Carburettor choices

Lots of discussions on replacements for Amals, with CRs & FCRs being the popular choices for performance oriented motors.
I'm looking for something for my 920 with 36mm manifolds.
What I haven't seen is any mention of using pumper Dell'Ortos, and was wondering if there's a reason for this, or is it just that I haven't been paying attention?
Any other options out there?

Ta

B+Bogus said:

Lots of discussions on replacements for Amals, with CRs & FCRs being the popular choices for performance oriented motors.
I'm looking for something for my 920 with 36mm manifolds.
What I haven't seen is any mention of using pumper Dell'Ortos, and was wondering if there's a reason for this, or is it just that I haven't been paying attention?
Any other options out there?

Ta

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Carb width is the problem. Dell Ortos must be splayed outward at an angle which hurts the flow. Its hard to get them to fit. Same with Mikunis. The only available carbs that fit side by side are Amals. Keihins and PWKs.

I modified the frame in the racebike below to accept Dell Ortos by removing the gusset and changing the oil tank etc so I could extend the manifolds. The extra length allowed them to be splayed outward from each other. A tight fit.

jseng1 said:

Carb width is the problem. Dell Ortos must be splayed outward at an angle which hurts the flow. Its hard to get them to fit. Same with Mikunis. The only available carbs that fit side by side are Amals. Keihins and PWKs.

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Thanks for this Jim, I suspected it may be the case.
Any chance of you offering your carbs in larger bore sizes?

Not sure but I believe LCR Ken uses Delortos on his 920 LSR bike?

I bought a pair of 39mm FCRs from Allen’s for my 1007. May be worth giving them a call?

The larger sizes of CRs, 35mm+ have larger bodies & may be a tight fit.

I have used Dellorto carbs on 750 and 920 Commando engines in my Commando PR, my wideline Featherbed racer, and my ex-Jim Schmidt monoshock racer. As Jim says, they are too wide to fit parallel, and have to be used with splayed manifolds. But they do work very well. I really doubt if there is any significant flow loss from the the mild curve in the splayed manifolds. I've also used a pair of Amal 36 mm MK2s on 750 and 920 race bikes, as well as on an 883 Commmando street bike, with a lot of success, and they do fit well in parallel in the Commando frame.

Fitting 35 mm FCRs is straightforward with the CNW kits, but you might find it more difficult if you fit the 37 or 39 mm carbs. I'm fitting a pair of 37 mm FCRs to my MK3 restomod project, and had to do some frame mods to get them to fit, shown below.



Ken

I suggest the major deciding factor in choice of carburettor besides price, must be the amount of fuel mixture control they offer. A pumper carb is a different way of richening the mixture to compensate for loss of vacuum when accelerating. I think the tapered needles is a better way of doing it. In the 70s, Lectron carbs gave good results on two-strokes, and two-strokes require better mixtrure control than four-strokes. I tune and use my 850 motor as though it is a two stroke - it makes a difference. With a two stroke motor, the throttle must always be fed on in a controlled manner. With Lectron carbs, mixture can be adjusted by turning the needles.
If you can get a carb which compensates perfectly for loss of vacuum, you would be able to whack the throttle open and get better acceleration, instead of needing to feed the throttle on.

lcrken said:

I have used Dellorto carbs on 750 and 920 Commando engines in my Commando PR, my wideline Featherbed racer, and my ex-Jim Schmidt monoshock racer. As Jim says, they are too wide to fit parallel, and have to be used with splayed manifolds. But they do work very well. I really doubt if there is any significant flow loss from the the mild curve in the splayed manifolds. I've also used a pair of Amal 36 mm MK2s on 750 and 920 race bikes, as well as on an 883 Commmando street bike, with a lot of success, and they do fit well in parallel in the Commando frame.

Fitting 35 mm FCRs is straightforward with the CNW kits, but you might find it more difficult if you fit the 37 or 39 mm carbs. I'm fitting a pair of 37 mm FCRs to my MK3 restomod project, and had to do some frame mods to get them to fit, shown below.

