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looking for some fresh ideas - have a 66 spitfire - friggin engine won't rev past 5500 RPM. at 5500, the engine's wound tight - no way this thing will turn 7-grand. verified the tach reading with a remote digital tach, so pretty much can eliminate a faulting tach reading. timing's spot on, valve clearance double and triple checked, double checked cam timing - idler gear between the crank and cam - all is good. carbs good, no restriction on air cleaner and exhaust. factory 21/47 sprockets. I'M STUMPED AND OUT OF IDEAS. - only thing i can think of is there's something going on with the camshaft - this bike may have been an ex-track bike, something' not letting this thing to rev past 5500. anyone have any suggestions, ideas, imput?

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points ignition ?

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Didn't the Spitfires have 19 or 20 tooth countershaft? Not sure why that might limit rpms but a 21 on my tbolt caused it to need more throttle opening at 70 mph. Does it still have the original mufflers? You can see through them?
And as Quinten said, if it still has 4ca it would be a problem but doesn't seem likely if it was a "trackbike"


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yes.

points ignition?

my first thiught was wonky cam timing, but youve checked that.

sounds like inadequate spark.


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I had this happen with a car. The problem was not enough fuel.

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no - Pazon electronic.

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Originally Posted by No Name Man
Didn't the Spitfires have 19 or 20 tooth countershaft? Not sure why that might limit rpms but a 21 on my tbolt caused it to need more throttle opening at 70 mph. Does it still have the original mufflers? You can see through them?
And as Quinten said, if it still has 4ca it would be a problem but doesn't seem likely if it was a "trackbike"
from the specs, 66 spitfires came with 21 tooth sprockets, but i'll doubt check. OE stock mufflers - i can see straight thru. new aircleaners, throttle slides go full up at WOT - sync'd perfectly. fuel - no difference if i run one petcock open or both open. don't think it's starving for fuel. over on another forum, someone metioned my main jet - presently running 190's - i think those may be a bit lean - i have some 210's in my parts box, may bump them up a bit. ordered up a couple 107 needles - just waiting on the UPS guy. plug reading look decent around 1/3 throttle - 50-60 MPH cruise. it's just when you bust the throttle WO - tachs up to 5500 - sounds like it's coming unglued and just won't rev higher.

Last edited by joe czech; 06/11/22 5:51 pm.
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i would be looking at main jet size ... 210 should make a difference... golden rule with carbs is replace one thing at a time


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What carbs are you running? the workshop manual for 1962-66 suggests that the Spitfire Mk II used AMAL 516 GP carbs with a 250 main jet. The A65 lightening from the same era used AMAL 389/689 (left/right) with a 270 main jet.

So if you are using the above carbs then a 190 main would be too lean and might explain your problem.

You can find a copy of 1962-66 A65 workshop manual on the CBS website Here.


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if your cams and spark are okay then fuel is the next limiting factor.

put in a stupidly too big main jet and see if anything changes.

test, dont wonder.


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Had this with a borrowed bike once, what is the bike like when you switch the lights on? The problem with this one was a duff battery, the bike ran great most the time apart from high revs.


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Couple thoughts:

1)7k rpm is past redline for that bike, right? Did the bike rev that high and now no longer?. To rev that high I would guess the valve springs would not be stock, the crank would be balanced and the engine modified for high performance.

2) When the slides are synched perfectly a next step might be to compare vacuum readings at various rpm. If not reasonably matching up, maybe a broken or weak valve spring or other issues affecting the fuel/air charge.

3) Compression good, right?

4) Doesn't that electronic ignition rely on the ancient advance weights/springs? If not, great, but checking the dynamic timing at about 3200 rpm would be a good exercise.

5) One weak coil maybe.

Hopefully its something simple like the wrong jets as Gunner suggested.

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Another thought is whether the ignition timing is too far retarded, this might prevent the engine from revving out.

I don't know how you timed your ignition but the factory way is to use the special plug which fits into a slot on the crankshaft.

