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A65 Thunderbolt pinking #782854 08/28/19 10:26 am
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Fraz Offline OP
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My Thunderbolt had a a50 top end on it which I have replaced with an a65.
I purchased a new AMAL Premier carburettor with;

3.5 Cutaway Slide Hard Anodised

230 Main Jet

106 Needle Jet

Size 17 Premier Pilot Jet

It has a lead replacement additive added when I fuel up.

Since I've put on the new carb it's pinking when hot and I open the throttle.

I played about with different main jets but it still pinks.

I've read this forum and carbon build up is usually the main culprit but it's had new barrels and head.

Can anyone give me any pointers what I should try for next?

Thanks

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Re: A65 Thunderbolt pinking [Re: Fraz] #782855 08/28/19 10:31 am
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kommando Online Content
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Did you remove all the sharp edges from the piston and the head when they were apart, doing so reduces the ability to pink.

What ignition are you running, do you know its advance curve and is the engine properly strobed at fully advanced.

What fuel are you using.

What revs and throttle openings is the pinking appearing.

Re: A65 Thunderbolt pinking [Re: Fraz] #782859 08/28/19 11:05 am
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Allan Gill Offline
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Agree with everything Kommando says, addition to that,Scrap the lead additive off also, I used some when I couldn’t get decent fuel once when in another country (my lightning runs around 10:1 compression and normally in the UK doesn’t ping for anything) the lead additive did stuff all, if anything it put a coating on the head and pistons which required a top end strip to remove the amount of fur that was on there.

Also why did you change the main jet? Unless your throttle is fully open this will do stuff all to help, it is operational throughout the throttle position past idle but until it is 3/4-WOT is it used in its entirety and Bedford that only has a minimal effect.


Also what air filter and exhaust setup are you running? If you have a velocity stack and reverse cone Megas for example the standard carb settings won’t help.


beerchug
Re: A65 Thunderbolt pinking [Re: Fraz] #782862 08/28/19 12:02 pm
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Fraz Offline OP
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Many thanks for the responses, I'm new to classic bikes so bare with me.

It had new hepolite pistons and a reconditioned head, I do not recall sharp edges.

It's got an electronic ignition installed, which was with the bike when I got it. It didn't pink when it had the old 626 AMAL carb installed which is why I didn't think it was timing related.

I used regular 95 unleaded but will try and use the 97 when I can find it.

It happens once the engine is hot and I open the throttle, not excessively but enough to pick up speed

It does have a velocity stack and straight through pipes.

The bike didn't pink before I changed the top end and it didn't when I had but ran the old carb.
I had read somewhere that someone with a thunderbolt that was pinking and they changed to a 3" slide and upped the main jet to a 250?

Re: A65 Thunderbolt pinking [Re: Fraz] #782866 08/28/19 1:01 pm
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Lannis Online Content
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As Alan said, forget about the main jet.

One solution might be to retard the ignition a bit until the plinking stops, then slowly try leaner slide/needle combinations to maximize power. With modern fuel and new parts, the old timing settings are obsolete...


I'll believe that it's a criss when the people who tell me it's a crisis start acting like it's a crisis ...
Re: A65 Thunderbolt pinking [Re: Fraz] #782871 08/28/19 2:19 pm
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kommando Online Content
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At low revs and partial/large throttle openings you are in the pinking danger area, follow Lannis's advice plus also try raising the needle to increase fuel and cool the combustion chamber at partial throttle. Main jet only affects full throttle, you should not be giving it full throttle at low revs, open the throttle progressively as the revs rise. Its not a CV carb and it does not have a knock sensor to retard the ignition automatically, you have to do more of the thinking for the engine.

Re: A65 Thunderbolt pinking [Re: Fraz] #782874 08/28/19 3:28 pm
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triton thrasher Online Content
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As K says, your problem may be as simple as the needle clip in the wrong notch.


