Service Bulletin: A65 Overheating Carburetor Problem

Service Bulletin: A65 Overheating Carburetor Problem

A few years back when I owned my 1969 BSA A65 Lightning, a problem developed which caused my Amal Premier Concentric carburetors to overheat

My Lightning would not hold idle and would in turn have poor throttle response after long periods of riding

I especially noticed this problem during the summer when I would come to a red light or re-starting my motorcycle when at the gas pumps 

During my ownership it took some time to isolate and locate what was causing this “overheating” carburetor problem and the symptoms that went along with it

The first telltale sign I found about my overheating carb issue was when I pushed the tickler down, fuel would spray / burst out, rather than trickle down when cold

I ultimately pinpointed the overheating problem due to the fact that the A65 twin carb (IE Lightning / Firebird Scrambler) heads do not have “bolt-on” manifolds which means your carb is bolted directly to the cylinder head - this allows heat to transfer to the carburetor and not allowing the heat to properly dissipate

Speed up to 2021, bike is long gone, however I found this service bulletin recently that covers exactly what I experienced

Let’s cover it below..

Service Bulletin

BSA A65 Overheating Carb Service Bulletin - 17/71

Click on the image above to enlarge

This 1971 dated bulletin states that the "Overheating Carburetor issue" happens when the surrounding air temp is high and while the engine is running at low RPM's for long periods 

To combat this problem, BSA recommends that you must use the 70-2968 insulator block to help prevent heat transfer to your carburetor

BSA does not state this, but the 70-2968 spacers now comes in different thicknesses, bore sizes for different (universal) applications and material (phenolic) 

On my A65 I did use the 70-2968 blocks with a thickness of .125 (1/8”), however it did not fix my problem, I ended up going with x2 spacers .125” on each carb with thin carb nuts - that did fix my problem

Not mentioned in the service bulletin, you can also help with lowering the temps of your carburetor and engine by checking your state of tune, spark plug choice and ignition timing within your engine 


If you are experiencing this problem and need to purchase an insulator block, I recommended clicking on the links below to view our stock

As stated previously, we offer different bore sizes and thicknesses to work with most applications

If you plan on running a thicker insulator block, be mindful of the stud length you have to work with…

 Part type  Part Number Bore Size Link
Insulator Block 70-2968 26(mm) Click Here
Insulator Block 70-2968 28(mm) Click Here
Insulator Block 70-2968 30(mm) Click Here
Insulator Block 70-2968 32(mm) Click Here


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Should you have any questions, please drop us a comment using the comment section below 

 Ride safe! 

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  • Darren

    Burlen sell the phenolic spaces for Amal carbs. Cheers.

  • Classic British Spares

    @Tim You can use it on either carb. See the link below. Thanks for reading today!

  • tim

    Where do you get the block that you used.
    I have 2 A65 Ts one 67and on 69 road bike.
    Does it differ in the heat depending on which carb you use. Cosentric or Monoblock ?
    Thanks for the Info.
    Tim G.

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