The manual I posted earlier says the timing plug must have "A50" upwards and "A65" downwards, this sets the ignition timing to the full advance position of 34 degrees.

With Pazon and Boyer ignitions you then move the stator and/or rotor until the red dot is visible in the stator.

Make note of any timing marks on the alternator rotor or paint a line on if needed, you can use this line later on using a strobe to check for full advance.


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Originally Posted by kevin
if your cams and spark are okay then fuel is the next limiting factor.

put in a stupidly too big main jet and see if anything changes.

test, dont wonder.
to answer several questions - original GP carb are MIA and have been replaced with (new) AMAL premier 930 Concentrics. i also lowered the compression to 9:1. best guess on jetting. 190 mains mostly affect WOT. going to take your advice - stupid crazy big mains - a couple 260's next.

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you have a puzzle for sure. the 190s ought to work. if the engine blubbers with the bjg jets but did not blubber with the 190s then i would conclude that the carburation is working as designed, becsuse you were able to make it too rich.

if it doesnt blubber with the 260s then something is keeping the big jets--and most likely the 190s too-- from flowing as much fuel as they should

Last edited by kevin; 06/12/22 12:31 pm.

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What do your plugs look like after attempting WOT ?

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Originally Posted by KevRasen
What do your plugs look like after attempting WOT ?
never read the plugs after WOT - at around 30% throttle, plugs look good. plug reading always puzzled me - where do you read them in relation to throttle position, and how long at a steady state throttle position when reading? since i do most of my riding around 30% throttle, i figure thats the best place to read them. i usually try to get at least a minute at a given speed/RPM. although i could be all wrong on my plug reading method.

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I'm no "plug reader" , light brownish = near enough and OK for me, sooty = bit rich maybe lean up relevant circuit, oily = possible problem beyond jetting.
Does it rev out in lower gears ? i.e. are you just struggling with top gear revving out or all gears ?
When trying to rev out does it pick up revs when you ease off the throttle slightly ?
I'd stick a 210 MJ in and try that if it's OK in all lower throttle openings.

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Originally Posted by joe czech
Originally Posted by kevin
if your cams and spark are okay then fuel is the next limiting factor.

put in a stupidly too big main jet and see if anything changes.

test, dont wonder.
to answer several questions - original GP carb are MIA and have been replaced with (new) amal premier 930 concentrics. i also lowered the compression to 9:1. best guess on jetting. 190 mains mostly affect WOT. going to take your advice - stupid crazy big mains - a couple 260's next.

With 9:1 pistons, 930 carbs fitted and a standard exhaust your bike is basically a Lightning. Lightnings we’re fitted with a 180 main jet, back when my bike was stock, I found that this gave the best performance. Any richer limited the revs.

Originally Posted by Bustednukel
Couple thoughts:

1)7k rpm is past redline for that bike, right?.

… nope, if you use the MAP calculations for redline, an A65 would be 9000rpm. 7000 should be easily achievable and repeatable without causing harm to the motor.


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Originally Posted by KevRasen
I'm no "plug reader" , light brownish = near enough and OK for me, sooty = bit rich maybe lean up relevant circuit, oily = possible problem beyond jetting.
Does it rev out in lower gears ? i.e. are you just struggling with top gear revving out or all gears ?
When trying to rev out does it pick up revs when you ease off the throttle slightly ?
I'd stick a 210 MJ in and try that if it's OK in all lower throttle openings.

initially i had a timing advance problem. the spec on the Tri-Spark had a max advance of 25° - the spitfire's max advance is 34° - there's 9° retard without any tolerance error. best i could do with the Tri-Spark was about 4500 RPM. switched to a Pazon ignition which has a max advance of 35° - with that change, i picked up about 1000 rpm - now will rev to 5500. still won't get near 7000. i kind of thinking i may be getting pre-ignition with the leaner mixture at the top end - not enough to cause engine knock, but enough to cause an engine performance issue - i'm by no means an expert here - just grabbing at straws. i think everything's good up to and before WOT.. going to try Kevin's suggestion and go with a stupid large main jet.