Amateur Loctite enthusiast.
Re: A65 Thunderbolt pinking [Re: Fraz] #782876 08/28/19 5:14 pm
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i doubt if this is the problem, but the books say a 20 pilot jet ( for the 68 carb with the separate jet, no numbers listed for the fixed later types, the premier has the option to change). Not a 17. Did you fit a 928? A 930 needs very different settings.
Modern fuel is different, on the strobe set the timing a degree or so retarded. See if that helps.
Book says needle position1 ( top notch ,leanest) , trying 2 or 3 is a free experiment and like previous replies, may be all thats needed.
Too late now, pistons as supplied come with sharp edges on the valve pocket cut outs, these benefit from rounding over and polishing, this takes away a potential hot spot.
When climbing hills keep it over 3.5 K RPMS, opening the throttle to climb hills at lower RPMs will provoke pinking on an otherwise healthy motor, use the gearbox, these motors are flexible but they are not tractors.


71 Devimead A65 750
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Re: A65 Thunderbolt pinking [Re: Fraz] #782894 08/28/19 7:02 pm
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A few things to try which may work:-
- try retarding the ignition slightly and see if there's any improvement
- what plugs are you using, should be something like Champion N4C or equivalent
- use a high octane fuel like Shell V-Power Unleaded, which has an octane rating of 99 RON, Tesco also supply a 99 octane unleaded called momentum


1968 A65 Firebird
1967 B44 Shooting Star
1972 Norton Commando
Re: A65 Thunderbolt pinking [Re: Fraz] #782915 08/28/19 9:44 pm
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The important bit is the straight pipes and air intake, this will change the settings quite a lot although normally it will demand a leaner air mixture than stock....

If your running a velocity stack (not bell mouth they are different again) identifies as Link to velocity stack note the pilot air circuit is entirely separate from the main carburereter, the Venturi is tuned better to the flow of the carb.

A bell mouth being Bell mouth

Velocity stacks by experience require leaner slide settings usually by 1, (so a 3 becomes a 3.5) and leaner needle jet settings (so a 106 needle with clip in position 1 becomes a 105 needle with clip in position 2) and about 5 sizes bigger on the main jet.

With a bell mouth the an increase in main jet by 2 or 3 sizes is normally all that’s needed.

Straight pipes normally demand a leaner set of jetting also throughout the range

Megas demand richer jetting.

So with your settings you will need to start a fresh using the AMAL carb tuning guide. Your bike may run but it’s loosing a lot in different ways unless it’s jetted to suit.

The sharp edges Kommando was mentioning don’t need to be raze right sharp, basically any corners of the piston need to be addressed, valve pockets being a main one. The technique is called radiusing, as the idea is to put a radiused or rounded edge to the piston to eliminate the corners glowing under the heat.

I’m guessing that no portwork has been done to the head?

What sort of rev range are you changing gear at?


beerchug
Re: A65 Thunderbolt pinking [Re: Fraz] #782936 08/29/19 12:19 am
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The blokes on here are right about the pistons, jetting etc.
On 95 fuel it will pink if the timing is at the book setting. My one does.
Knock a degree or two off the timing, you won't notice the performance difference.
Try 98 fuel, it's all i use when i can get it. Use the gearbox, that's why it has 4 gears.

Re: A65 Thunderbolt pinking [Re: Fraz] #782974 08/29/19 9:03 am
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I missed the bit about the straight pipes and velocity stacks, Allan covers it well, it will change the stock settings quite a bit. IME velocity stack are not worth the bother for a road bike, short term potential performance gains are quickly offset by much increased ring and valve guide wear thanks to unfiltered air..


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Re: A65 Thunderbolt pinking [Re: gavin eisler] #782978 08/29/19 10:54 am
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Allan Gill Offline
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Originally Posted by gavin eisler
I missed the bit about the straight pipes and velocity stacks, Allan covers it well, it will change the stock settings quite a bit. IME velocity stack are not worth the bother for a road bike, short term potential performance gains are quickly offset by much increased ring and valve guide wear thanks to unfiltered air..


