Rubber mounting Amals

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SteveBorland

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Wondering if anyone has tried using the adaptors from the Amal Mk2 carbs and used them to rubber mount Commando carbs?
The only commercial kit I've found is from RGM, but its rather expensive and they are currently out of stock.

/Steve.
 
SteveBorland said:
Wondering if anyone has tried using the adaptors from the Amal Mk2 carbs and used them to rubber mount Commando carbs?
The only commercial kit I've found is from RGM, but its rather expensive and they are currently out of stock.

/Steve.
Click to expand...

Is there a problem you'd be looking to solve/improve with this mod?
 
Looking at the way the carbs are bounced around on a standard Norton engine, I suspect that a form of rubber mounting to dampen the vibs would be a Good Thing. It must also accelerate the wear problem 'twixt slide & bore.

I recall an article in Motorcycle Mechanics in around 1978 where Mick Hemmings was quoted as saying that this was one of the many good things he did on his bikes.
 
The rubber mounts isolate engine heat better than the stock setup.
Carb removal is greatly simplified as well.
My PWK's use a similar mounting system and it works well.
 
Last year I briefly tried a pair of Mikunis on my Slimline racer, but went quickly back to Amals. However, I DID keep the manifolds and rubber adaptors that came with the Miks. The bike starts much better and runs like an ICBM missile. No more wear and tear or adjustment on the carbs since then!!!
 
Jagbruno said:
Last year I briefly tried a pair of Mikunis on my Slimline racer, but went quickly back to Amals. However, I DID keep the manifolds and rubber adaptors that came with the Miks. The bike starts much better and runs like an ICBM missile. No more wear and tear or adjustment on the carbs since then!!!
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I very strongly agree. I rubber mounted the Amal carbs on my Dommie race bike because I had fuel frothing and starvation at the end of long straights. (Around 7500 rpm with carbs mounted on very long intake manifolds)

I found the Amal Mk 11 type rubber components to be TOTAL rubbish. They cracked, seemed to not like ethanol and methanol and sometimes lasted only one or two races before the carbs were hanging off on the cables.

The Mikuni rubber adaptors lasted a season of racing. More expensive but they worked - which is always useful :D

The ones that look like this. You need to turn up some manifold adapters.

http://www.amazon.com/Mikuni-Rubber-Mou ... B000GZO4V0
 
The Amal MK2 mounting rubbers are resistant to ethanol / methanol if you use the correct type

2928/123 for petrol 2928/119 for alcohol
 
Fuel frothing causes rich running which can slow the bike. All it takes is for a carb to touch the frame or a bolt attached to the frame even if the carbs are flexibly mounted. If you are using a rigidly mounted air cleaner where the carb inlet is too rigidly fitted to the cleaner, can cause frothing. It might not matter much with a road bike, however on a racer it is critical. Even the slightest bit too rich and the motor will become sluggish especially when accelerating, and that is more devastating if you are using petrol rather than methanol fuel.
The rigidly mounted carbs on the commando are simply cheap and nasty. Like a lot of things, they look OK so many people would not change them. I think I bought the mounts on my bike from Mick Hemmings along with a pair of 34mm alcohol kitted Mk2 Amals, and even then the jetting as received was ridiculous.
 
Yet another go faster fad from the 1950s / 1960s??
Without doubt a correctly rubber mounted carb system will reduce frothing and will isolate the carbs from the engine to some extent although air passing the motor them passes the carbs...Although exactly how one ensures the rubber mounting results in less vibration and frothing and not an increase......
To heat isolate the carbs use those solid black was it TUFNEL?? spacers between the head and inlet tract ALONG with tufnel washers beneath the heads of the bolts fixing the inlet tract to the head.
Personally the only reason we employed rubber mountings to the carbs was to increase inlet tract length to 'tune' it to suit the cam etc being employed.
NOTE... Modern fuels and avgas DID NOT agree with the tubing we employed all those decades ago. It took us a long time to realise the loss of performance was because the tube had expanded into one inlet tract partially blocking it off ...so simple a reason no one though to check it!! We did from that point on and replaced the tubing on a regular basis as we bought it by the yard.......
 
