| Author |
 Topic  |
|
|
Simon
United Kingdom
450 Posts |
 Posted - 22/04/2008 : 18:30:23
|
Rather a "tired" topic but in spite of using "search" I am still a little confused on correct set-up. The flywheel on my J2 is 10" diameter = 31.4" Circ.(798mm) The "IGN" mark is 2 1/4" BTDC = 26 Degrees. Is the distributor (13 degree auto advance)set for points to open at that point? The book says set at 1 1/2" BTDC ( 17 1/2 degrees) I note similar question on the PB indicate the flywheel is 26.3" in circ.,odd it's smaller than the J2.
Expert advice gratefully received! Simon. |
|
|
Peter Green
United Kingdom
1682 Posts |
Posted - 22/04/2008 : 20:02:17
|
Simon
The IGN mark on the flywheel was used for static timing, the distributor was set so that the points were just opening at this position. It is not advisable to accurately time an engine these days with these marks (although it is a good starting point) for two reasons.
1) Engines need less advance than they did in the 1930Æs.
2) A lot of cars have had their distributors changed over the years and many do not now have the correct mechanical advance. As a guide, the engine should be timed so that the maximum advance is about 35/36 degrees BTDC
Peter
|
 |
|
|
Keith Wallace
United Kingdom
367 Posts |
Posted - 09/05/2008 : 07:51:16
|
Can any one confirm that the original "IGN" timing mark on the flywheel of a PB is equal to 20.54 degrees of advancement?.
(26.3" Cirm of flywheel divided into 360o = 13.69o per inch x IGN mark @ 1.5" BTDC = 20.54o) ?
Is the additional centifugal advancement a maximum of 13o?, giving a total maximum advancement of 33.54o?
If these calculations are correct, Peter's total of 35/36o maximum advancement is only 2 or 3 degrees above that originally set?
Sorry to continue this subject, but I can't find any previous topics on this subject. |
Edited by - Keith Wallace on 09/05/2008 07:56:36 |
|
|
|
David Allison
United Kingdom
665 Posts |
Posted - 09/05/2008 : 08:49:25
|
Keith
I am sorry to say that this is a case of RTFM (Read The Flamin' Manual) - Peter made it very clear that the markings on the flywheel are there for guidance only.
Sorry if this is a bit on the blunt side - but the advice is very clear and concise (although maybe I am being too harsh?).
When these engines were built there was none of the easily available electronic equipment that we have today (there was certainly no Halfords).
The markings allowed people to build their own engine and fit the distributor in vaguely the right place to allow the engine to start.
We all still use these markings today to allow us to position the distributor in the right place(ish)for the "first start".
Once the engine is running the ignition timing can be set up to suit the individual engine.
Pre-war a lot of engine tuners (I still do) set the ignition timing by ear, it is a similar technique to the carb tuning and can be done by a practiced ear quite quickly.
I also tend to check my handiwork afterwards because there are influences on the ignition timing that it is not possible to check by ear.
The dwell angle (the period the contact points are closed) can influence the ignition timing by 10 degrees if it is incorrect - I normally set the points gap and then adjust it electronically (the method is well covered in tuning manuals).
The ignition timing is very sensitive to climate, altitude, engine tune and fuel - the ability to time an engine by ear was well practised before the strobe electronic timing lights became available and it was a technique taught to me at an early age (it does help if you have a musical ear too).
Once set up I tend to check the ignition timing against a mark on the blower drive and front housing of the NA just to make sure I am right - for Tesco 99 octane I run at about 34 degrees - for the lower octane unleadeds I pull the timing back by as much as 7 degrees depending on climate and how soon I think I can get the next tankfull of decent fuel.
Last year at Dunsfold I got hold of some Avgas 105 octane and the car liked it too much! I was able to run full advance (around 36 degrees)and sustain long throttle openings without overheating or plug problems that I sometimes get with the 97 octane and running retarded to prevent detonation.
Detonation is a very serious problem - it is caused by the fuel mixture burning slightly prior to the spark (sometimes also called Pre-ignition or dieseling)and many people dont hear it when driving.
It takes some ear training to hear it and it is common with cars set up to run within a test cell.
If the mixture is slightly weak and the ignition timing overadvanced your get a light tinkiling noise on long throttle openings or high load (going up hill).
Too rich and overadvanced you get a heavier (sounds like a ball pein hammer on aluminium) knocking.
Un-checked the result can be a hole in a piston.
I prefer to set engines slightly rich and retarded - this is a safe set up based on the fact that fuel is cheaper than pistons!.
You do need a degree of mechanical sympathy to drive and maintian these cars (although this can result in a paranoid ear) all of which is gained over time and familiarity.
The best information on ignition timing for MMM cars is in Blower - but the varous tuning manuals are very good too and give advice on non-standard engine tuning principles (if you want to be an anorak like me that is).
Hope this helps - but Peter is right do not rely on static timing marks to set ignition timing (how can you be sure they are in the right place?).
This has probably turned into a bit of a rant and is certainly too long.
