Race Engine Forum UK

I recently came across an Alfa romeo manual with instructions of how to modify and adjust the advance curve of a Bosch distributor.

After removing several parts, (interrupter,stator plate, circlips etc) you have access to the centrifugal weight and springs.

After removing the 2 springs, you set up a protractor attached to the distributor housing so it's in line with the shaft.
Then with a pointer on the rotor arm(simple wire) pointing to 0 degrees while its at its furthest CCW position, you carefully move the weight outwards until it hits the stop peg.
Take a reading on the protractor where the pointer is.

Mine measured about 16.5 degrees!! So crank advance is ~32 + static(8) = 40 degrees.

MAX advance for this engine should be 36 degrees!!!

Has anyone had experience of measuring the max advance using this method?

If this is a valid and accurate method my distributor needs a serious adjustment to reduce the max advance.

Any opinions would be appreciated.

Thanks.

Interesting post, seems to make sense, though you'd want to check on-engine with a strobe..

G

Thanks Guy.

I was expecting to get interest on my Alfa forum but to no avail.

Timing light on the flywheel appears to show up fine on the max mark at 36 degrees.
But a little confusing why the degrees on the rotor shows a much higher advance.

How accurate is the flywheel timing and timing light I do not know.
I would imagine the protractor method would be a good indication as outlined in the manual.

Curious if anyone else has done the same.

Eager to receive the H&H springs to get a stronger secondary spring in the distributor.
After replacing the primary spring for a stronger one the detonation on WOT moved from 2900 range up to 3300 range! After 3400-3450 rom there is no detonation.
But this is just the ideal rev range to overtake fast, in 3rd gear. I change down from 4th to 3rd and blast away from 3200 to 5500. Lovely sound from the boxer.
Once this is fixed I will be so happy.

Regards

You must spin the distr up to check the calibration - either in a distr testing machine or on an engine.

I don't know why but you must!!! (GC says...!)

In my exp strobe lights are only as accurate as the timing marks. If the timing marks are right the timing will be..!!


G

Guy Croft wrote:You must spin the distr up to check the calibration - either in a distr testing machine or on an engine.

I don't know why but you must!!! (GC says...!)

In my exp strobe lights are only as accurate as the timing marks. If the timing marks are right the timing will be..!!


G

Yes it would be ideal to spin the distributor to check the calibration in a machine like the SUN but as you know I'm in the middle of nowhere.
I have to resort to trial and error methods - changing springs and testing on the road.
I'll get there eventually.

I'm pretty sure the mid-range part of the curve needs to be tuned down in terms of advance by using a thicker secondary spring. Higher revs I have tonnes of torque without any pinging or overheating. Same goes to rpm range below 3000 rpm.

Will keep you posted. (my situation may change dramatically very soon - I'm in the final line up for a new job in Texas. Should hear something this week I hope. 4 interviews and 2 months later, still waiting anxiously for an answer. Have I been chosen or not? I'LL BE IN CAR HEAVEN!)


Chris

very best of luck there Chris!

G

Guy Croft wrote:very best of luck there Chris!

G

Thanks GC.

Same company but country transfer opportunity (family move etc all included) to the HDQ over there.

Just waiting to hear from HR. Rejection letter or THE phone call!

If you get a chance could you post a sketch of the factory advance curve of crank deg v rpm, maybe mark on it what you think yours currently has?

Do it as a jpeg, OK?

I'm concerned you might be overcomplicating this a bit or altering adv to improve perf but actually masking another problem...

dunno, over to you when you get time.

G

Here's a first attempt at a drawing. Sorry had to erase some other curves on it.

Red line details the curve the distributor should have.
Blue line is what I think is happening, beginning to over advance at 3300 rpm and tailing off over 3500 rpm.
Maybe it stops detonating over 3500 because of the increased emulsification of the fuel from the mains kicking in.

Or the second curve in purple as it's more linear could be a possibility.

Could be another issue yes. But what would be the most likely cause? I have tried colder plugs, richer mixtures, colder air intakes. Symptoms were the same.

Before I installed the thicker primary spring the detonation was over a wider rev range starting earlier, 2900-3400 or so under WOT. Interesting.


Thanks

I wonder if my mains are just too rich and burning too fast when hitting the 3300 mark.
I know that lean mixtures burn slower (but can also contribute to detonation!). Maybe return to the factory spec'd mains (142) (designed for the original power curve).

I also heard today that Dellorto jets are actually smaller than the stamped size. 142 original mains are I think 140 real size.
I had a spare set drilled to 145, so this is a 0.05 jump.

As you said maybe I was masking a rich main jet issue that was burning too fast in the cycle.

What do you think?

Chris

I think nothing wrong with your distributor. Even if theorically it advances up to 40 degrees on protractor, it will never actually go that far when retained by the set of springs and spinning on the car. At their maximum extention, the springs generate too much force that they will resist going further, even when centrifugal force is great on higher RPMs. They just reach their limit and resist. It is even rumored that on higher RPMs, when engine momentum gets slightly slower, they cope with the centrifugal force and start reacting to back off the plate -and the timing- couple degrees.

N

Thanks for your input N.

Well I have 2 possibilities that could be causing the detonation.
As N mentions he doesn't think it is the distributor but it's still one cause I want to rule out by trying a stronger secondary spring.

Other option could be that the main jets are just too rich that is burning too fast. (really can't be the other way being lean as the mains are already fairly bigger than the factory size). The factory sizes of the main jets I believe are usually a little oversized anyway to avoid lean mixtures at high speeds.
If the mixture is a little leaner it's going to burn slower and reduce the early burning of the fuel.(theoretically no?)

I will try altering both variables to see if either make a difference.

So tomorrow I will try the factory sized main jets first (as I don't have the new distributor springs yet).


I could even try using the vacuum advance from the manifold port and reduce the overall timing if its not a jetting issue. So very good advance at idle and under light throttle/load and less advance under load and WOT.
Ported vacuum advance was used to reduce emissions. I'd like to see how the engine reacts with a manifold vacuum advance. On the Dellortos there are the adjustable ports below the throttle plates.

Tonight I will change the main jets back to original 142 dellorto jets (factory spec'd), give it a test run and report back.

Will have another look at my AFR monitoring system, but I'm suspecting the lambda is at fault. I will try and measure the output with my DMM.
Really need it to see what's going on under different engine loads/rpms.

Factory main jets in and tested. Very little difference - in fact I lost some power. larger 145's will go back in.
OR maybe I need larger main jets. a 150. Maybe its going lean at 3300 rpm.

Was speaking with someone else who has 160 mains with his short trumpets.

I will try feeding in colder air to the airbox next to see if its a hot air issue also.

Then the last resort will be the stronger distributor springs.

If all of those fail I'm at a loss!!

hmm looking at this reference chart I think the emulsion tubes may not match what is here.

http://www.Alfa-pages.co.uk/carbjet.php

The chart here says they should be 9164.2
I have 9164.4

Could this something to look at?
What would the difference be between the 2? I believe that the higher the number the sooner the main kicks in. Is this true?
if this is the case then I wouldn't want a smaller one than what I have.