Timing Dual Plugged MG

[BillN]

I own an 83' LMIII that has dual plugged heads. I had some flywheel/clutch work done (lightening and splines) and now have been "playing" with the timing. I've read that the burn should start later as it is faster with dual plugging, but I haven't found any information that is specific to MGs. Lot's of the "seat of the pants" or dyno testing suggested. I've set the timing at two ring gear teeth after TDC, the S and D markings, but am wondering if anyone has some experience to share here.
TIA,
Bill
83'LMIII
73' Eldo
00' Sport
07' Norge

 

[Morizzi]

Bill,

I can't give you a definitive answer but I can tell you the timing won't be AFTER TDC.

Each tooth is roughly 4 degrees. Its a bit less but irrelevent for this post. A standard LM III has static timing at 8 deg BTDC. You have it at 8 deg ATDC, a 16 degree difference! :blink:

The reasoning of 2 plugs is to give the combustion 2 points of ignition so the pressure builds in the combustion chamber faster. The location of the plug also helps direct the pressure down the bore. On engines with the spark plug centrally located twin plugging doesn't make much sense. On the LM III the plug is off centre but not by very much. The maximum distance to the far wall is probably no more than 50mm in an 83mm bore. (Guess from memory)

You are trying to time the burn so the pressure is just starting to build as the piston reaches TDC. This means that you will get maximum power without the piston having to fight against the building pressure on the way up.

There are a number of ways to adjust it and a number of clues.

I'd be guessing and it is a guess that it will be around 4-6 deg BTDC. That is what I would expect.

Fuel isn't the same as it was in 83 so the timing marks are just a starting point these days anyway but here is what I would do.

With the tank off and using a remote fuel source I would have the bike idling and adjust the dizzy for maximum revs. I would then back it off a few degrees.

I would ensure the advance is working properly as per spec and it reaches maximum advance about 5000rpm. If it doesn't then get new springs and oil the weights.

With new plugs in I'd ride it as I would normally ride and do a few plug chops. You shouldn't labour the engine. 4-6k is its normal happy zone. A lot will depend on your riding style and when you change gear, not to mention the quality of the fuel.

You are looking at the earth strap. The annealing line should be about half way, in the middle of the curve. If it is too close to the tip the burn is too cold. If it is close to the threads the burn is too hot and either the timing too advanced or the mixture too lean.

If you hear any pinging then that is probably telling you the timing is too advanced. There can be other causes but retard the timing by a few degrees and see if that helps. Pinging is a sign that the timing isn't just advanced but way too advanced.

Hopefully someone else who has butchered a LM III will chime in and give you a more definitive mark.

Rod with 2 LM III's that have standard engines and an 8 deg BTDC static timing.

 

[Chris R]

I gather that the engine was only twin plugged and the previous owner did not recurve the ignition.

The manufacturer figures out the curve to begin with by putting the engine on a dynamometer and playing with the advance at different RPM/Loads points until the they get max power without detonation. As Morizzi pointed out this is very dependent on fuel - which in the case of Guzzi in the 70s was high octane leaded. They then fiddle with springs and weights to get an approximation of the curve they plotted.

The plot will not be a straight line as it follows the volumetric efficiency which changes depending on the cam installed/port lengths etc.

As the distributor is a simple device and does not have any kind of vacuum advance all you can hope to do is come up with a reasonable curve that works in most instances - which is why engine management systems are really fantastic as you can change these things on the fly in real time on the dynamometer. The simplest/practical thing you can do is dial in two points: idle and max advance.

The beginning point is as Morizzi says around 6-8 degrees at idle or static and 30-34 degrees at full advance (My guess based on my modified lemans1). Full advance is the critical adjustment as if you dial in too much advance you will melt things, detonate etc. So check that first as Morizzi says - dial in the advance in small increments, do plug chops, measure the temperature at the exhaust as a baseline and listen very carefully for detonation. Detonation sounds like high voltage jumping a gap - if you see little aluminum specks on the plug you also have detonation: it is the piston breaking up!.

Then set the advance at idle to the point where with minimum advance you get a reasonably smooth idle. Timing and fuel interact so you have to play with both at the same time: start retarded and rich and move slowly to advanced and lean. Now recurve the distributor to give you those two points.

I think Guzziology has a table with different weights and springs that lets you figure out advance curves. If you can still find a Sun Distributor Tester you can recurve the distributor much more quickly but they are rare these days: they either cost nothing or a lot as they are funky looking things but useless for most folks.

 

[Carl Allison]

At some point, I'm going to have to go through this same routine with my Sport 1100i. I had Mike Rich Stage II port the heads, dual plug them and recess the valve pockets to allow a more radical than typical street cam. That's as far as I got. Cams are in short supply (so is funding ) so the bike has gone basically untouched for longer than I want to think about. As I recall though, a dual plugged head is retarded about 7˚ over stock through the middle of the rpm range. All that provides is a starting point for finagling the best advance curve positions. If you're willing to spend an extra hundred dollars or so, you might want to look into Cliff Jeffrie's Rec-Ignition Module. It takes the all-electronic approach to changing the curve on top of the static advance the bike has. While it only provides four different curves, at least you can change them with a switch and not fiddle with springs etc.

 

[DAMMAG]

I've got a twin plugged LM1000 motor in my LM3.

If I recall correctly I modified the distributor to advance only 16 degrees, with timing at 12 degrees BTDC initial and 28 degrees BTDC at full advance as per the Raceco website:

"At the same time as having your heads converted you will probably have had your distributor recurved. The new approximate timing figures are as follows: 12 degrees initial advance, 28 degrees full advance. Final full advance ignition figure will have to be determined either by seat of the pants testing (i.e. advancing the ignition timing until the engine pinks at high rpm and then backing off 2 degrees) or dyno testing. If re-marking your flywheel, remember that each tooth on the ring gear is 3.75 degrees, so your initial timing mark will be in the region of 3 teeth before tdc and your full advance figure will be in the region of 7 1/2 teeth before tdc. When setting your igntion timing it is important that you strobe it at full advance as this is the most important figure."

Damian

 

[OldJock]

In addition to Carl's post mentioning the Jefferies unit. Here are some more, I am not recommending one over the other as they all get positive reviews, just offering some alternatives.

Volker Sachse's ignition system has a bunch of curves programmed in and is changeable via a dip switch.

http://www.elektronik-sachse.de/index_en.htm

Drop a line to the guys at Ignatech and tell them the position. They have units where the curves can be adjusted via software to map the advance curve.

http://www.ignitech.cz/english/aindex.htm

Although initially costly (and I have no clue about your preferences re points vs electronic ignition). I think they would make life a good deal simpler and if paying Dyno time ultimately cheaper in the long run.

John