How many HP on my MK3

Madnorton said:
A bog standard MK3 850 I suspect would be pushing its luck around mid 40's - there was a well known tuner in the UK who suggested a MK3 in standard trim would at best hit 40. I run a similar set up to the OP and mine pushes out 50, I also know another owner in the UK who tested his well built and expensively modified 850 engine with all the nice tuning parts and large valves etc and that was hitting 58 and that was on the same dyno with same settings. Realistically the 850 will benefit massively from larger valves and the other parts to accommodate them. The 850 capacity versus the valve size is at the very limit for the standard valves. What the OP has noticed is that the torque curve is most probably raising quicker at lower revs and remaining pretty flat through the rev range which makes for a comfortable and easy riding bike. Having max HP but only using it less than 5% of ride or not at all on public highway is pretty pointless, try and get the power and the torque to where you are going to use it, and if I had the one answer that suits all for road and race bikes I would be selling it. A dyno run is well worth it, you can see the curves and monitor the fuelling mixture and carburettor response when changing gear, if you are scared of wrecking it the tester will not thrash the balls off it.
Click to expand...
A standard MK3 will produce the same power as any standard 850, provided all have the same silencers.
They are also the same weight as any standard 850/750 that has been fitted with an Alton e start. I weighed an estart fitted 750 Interstate, an estart fitted 74 850 Interstate and my MK3 Interstate, all full of fuel.
The bikes were all within 5 lbs of the same weight.
So much for the " very heavy MK3 " myth.

Broken record on this, but we tried my 850 on full acceleration against Jim Bush's modified and very fast 750. Jim has had many Commandos and definitely knows his way around them. He is the "Bushman's carb secrets" man. He told me that this 750 is the fastest Commando he has ever owned. It has a Comstock ported head with D exhaust ports, raised compression and a PW3 cam.
My MK3 just has earlier type silencers replacing the factory black caps. Internally it's as it left the factory in 1975, 46,000 miles on the original parts, save a new head gasket or two.

The bikes stayed dead even to 80 mph , where we had agreed to stop the test.
In the heat of the moment Jim kept going, so I did too. As my 850 went out of its power band his went into its power band, the 750 pulled away a bit 85-95 mph
I finally put it in top and 95 -110 we stayed even again.
So not at all the expected result on the actual road dyno.
I'm sure the MK3 got it's reputation for low power from those stuffed up factory black cap silencers. Some time ago Jim gave me a set of factory black caps to try. I made the jetting change and tried the bike on Dyno Hill. I don't recall the numbers, but they were very sad. It was so gutless!
The strange thing is that the MK2A 850s had the same black caps and the same poor performance ( shown in period road testing) but don't have the same reputation for low performance that the MK3 got stuck with.

Glen
 
Last edited:
worntorn said:
A standard MK3 will produce the same power as any standard 850, provided all have the same silencers.
They are also the same weight as any standard 850/750 that has been fitted with an Alton e start. I weighed an estart fitted 750 Interstate, an estart fitted 74 850 Interstate and my MK3 Interstate, all full of fuel.
The bikes were all within 5 lbs of the same weight.
So much for the " very heavy MK3 " myth.

Broken record on this, but we tried my 850 on full acceleration against Jim Bush's modified and very fast 750. Jim has had many Commandos and definitely knows his way around them. He is the "Bushman's carb secrets" man. He told me that this 750 is the fastest Commando he has ever owned. It has a Comstock ported head with D exhaust ports, raised compression and a PW3 cam.
My MK3 just has earlier type silencers replacing the factory black caps. Internally it's as it left the factory in 1975, 46,000 miles on the original parts, save a new head gasket or two.

The bikes stayed dead even to 80 mph , where we had agreed to stop the test.
In the heat of the moment Jim kept going, so I did too. As my 850 went out of its power band his went into its power band, the 750 pulled away a bit 85-95 mph
I finally put it in top and 95 -110 we stayed even again.
So not at all the expected result on the actual road dyno.
I'm sure the MK3 got it's reputation for low power from those stuffed up factory black cap silencers. Some time ago Jim gave me a set of factory black caps to try. I made the jetting change and tried the bike on Dyno Hill. I don't recall the numbers, but they were very sad. It was so gutless!
The strange thing is that the MK2A 850s had the same black caps and the same poor performance ( shown in period road testing) but don't have the same reputation for low performance that the MK3 got stuck with.

