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Discussion Starter #1
As some of you may know I've been experiencing some detonation issues.

I have the 6A running now on a stock coil.

I tested the car up a very steep incline yesterday and with a full load of passengers!

In 2nd and 3rd on its limit it experienced no detonation.

ONLY when I accelerate fast 3/4-full throttle I hear detonation even with a light load on a level road. (2500-3200 rpm).

Do you think the detonation is occurring due to a lean mixture under the wide throttle opening? (due to the steep hill load test showing no detonation).
Or the dizzy springs are advancing too quickly under fast acceleration?

If its a lean mixture under 3/4-WOT what's the best way to cure this?

I have some stiffer dizzy springs in the post. I can try a stiffer primary spring maybe.

Thanks
 

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Your AFR gauge could provide the answer to the mixture question.

At light throttle the vacuum chamber will adds extra advance to the actual advance value.
If the engine is stumbling around 2000 RPM with constant & (very)light throttle on the road (in 2nd gear),
that's also a sign of too much advance.
If you disable the vacuum advance by disconnecting the vacuum hose from the chamber (and seal the
hose towards the carb), and the detonation / stumbling remains, then the advance is too much.
 

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Discussion Starter #4
Your AFR gauge could provide the answer to the mixture question.

At light throttle the vacuum chamber will adds extra advance to the actual advance value.
If the engine is stumbling around 2000 RPM with constant & (very)light throttle on the road (in 2nd gear),
that's also a sign of too much advance.
If you disable the vacuum advance by disconnecting the vacuum hose from the chamber (and seal the
hose towards the carb), and the detonation / stumbling remains, then the advance is too much.
vacuum advance is already disconnected:thumbs:
no stumble before or after disconnection.

Only detonates on sudden throttle opening, very fast acceleration.

any further retard then it is less than 7 degrees static and that has an effect of horrible take off and fats heating when idling.
its just under 8 degrees static now.

Thats normal detonation dont worry ......
hope so.

well I's still like to try as slightly stonger spring.
As I have a new assortment set coming. new ones with different tensions.
Maybe the old ones in there have just lost some of their tension.

I would prefer a higher static advance (9 degrees or so) and slower advance curve to its max at 4500 rpm. I know it doesn't feel quite right.
 

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Look , there will be always some detonation especially on sudden acceleration due to old school electronics. Since your performance is good , your carbs are tuned and your current is strong then this is normal especially if you dont have performance issues or high consumption.
 

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Discussion Starter #8 (Edited)
Do you know at what rpm the secondary spring comes into play on the Bosch?

If the detonation is occurring mainly at 2500-3200 rpm is the secondary spring starting to come into action here or just the primary?

There is a small amount of slack in the second, which I think comes into play after about 6-7 degrees of advance.
But what revs? Not sure.
maybe just try a stiffer primary spring first. I know it never pings at full revs (max advance around 34 degrees)

Trial and error is tricky here because they are difficult to change.

Says below the 2nd one comes in at a much higher rpm.

Mechanical Advance - Inside the distributor are a set of springs and weights. The weights are held in place by springs and swing outward as the shaft rotates with the engine. As the weights move the plate that holds the points twists counter clockwise relative to the shaft. This has the effect of advancing the timing because the points are now closing earlier than before (the shaft rotates clockwise with the engine). This is called mechanical (or centrifugal) advance and the reason we need more advance as the engine spins faster is that the piston is travelling faster but the chemical reaction in the combustion chamber still has a relatively fixed reaction time. If the advance comes in too soon the pressure from combustion might try to force the piston down during it's up travel. If it comes too late, less torque will be produced as the pressure will be wasted as heat absorbed by the cylinder walls.
dizzy curves

Most Bosch distributors (except for the 009) have two types of springs inside. The first is fully extended and reacts as soon as the engine speed increases from idle. The second spring is elongated so that it begins to stretch at a much higher rpm. Not only different in length but this spring is also stiffer and the overall effect is an advance curve that has two slopes: at first a quick advance then it tapers off. The maximum advance is limited by two stops inside the body that prevent the weights from moving further outward at higher rpms. The required amount of advance depends on the combustion chamber design and the camshaft selection. 20-24 degrees is typically required by 3500rpm. After that the turbulence of the intake mixture is enough to promote combustion at the maximum advance.
 

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For the 305.50 engine the max centrifugal advance is reached between 3400-3500 crankshaft RPM
[Marelli and Bosch 0237601017(?)] these are providing at least three slopes in the advance curve.
The engine 305.58 was equipped with a differently calibrated dizzy, which provided higher centrifugal
advance values (probably it has 0237601014 Bosch number) also with more than two slopes.
 

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A nice tool to tune the centrifugal advance (called Sun 600), only a few left since
the ignition advance is determined by digitally stored ignition maps.
 

