Carburetor spacers

Anvill

Anvill

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Feb 26, 2022
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Hi all,

At long last I’ve finally got my MKIII’s rebuilt head back from the very gracious Leo Geoff!

He took off my 2 to 1 Mikuni adapter in the process, and so I need to reorder gaskets. I see that the previous owner had quite a thick set of gaskets on it, perhaps more like spacers.

I’ve seen things like this used to isolate engine heat, but was curious how necessary they are. I wanted to know if any other folks have used spacers and if so what they think of them. Do they serve their purpose well on a Commando or are stock gaskets generally fine for a single Mikuni setup?

The previous owner also had an oil cooler equipped which I’ve deleted. Generally I’ve been trying to remove unnecessary upgrades, though I think I will keep the single Mikuni for the time being.

Thanks for your help,
Anthony
 
When I was running a single Amal concentric on my Commando, I had a thermal lock of the throttle slide one day after a spirited bit of riding. Had just stopped for fuel and just prior to restarting, I gave the grip a quick twist out of habit. Well I was confounded when the grip would not return to fully closed position on its own. Inspected bars, cables and then cable entry at carb top. All looked/felt good. Hand grazed across cab body and found it too hot to touch. Bowl also too hot to touch. After a few more minutes past, I heard the throttle slide snap shut by itself. The heat had soaked from hot head to carb body and expanded the body enough to sieze the slide. There were only the thin phenolic type "spacer" insulators btwn head and 2into1 manifold. Once fired up and moving, air flow kept things happy until I got home. Fitted the thicker phenolic insulators btwn head and mani ports and also put a thin one btwn carb and mani. Never had that seizing again and body stayed much cooler to touch.
This is likely less a concern on two carb setups as having two inlet manifolds increases cooling surface area before reaching carbs.
 
The Mikuni mounts to the manifold on a rubber flange fitting, so is isolated from heat anyway, so no need for other insulators IMO.

There may be a theoretical discussion to be had about performance gains to be made by keeping the manifold, and therefore intake charge, cooler, but without being rude, if you run a single carb you ain’t havin’ those thoughts anyway.
 
Tornado said:
When I was running a single Amal concentric on my Commando, I had a thermal lock of the throttle slide one day after a spirited bit of riding. Had just stopped for fuel and just prior to restarting, I gave the grip a quick twist out of habit. Well I was confounded when the grip would not return to fully closed position on its own. Inspected bars, cables and then cable entry at carb top. All looked/felt good. Hand grazed across cab body and found it too hot to touch. Bowl also too hot to touch. After a few more minutes past, I heard the throttle slide snap shut by itself. The heat had soaked from hot head to carb body and expanded the body enough to sieze the slide. There were only the thin phenolic type "spacer" insulators btwn head and 2into1 manifold. Once fired up and moving, air flow kept things happy until I got home. Fitted the thicker phenolic insulators btwn head and mani ports and also put a thin one btwn carb and mani. Never had that seizing again and body stayed much cooler to touch.
This is likely less a concern on two carb setups as having two inlet manifolds increases cooling surface area before reaching carbs.
Click to expand...
I had exactly the same thing happen with twin amal's on an extremely hot day, since then I have had ongoing issues with sticking slides (premiers).
I took the carbs off and polished the carb bores with autosol to make everything ran freely up and down which it did.
I could not see anything obviously wrong with the carbs & slides and are now running the thickess carb phenolic insulators. Any ideas ? 🤔
 
I would always try to have carburetors mounted so they vibrate less. If the fuel froths in the bowls, the motor can run rich - and slower.
 
Desmopower said:
I had exactly the same thing happen with twin amal's on an extremely hot day, since then I have had ongoing issues with sticking slides (premiers).
I took the carbs off and polished the carb bores with autosol to make everything ran freely up and down which it did.
I could not see anything obviously wrong with the carbs & slides and are now running the thickess carb phenolic insulators. Any ideas ? 🤔
Click to expand...
Try putting insulators at both forward and rear ends of each manifold, thin/thin, thick/thin or thick/thick if you think it will sort it. If your fuel bowls are too hot to touch after a brief stop up, there will be consequences I'm sure. I have also seen trick cooling-finned manifolds from somewhere.

Any chance you warped the carb bodies from something being too tight?
 
acotrel said:
I would always try to have carburetors mounted so they vibrate less. If the fuel froths in the bowls, the motor can run rich - and slower.
Click to expand...
That's a good idea. Heard some folks using flat rubber in place of the phenolics to take heat and shake out at head/manifold flange.
 
Tornado said:
Try putting insulators at both forward and rear ends of each manifold, thin/thin, thick/thin or thick/thick if you think it will sort it. If your fuel bowls are too hot to touch after a brief stop up, there will be consequences I'm sure. I have also seen trick cooling-finned manifolds from somewhere.