Glen
Click to expand...
I suspect the difference in weight was in the fuel load, the pre MK3 tank Interstate tank is larger. This also ties in with what a great UK tuner told me that the original trim MK3 is not making that much at all.
What would be interesting is to run an 850 with RH10 head and one with RH4 head and see if there is much difference.
 
Madnorton said:
Yep, they will do exactly as you ask for as you are the paying customer.
Click to expand...
The only real way they can wreck an engine is if they smash it down through the gears overreving it to slow down the drum. Other than that you need to tell them the red line and watch the match between his computer rpm readout and the bikes tacho.

Plus any stupid mistakes like dropping it getting it up on the dyno. It happens. And tangling tiedowns in the wheel and chain.

If your going there with the objective of tuning the carb or ignition timing then make a plan and write it down. Take spare jets and needles. Your strobe and battery if testing timing changes. Most shops only do modern Japanese bikes. They will have zero idea on how to tune a Commando.

Keep very careful notes making sure his run numbers match exactly your changes. Beleive me in the heat of the moment this is very easy to screw up. Take ear protectors, heavy gloves for when you touch a hot bike. And tools you need. Most shops these days only have metric tools.

If your getting really fussy ask him what environmental correction he is using. There are different SAE, DIN standards that can change the result by up to 5 % worse case.
 
Last edited:
worntorn said:
A standard MK3 will produce the same power as any standard 850, provided all have the same silencers.
They are also the same weight as any standard 850/750 that has been fitted with an Alton e start. I weighed an estart fitted 750 Interstate, an estart fitted 74 850 Interstate and my MK3 Interstate, all full of fuel.
The bikes were all within 5 lbs of the same weight.
So much for the " very heavy MK3 " myth.

Broken record on this, but we tried my 850 on full acceleration against Jim Bush's modified and very fast 750. Jim has had many Commandos and definitely knows his way around them. He is the "Bushman's carb secrets" man. He told me that this 750 is the fastest Commando he has ever owned. It has a Comstock ported head with D exhaust ports, raised compression and a PW3 cam.
My MK3 just has earlier type silencers replacing the factory black caps. Internally it's as it left the factory in 1975, 46,000 miles on the original parts, save a new head gasket or two.

The bikes stayed dead even to 80 mph , where we had agreed to stop the test.
In the heat of the moment Jim kept going, so I did too. As my 850 went out of its power band his went into its power band, the 750 pulled away a bit 85-95 mph
I finally put it in top and 95 -110 we stayed even again.
So not at all the expected result on the actual road dyno.
I'm sure the MK3 got it's reputation for low power from those stuffed up factory black cap silencers. Some time ago Jim gave me a set of factory black caps to try. I made the jetting change and tried the bike on Dyno Hill. I don't recall the numbers, but they were very sad. It was so gutless!
The strange thing is that the MK2A 850s had the same black caps and the same poor performance ( shown in period road testing) but don't have the same reputation for low performance that the MK3 got stuck with.

Glen
Click to expand...
I’d say there are 3 factors on a MK3 vs an earlier bike that have question marks over their performance impact:

1) the black plastic airbox
2) the black cap silencers
3) the RH4 head.

With the airbox, there are many who say it’s fine, perhaps even better, than the earlier one, so this is less clear.

With the black caps, again there are those who claim they’re fine, but I’d argue that the vast majority seem to acknowledge they are a significant limiter.

And the head, Commoz was quite clear in his posts that the ports were a big step backwards on the RH4.

A 4th factor when comparing 850 to 750 is the valve sizes. The valve sizes are good for a 750 but not an 850. On an 850 they really do become the bottleneck to performance, as I believe my Dyno results demonstrate quite strongly. Tony Smith (RIP) the ex Guz Kuhn racer told me similar, he said they could not understand why the 850s didn’t respond to tuning like the 750s , until they fitted bigger valves, then ‘boom’ !

This does not necessarily mean that bigger valves will unlock performance on a stock 850, but that any tuning done will be severely restricted unless bigger inlet valves are used.

As Ashley says, it would be very interesting to see how your bike responded to a RH10 head Glen; especially one that Comnoz had breathed on !
 

Similar threads

D
  • Locked
Photos of the SU and things on my Mk3
Replies
10
Views
1,604
Hobot
Hobot
powerdoc
  • Locked
Evaluating the old parts on my Mk3
Replies
6
Views
1,148
Josh Cox
J
Back
Top