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Discussion Starter #11 (Edited)
For the 305.50 engine the max centrifugal advance is reached between 3400-3500 crankshaft RPM
[Marelli and Bosch 0237601017(?)] these are providing at least three slopes in the advance curve.
The engine 305.58 was equipped with a differently calibrated dizzy, which provided higher centrifugal
advance values (probably it has 0237601014 Bosch number) also with more than two slopes.
thks. As mine is the 30550 engine your first figure helps a lot. (but I did think that the 2 final marks on the flywheel were for advance curves at 3500 and 4500 rpm so max comes in at 4500)
It seems to ping more at around 3000 rpm if I hit the pedal hard.
So maybe the secondary spring is a little weak.
What do you think?

Yep seen pics of the Sun 600. not sure if any exist here.
:thumbs:
 

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IMHO both springs are weak or one of them is broken.
The weaker spring (and it's counterweight) would be responsible for the 1degree/100 RPM advance steepness,
in the ~950 to ~1500 RPM range.
Around 1500 RPM the stronger spring is start to kick in, changing the advance steepness
about 1degree/200 RPM. This steepness remain constant until the crankshaft reaches 3400-3500 RPM,
after that the (centrifugal)advance will not increases further.

I think the max advance is reached before 3400 RPM, and the rich mixture starts to detonate
during hard accelerations and high engine loads.
 

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Discussion Starter #13
IMHO both springs are weak or one of them is broken.
The weaker spring (and it's counterweight) would be responsible for the 1degree/100 RPM advance steepness,
in the ~950 to ~1500 RPM range.
Around 1500 RPM the stronger spring is start to kick in, changing the advance steepness
about 1degree/200 RPM. This steepness remain constant until the crankshaft reaches 3400-3500 RPM,
after that the (centrifugal)advance will not increases further.

I think the max advance is reached before 3400 RPM, and the rich mixture starts to detonate
during hard accelerations and high engine loads.
Great info thanks lonewolf.

Yes the carbs are set up with a richer mixture now. 55 idles and 145 mains.
This was to eliminate a flat spot. Maybe the mains need to be 142 again but I am running with MSD 6A so it does burn well.

Maybe the detonation was less evident with the leaner mixture as it burns faster but had a horrible flat spot.
 

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Discussion Starter #14 (Edited)
Has anyone tried the vacuum advance from the manifold instead of ported?

I could have a higher idle advance and lower overall advance once the throttle is opened.
Just curious of how much it advances the timing at idle as I don't have the vacuum canister attached right now.
 

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Discussion Starter #15
a neat trick could be to use the vacuum port from below the throttle plate for the advance.

This is adjustable. So I can retard the idle advance to about 6 degrees or enough to compensate for the weak springs and open this vacuum port just a little to raise the static idle advance to about 9 degrees.
= good advance at idle which will drop to zero once throttle is opened.:)
Not sure how it will respond but an easy trick to try.
 

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Discussion Starter #16
:rant::mad:

new dizzy springs never arrived! 5 weeks now and i bet they are lost foreever! (also had a box of Christmas presents go missing!!)

Anyway the company has offered to send me another pack:). Will get them sent to my office this time (international free trade zone!).

But so much time waiting to get my timing working as it should. I miss being able to floor it WOT while overtaking etc.

(even if I sell it and change it for the 33 ie series 3 I'd like to get it working properly first of course)
 

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Discussion Starter #17 (Edited)
:cry:I changed the filters back to the original airbox with an extra big cold air intake tube to rule out the detonation occurring due to hot air soak.

No improvement!

It doesn't occur under load at any other rpms in any gears other than the range between 3200 ish to 3600 WOT.
Mostly in 3rd, a little in 2nd but none in any other gears.

So next variable could be the secondary dizzy spring? (hasn't arrived yet!)

It has also been suggested my compression maybe a touch high if I have tried all other possibilities!!
A thicker head gasket would be needed!!:cry:
 

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I also get some detonation from my 1.4 ie Imola around 2500 RPMs on 2nd & 3d gear. It doesn't do it always, mainly detonation occurs after the engine get warm, but differently from brit01's 33, mine does it even at higher revs (when accelerating hard or floor the pedal). It is not very loud but I still can hear it.
I'm afraid it is running a bit lean, but WHY? Could this be a sign of a worn fuel pump? Car also takes a while to fire up (not the first start in the morning)

Is it normal for the fuel injected 33 to run a bit lean, as my spark plugs are more whitish then brownish?
Could detonation also be caused by a slightly bigger gap than normal, in the spark plugs?


No oil consumption, no lack of power...
 

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Discussion Starter #19 (Edited)
MON!!

MON=83 :cry:

Extremely low grade octane rating here (and I'm using the best available on the street)

Should be at least 87 for an engine with my compression!

Have to look into this further.
 
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