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#443036 07/03/12 7:40 pm
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I have been testing out my '65 Thunderbolt Rocket and it seems to be running hot and I'm getting what sounds like a spark knock on acceleration once it's warmed up. When it's cold, no knocking.
I double checked the timing and lowered the needle one notch, what else could be the cause?


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Lowered the needle or lowered the circlip? Lowering the needle makes it leaner which is the opposite of what you might need. Knock on light acceleration or full throttle? In what RPM range? When was the last time you looked foe cabon build-up in the cylinders? If you have a spray bottle you can spray water in the intake whilst running, that will sometimes knock any carbon loose and out the exhaust.

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Originally Posted by DMadigan
Lowered the needle or lowered the circlip? Lowering the needle makes it leaner which is the opposite of what you might need. Knock on light acceleration or full throttle? In what RPM range? When was the last time you looked foe cabon build-up in the cylinders? If you have a spray bottle you can spray water in the intake whilst running, that will sometimes knock any carbon loose and out the exhaust.


Sorry, I lowered the clip, raised the needle! I was thinking carbon build up but wanted to get some nore opinions. When I static timed it I lightly oiled the AAU and it seemed to be working fine.
It knocks the worst on light acceleration under load.
This bike was a barn find that sat for 20+ years.


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Search the forum for "pre-ignition" or "pinking" or "pinging". This has been discussed a LOT. A65's seem to be overly susceptible, most likely due to combustion chamber design. The problem is that (when ignition and timing are in order) the combination of high compression, inadequate octane and hot spots on the piston cause the fuel to spontaneously combust or pre-ignite before it's supposed to. You can eliminate it by lowering compression, raising octane, or doing something about the hot spots in the combustion chamber. Changing timing or jetting beyond where they are correct seems to solve little in this case.


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Thanks Alex, I'll do some reading...and maybe buy some octane boost.


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Knocking on light acceleration might mean the springs on the AAU are weak. The cheap fix is to bend the tabs holding the springs (unless you have the AAU with posts on the base). Otherwise measure the spring O.D> and wire size and find a slightly stiffer spring at the hardware store. Use a pair of needle nose pliers to bend a loop from the coil at each end.

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I've had little success with octane boosters...but I haven't tried them all...

And yes, make sure jetting and timing are right first.


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I've had no luck with octane boosters either. 110 octane racing fuel mixed 1:3 with pump gas does wonders. Also the Boyer electronic ignition has a more gentle acceleration curve and helped me a lot. But the motor is prone to "pinking" because of its head design and the lower octane fuels that replaced the 96 octsne leaded stuff we had in the sixties. If I had to rebuild I would go sith lower comprssion.

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Originally Posted by Mr Mike
I've had no luck with octane boosters either. 110 octane racing fuel mixed 1:3 with pump gas does wonders. Also the Boyer electronic ignition has a more gentle acceleration curve and helped me a lot. But the motor is prone to "pinking" because of its head design and the lower octane fuels that replaced the 96 octsne leaded stuff we had in the sixties. If I had to rebuild I would go sith lower comprssion.

Mr Mike


I've had no luck with the Octane Boosters either. The 'Super Unleaded' Gas we get here in the UK is normally 97 Octane. But even with Booster my A65 still 'Pinks'. I was advised to go for lower compression pistons but didn't listen to the sage advice!!! crazy


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I am still running points ignition. When I drug this bike out of the barn it had been in for the past 25 years it was like a time capsule. All the lights worked, speedometer works, it still has the original tool kit!
Maybe I will retard the ignition a little. Has anyone tried 100 LL AVGAS?


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If it's been sitting the past 25 years, I'd suspect weak springs on the AAU as Mr. Madigan suggests.


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Originally Posted by JD
If it's been sitting the past 25 years, I'd suspect weak springs on the AAU as Mr. Madigan suggests.


Maybe I will try to find some springs at the local harware store as was suggested earlier. I was tempted to put an eloctronic ignition but am trying to keep it original.


