convert back from mikuni to 2 amals

comnoz · Feb 28, 2020

gortnipper said:
This is very interesting. I have one of Mike's 2EX1 exhausts, and the right pipe takes a sharp bend across the front if the barrel while the left one extends out before bending down.

There is quite a difference in how the Amals are jetted to get a similar plug color. The right carb has a larger needle and main jets.

I had wondered if it was a porting difference, or something else.

I wouldn't be surprised if the pipes were the reason.

The best horsepower I ever had on the roadrace track on a stock stroke motor was made with 1-1/2 inch copies of SS pipes. With reverse cone megaphones.

tri-man · Mar 7, 2020

comnoz said:
There is no question, going up to the 32mm port with the same 1.5 inch valve was a bad move.

The small valve limits the airflow so the large port ends up with low velocity and poor cylinder filling efficiency.

The carb size doesn't make a big difference, it will just maintain the torque to a little higher rpm with the 32's and a tapered manifold. But it does loose a little throttle response down low with the 32's.

I've been away from this conversation for a while, busy days. My experience with Norton isn't huge, or rather wasn't, as I've mainly worked on Triumphs for the last 45 years. However, this has changed a lot lately, as so many of the trusty old Norton hands packs it in due to old age, poor health, or the purchase of an RV...
I'm fascinated by the reasoning behind the difference between RH4 vs the RH10 heads, why did they do it? I learnt from this forum and Comnoz in particular, that the smaller port heads gives the engine a substantial increase in midrange power, which is where most people spend their riding time. As the experience quoted above didn't include running the engine with twin Amals on 32mm ports, it wasn't really conclusive to me.
Now I find myself with 3 Commandos in my garage, a Mk2A 850 (a general work-over), a Mk3 (cam change...) and an early ca 1970 Roadster 750 (tossed the single Mikuni for twin 930 Concentrics).
Riding these bikes back to back these last few days have left me with the following "seat of the pants"impressions: The Mk2A stomps away from the Mk3 in a roll on from almost any speed, in fact the Mk3 needs around 5000 rpm before it matches the other.
The 750 is stronger than the Mk3 at any speed, though a little softer than the Mk2A below say 4000 rpm. Beyond that it feels faster.
If I was to choose between them I'd have the 750, light as a Bonnie, fast, and smoother than either 850. (with the Mikuni my mother could have outrun it)
Back to the sleeve trick, I learnt a great lesson some 3 decades ago when I built a super quick Triumph TR7 1973. I used the standard inlet cam, a 1/2 race Hyde exhaust cam, standard pistons and carb. Apart from some cleaning up of the really rough ports, I left them at 1 1/16" or 27mm. Acceleration was almost brutal, and so much harder than any T140 I had tried similar tricks on. It was a bit peaky though, useful power from 4000, it needed 5000 before the party started, and by 6500 the party was over. Anyone who has ridden a T140 will recognize the shape of the power curve, only this one gave so much more.
I've since tried a similar trick on a 66 Bonnie, using Norton inlet sleeves, taking the ports from 1 1/8" to 1 1/16", or just under. It gave the engine a useful boost in the midrange, and I think the top end was at least equal.
Fitting a dual carb set-up to a 1971 Triumph TR6 (small ports) for a customer nailed it for me. I've had many Bonnies including 71/72 model 650s, and they were no match for the twin carb TR6 at any speed.
Without a dyno it's easy to dismiss this as wishful thinking, and it would be foolish of me to claim they're anything other than MY impressions.
Still, they ARE my impressions, and fueled by my "success" I made up a triplet of sleeves to take my Trident inlets from 1 1/16" to 1". Any standard Trident will be found a bit lacking in oomph below 5000 rpm, long cams, big ports and valves see to that. Short of a re-design, different cams, or a big bore kit, there isn't a lot to be done about this, but a set of sleeves is relatively easy to make. Again, no dyno runs (yet) but my impression is more power through the rev range, and if there's any loss up top it can't be much. Very similar results to the RH4/RH10 conversion.
I never radius or blend the sleeves in, I leave them stepped. I try to determine how far in they need to go and notch them for the valve guides. Very unscientific and "eyeball" engineering, but also simple to do, and to my mind, well worth it. After all, where I live and ride it's all about turns, bends and hills, a solid midrange is what you want, and by the time your mate on a 130 bhp 600 finds the right gear, you're through the bend, and long gone.