View attachment 115266

Ken

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Not knowledgeable and certainly not an expert, just wondering.

Shouldn't you, in effect, replace the crossmember by putting one behind the tubes. I would think that would maintain the rigidity of the frame and still give you clearance. It probably would not need to be as "tall" as the original if it gets in the way of the air cleaners.

B+ Bogus - I'm not sure the 35 or 37mm CRs will fit in the frame. The 33s are tight as it is. Same with the 37 or 39 FCRs as Ken mentions. Splaying the Del Ortos or Mikunis when you have them on long manifolds or rubber manifold extensions should not impede the flow as Ken mentioned. That's what I did with the 36mm Dell Ortos on the custom racer in the photo in the 2nd post. But I don't like carbs splayed out at a sharp angle as you see with short steep angled manifolds such as the dual Mikuni setup shown in the photo below. Be aware that carb supply is a bit weird right now because Sudco just closed its doors.

The amount of squirt on the phf and phm can be adjusted down , my old B33 has a phf30 it was quite rich when you reved it , I just reset the pump screw , it's getting a phf36 soon which might help the 600cc .

Shelby-Right said:

The amount of squirt on the phf and phm can be adjusted down , my old B33 has a phf30 it was quite rich when you reved it , I just reset the pump screw , it's getting a phf36 soon which might help the 600cc .

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It would make sense that the pump rate would be adustable, and if is done by turning a screw, that is much better than opening a carb and changing needles and needle jets, The trouble might be in getting two carbs to be identical in adjustment.
If you are seeking performance with petrol, the margin for error is very small.

I think , like on holley V8 carbs the pump volume is measured over 10 pump shots in cc , so if you could ? Squirt into a hose , maybe you could set them the same .

Some great information there - thanks!

@ Eddie - I'm guessing those 39s are for the Seeley?
The current plan is for the 920 to go in a standard frame, and keep the Seeley as a 750.

I originally intended fitting 36mm Mk2 Amals, but they're nowhere to be seen these days.
surfing through other posts it looks like it's not a new problem:


Interesting comments about boring out 34mm Mk2s to 36mm though - I happen to have a pair of those lying around too

B+Bogus said:

Some great information there - thanks!

@ Eddie - I'm guessing those 39s are for the Seeley?
The current plan is for the 920 to go in a standard frame, and keep the Seeley as a 750.

I originally intended fitting 36mm Mk2 Amals, but they're nowhere to be seen these days.
surfing through other posts it looks like it's not a new problem:

Thread 'AMAL MK2 Carbs Availability?'

Apr 25, 2019

Does anyone with closer contact with Amal/Burlen know if they are ever going to produce MK2 carbs again, Particularly the larger size 2000 (36mm, 38mm, and 40mm) and Smoothbore series? I sent them an email, but got no answer.

Ken

• lcrken
• Replies: 46
• Forum: Norton Commando Forum

Interesting comments about boring out 34mm Mk2s to 36mm though - I happen to have a pair of those lying around too

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Yes, 39s on the Seeley (1007) and 35s on the Commando (920).

Ken has shown how FCR 37s / 39s can be fitted to a Commando frame by modifying / removing the gusset. So it’s definitely an option. When I bought my 39s one reason for choosing them over 37s was that 37s were simply not available… anywhere !

Another option is to fit FCR 35s. Thats what I have on my road going 920 with 36mm ports, you’re talking half a mil difference on the circumference, which I think pretty much gets ‘lost’ anyway when mounting on the rubber stubs. This was not my preferred option as it’s not ideal in theory, but I did it to ‘get me going’ and seems to work fine, so I’ve not changed it!

However, be warned, even FCR 35s won’t fit using Steves manifolds as they’re ‘just’ too long and the carbs will foul the gusset. Matt made his cNw manifold specifically for the purpose. I used one of Matt’s manifolds and opened it out to suit.

36 / 38mm MK2s do pop up on eBay, just a case of keeping a regular eye out for them.

