Mikuni questions.

After reading a recent post re: dual 34 mm Mikunis , I pulled out my single Miks. , not dual , with manifolds from the crypt. Yes I have 2 single 34 mm. Miks in bags with larger manifolds ( both are 40 mm. carb. end , to both intakes of 32 mm. and the single carb. rubber adaptors. Sitting for years in storage. One is very old , flip down/up choke. The other is newer with pull up/down choke. I unscrewed jets tonight and found 6DH3 needles , P0 and P2 needle jets , 35 pilot jets and a 1.5 air screw in the bag and one size 3 slide. The bike build will be a MK111 850 c.c. on a rebore to .40 oversize. The head is an RH 10 ,with the smaller 30 mm. intakes. Non crossover peashooters. MK1v Boyer. 6 Volt coils. Stock cam, but may purchase a stage 1 cam in future. Presently using 30 to 32 manifolds for the 2- 32 mm. Amal Concentrics.

Several Questions are : Winners Circle are/were my parts source here in Toronto. Might be closed from Covid ? Should I just buy the TM 32 from them, pre- jetted ? A new flat slide interests me. If so I'll need a cable to fit the MK 111. I'm old so it's about slower driving style. Need the torque pull and not the top end anymore.

Where to get throttle cable for the single Mik. for a European bar MK 111, either for V or TM series .

Obviously the 2 manifolds I have to choose from are both too big ? I measure 40 mm. at the air filter mouth and of course 32 mm. at both intake ports. The RH 10 head is 30 mm. at each intake port I believe. Will this be a problem ? I think so due to air dam effect ? I'll buy a smaller manifold if need be. BTW. I have another 850 head , RH 4 , which I believe is 32 mm. intake ports. Again , I'm after torque and rarely pass 5000 R.P.M. , only to pass when feeling empowered or foolish.

More questions soon.

The RH10 head is definitely the better head unless you‘re tuning for max power at high revs.

My understanding re single carb manifolds is that unless you cut out the gusset in the Commando frame to make more space, you’re stuck with a manifold that has very tight bends and is quite restrictive to flow above 5000 rpm. Largely irrespective of the carb feeding it.

Your riding style (below 5000rpm) suits a single carb set up. But I’d question the value in putting a performance orientated flat slide carb upstream of a restrictive manifold.

IMHO I’d either go the normal ‘off the shelf’ Mikuni kit route. Or, if I wanted a flat slide, I’d look into removing the gusset and fabricating a one off manifold (like Baz did on this forum). The former is an easy bolt on job and you can tap into a wealth of jetting knowledge if you get into difficulty. The latter is quite an engineering project and you’re pretty much on your own with carb set up !

Going from a 32mm intake manifold hole to a 30mm intake head port won't make that much difference with a single carburetor. I would do a simple port match blend on the intake ports myself, but you could just leave it and it would work for what you plan to do. Would it be optimal? No, but you said you don't want to go fast anymore.

The 40mm carburetor is big, but not too big in a single carburetor configuration on an 850. It can be made to work. That said, not everybody has the touch for tuning, and it could be more work than you'd be comfortable with.

I agree with Eddie
If you want to reduce the guesswork and time it takes to get a good tune with an old 40mm carburetor, MAP Cycle sells complete single kits for the Norton Commando with a 36mm VM Mikuni carburetor. A 36mm carburetor will give you all the low end performance you want. MAP has been around a while, and their Norton Mikuni kits are a little less expensive than Sudco kits. They also sell a 34mm kit for the MkIII e-start bikes with throttle cable. They have a toll free number.

Schwany said:

Going from a 32mm intake manifold hole to a 30mm intake head port won't make that much difference with a single carburetor. I would do a simple port match blend on the intake ports myself, but you could just leave it and it would work for what you plan to do. Would it be optimal? No, but you said you don't want to go fast anymore.

The 40mm carburetor is big, but not too big in a single carburetor configuration on an 850. It can be made to work. That said, not everybody has the touch for tuning, and it could be more work than you'd be comfortable with.

I agree with Eddie
If you want to reduce the guesswork and time it takes to get a good tune with an old 40mm carburetor, MAP Cycle sells complete single kits for the Norton Commando with a 36mm VM Mikuni carburetor. A 36mm carburetor will give you all the low end performance you want. MAP has been around a while, and their Norton Mikuni kits are a little less expensive than Sudco kits. They also sell a 34mm kit for the MkIII e-start bikes with throttle cable. They have a toll free number.