SR

EstuaryBoy · Mar 8, 2020

Re: The Mikuni carb.. What I couldn't live with was the on/off cold start device. Maybe okay if the bike can sit awhile, or you're straight into wide open spaces, but drive headlong into stop/start commuter traffic and I found it very frustrating. Ended up with a Phoenix SU kit, reconditioned by Burlen and love it!

tri-man · Mar 9, 2020

EstuaryBoy said:
Re: The Mikuni carb.. What I couldn't live with was the on/off cold start device. Maybe okay if the bike can sit awhile, or you're straight into wide open spaces, but drive headlong into stop/start commuter traffic and I found it very frustrating. Ended up with a Phoenix SU kit, reconditioned by Burlen and love it!

I agree about the on/off start, and it's a comment I hear all the time, also from Concentric Mk2 owners. I have no experience with SU on bikes, but like them on cars.
By the way, I suspect the downfall of the MAP single Mikuni set-up is the rather flat manifold with its cramped bends. I'd be willing to bet that a manifold more like the old Norton or Triumph style ones, and a Concentric would yield much better results.

Fullauto · Mar 9, 2020

EstuaryBoy said:
Re: The Mikuni carb.. What I couldn't live with was the on/off cold start device. Maybe okay if the bike can sit awhile, or you're straight into wide open spaces, but drive headlong into stop/start commuter traffic and I found it very frustrating. Ended up with a Phoenix SU kit, reconditioned by Burlen and love it!

Quite simply the Mikuni is not tuned correctly. And you can get a cable conversion.

EstuaryBoy · Mar 9, 2020

Fullauto said:
Quite simply the Mikuni is not tuned correctly. And you can get a cable conversion.

To my understanding, all the cable conversion does is put the on/off capability in a more convenient place.. Once warm, the bike ran fine, within the limitations of a single carb. But straight into traffic from cold, a choice of too weak or too rich, neither of which were conducive to smooth running.... But either way, the SU (though hardly a cheap option) trumps it every which way in my estimation, though it's not a 'performance' upgrade..

Fast Eddie · Mar 9, 2020

EstuaryBoy said:
To my understanding, all the cable conversion does is put the on/off capability in a more convenient place...

Not really. The lever assembly on a Mikuni is very much an on / off switch. Whereas with a cable, you can regulate it between the max on / fully off positions. Same with mk2 Amal’s.

Plus, having a lever on the handle bars means you can regulate on the move, at traffic lights, etc.

Nevertheless, if correctly set up, as Fullauto eludes to, it shouldn’t be problematic at all if given a short time to run before setting off.

baz · Mar 9, 2020

Fast Eddie said:
Not really. The lever assembly on a Mikuni is very much an on / off switch. Whereas with a cable, you can regulate it between the max on / fully off positions. Same with mk2 Amal’s.

Plus, having a lever on the handle bars means you can regulate on the move, at traffic lights, etc.

Nevertheless, if correctly set up, as Fullauto eludes to, it shouldn’t be problematic at all if given a short time to run before setting off.

Don't know about a cable on the mikuni I only had the on off lever never a problem
But it made a huge difference on my t140e with mk 2 amals when I went to cable
It had been revving to 3500 rpm or nothing before the conversion
Still prefer the tickler buttons on mk 1s though

EstuaryBoy · Mar 9, 2020

Fast Eddie said:
Whereas with a cable, you can regulate it between the max on / fully off positions.
….if given a short time to run before setting off.

Well.... we live and learn, which is why we're here, no?

I was fortunate to find the SU kit cheap and have it reconditioned FOC (thank you, Burlen) and it does make the bike a much nicer drive IMHO... and good mileage too.

baz · Mar 9, 2020

EstuaryBoy said:
Well.... we live and learn, which is why we're here, no?
I was fortunate to find the SU kit cheap and have it reconditioned FOC (thank you, Burlen) and it does make the bike a much nicer drive IMHO... and good mileage too.

What's it like with regards to starting?

Fast Eddie · Mar 9, 2020

I rode a hot rod Triumph with an SU carb once, I was staggered how well it ran. Also, ref t-man’s point, it was on a long Y shaped fabricated manifold, not a Norton style squashed, stubby thing. I was very impressed by the overall set up.

EstuaryBoy · Mar 9, 2020

baz said:
What's it like with regards to starting?

I've never found it a problem. I have read where people find theirs need priming, or a full fuel tank, but I've had no issues. Sadly my initial correspondence with Burlen was via email and lost when I changed internet supplier, because I seem to remember them mention an 'improved' starting circuit.. But don't quote me....

EstuaryBoy · Mar 9, 2020

Fast Eddie said:
I rode a hot rod Triumph with an SU carb once, I was staggered how well it ran. Also, ref t-man’s point, it was on a long Y shaped fabricated manifold, not a Norton style squashed, stubby thing. I was very impressed by the overall set up.