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If it will only rev to 5.5k in the Lower gears something is far wrong.
Regardless of which ignition system you use max advance should be around 34 degrees BTDC, this was the BSA figure , modern fuels burn faster , many folks here reduce the max advance a degree or so, start with stock , if it pinks under load retard a baw hair.

BSA considered the red line as 7.5K

A65 L with AMAL 930 used 180 or 190 dry frame depending on year, OIF same carb 220.

Try taking the silencers off for a test and see if it makes any difference, some pattern models are restrictive.
Same goes for air filters.

Last edited by gavin eisler; 06/12/22 8:22 pm.

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Check cam timing. My '66 had a bad cam from the factory. Intake on primary side opened 20 degrees late and closed 30 degrees early. Took me forever to figure it out.

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Hi Joe,

Is the ignition system a Pazon Sure-Fire? If it is did you set it up as stated in the the instructions. First of all set the crank at 34 degrees. BTDC. Fit the magnetic rotor into the pinion and tighten up to finger tight( remember the pinion goes around Anti- clockwise). Put the trigger mechanism in place with the adjustment slots mid way with the screws. Turn the magnetic rotor around until the red marker on the rotor appears in the Anti-clockwise timing hole on the trigger mechanism. Tighten up the screws and bolt. You should now be able to start the bike reasonably easy and rev it up to way past 5000. You may have to adjust the trigger mechanism slightly backwards or forwards until you get it accurate. Using a strobe light is the best way and the engine needs to be running aat 4000 rpm to achieve maximum advance.

You may well have already done the above, if so I can't help any more than others have. I also have a 66 Spitfire with Pazon ignition and have no problem with it and it does 16.5 mph per 1000 rpm with a 20 tooth main sprocket. I have 10 year old 32mm Concentrics with 190 main, .107 needle jets , 3 slide and position 2 for the needle. Hope you find a solution to the problem,

Keith.

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Poor valve springs can cause this, but they would have to be pretty bad.
I take it you are referring to revving in lower gears IE 1st+2nd The a65 will
normally happily rev it's nuts off in those, 3rd can take a while if you have
tall gearing but 7k is not a lot for a good a65 lump.

Timing the a65, you are setting the fully advanced point as the default by
working on the markers. All the EI will do is RETARD from that point
dependant on revs, SO if you set the mark at 34 deg and you have any
one of the standard ignition packs they will achieve that at around 4k rpm.
Pazon Trispark Boyer etc will only retard from that point at lower rpm.The
range that they retard is what may differ IE, some have a doglegged curve
that advances at around 1k to prevent idle stalling. Most of the ones on
the market have a maximum retard of around 25 degs engine 12 degs cam.

Last edited by NickL; 06/13/22 1:08 am.
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Yet another thought is whether the carbs are balanced if you haven't previously balanced them, the procedure is fairly easy and you just need to get hold of a decent quality balancer, I like the ones sold by Morgan Carbtune in NI which use steel rods instead if mercury.

Whatever balancer you choose the 2 rubber hoses are attached to the manifold in place of the balance pipe, right and left.

With the engine warmed up and the correct mixture determined, blip the throttle and watch how the slides behave, ideally, they should mirror each other. If they don't you have to adjust the throttle cables until the slides move equally. You could try a WoT test and see what happens, it may be that one piston is reluctant to rev whilst the other wants to but cant as its being held back by the other, this should be apparent on the balancer. where one piston is pulling more vacuum than another.

Also when the throttle is shut but the engine running the slides should drop equally, if not some adjustment of the throttle stops is needed.

When finished, put back the original balance pipe.

This was a simple explanation of how it's done, I'm sure others can chime in with better info.

Last edited by gunner; 06/13/22 6:39 am.

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