BTDC question

[Ramjam]

Hi
Can anyone tell me from which point do you measure the number of mm for the timing? Is it from the centre of the piston or the side near the wall?
Mine is supposed to be 3.6 mm and i m not even sure how to get a decent measurement

any help appreciated


mj

[Alan H]

It doesn't matter if the head is off. 3.6mm is 3.6mm. If the head is on and you're measuring down the plug hole, then make sure the plug is central and vertical or the measurement will be different. {Personally, I always used to do mine from the timing marks and that was OK.

[tz375]

I prefer to set timing with the head off and a DTI rigidly mounted. I use that to check the actual timing and I also use it to move/tweak the propeller so I can use a timing light later on should I feel so inclined.

3.6mm sounds like the measurement on an angled spark plug on a GT750, rather than a vertical one on a 380. My copy of the service manual suggests timing of 2.05 - 2.93 depending on year and left/right (angled) or center (vertical).

[Ramjam]

sorry, yes, i was looking at he 750. But i still dont understand how to measure the 2.09 mm. Is it from the top of the head as if you put a ruler accross the top over the pots? In this case, 2.09 mm would be down the side of the bores ? Or from the centre of the bores? It makes quite a difference no?

thanks
mj

[Alan H]

It makes no difference at all.
If the piston moves down any distance at all, it all moves.
Not just the centre or the sides.
Take the head off and measure vertically down anywhere you like.
As the piston moves, tbe point that your dial gauge is touching does not change.

[Cliff]

I think the confusion on the measurement has to do with the dome on the piston. With the head off, if you measure from the top of the cylinder to the top of the dome, or center of the piston, you're timing will be too far advanced because at TDC the top of the dome is ABOVE the top of the cylinder!
To measure the distance correctly, set up your dial indicator to contact the piston. This can be at the center or the side of the piston, doesn't matter just as long as it's in a straight line. Roll the engine over to find Top Dead Center (TDC) and ZERO the dial indicator. Roll the engine backwards, past where your measurement should be then roll it forward slowly until you hit your measurement and the points should just open there! On the 380 DO NOT use the nut on the end of the points cam to roll over the engine even with the head off!
Like Alan H said, use the timing marks! I've done both my 550 and 750 with the dial indicator and then rechecked with a timing light after and they were both spot on the marks!
Hope this helps!!
Cliff!

[tz375]

I see where the confusion is coming from. Don't think in terms of an absolute position of one part of the piston relative to the head.

It is the distance that the piston has to rise to reach TDC. So if you zero the dial gauge at TDC with the pointer almost fully raised and the DTI bolted to the barrels with a bracket of some sort and rotate the motor backwards by say 3mm and then forwards until it's at teh correct reading, you will be where you need to be.

Something like this...

[Ramjam]

I think the confusion on the measurement has to do with the dome on the piston. With the head off, if you measure from the top of the cylinder to the top of the dome, or center of the piston, you're timing will be too far advanced because at TDC the top of the dome is ABOVE the top of the cylinder!
To measure the distance correctly, set up your dial indicator to contact the piston. This can be at the center or the side of the piston, doesn't matter just as long as it's in a straight line. Roll the engine over to find Top Dead Center (TDC) and ZERO the dial indicator. Roll the engine backwards, past where your measurement should be then roll it forward slowly until you hit your measurement and the points should just open there! On the 380 DO NOT use the nut on the end of the points cam to roll over the engine even with the head off!
Like Alan H said, use the timing marks! I've done both my 550 and 750 with the dial indicator and then rechecked with a timing light after and they were both spot on the marks!
Hope this helps!!
Cliff!

Exactly!!!! The dome of the piston.
Unfortunatly, i dont have timing marks. That's WHY it's important i get it right this way as i dont have options.

[Alan H]

If you don't have timing marks then the 3 bladed 'windmill' behind the points plate is missing. This is held in place by a small 3mm diameter pin. Make sure this is OUT completely or it could get in the seal and damage it, or better still, fit a windmill plate and timing in the future will be easy.

[Ramjam]

It's still confusing as because of the dome shaoe of the piston i still cannot determine what is TDC.
Is it when you bring the outside rim of the piston in line perfectly with the outer edge of the cyclinder block - ie so that the dome of the piston sticks above the line? OR is it that the top of the dome is perfectly in line with the top of the cylinder block, therefore making the outer edge of the piston about 2mm lower than the line?
confusing, but im sure we'll get there with more beer

mj

[Ramjam]

If you don't have timing marks then the 3 bladed 'windmill' behind the points plate is missing. This is held in place by a small 3mm diameter pin. Make sure this is OUT completely or it could get in the seal and damage it, or better still, fit a windmill plate and timing in the future will be easy.

yes, it's missing. So therefore i need to do it with piston number one

[tz375]

This is not that complicated. TDC is when the piston is at the top. ie not part of it somewhere close to the top . When it cannot go any higher - that's TDC. At that point the sides, middle and everything else will be as high as it can go. You are still thinking of what part relative to the barrel. It's when the whole piston is as high as it can go.

Take the head off and rotate the motor by hand with a wrench on the alternator - NOT the Points nut, or stick it in gear and turn the engine over a few times and watch as the pistons rise and fall. Then rotate it so that the left pot is at TDC - as high as it goes. Not fit the DTI and adjust it so that it reads zero.

Rotate the motor slowly a little either side of TDC to confirm that you have the exact point as it rocks over the highest point. That's TDC.

[Alan H]

If you don't have timing marks then the 3 bladed 'windmill' behind the points plate is missing. This is held in place by a small 3mm diameter pin. Make sure this is OUT completely or it could get in the seal and damage it, or better still, fit a windmill plate and timing in the future will be easy.

yes, it's missing. So therefore i need to do it with piston number one

You will have to do all three separately.

LOOK HERE FOR DETAILS

The part is available HERE, by the looks of it, as it didn't sell.
Try an email and if they still have it they will sort you out. Crooks Suzuki are one of the original Suzuki dealers established in the early 60s and know what they are on about.
If you get one of the timing plates, you won't have to go through all this again when you need to check the timing.
You should be able to get some 3mm steel rod locally to make a locating pin.

[pull the wire]

I blueprinted all of my port timing with the head off obviously mounting my dial indicator as vertical as possible. The specs for ignition timing in the service manual take into account the angle of the outer 2 spark plug holes. A very small diameter dial indicator stem/touch point can be mounted vertically into the spark plug holes for the left and right cylinders but the service manual spec is still for an indicator installed at an angle. That's why the measurement is greater for left and right to account for the angle of the touch point. With the indicator on an angle the actual travel of the tool is more. I am not sure but if you do set up all 3 cylinders with a vertical measurement, like the center cylinder spec, you should be good. I did read somewhere that the spec for the center cylinder is supposed to be slightly retarded versus the outer cylinders to reduce the possibility of detonation due to the heat factor. Can anyone verify this because I will setting up my ignition soon.

[Suzukidave]

Did you get the TDC worked out ?