"Without doubt a correctly rubber mounted carb system will reduce frothing and will isolate the carbs from the engine to some extent although air passing the motor them passes the carbs...Although exactly how one ensures the rubber mounting results in less vibration and frothing and not an increase...... "

Exactly. :idea:
 
Apart from anything else, just the reduction in the buggeration factor involved in removing the carbs seems to make this worthwhile. With a standard setup, just changing jets becomes a task requiring considerable patience and dexterity.

The people contributing with constructive suggestions here are to be thanked. Yet another small project added to the list....
 
Jagbruno said:
Yes, those are the Mikuni adaptors that I'm using. Strongly recommended.
Click to expand...

They will not fit a standard intake manifold without an adaptor plate which moves the carb further to the rear. The bolt spacing of the Mk1 Concentric and the manifold is 2" while the Mikuni features a 60mm bolt spacing. Moving the carb backwards could result in an interference with frame tubing. A nicer solution is the kit sold by Old Britts; here, a special short manifold (30 to 34mm) is used together with the Mikuni rubber mount which I guess is M-VM28-200K.
Carb spigot of the Concentric Mk2 is 40mm and fits the rubber mount nicely.
I have not tested this setup for frame clearance.

https://www.oldbritts.com/tm.html
https://www.jetsrus.com/mounting_flange ... anges.html

Now all we need is a proper throttle linkage to ensure syncronization of the carbs :)

-Knut
 
With my old 900 cc roadracer with the isolastic mounted motor there was nothing I could do to make the engine run cleanly throughout the rpm range. I must has spent $1000.00 on jetting for the downdraft FCR's.

Finally I made mounts and mounted the carbs solidly to the frame and used flexible manifolds. After that it was a different machine. The jetting was easy and predictable. It ran like a modern streetbike from idle to 9000. Jim
 
acotrel said:
The rigidly mounted carbs on the commando are simply cheap and nasty. Like a lot of things, they look OK so many people would not change them. I think I bought the mounts on my bike from Mick Hemmings along with a pair of 34mm alcohol kitted Mk2 Amals, and even then the jetting as received was ridiculous.
Click to expand...

Acotrel, I'd be interested to know which jets you selected for Concentric Mk2 carbs. Les Emery once claimed it was impossible to tailor the Concentric Mk2 for the Commando making it run as well as with the Mk1 carbs. Are yours on a street bike?

-Knut
 
comnoz said:
< ... > Finally I made mounts and mounted the carbs solidly to the frame and used flexible manifolds. After that it was a different machine. The jetting was easy and predictable. It ran like a modern streetbike from idle to 9000. Jim
Click to expand...

Here is a guy who did the same to the Mk2 Concentrics of his late model Triumph.

http://www.triumphrat.net/classic-vinta ... lters.html

-Knut
 
comnoz said:
After that it was a different machine. The jetting was easy and predictable. It ran like a modern streetbike from idle to 9000. Jim
Click to expand...

Exactly my own experience on my 850/PW3/lightened crank Slimline racer

-never tried to rev it above 8000 though :mrgreen: -
 
Knut,
The MK2 Amals are on my Seeley 850 running methanol fuel. The carbs and manifolds are 34mm and the ports are tapered back to standard (30mm ?) in the first 25mm. If you want to run methanol at standard comp. I use 34 degrees ignition advance, and the carbs as follows - No 3 slide with 6D Mikuni needles, 0.116 inch needle jets and 670 Amal mains. I made my own needle jets out of brass hex using number and metric drills.
If you are jetting for petrol, you'll need to at least buy a range of needle jets and probably a couple of Mikuni needles to get the tuning fine enough. Don't worry too much about the main jets as long as they are slightly over-rich. You won't normally burn a piston if the bike is not coughing, unless you are at full throttle ((on the mains).
 
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