Regards David
|
|
|
|
Peter Green
United Kingdom
1682 Posts |
Posted - 09/05/2008 : 11:38:05
|
Keith,
Just for clarification, the IGN mark on a PB flywheel is 20 degrees BTDC (give and take the odd half degree due to the line thickness), and originally the centrifugal advance in the distributor related to another 20 degrees on the flywheel making a total of 40 degrees. I must point out that the actual advance in the distributor was only 10 degrees as it rotates half engine speed. This maximum advance (40 degrees) is too much for modern petrol, as I said earlier I have found that 35/36 degrees is the optimum advance for todayÆs petrol.
Where have you got your figure of 13 degrees centrifugal advance from? Is it stamped on the centrifugal advance plate inside the base of the distributor, if so this means that the distributor has 13 degrees centrifugal advance which relates to 26 degrees on the flywheel, so it is not the original distributor for the car.
I hope I have made myself clear, if you have any queries I will try and answer them.
Peter.
|
|
|
|
Keith Wallace
United Kingdom
367 Posts |
Posted - 09/05/2008 : 12:36:42
|
Thanks Peter that's just the information I was looking for!
(I have a little way to go before I can tune by ear).
By they way I had used Simon's "13 degree auto advance" for my calculation.
|
|
|
|
Gordon
United Kingdom
692 Posts |
Posted - 09/05/2008 : 17:00:12
|
I was told by Barry Foster that he uses 37 deg advance and fine-tunes this by putting a temp sensor on the exhaust manifold and changing the timing to get the LOWEST temp at fast cruising ie 4500 to 5000 rpm. Please be aware that this is my memory of a conversation with Barry and that I may be miss-reprenting his advice. Of course his 37 degree figure is particular to his engine and the points made by David Allison re climate, altitude, engine tune and fuel are wholly relavant but I was interested in Barry's fine tuning method as I had never heard of it before. It appeals to my scientific training, tone deafness and inexperience!
Gordon |
|
|
|
kimber
United Kingdom
1529 Posts |
Posted - 09/05/2008 : 17:50:26
|
| I seem to remember from my J2 days of long ago that some J2's had 13 (=26) degree advance distributors? No doubt Blower will confirm. |
|
|
|
Keith Wallace
United Kingdom
367 Posts |
Posted - 11/05/2008 : 06:38:07
|
| Please to report timing now set at a maximum of 36o and she defenitly smells happier. (perhaps the ear will come latter). |
|
|
|
David Allison
United Kingdom
665 Posts |
Posted - 12/05/2008 : 08:51:26
|
Exhaust manifold temp sensors are widely used for engine set up - indeed modern ECU's use a sensor in the manifold to constantly vary the ignition timing to suit conditions.
Fred Dixon (the pre-war Riley wizard) was an ex motorcycle rider and tuner too.
He always championed tuning by using one carb per pot - and used to carry out engine set up using either stub exhausts or even no exhaust at all.
In this way he tuned the engine by the colour of the exhaust flames - this technique is still used by a number of well respected engine tuners.
Barry is using modern technology to assist in a well proven engine tuning technique - he also does not incurr the wrath of the local population by running engines without an exhaust!
I always use the rule of thumb that retarded is safer than advanced - and that fuel is cheaper than pistons.
It is boring and safe to run a couple of degree back and a flat or two too rich for road cars running on a variety of lead free fuels.
You could tune the car to run on E85 (bioethanol) but that is another story!
Regards David |
|
|
|
Keith Wallace
United Kingdom
367 Posts |
Posted - 12/05/2008 : 17:28:01
|
Can any one help or advise on a timing fault?
The problem is that whist number 4 plug give a strong pulsed light spot on the timing mark, number 1 plug wavers back and forward of the timing mark, why should this be?
Is the use of sparking plug cap supressors recommended? |
|
|
|
David Allison
United Kingdom
665 Posts |
Posted - 15/05/2008 : 09:07:10
|
Keith - there are a number of possible causes.
Spark plug suppressors will neither cause nor cure the problem you have (I could be wrong) all they do is cut out the electromagnetic radiation that scrambles radio and TV reception.
I prefer un-suppressed ignition for the childish reason that I enjoy watching people trying to work out why their mobile phone has stopped recieving - tee hee!
It can be wear on the cam inside the distributor - although this would also give a waivering signal on number 4 cylinder too with all probability.
A problem with the distributor cap or plug lead on number 1 cylinder.
Can cause a wavering trigger command to your timing light.
I would check inside the distributor cap for any damage around number 1 cylinder - also check the connection from the cap to the HT lead, make sure it is a clean connection.
If all else fails change the number 1 plug lead - or time the engine uing number 4 cylinder.
Sorry without seeing the car and the problem and I can not be much more help, but I think this gicves some reasonable things to look for and check.
Regards David
|
|
|
|
bahnisch
Australia
674 Posts |
Posted - 15/05/2008 : 10:33:53
|
| My F-type was missing under load (eg long hills, etc) and at higher revs and it turned out to be the distributor rotor. It had eroded to the extent that there was too much of an air gap between it and the terminals in the cap and even my "Sports Coil" couldn't bridge the gap when the going got tough! |
|
|
| |
Topic |
|
Forum Locked
Topic Locked