Any chance you warped the carb bodies from something being too tight?
Click to expand...
The carb bodies are not warped as I am always very careful not to overtighten them. The slides drop down freely when cold, not really sure to check next....
 

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Tornado said:
So the saying "That which can be asserted without evidence can be dismissed without any. " springs to mind.

Why would reducing heat soak and or vibrations be a bad idea?
Click to expand...

I’m not say it is !

I’m saying sticking some rubber in the joint (“flat rubber in place of the phenolics”) sounds like a bad idea to me.

Firstly, as the manifold will still be held on by studs / bolts, they will still transmit vibration. So I doubt the rubber would reduce much.

But, Commandos generally don’t suffer from carb vibration as far as I’m aware, so what’s the aim?

Secondly, if heat is your issue, the stock heat spacers will work well.


But more importantly, I’d be concerned about potential new issues caused…

What kind of rubber would you use? How thick? Is it going to crush with heat cycles and cause a leak? Is it going to become brittle and crank? Will it be petrol / ethanol proof? Being a ‘soft‘ joint, how will you keep the fasteners tight? Also with the soft joint, what about flange face disortion?

Etc.

Hey, you asked !
 
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Here is a post on rubber mounting Amal's:

Tricks to mounting carbs with hex nuts

I've seen MANY slotted manifold inside holes, so the inside 2 allen head bolts only need to be loosened, not removed. I did that to my Blue monoshock bike. Saves a few minutes and possible busted knuckle...
www.accessnorton.com www.accessnorton.com
 
Tornado said:
Here is a post on rubber mounting Amal's:

Tricks to mounting carbs with hex nuts

I've seen MANY slotted manifold inside holes, so the inside 2 allen head bolts only need to be loosened, not removed. I did that to my Blue monoshock bike. Saves a few minutes and possible busted knuckle...
www.accessnorton.com www.accessnorton.com
Click to expand...

Interesting, had not seen that before, good find.
Carburetor spacers

The Bakelite insulators still do good job. You can find them at various parts sources or buy a sheet and make your own.
 
@phippsy Hi do you have a update in regards to your T140 ? alloy/rubber adapters ?
Or why you think they are better than using MKII manifold rubbers for heat and vibration insulation ?
Of course with the correct inlet stubs
 
Tornado said:
Here is a post on rubber mounting Amal's:

Tricks to mounting carbs with hex nuts

I've seen MANY slotted manifold inside holes, so the inside 2 allen head bolts only need to be loosened, not removed. I did that to my Blue monoshock bike. Saves a few minutes and possible busted knuckle...
www.accessnorton.com www.accessnorton.com
Click to expand...
Obviously there is nothing wrong with properly installed ‘correct’ rubber stub mounts as discussed here. My comments above were regarding your idea to use rubber in place of the phenolic spacers (but I’m still not sure how much your average road going Commando suffers from vibration induced carb issues).

The Mikuni type combined flange / stub is a good choice as it removes one joint, they are well made, and they will provide a shorter installation, which might be a useful consideration on a Commando.

Also, I believe that rubber mounting the carbs would be a good idea for those using the Pender gantry. When bolting up rigidly the gantry can expose a ‘stack up of tolerances’ that simply doesn’t matter when the carbs are independent of each other. Rubber mounting them would resolve this, allow the gantry to perform as intended, and prevent any stress on the carb bodies.
 
A post from the past...

Rubber mounting Amals

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.
www.accessnorton.com www.accessnorton.com

 
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Fast Eddie said:
Obviously there is nothing wrong with properly installed ‘correct’ rubber stub mounts as discussed here. My comments above were regarding your idea to use rubber in place of the phenolic spacers (but I’m still not sure how much your average road going Commando suffers from vibration induced carb issues).

The Mikuni type combined flange / stub is a good choice as it removes one joint, they are well made, and they will provide a shorter installation, which might be a useful consideration on a Commando.

Also, I believe that rubber mounting the carbs would be a good idea for those using the Pender gantry. When bolting up rigidly the gantry can expose a ‘stack up of tolerances’ that simply doesn’t matter when the carbs are independent of each other. Rubber mounting them would resolve this, allow the gantry to perform as intended, and prevent any stress on the carb bodies.
Click to expand...
To be clear it was not my idea to mount with rubber, just that I have heard of others doing so. I have a gasket sheet sampler pack which has a rubberized synthetic cork sheet. I recall a post from an owner saying they used similar on their carb manifolds to good effect.
 
I agree with FE, but not for the stated reasons.

The carbs, hung on the end of the manifolds will flop violent up & down if a rubber gasket of same thickness as the original hard gasket.
The mounting hardware and holes will gnaw it self to death.
 
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