Welcome to our resource for MG Car Information.
MG Midget and Sprite Technical - 1275 Midget ignition timing
|I have a 1275 midget, standard cam, but with high lift rockers, higher comp. ratio, perf. exhaust manifold and magnetronic ignition. When ig. timing set to 10 deg. advance at 1000 rpm the car runs badly and lacks power- all the symptoms of retarded spark. When set to 20 deg(!) advance it runs properly - no pinking. Have checked everything else. Its a mystery - any suggestions?|
|G M K Hunt|
|Is that measure with or without vac connected?|
If vac connected, the vac unit will also contribute to advance, so would expect more than 10 deg under these circumstances. But very hard to calibrate under these circumstances, so vac normally disconnected.
Why not check static is ~7 BTDC? I prefer static setting, since it's independent of rpm and so repeatable.
Bob (Turbo) likes dynamic setting... he'll be along shortly.
|Timed with the vacuum disconnected.|
Not sure how to do a static timing with a magnetronic ignition,
|G M K Hunt|
What max advance do you get? If the dizzy is tired it may have reduced advance so 20 at idle might give say 26 max so 10 would only give 16 max and be as flat as a f**t :)
|To do static with magnetronic, simply use the timing light connected in the normal way the turn the engine over slowly by hand until it flashes.|
When distributors get old and tired, they get a bit of slack in the advance springs and their mounting pins. This can cause the advance to kick in earlier than it should do, and give a misleading steer if setting the timing by reference to one engine speed only. The end result is that the timing will be right for one engine speed and wrong everywhere else.
This is quite common now, given the age of the cars. To confirm or eliminate this from your list of possible causes, you need to check the total advance at several engine speeds, compare with what it should be doing, and adjust the advance mechanism until it ought to.
I have described it on my website here http://www.mgparts.co.nz/distributor.html using the MGB as an example but the principle is the same.
The Midget is not as prone to the problem as the earlier B (due to where the second spring takes up) but it does still happen.
The relevant figures for total advance, vacuum disconnected, to compare with for the 1275 (from the
official BMC manual) are
static 7 deg
1000 rpm 13 deg
2000 rpm 19 deg
3600 rpm 29 deg
An extreme cause of the symptoms you describe can occur if the distributor has not seen enough lubrication between cam and spindle and has lain idle. The cam seizes onto the spindle and all mechanical advance ceases. I have seen this a surprising number of times!
Snap! and much more succinct!
|The distributor is "new", reconditioned one from MGOC.|
The spark advances by 10 deg from 1000 to 3000 rpm.
I may buy a gunson timer, since the measurement were done by a very experienced (on older cars) mechanic while I watched and discussed it with him. We are both baffled.
|G M K Hunt|
|Nice one Ant.. :)|
Greg you need to disconnect the vacuum and rev your engine to above 4.5K RPM (at this speed the mechanical timingis fully advanced) then set the timing at about 32 degrees BTDC give or take a degree or 2!
If the car still does not run well then something else is wrong.
This is the only method suitable for setting up ignition timing when the mech advance of the distributor is unknown. If it is known and is accurate then the ignition timing can be set statically by setting it at 32degrees BTDC less the mechanical advance.
|Bob Turbo Midget England|
|Many thanks indeed to all concerned. I will buy a strobe light and get down to serious business, cross checking the static & dynamic settings to unravel the mystery. Will get back to everyone when the light dawns.|
|G M K Hunt|
|It is quite possible that the harmonic balancer is showing a false reading. As the rubber sleeve gets worn, the outer pulley can slip on the hub. I had this happen on my Volvo 240 (terrible screeching noise). On the 1275 Midget, it seems that the mark is off.|
I recently retarded timing slightly as I leaned the mixture to obtain proper CO reading on the exhaust gas analyzer. Interesting how the timing advance increased CO.
I to found I was a victem of this recently myself as I had powder coated the pulley ... so looking forward to my next attempt this next weekend.
|Im about to go down this path in the vary near future, but I have a few questions Ive been pondering for several months|
I have a 27 series (YH???) Mallory duel point Dissy with a re-curve kit. so its completely adjustable. the engine is radically modified so obviously the factory curve wont apply.
I know the "inital timing" (idle) is 10 degrees BTDC at 1000 rpm
The "Total Timing" will be 4500 rpms and set at 32 BTDC. (strange, Ive now heard that 3 times in 2 weeks)
This Is where Im confused...
1.) Can I have both specs above (the Intial and the Total timing) installed in the dissy or is it one or the other?
My thinking is yes because the dissy is completely adjustable... Including the guts with special keys, springs and wieghts.
2.) The rate of advance .... The speed at which the advance takes place.
How do I figure that? With the engine being heavy modified Do I want "The rate/Speed" to be faster or slower then the stock dissy on a stock engine?
In My thinking I can see an arguement for both sides of the coin. Both slower rate of advance as well as faster speed of advance.
Prop... Im not high jacking this thread
But How do I go about accomplishing this key part of the artical at say 85 mph... perhaps a small child duck taped to the valve cover...LOL
>>>>>Without special equipment, the most practical way of setting up lower speed advance is to road test the car, driving it in top gear on full throttle at the target engine speed. Advance the distributor 2 deg between each run and note the setting at which pre-ignition is first heard at the target engine speed. Measure the total advance at that speed, note it down, then road test for the next target engine speed. . Do this at 1500 RPM, then at 2000 RPM and finally at 2500 RPM The optimum ignition timing at each target engine speed will be 1 - 2 deg less than the figure you recorded as when pre-ignition appeared. <<<<<<<
Btw... Ive got the SW-05 swiftune cam with high lift rockers 1.5 ratio so the valve lift measurement is .495 ??? and so far the compression is 230 psi, but the rings are only partially broken in, hoping to have that accomplished next week... I do have the camshaft (Advanced to 102.5) instead of the 106 to 108 due to the roller rockers
Generally, the more tuned an engine is, i.e.
- improved breathing fills the cylinder better
- higher CR adding to compression
... the slower should be the rate of advance with rpm. Most 'high performance' dissies use thicker springs to slow the rate of advance with revs and raise the revs t which the max figure is reached.
The main reason this is: the rate of burn is determined by the speed of the flame front; higher compression means more swirl and faster flame front; therefore you need less advance to have the spark at the right time.
This thread was discussed between 08/06/2010 and 16/06/2010
This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.