TUNING & TIMING

TUNING: CARBS |

TIMING: TIMING MARKS | TIMING MARKS: DEAD STOP | BUBBLE-O-METER | DISTRIBUTOR | SPECIAL TOOLS

TAPPET ADJUST |

Subject: Timing Marks

Guzziology gives measurements in mm from Top Dead Centre for timing marks on various big twins. Two problems: it's difficult to measure while the bike is assembled, and secondly all the flywheels I have looked at so far (admittedly only 3) have had TDC incorrectly marked (about 2 deg retarded), so if you measure from the factory TDC mark, you'll be out. This is how to set it up accurately:

First you need a piston stop. Take an old spark plug and bash the guts out of it. Patience and eye protection are essential. Don't damage the thread. Insert a longish 6mm screw through it with a nut & washer either end so the head of the screw protrudes about 30-40mm past the plug thread. Those cheesy japanese philips head case screws are ideal cos the heads are rounded and won't hurt the piston.

Put the (whichever) piston somewhere near top. If you're starting from scratch, use a screwdriver in the plug 'ole.

Remove the alternator rotor bolt and bolt on a plastic degree wheel - a few bucks at a speed shop. You need a shortish 8mm bolt and a couple of large washers. Put the TDC mark where you can get a good straight look at it.

Fix a piece of wire to one of the alternator cover screw holes and bend it so it points vaguely to TDC.

Turn the motor past top about 90 deg and screw your piston stop firmly into the plug 'ole. Turn the motor *slowly* backwards until you feel the piston come up against the stop. The pointer should say something like 20 or 30 or 40 degrees after top. Whatever it says, write it down *exactly*. Don't be afraid to guess fractions of degrees. Bend the wire if you want at this point but thereafter do *not* bend the wire, *or* move the piston stop! Now turn the motor forwards until you hit the stop again and write down the reading. This time it will be before TDC.

Now for a little arithmetic. Let's say the first reading was 32.5 deg after top, and the second one was 29 deg before. Add them together for a total of 61.5. Halve this and you get 30.75. This is how many degrees after the second reading OR before the first TDC is. Instead of counting, just subtract the split figure from either of the initial readings. eg, 29 - 30.75 = -1.75. That is, 1.75 *after* top (as shown on the degree wheel). If this is confusing, think of it this way: TDC is exactly halfway between the two initial readings. Work it out any way you can.

Remove the piston stop and turn the motor to show this reading. This is actual TDC. Bend the pointer so it's bang on TDC. Now you can mark the flywheel wherever the index mark on the bellhousing lines up. If that was the RH cylinder, turn the motor exactly 90 deg and mark TDC for the left. Otherwise go 270. From here it's a simple matter to make the static and advanced marks at the appropriate points (see your manual for details), since your degree wheel is now accurate. Or if you can get to it, just measure on the FW 2mm per degree from TDC.

That's it. Accurate timing can now be yours. A couple of additional points: Avoid turning the motor backwards any more than you can help. It may or may not be bad for the camchain/tensioner. Best to play it safe I reckon. Don't use the bolt at the front to turn the motor or you risk dislodging the degree wheel and having to start again. If the bike is assembled, put it in *top* gear and turn the rear wheel *slowly*. A sharp white wax pencil is good for making marks. Paint tends to splodge and may not stick. Don't be tempted to clean the FW rim first - you want it as unreflective as possible. Wipe the excess dust off, but don't make it bright. I painted mine *flat* black before assembly. Gloss paint, even black, will just reflect your strobe.

Bryan

Well done Bryan, this method is commonly known as the 'Dead Stop' method and is probably the most accurate way of finding TDC you can get.

The fact that Guzzi can get away with sticking the marks on the flywheels *somewhere* around TDC just goes to show what a loose old bit of kit they are.

On the topic of winding the motor backwards *against* the tensioner, don'ta worry, it won't do any damage. I suppose it might if you ran the engine backwards but this is a bit difficult :-) and a bit of low stress pushing and pulling won't harm it one little bit.

Ta-Ra.

Another method that works in a non contact way is as follows:

Having removed all non metalic bits from said spark plug, weld [or glue] a pipe into the resultant hole. A pipe size as small as posable is recomended, about 4 mm ID is the one that I use.

Use the visual or screwdriver method to get the motor near TDC.

Using soapy water, form a bubble over the end of the tube.

Turn the crank in whatever direction makes the bubble bigger, as in piston coming up.

If the bubble bursts, just reform it with the soapy water again. Surprisingly small amounts of piston displacement can be observed using this method.

When the bubble stops getting bigger, that is TDC for that piston, and can be marked acordingly.

Small back and forward movements with the crank will confirm TDC.

Repeat in the other spark plug hole and both cylinders are now marked for TDC.

Andy Henzell