Service Bulletin: A65 Overheating Carburetor Problem
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
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
Purchase
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 |
Thanks for reading
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Should you have any questions, please drop us a comment using the comment section below
Ride safe!
Thanks for reading
Darren
Tim,
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!
https://www.classicbritishspares.com/products/triumph-bsa-norton-30mm-phenolic-carb-insulator-spacer-70-2968-tr6-t120-a65
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.