Lemon Drizzle Gang on a nice 73 850

They mention a top speed of 85 mph with a Commando fitted with a single Mikuni.
This is wonderful as I can honestly say my single carb BSA 650 Super Rocket should outrun a single Mikuni 850 Commando!
The BSA does an honest 92 mph GPS with me flat on the tank. We won't talk about how long it takes to get up there or how bad the vibrations are at that speed.

Back to the 850 on test- it has new twin Amals and is said to go very well.

At 13:17 ish, they should have stopped at the Granary cafe (green awning on the right). They do nice tea & cakes.

My 850 will sit all day happy at 85mph, with a single mk2 Amal (34). Still has plenty left to squirt past on overtakes.

worntorn said:

They mention a top speed of 85 mph with a Commando fitted with a single Mikuni.

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They might mention that but I’ve had my mk3 over 100mph.

Well I wont race you with the BSA then!

Power Barn says the Mikuni single is "easy bolt on horsepower"
I recall that Jim's dyno did not agree!

The single Mik vs twin Amals debate is like oil threads, they come up from time to time and are never resolved! IMHO it’s not about right or wrong, it’s different horses for different courses.

Mikunis are good, well made instruments that meter fuel well. They are also very cheap to buy, especially when only buying one!

As such, a single Mik gives good low to mid rpm running. Being a single carb, the ‘hassle’ of synchronising etc is removed (I used inverted commas there because for many, in fact most, synchronising carbs is not really an issue). The single carb is therefore definitely easier to get the bike idling well, getting good off idle throttle response, etc.

But there’s no question that they reduce performance above 5k ish. Comnoz showed it to be the manifold design that’s the bottleneck rather than the carb itself, hence different sizes of carb make little difference.

Its worth bearing in mind that when folk say there’s a loss of ‘top end‘ performance, it doesn’t just mean top speed, it means performance above 5k in all gears.

So, if you don’t ride above 5k anyway, then a single carb will not reduce your performance at all and of course, if replacing worn out or out of synch carbs, you’ll actually notice an improvement in performance.

However, if you’re someone who enjoys the benefits of revving their Commando to 6k, a single carb is gonna impact your fun factor somewhat.

Some people seem to struggle with the cold start on Mikunis as it’s a bit ON/OFF. I’ve got Mikunis on another Brit and I always start it and ride off ASAP not leaving it to idle, that seems to help avoid hassles when the engine temp is between the ON/OFF settings of the cold start system.

It would be interesting to mount a single Mikuni on my MK3 to see the effect on Dyno hill.
Who knows, the single carb might pull harder in the 4-5 k rpm band.
I would hate to lose or detract from the 5 to 6200 part of the rpm band though.
That is where the fun is on that bike , and of course the best soundtrack along with the strong acceleration.

Glen

Experts with an iphone. I spent my career in the movie biz so I can be a real snark when it comes to the vast offerings of one man videos.
The scenery/countryside is beautiful, always one of my favorite aspects of England.

worntorn said:

It would be interesting to mount a single Mikuni on my MK3 to see the effect on Dyno hill.
Who knows, the single carb might pull harder in the 4-5 k rpm band.
I would hate to lose or detract from the 5 to 6200 part of the rpm band though.
That is where the fun is on that bike , and of course the best soundtrack along with the strong acceleration.

Glen

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Years ago before the internet and being able to tap into other people's font of knowledge I did some work on my mates combat
It hadn't exactly been reliable and he wanted me to detune it a little
He had a spare 850 bottom end but no barrels or head
I'm not sure what cam was in the bottom end but it probably wasn't stock
I fitted the combat 750 top end but I made a 1mm decomposition plate to put under the barrel
All went well
I refitted the twin carbs and they had the usual wear on the slides and the needle jets were really badly worn
I'd heard about single mikuni conversions but never seen one
I decided to make my own using a carved up Norton atlas manifold and a 34 mm VM mik from a Suzuki GT 500
I had the pilot air adjustment screw almost hanging out in my attempt to tune it
What shocked me was the bikes ability to pick the front wheel up if you wanted to
It never did that with the twin amals
But I just didn't know enough then to make it work right
So it went back to twin amals
My mate still owns that bike although he hasn't ridden it much in the last year or so

I don't doubt twin carbs will give more power at higher revs. I reckon the 'debate' centres around what you want the bike for, how it's set up and riding style. That dictates whether we're happy with a single or twin carb set up.

For me, day trips with mates on fast B and A roads, no in town work and regular solo quiet country lane runs. The gearing on my Commando is higher than stock, so 6,000rpm in 2nd gear is over the speed limit. For some, high rev shove is the thrill. I have always preferred big twins with good low down torque, I get a thrill from my Commando firing me out of corners with a deep rumble. It does pull really well at 3,000 to 4,500rpm. Just what I like . 4,500 in top, is 90mph (indicated), still smooth and more to go.

I bought a CB750K6 4cyl, to try a bit of higher rev motoring. It's quite good. I definitely prefer my Norton. As a consequence, a single carb feels good. I don't feel the need to change to 2 carbs. But I'm not saying they perform the same.