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From Bruce Anderson's 911 Performance Handbook:
100/130 Av gas + premium unleaded R+M/2 Spec Grav
0 100 91.3 0.7579
25 75 94.8 0.7416
50 50 97.6 0.7256
75 25 100.6 0.7093
100 0 102.6 0.6926

Aviation fuel has a lower specific gravity so your settings will have to be enriched proportionally.
If your AAU has the tabs on the base plate for the springs you can try bending then slightly. Otherwise, use a caliper to measure what you have and find a slightly larger diameter wire or same and start with half a loop shorter.

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Originally Posted by dirtymartini
...Has anyone tried 100 LL AVGAS?

I use it all the time in all my machines. Never a knock, ping or pink. I've use it 50/50 with premium pump gas when out of town. Same success.


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Originally Posted by dirtymartini
I was tempted to put an eloctronic ignition but am trying to keep it original.


Electronic ignition is nice...set it and forget about it. You could always keep the points and condensors and put them back in when you have some free time, or if you decide to sell the bike and post it as original.


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Originally Posted by Gary E
Originally Posted by dirtymartini
...Has anyone tried 100 LL AVGAS?

I use it all the time in all my machines. Never a knock, ping or pink. I've use it 50/50 with premium pump gas when out of town. Same success.


Thanks, I have a small(podunk) FBO a few miles away.


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Originally Posted by JD
Originally Posted by dirtymartini
I was tempted to put an eloctronic ignition but am trying to keep it original.


Electronic ignition is nice...set it and forget about it. You could always keep the points and condensors and put them back in when you have some free time, or if you decide to sell the bike and post it as original.


Yes they are...I have a Pazon Sure Fire in my Mark III Commando and a Power Arc in my '69 Commando "S"


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Lowering compression works. BTDT. But the best way to cure the pinging IME, is to clean up the combustion chamber. Slightly radius the sharp edges around the valve cutouts on the piston and polish the piston crowns and combustion chamber. My only bike that pings is the only one I haven't done this to. The transformation can be dramatic. I run my C15 race bike with 10.2:1 compression on 93 octane unleaded. It hasn't pinked or knocked once.


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Originally Posted by Alex
Lowering compression works. BTDT. But the best way to cure the pinging IME, is to clean up the combustion chamber. Slightly radius the sharp edges around the valve cutouts on the piston and polish the piston crowns and combustion chamber. My only bike that pings is the only one I haven't done this to. The transformation can be dramatic. I run my C15 race bike with 10.2:1 compression on 93 octane unleaded. It hasn't pinked or knocked once.


What did you use to radius the valve cutouts?


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Originally Posted by JD
Originally Posted by dirtymartini
I was tempted to put an eloctronic ignition but am trying to keep it original.


Electronic ignition is nice...set it and forget about it. You could always keep the points and condensors and put them back in when you have some free time, or if you decide to sell the bike and post it as original.


Does anyone make an electronic ignition for a 6v system?


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Originally Posted by dirtymartini
What did you use to radius the valve cutouts?


A small sanding drum on a dremel tool to knock it down then successively finer grits of sandpaper down to 320 grit by hand. Then a run on a polyshing bob to really get it smooth and shiny.


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Following this closely as I am having the same problem with pinking. What's the best method of removing the carbon buildup before addressing the sharp edges ?

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is the mechanical advance working not stuck? this is complicated but a backdown timing lite will show the actual timing of both cyls.ck'd individually.points with different gaps can make it run wonky. with all the ethanol fuel it is causing many issues. mite be time to get some low compression pistons and solid state ign

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Originally Posted by bodine031
is the mechanical advance working not stuck? this is complicated but a backdown timing lite will show the actual timing of both cyls.ck'd individually.points with different gaps can make it run wonky. with all the ethanol fuel it is causing many issues. mite be time to get some low compression pistons and solid state ign


In an earlier post I mentioned taking the AAU out and checking it over. I gave it a little 3-in-1 oil and put it back in. I was thinking a little more about this...It doesn't start pinging until the engine is fully warmed up. If it were the AAU springs causing the pinging wouldn't it ping when it's cold?


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Pour water slowly into the carburettor,with the engine running at very fast idle.One glass of water will clean a lot of carbon from the heads and exhaust of a hot engine.

Scotchbrite and kerosene will clean off what remains.You can clean up by lightly greasing the cylinder and cranking the engine,to push any rubbish to the top of the cylinder.

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