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 |
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Ride safe!
Thanks for reading
Classic British Spares
@JCS – Thanks for reading and for your comment. I have not thought about making the studs longer but I think its a good idea. I would have to take into consideration air filters and the side cover clearance
James A Schaible
Riding Chicago streets during the summer, I had this problem with my ‘71 A65 Thunderbolt (single carb). A phenolic spacer helped, and I fabricated a heat shield/heat sink from aluminum sheet. With the added thickness of the sheet and gaskets I had to be aware of stud length and couldn’t install longer studs because the OIF bikes have no “wiggle room” to remove the carb. I cut two nylock nuts down and have had no problems. As with most vintage Britbikes, mine still doesn’t like really hot 90+ days but it’s much happier with the modifications.
JCS
I solved the problem on my ‘68 A65 with some 3/4" phenolic spacers (can’t remember where I got them…). However, this required replacing the carb studs with longer ones, which I had to make myself since the threads into the head are BSF 5/16-22. While not rocket science, it was a bit of a PIA to have to make the studs (I have trouble keeping the threads perfectly concentric when using a die). Have you guys considered offering longer studs to go with thicker spacers? (I made mine from stainless steel since this has a lower thermal conductivity than carbon steel.)
Andy Ryan
Is there a spray that can cool the carburettor
Classic British Spares
@Tim – Yes, so do we. Cheers