Completely agree, I use them on mine although in reality they cause more headaches than the performance you will see. On a stock bike you’ll probably see 5mph at top end but that is about it. A good set of air filters will make life a lot easier. It’s more for aesthetics than anything.


I’ve been using mine since 2012, and only changing back to bell mouths or air filters when it was impossible to get the mixture any leaner with my various set ups. But even now I’m thinking of going back to air filters.


beerchug
Re: A65 Thunderbolt pinking [Re: Fraz] #782996 08/29/19 3:14 pm
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My recent Spitfire rebuild has the same pinging issues. I suspect part of the problem is the compression ratio is over 10:1, a result of repeated head skimming over the years. I am using 9:1 Hepolites.

I retarded the timing a couple of degrees, this definitely helped a lot.
I also tried using 100 octane aviation fuel, which eliminates the problem altogether....problem is availability! So it's back to using 98 octane LRP available at the pumps in Western Australia.

Next steps will be to try Champion plugs; getting a lot spluttering while the engine is warming up using NGKs. I will also raise the needles as suggested in this thread, like the idea of a free fix!

Re: A65 Thunderbolt pinking [Re: Fraz] #783160 08/31/19 3:52 pm
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Fraz Offline OP
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Thank you all for the excellent replies, I am learning a lot.

Yesterday I raised the needle, emptied the tank of the regular fuel and refilled with 97 fuel. Took it for a spin and no pinking! I haven't had a chance to go for a proper ride yet but this is a massive Improvement.

The plugs I have are Brisk L14c which are equivalent of Champion N3C

The plugs are a little rich so will need to play about with the carb a little more. I will also retard the timing slightly

Re: A65 Thunderbolt pinking [Re: Fraz] #783162 08/31/19 4:28 pm
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Might be worth getting some proper champion N3 plugs and trying again. Just because something is supposed to be equivalent to another maker doesn’t always mean it is.


beerchug
Re: A65 Thunderbolt pinking [Re: Allan Gill] #783170 08/31/19 6:09 pm
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Fraz Offline OP
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I had been recommended Brisk plugs as being very reliable. I will get some N3 and see if I notice a difference

Re: A65 Thunderbolt pinking [Re: Fraz] #783174 08/31/19 8:23 pm
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Reliable and equivalent are not the same, could be the best spark plug in the world but out by 1/2 a grade.

Re: A65 Thunderbolt pinking [Re: Fraz] #783204 09/01/19 5:48 am
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NGK B8's are fine. I like the B8EV's, but that's because i had a box of 'em from years ago.
The book says N4's for a t'bolt as i remember, that's nearer to a B7 (slightly hotter plug) But B8's are fine.

Plug threads are a bit like oil threads i think.......................................

Re: A65 Thunderbolt pinking [Re: Fraz] #783310 09/02/19 8:46 pm
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Fraz set up is faaar from stock with no air filters and empty silencers.
So he is much leaner everywhere than stock bike.
All the above posts are valid because they are advice getting richer mixture.
I'd just come back to air filters and original exhaust, stock set up is so easy with starting and riding.

Re: A65 Thunderbolt pinking [Re: Fraz] #783317 09/02/19 10:16 pm
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I used B8's even when racing with 11-1 and open megga at 900cc on my A65.
If the mixture is somewhere near correct they will be fine. If it's too lean, fitting
colder plugs is not really the answer. Visions of melted pistons come to mind.

Re: A65 Thunderbolt pinking [Re: Fraz] #783590 09/06/19 8:41 am
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Fraz Offline OP
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My Thunderbolt is far from stock, its taken me a long time to get to this point. The pinking seems to have stopped and the plug is looking close the what it should be. I'm hoping to get some more long runs on it before winter kicks in and get it set up just right. I'm really impressed with its pulling power for a 65 bike!

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