There was a guy on ebay selling an old Hillman camper for which he'd fabricated a 4-2-1 inlet manifold for an SU.. The carb must have been a good 18" from the head but he reckoned it ran fine!

baz · Mar 9, 2020

Fast Eddie said:
I rode a hot rod Triumph with an SU carb once, I was staggered how well it ran. Also, ref t-man’s point, it was on a long Y shaped fabricated manifold, not a Norton style squashed, stubby thing. I was very impressed by the overall set up.

Did the Y shaped inlet manifold have a bump with "shorrocks" written on it?

Fast Eddie · Mar 9, 2020

baz said:
Did the Y shaped inlet manifold have a bump with "shorrocks" written on it?

Ha! Sadly not.

It was a Dresda that Dave Degens built, he had this idea that with rising fuel prices and green agendas, a Classic Cafe racer that could do over 100mph AND over 100mpg was just what folk needed (this was 20 years ago, to be honest, he was ahead of the times).

It was a 650 with his bathtub squish head, his own cams, very lightweight valves and springs, lightweight crank, and the SU.

It was fabulous, it revved like an MV or Honda multi. I had it at 9,000rpm (much to his annoyance). He was certainly onto something with the idea, but sadly he got bored and moved on !

baz · Mar 9, 2020

Fast Eddie said:
Ha! Sadly not.

It was a Dresda that Dave Degens built, he had this idea that with rising fuel prices and green agendas, a Classic Cafe racer that could do over 100mph AND over 100mpg was just what folk needed (this was 20 years ago, to be honest, he was ahead of the times).

It was a 650 with his bathtub squish head, his own cams, very lightweight valves and springs, lightweight crank, and the SU.

It was fabulous, it revved like an MV or Honda multi. I had it at 9,000rpm (much to his annoyance). He was certainly onto something with the idea, but sadly he got bored and moved on !

Sounds very interesting , shame he got bored

baz · Mar 9, 2020

baz said:
Sounds very interesting , shame he got bored

Maybe I'll try the phoenix kit out I have sitting on the shelf
I'm just concerned it might run like a single mikuni!!!

Bernhard · Mar 9, 2020

comnoz said:
Air can definitely do unexpected things. A couple of things I have noticed.

With a head on the flow bench and a 34 mm single carb manifold mounted, the flow is miserable- as expected.
But if you take the bolts out and hold the manifold against the head nothing changes until you slide the manifold about 1/8th inch to the side so the ports are mis -matched, the airflow increases by ~15% on the side you slide the manifold toward, and drops on the other side.

Another mystery,
With a head on the bench with an exhaust port flowing with a 24 inch exhaust pipe with a 90 degree angle a few inches from the port, similar to what you would see on a motor.
If the pipe is pointed down like you see with low pipes you get one number.
But if you rotate the pipe so it is pointed to the side, like with SS pipes, the flow increases considerably.
This shows up on the dyno also.

Norton solved this problem on the 1960s 750 Atlas with the single carb model, the 1 1/8 Monoblock fitted to a single carb manifold, but, on the cyl head ports, a removeable split sleeve was inserted to speed up the flow and give a top gear performance from 15 mph to 105 mph, the bike wasn't the fastest, but it had by far the widest top gear performance of any other British parallel twin.

eskasteve · Mar 9, 2020

Since your (Alan) original post has been up for a while I was wondering if you ever pulled the trigger. Talking to Matt, while always pleasant, can get a bit expensive. I called him a few years back to inquire about the machined from billet primary. Just like the Accessnorton posts our questions and answers drifted off point. Before I knew it I'd ordered up the primary and a single Mikuni kit and as I recall, a few other bits and pieces. I'd always wanted to try a single carb and Matt had all the right answers to my questions. Even though I had a new set of Premiers on the 850 I decided to give it a try. It worked just as Matt said it would. Yes, maybe my top end was compromised a bit, but not as bad as some posters would have you think. My low to mid range performance did seem to be better. When I did up the engine a few years earlier I built it to stock specs as I really have never tried to wring the last bit of HP out of the old beast. I had a mildly hot rodded 2011 BMW S1000RR to satisfy my need for speed. Now there's a bike that will make you forget all about trying to boost your Norton's performance. Probably cheaper in the long run, too.
Oh yeah, the primary was and is a work of art.

alan hodge · Mar 9, 2020

eskasteve i am a low to medium speed rider sticking with the single carb gave the $$ to church for lunches for poor folks