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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 Ride safe! 

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  • 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

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