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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  • Good Country

    With lead being removed from our fuel and all the additives they use to dick around with octane and lowering supplier cost do you think the boiling point of fuel today has been lowered causing some of the problem?

  • Classic British Spares

    @Steve – 70-2968 originally came in 1 bore size. That is why BSA recommends that you open the bore to port match the head and carbs. Common practice even in todays standard… Die grinders / Dremel’s work too. Thanks for reading

  • Steve

    Great article and explains why my 69 lightning cuts out at the lights. Not sure about the suggestion to file asbestos. Presumably when the service bulletin was written we were none the wiser

  • Peter W

    Yep, same as all the rest of the riders here in that my A65 OIF T/bolt did not like temps above 90 F.
    But the T/bolts had the removeable inlet manifold with gaskets onto the head.
    So that helped insulation but I still needed the two (2) .125" insulation blocks with 3 gaskets.
    This worked but pushed the Amal back into the single air filter assy rubber mount and it is a PITA to fit!
    My ‘69 B44VS arrived from Tucson with a 30 mm JRC PWK flat slide. Took a number of runs to get the jetting etc right but ’seems’ to run far better on the hotter days. But I still fitted 2 insulators!

  • Don J

    Hi All, We ride no matter how hot. 100f is common. Riding up Central Valley & Sierra foot hills 110f not uncommon.
    I’ve done many tests feeling carbs. Even the thin insulator is huge help. Thick insulator is much better. This applies to both BSA & Triumph. I’ve done tests with both thick insulator & air gap with fat oring on Triumphs. It’s a wash which which keeps carb cooler.
    100+f moving constantly not much problems. Stop/go traffic with little wind & repeated stop lights in city motor heat is very high. The entire fuel tank & fuel gets very. Let the rpms down at red light can/will stall, & can be hard to start. Worn slide makes this much worse. California fuel is made to always be under some pressure. In a carb you must keep blipping throttle. You’ll feel it when that’s needed as it will want to stall. A higher idle really helps. But who likes too high of idle?
    Hot start issues will really scare you. You’d swear motor has no spark.
    What my riding buddies & I do is tickle carb like it’s cold. Hold full throttle, kick hard for faster cranking rpm. Be ready to back off throttle, but keep it 3-4000 rpm until carb cools.
    Be mindful to keep idle extra high for about 2-3 minutes.
    Motor stopped high heat comes up open intake valve & from head in general. No insulator will stop this. On hottest days you stop bike for restroom break. Tickler button is too hot to push with bare finger.
    We often leave fuel tap on as evaporating fuel is replaced by cooler tank fuel. Hottest I’ve ridden in is 117f. Either figure out how to deal with the heat or watch TV. I’ll take heat any day.
    T140E carbs will do similar hot start problems. Use enrichment lever & hold full throttle. I’ve started many bikes like this.
    If either carb then floods. Hold full throttle several kicks. Then kick no throttle then try tickling no throttle. Sometimes they really scare you.

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