750/850 weight comparison

baz · Oct 25, 2022

worntorn said:
Another friend with a modified 750 would like to have a go at the MK3. His bike has a lightened crank and other enhancements.
I might do that for fun but it doesn't really answer the question " Is a stock 750 faster than an 850"
That is the claim I've heard over and over for years and I'm now almost certain it is incorrect, just as the weight claims (750 much lighter than 850) were incorrect.

We can see that the hotrodded 750 with raised compression , much modified head and hot cam is dead even with the Pig of all Pigs MK3 850 to about 85 mph, then has a slight advantage for a couple of seconds, then is even again in top gear.
So, assuming the power enhancements work, which the owner tells me they do in spades, it's probably safe to assume that a stock 750 is a bit less powerful than a stock 850. I expect in the real world the difference isn't great, the 850 would only pull away by a small amount.
But it is the reverse to the commonly held belief.
I think that myth came from those horribly restrictive black cap silencers that were on the later 850s.
With those in place, a standard 750 would slaughter the poor wallowing 850.

Glen

The difference in weight as you know is the starter,starter sprag bigger battery, primary chain tensioner and all the gubbins that goes with the starter , thicker crankcase, heavier inner primary case
Plus the gear change crossover shaft ,rear m/cylinder etc etc
So we know the MK3 is heavier
But exactly how much I couldn't tell you
I know back in the day many were not amused by the electric start (electric tart ) as they were known
And the lh gear change
A lot of bikers at that time said Norton had sold out
So I'm guessing that's where the overweight MK3 talk would have stemmed from
I've said before that at the time I wouldn't have stood in the same street as an electric start bike
My Japanese bike riding mates used to say 'when was the last time you started a car with a starting handle '

Fast Eddie · Oct 25, 2022

baz said:
I've said before that at the time I wouldn't have stood in the same street as an electric start bike

I found you had to Baz in order to get close enough to properly take the piss…

Inspired by Bike Magazine, I had a T shirt printed with “GPZ’s”on the front and “SUCK!”on the back

baz · Oct 25, 2022

Fast Eddie said:
I found you had to Baz in order to get close enough to properly take the piss…

Inspired by Bike Magazine, I had a T shirt printed with “GPZ’s”on the front and “SUCK!”on the back

We always took the piss out of electric start bikes
Not so much now!! Oh well

worntorn · Oct 25, 2022

baz said:
The difference in weight as you know is the starter,starter sprag bigger battery, primary chain tensioner and all the gubbins that goes with the starter , thicker crankcase, heavier inner primary case
Plus the gear change crossover shaft ,rear m/cylinder etc etc
So we know the MK3 is heavier
But exactly how much I couldn't tell you"

I can tell you exactly how much that all weighs as we just measured the difference.

1973 850 Kickstart Inter - 447#

1975 850 MK3 Inter 477.

Its just 30 lbs.

The 750 Inter with Alton was 471, only six pound different, 2lbs of that in the skimpy swingarm that some upgrade to MK3. So essentially the same weight +- a few pounds.

Glen

Fast Eddie · Oct 25, 2022

baz said:
We always took the piss out of electric start bikes
Not so much now!! Oh well

Indeed.
I don’t have the T shirt anymore either …

baz · Oct 25, 2022

worntorn said:
I can tell you exactly how much that all weighs as we just measured the difference.

1973 850 Kickstart Inter - 447#

1975 850 MK3 Inter 477.

Its just 30 lbs.

The 750 Inter with Alton was 471, only six pound different, 2lbs of that in the skimpy swingarm that some upgrade to MK3. So essentially the same weight +- a few pounds.

Glen

Sounds about right
My Alton equipped 750 is 442lb with 2 1/2 gallons of petrol
I'm not sure what an inter tank holds ,but that'd be the difference

triumph2 · Oct 25, 2022

I vaguely remember reading a road test of a then new Mk III Commando by the late Jim Greening. Motorcyclist magazine?
He used a personal route that he used for all his road tests. The main thing I remember was his surprise that it was the fastest time of any bike he had tested to date though it didn't feel liike it.

worntorn · Oct 25, 2022

Perhaps the dealer had replaced the Black caps with open Pea Shooters.
That's all they need to get them going.

