Mk2A 850 Interstate (cNw) with 35 FCRs, Fullauto head and 22t front will pull 7300ish in top (a bit over 125mph)
I don't care about 750s, I'm just happy with what I've got!
Good on you Matt (@cNw )!!

Edit: Standard box, Euro bars and "halfway rearsets" (4" back, like all Interstates should have!)

Deckard said:

Hmm, anyone tried that and regretted it?
VR880's had lightened cranks and performed well. IIRC, steel rods would have made them more reliable.
I will be upping the CR at least with Carrillo rods anyway.

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A little searching the Forum yielded someone stating the 850 counterweight is 5lbs heavier than a 750.
Seems a little high.
OP said 1lb difference
That said, is there a reference as to actual weights of 750 & 850 counterweights?
As well as the weights of the new Andover 1 piece crankshafts now offered for both
I weighted my 750 crankshaft on the bathroom scale and it's 23lbs.
I wonder what an 850 crankshaft weighs.

Makes sense what Lugwig advised about a 750 crankshaft in a 850 if an 850 crankshaft weighs 5lbs more.
I believe Kenny Dreer lightened VR880 cranks by 3 lbs.

Deckard said:

Hmm, anyone tried that and regretted it?
VR880's had lightened cranks and performed well. IIRC, steel rods would have made them more reliable.
I will be upping the CR at least with Carrillo rods anyway.

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A VR880 owner posted on here some years ago. He found his VR880 to be very prone to stalling on take off at traffic lights. Iirc he was accustomed to riding a standard 850 and found the transition tough.
Also Iirc the problem was solved when the conrods went out thru the bottom of the crankcases.


Glen