I have a set of NOS MK2 36s (no they’re not for sale) that I intend to try at some point along with the Maney manifolds.

Depending how brave you are, you could try a pair of Chinesium 36mm PWKs. They’re only around £40 each on eBay, so you won’t lose much when you give up and throw them in a skip …

If you wait until October you can borrow my FCR 39s to try if needs be.

Something to remember, well on holleys anyway but I am sure this would be the same , is , a smaller jet/nozzel gives a longer duration squirt and bigger gives a shorter dump of fuel . Probably not the best wording, but hopefully you get idea .cheers.

marshg246 said:

Not knowledgeable and certainly not an expert, just wondering.

Shouldn't you, in effect, replace the crossmember by putting one behind the tubes. I would think that would maintain the rigidity of the frame and still give you clearance. It probably would not need to be as "tall" as the original if it gets in the way of the air cleaners.

Click to expand...

I don't have the gusset on my commando
It'd been removed by a previous owner
I've never got around to replacing it in 30 plus years

acotrel said:

A pumper carb is a different way of richening the mixture to compensate for loss of vacuum when accelerating. I think the tapered needles is a better way of doing it.

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I would dispute that tapered needles are any sort of equivalent to accelerator pumps.

In a Concentric, the tapered section of the needle in the needle jet meters controls mixture when the throttle is steady at 1/4-2/3 open.

The brief loss of vacuum when the throttle is opened quickly is compensated for by the accumulated froth (“emulsion”) being sucked out from the upper part of the needle jet. One of my eccentric experiments was a shortening of the needle jets in twin 928s. That introduced a very annoying hesitation when opening the throttle from 1/4 to 1/2 or a bit more: as in a typical burst of acceleration on a public road.

I’m not saying that froth-sucking is as good as pumping, but the effect is real.

I ran 36mm Dellorto's on my race bike and they performed well. I used a featherbed frame, so it was easier to get them installed. But I don't remember having to splay them out much, maybe a bit by milling the manifolds to a slight angle.

I thought pumpers were not legal for vintage racing.

Sudco closing its door is probably the worst thing that could happen for vintage fuel burning motorcycles in the USA. Carburetion is going to get expensive and hard to get unless Amals are your jam. FCRs are out of stock at CNW.

I got my 35mm FCRs in Sudco kit form and made manifolds up for a straight up engine from Commando MkII manifolds. Did that before Jim Schmidt started making manifolds for the CRs. Jim's manifolds would be easy to modify. You might be surprised how well JB weld works. That said I've not worked around a Commando frame for 40 years, so am clueless about what you would have to deal with. If I were building a clip-on cycle, I'd start with a Seeley frame so I had some room.

Fast Eddie said:

........

Depending how brave you are, you could try a pair of Chinesium 36mm PWKs. They’re only around £40 each on eBay, so you won’t lose much when you give up and throw them in a skip …

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The Chineze PWKs usually need different needles because they might be 2 stroke needles and too narrow on the small end. The fuel passages may also need to be opened for WOT to prevent starving. That's a PIA but the truth is they are great carbs if you get past those hurdles and figure out how to adjust the idle (different than Amals).

Someone mentioned float sticking on the PWKs and there are 3 causes and solutions. First shake the carb and listen - if the float rattles then its free and you're OK. If not then some carbs have a brass tube at an angle rubbing the float that needs to be bent out of the way. Or the ribs on the sides of the float needle can occasionally hang up on a hole in the float needle recess. The corners of those ribs need to be filed on an angle so they don't hang up. Not that big a deal. The 3rd problem is guming up the needle from fuel lines degrading from ethanol laced gas - clean the needles and change the fuel lines to yellow tygon hoses.

The other question is if you can set them up for left and right by moving the idle speed screw over to the other side. Another PIA but its doable. I've worked with the 32 & 34mm PWKs and have needles and specs for them but I haven't worked with the 36mm PWKs Photo below shows a couple modified and mounted PWKs. They are very small and fit side by side.