Glen

EstuaryBoy · Oct 25, 2022

triumph2 said:
I vaguely remember reading a road test of a then new Mk III Commando by the late Jim Greening. Motorcyclist magazine?
He used a personal route that he used for all his road tests. The main thing I remember was his surprise that it was the fastest time of any bike he had tested to date though it didn't feel liike it.

I have a reprint of an August '75 'Motorcyclist' road test of the Mk3 by Bob Greene...
He took it over his personal test route at the quickest time ever (black caps, balance pipes et al!), beating the previous best time he'd set shortly before on a T160, which itself took 'best time' away from a Guzzi V-7 Sport.
Alas no details of his test route ('over the mountain'), but the course was covered in just over half an hour.

triumph2 · Oct 25, 2022

EstuaryBoy said:
I have a reprint of an August '75 'Motorcyclist' road test of the Mk3 by Bob Greene...
He took it over his personal test route at the quickest time ever (black caps, balance pipes et al!), beating the previous best time he'd set shortly before on a T160, which itself took 'best time' away from a Guzzi V-7 Sport.
Alas no details of his test route ('over the mountain'), but the course was covered in just over half an hour.

that would be the one! I was close which is the best I can hope for these days. It stuck in my mind that the "porky" Mk III was the quickest.

EstuaryBoy · Oct 25, 2022

triumph2 said:
that would be the one! I was close which is the best I can hope for these days. It stuck in my mind that the "porky" Mk III was the quickest.

Reading between the lines, the Guzzi took 33 minutes to cover the course, the Norton was 72 seconds quicker, averaging 63.6mph.
Without the benefit of a slide rule the course must have been over (as in 'more than') thirty miles??

worntorn · Oct 25, 2022

Sounds like the MK3 was going "far faster."

Glen

baz · Oct 25, 2022

worntorn said:
Sounds like the MK3 was going "far faster."

Glen

I think I know what happened
Once he'd got all that weight up to speed he just couldn't slow down because of the iffy brakes ???

worntorn · Oct 25, 2022

I just had to try out the new "head wound" emoji, Thank you Jerry.

Back to the hefty MK3 and it's feeble brakes..

Ok but, I'm wounded.

Glen

Schwany · Oct 26, 2022

The more you ride the same course, the faster you are bound to go on it. Was the next bike he rode over that course faster yet?

worntorn · Oct 26, 2022

Just recalled that the MK3 is still using the same pistons, rings and valves that it came with in 1975. The only internal engine work ever done on the bike from new was a head gasket replacement when the head studs pulled out.
It has about 40,000 miles on the clock.
It's probably not as crisp as it could be, but still seems to get rolling at a good clip.
Top speed for both of us (we were in lockstep again in 4th) at the end of the run was about 110 mph.
Yes it does make one wonder how much we really gain with performance mods. If I could get a proper run against a stock 750 we could answer that question as well.

Glen

bluto · Oct 26, 2022

FWIW an inexpensive and reasonably accurate way to weigh a bike is with a crane scale (about $40 IIRC)
the photos and weight are for my porky Sportster ;-)

worntorn · Oct 26, 2022

That's the type of scale I used. Only difference being I used a permanently mounted electric ceiling hoist to lift the bikes.

Glen

Schwany · Oct 26, 2022

Thanks bluto and Glen, now I know how to weigh my not so porky P11 easily. Will have to drag out the motor hoist, buy one more set of soft ties, and buy a crane scale. Looks simple enough. Plus it will add to my hoarding pile.

Performance modifications have made a positive difference on every bike I have owned. I'm addicted to doing it though, so the gains could all be in my head. The head with the ears on the side. lol

worntorn · Oct 26, 2022

I believe the mods are working on Jim's 750 , perhaps to the tune of 5hp vs stock 750 at peak?
The 850 grunt up to 6000 keeps it in the game.
From that we can figure that in the real world, a stock 750, in the same config as a stock 850, is going to get left behind on an acceleration test like this.
I would bet that the 750/850 difference is small though.

Glen

750/850 weight comparison

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