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Thanks , I'll likely use the newer 34 mm. VM Mik that I have then.
The 40 mm only refers to the cast manifold opening that accepts the rubber mount for the 34 mm carb. Sorry about any confusion there.
Both single manifolds I have are large , one says MAP on it , the other IS. I'd happily trade one for a smaller manifold if anyone is interested.
This morning I managed to remove the air screw on the newer carb after soaking the hole in WD 40 for an evening. It's a size 2 air jet , very small little thing. Apparently I'll need to fit a smaller bore one like 1 or 1.1 or 1.2 size.

I would not leave a step in the inlet tract. The have been claims the a BSA Gold Star is better with a step in the port. But I don't believe it.

acotrel said:

I would not leave a step in the inlet tract. The have been claims the a BSA Gold Star is better with a step in the port. But I don't believe it.

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Yes ,agree with that and I won't go there : hence the offer to trade my large manifold to any reader who wants to go bigger , I realize now the need to go to a smaller manifold to fit the head .
I'll still need some parts to make this happen properly. Jets and cables. I'm in Toronto , Canada. I'll call Winners Circle up now , but in Covid shutdown for non-essentials there may be a looong wait.

Torontonian said:

Yes ,agree with that and I won't go there : hence the offer to trade my large manifold to any reader who wants to go bigger , I realize now the need to go to a smaller manifold to fit the head .
I'll still need some parts to make this happen properly. Jets and cables. I'm in Toronto , Canada. I'll call Winners Circle up now , but in Covid shutdown for non-essentials there may be a looong wait.

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If I were you I would just try it with the manifold you already have
I'd be surprised if you could tell any difference on a single carb manifold

The 1mm step around the circumference of a 32mm intake manifold hole against a 30mm intake port hole is not optimal, but it's not going to make a huge difference on a daily driver.

Something that could be done if there is room and a person doesn't want to pull the head off and do a proper intake port match is take a pair of 30mm insulation spacers and put a 32mm to 30mm taper on them. It's only a 1mm taper all the way around. Much ado about nothing really.

Schwany said:

The 1mm step around the circumference of a 32mm intake manifold hole against a 30mm intake port hole is not optimal, but it's not going to make a huge difference on a daily driver.

Something that could be done if there is room and a person doesn't want to pull the head off and do a proper intake port match is take a pair of 30mm insulation spacers and put a 32mm to 30mm taper on them. It's only a 1mm taper all the way around. Much ado about nothing really.

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Yes , interesting idea to adapt. I could use my round file to taper file the 30 mm. fiber heat insulator spacers to match the larger manifold ports.

I’m pretty sure that I recall Comnoz saying that a small step is quite irrelevant to flow when tested on a flow bench.

I‘d also envisage that the larger manifold would offer less restriction to flow in what is known to be a restrictive manifold.

I guess what I’m saying here is that I agree with Baz, why not give it a go? You’ve nowt to lose.

acotrel said:

I would not leave a step in the inlet tract. The have been claims the a BSA Gold Star is better with a step in the port. But I don't believe it.

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Odd things can happen in the inlet ports aerodynamics, but like you I would like to see the proof right in front of my nose.

Torontonian said:

Yes , interesting idea to adapt. I could use my round file to taper file the 30 mm. fiber heat insulator spacers to match the larger manifold ports.

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Odd as it may sound, on the single carb 750 Atlas, Norton inserted split collars that reduced the size of the inlet port in the head.
This enable it to run on standard gearing all the way down to 15 mph in top gear.

Bernhard said:

This enable it to run on standard gearing all the way down to 15 mph in top gear.

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It does amuse me just how much emphasis period road tests put on this aspect.

Personally I’ve always been quite happy to change down a gear or two !

That and the smooth tickover at 500rpm , were a big deal back in the day ....

Fast Eddie said:

It does amuse me just how much emphasis period road tests put on this aspect.

Personally I’ve always been quite happy to change down a gear or two !

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Remember sidecars??

Bernhard said:

Remember sidecars??

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Thought outfits still had gearboxes

Fast Eddie said:

It does amuse me just how much emphasis period road tests put on this aspect.

Personally I’ve always been quite happy to change down a gear or two !

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Some people don't know how to use a gear box. So they ride Harleys.

Modern Harleys have got 6 gears.

In fact, with their big torquey engine and close ratio box, I had assumed you were on the design team Al

Harleys have 4 valves per cylinder too.

I'd like to be able to ride 15mph in 2nd gear and think I'd accomplished something special.

acotrel said:

I would not leave a step in the inlet tract. The have been claims the a BSA Gold Star is better with a step in the port. But I don't believe it.

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It does work on BSA Goldstars.