Safe level of performance from an 850 MK111

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Dec 22, 2006
I am wondering what performance enhancements can be done to the 850 without losing the wonderful low and midrange torque. Also, I would not want to greatly diminish the life of the motor by adding a few horsepower.

On this subject, what is the actual output of the MK 111 850?
I have one book that says 58horsepower, the same as early 750s, but 56 foot pounds torque vs 46 for the 750. My other book, the Haines manual claims that the power output of the 850 is greater than the early 750 and equal to the hi compression 750 combat but with much greater longevity and again, higher torque for better performance in most real life situations. So I don't know if they are supposed to put out 58 HP at the crank or 65. I suspect that the 58HP figure is correct and the Haines manual is saying the extra torque of the 850 thru the acceleration range makes up for any missing Hp(as compared to a combat) right at top.

I have low restriction peashooters, boyer, stock twin 32 MM Amals, everything else stock, no complaints but a bit more is always fun.
To give you an idea of real figures, my 750 has been on the dyno.
It has 10.5:1 compression, Axtell cam ( almost identical to the 4S) 36mm Amals and supertrapp exhausts, it makes 58 bhp at the real wheel.
I guess 58bhp at the crank for an 850 is possible.
My view would be that a bigger pair of carbs would be the most economical way to go. You can get a kit with a pair of 34mm Mikunis.
As for reliabilty, I doubt if you would have problems if you manage to squeeze a few more horsepower out of it.
John, did you make the changes to the bike, or did it come to you with the mods? If you made the changes, did you lose anything in the way of tractability or low end to mid range grunt?

The reason I ask is that I recently met a fellow who did major mods to his 750. He went high compression ,I believe it was 10.5 or 11.0 to one, lightened crank, lightened valve train, gas flowed heads and so on. He told me that he lost a lot of torque below 4500 RPM and also that the bike requires avgas. The bike flies at the top of the RPM range, but riding it has become a bit of a chore, especially in stop and start traffic.
It's a bit difficult to give a definitive answer because although I made the mods myself, I had never ridden the bike before the tuning started. It IS a race bike, but pulls well from low rpm, although it won't accept full throttle below 4500. I run it on a 50/50 mix of 98 octane unleaded and "shell racing 100" which I believe is the same as avgas. I only mix the racing 100 because it has lead in it, but I have been told that I can run a commando on unleaded and I intend trying it this season on BP 98 octane unleaded.
I used to own an 850 mk3 as a road bike and I recall it feeling like it was running out of steam around 80MPH, but I can inderstand why you wouldn't want to lose that low end grunt that commandos are so good at. I really doubt that a pair of 34 mm carbs would lose much bottom end torque, but would help out at the top end.
I am currently changing over from the 36mm amals ( mk2) to a pair of 34mm del ortos, in the hope of picking up some mid range. You may think this contradicts what I have just said, but I am trying to improve full-throttle response, which you wouldn't be using in traffic.

Here's the bike ... rton02.jpg
My Mk3 850 currently is fitted with 3S cam (noisy) a pair of 34mm Mk2 Amals, a Mk1 850 ported head skimmed to give 9.5:1, S&B filters and straight through silencers etc. I did expect to loose some of the bottom end, however, it has as much grunt as my mate's bog standard Mk3 until it hits 4000rpm then it just takes off.
How long will it last? time will tell.

Nice bike John, does it vibe much? Delortos are great carbs but grief were they heavy on the throttle on my old Guzzie
The bike pulls quite strongly to 90 MPH then acceleration drops off quite a bit. I did take it up to 115 sitting up, which I thought was excellent, more top end than I was expecting.
I use the bike for touring, so premium unleaded is the highest octane gas I can get, about 91 octane on average. Right now I run it on regular as it has standard 8.5 to one. It doesn't ping on the regular, and the power output is the same as with premium, since higher octane fuels do nothing for you unless compression dictates you must use them to prevent pinging.

Would the switch to larger carbs have a positive effect without opening up the intake stubs and ports to match?
I know that with the Vincent no power gain results unless the porting and stubs are done as well.

Interestingly, at most road speeds my mostly standard Rapide outdoes my modified Rapide, as near as I can tell from seat of the pants. It also outpulled a friend's severley hopped up Rapide last year when pulling up a steep 10 percent , six mile long grade in the Rockies. We were both loaded 2 up plus full gear with our overall weight being about 50 pounds or so greater. He has 9.4 to one comp., digital ignition, Lightning cams, 34 MM Flat slide Mikunis, intake and exhaust porting, oversized two into one exhaust going into a very loud Supertrapp, 5 speed Quaife and a bunch of other stuff I have forgotten. His bike runs only on Premium, whereas my touring Rapide does fine on regular (8 to one comp, 6.2 was factory stock)
He pulled out to pass the lead bike, and I gave chase. We caught him and passed at about 80 Mph to his 75. I could not hear my motor as we passed him, his Supertrapp is so loud, but we were both pretty surprised at this result. I think he is planning a supercharger for the bike now!
A well tuned stock 850 should run up to about 6800 rpm with ease.

If you build a motor to exceed 7500 rpm with the stock crank, you are asking for big trouble. :shock:

What rpm are you turning?

Another issue is a slipping clutch.
Easy to check. Do a high gear roll on around 70 mph.
The clutch holds fine on the bike, and power is really very good, however, like most people, no matter waht the power is, I always want a little more. I have forgotten the exact RPM at 115 mph, but I do recall glancing at the tach and noting that it was close to the redline. I think the bike must be in pretty good shape and tune to do 115, from what I have read some barely reach the ton, especially after a single carb conversion (correct me if I'm wrong here)

I know I should leave it alone and be happy, but idle hands tend to make trouble.....
With hindsight I agree with Worntorn just use the Commando's grunt.
""Would the switch to larger carbs have a positive effect without opening up the intake stubs and ports to match? ""

You would have to open up the stubs and possibly the ports depending on the port size. Mine has 32 mm ports, so I have just tapered the inlet stubs from 34 to 32 to match the head. That way I don't have to mess up the head. I think the mikuni kit comes with matching stubs that have the taper in them. They also have a bit of a splay on them because the mikunis are wider than amals and won't fit that close together.
""Nice bike John, does it vibe much? Delortos are great carbs but grief were they heavy on the throttle on my old Guzzie""

It doesn't vibrate much considering it's rigid mounted, but I only ride it in short club races. I reckon a couple of hours on the highway would change my mind!
As for the carb springs, see the thread "nylon lined cables". I am experiencing exactly what you said.
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