I'm curious...ACK made some turn out manifolds so dual Mikunis could be used. Did any one try these?
I am a big fan of Mikunis. The VM is an excellent carb, built better than Amals and much cheaper. Looking on Ebay 34mm carb sell between $90 and $120 while Amals sell for $300! Only problem is that they don't fit into the space. It really hurts to spend $600 for a pair of Amals when Mikunis would have cost $200

seattle##gs said:

I'm curious...ACK made some turn out manifolds so dual Mikunis could be used. Did any one try these?
I am a big fan of Mikunis. The VM is an excellent carb, built better than Amals and much cheaper. Looking on Ebay 34mm carb sell between $90 and $120 while Amals sell for $300! Only problem is that they don't fit into the space. It really hurts to spend $600 for a pair of Amals when Mikunis would have cost $200

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A forum member near me has dual Mikunis and the manifolds do turn out somewhat. IMHO, they should turn out much more. The drive side is not bad, but the timing side foam air filter is crushed against the oil tank. Since the head positions the intakes so close together on a Norton, it's hard to get the carbs splayed enough to have decent air filters and you certainly cannot mount them close together like Amals.

I know it's a real bear to fit them but curious how it performs. Does the bend cause flow problems?

Surprised this video has resurfaced, we filmed it over 2 years ago!
I didn't actually say 85mph was the top speed on a single mik, though Mike inferred it with his "that was it" comment. However, in terms of safe overtakes behind a row of slower bikes on the Isle of Man during Classic week (with potential for very fast oncoming traffic!) on a road I was familiar with from previous visits on a slightly hot-rodded T140v (with two Mikunis btw), that otherwise standard 850 really didn't want to punt past anything once above 80 with anything like the acceleration of the (slightly tuned as per description in the video) commando in the video, or my T140v. So it's not quite a direct comparison but the difference is/was very marked.

For me, I love snarling up to around 90 on the road and have the ability to overtake in as short an "exposure to danger" window as possible, the twin carbed set up helps with that. No surprises, dyno tests as mentioned, plus Cycle magazine's own testing with a single Mik setup in their March 1975 issue (also in the Brooklands Books "Ultimate Portfolio" commando book) all bear this out. No loss of low down torque either (Mart's comment) or in the 4-5k rev range (Worntorn's dyno hill comments) as that's the rev range I use all the time, I hardly ever take it over 5!
As others have also said, the single Mikuni setup is perfect for how others use theirs and the difference might be less marked on the same bike, but there will be a difference and my current commando is hardly heavily modified engine wise.

"Experts with an iphone. I spent my career in the movie biz so I can be a real snark when it comes to the vast offerings of one man videos." what the hell does that mean? Filming these takes ages and not on an iphone!

There's a later video filmed last summer where I let Mike (who doesn't own any classic bikes, that's the premise of the channel) loose all day on the same commando and my very very nicely sorted R90S. Similar performance, he would choose the commando any day of the week, he loved it!

I always enjoy your videos.
Hope I didn't put words in your mouth with my 85mph comment . That was one of my takeways from this one.

Same as you, when it is time to pass, I want to do that as quickly as possible, don't want to be hanging out there. Even a twin carbed Commando can be a bit anemic when in a short uphill passing lane while dealing with a strong headwind. I was on the Vin behind a clubmate on a 750 twin carb stock Commando in such a situation. The 750 couldn't seem do much over 75 mph with throttle pinned. The Vin had a little more, not a lot, but enough to get past the 750 and get by the line of cars.
With a 30 mph headwind the wind resistance at 75 mph is the same as at 105 mph in still air. And then there is the hill to add in!
I had to decide whether to be rude and pull around or pull back in as it was clear he wasn't going to make it past a tightly packed string of slow moving traffic.
Rudeness won out. I have never been forgiven, but I'm still here to talk about it
He eventually hit the brakes and pulled back in.

Glen

Funny how the single versus twin carbs debate pops up now and then

worntorn said:

I always enjoy your videos.
Hope I didn't put words in your mouth with my 85mph comment . That was one of my takeways from this one.

Same as you, when it is time to pass, I want to do that as quickly as possible, don't want to be hanging out there. Even a twin carbed Commando can be a bit anemic when in a short uphill passing lane while dealing with a strong headwind. I was on the Vin behind a clubmate on a 750 twin carb stock Commando in such a situation. The 750 couldn't seem do much over 75 mph with throttle pinned. The Vin had a little more, not a lot, but enough to get past the 750 and get by the line of cars.
With a 30 mph headwind the wind resistance at 75 mph is the same as at 105 mph in still air. And then there is the hill to add in!
I had to decide whether to be rude and pull around or pull back in as it was clear he wasn't going to make it past a tightly packed string of slow moving traffic.
Rudeness won out. I have never been forgiven, but I'm still here to talk about it
He eventually hit the brakes and pulled back in.

Glen

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Farting at the dinner table is rude. Smashing the throttle to stay out of harms way.... big boy rules apply.

Scary ride through the village. Much too fast for my liking.

- Knut

wavey_davey said:

Surprised this video has resurfaced, we filmed it over 2 years ago!


"Experts with an iphone. I spent my career in the movie biz so I can be a real snark when it comes to the vast offerings of one man videos." what the hell does that mean? Filming these takes ages and not on an iphone!

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What part of the observation do you not understand?

Mart UK said:

My 850 will sit all day happy at 85mph, with a single mk2 Amal (34). Still has plenty left to squirt past on overtakes.

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The bike is willing, but the flesh is weak. My neck starts hurting over 70mph.