how to measure your port timings

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tavspeed
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Sun Dec 01, 2013 3:26 pm

hi chaps,got my engine ready to measure port timings,its a race rapido with 60mm crank,mb reed valve,has been tuned by jb at some point not sure in what way,but exhaust and inlet look to have been opened up and polished.

so got a timing disc and going to make a reference pointer at work tomorrow with a old flywheelnut and welding rod.
so with the timing disc fixed to the engine,piston at top dead centre,i rotate engine clockwise,

what are the measurements i need i.e top and bottom of transfers,top of exhaust in degrees,then someone can tell me if they are any good.

is the inlet port timing important as i have a reed valve.

i have a squish of 1.4mm with a 1.5mm head gaket and no base packer,so have the option of a .5mm head gasket and 1mm base gasket,if the first readings are low.
going to use a pm 25 exhaust and a have a sx200 gearbox 15 tooth on the front,think it gives 4.9 final
drive.
so looking for timings to work with the pipe,which is quite revvy,also have a scorpion if i need it,cheers all.

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coaster
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Sun Dec 01, 2013 7:59 pm

You don't need to worry about the inlet as with the reed valve you will be looking at 360 degrees (assuming there are windows in the piston?) Rather than fitting a pointer to the flywheel nut, most people fit the timing disc under the nut and attach a wire pointer under one of the cowling screws.

tavspeed
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Sun Dec 01, 2013 10:36 pm

hi coaster,the timing disc i have i bought at a parts fair and is a bit big. so have had to cut it to fit round mag flange,making up a pointer at work is no problem,but need to know when i set off at tdc or 0 deg when the piston starts to open the exhaust and then the inlet,these are the values in degrees,that most tuners would understand,then they might say exhaust is a bit high or low,if you know what i mean,and also what the blowdown is,which i think is exhaust deg minus transfer deg,hope this makes sense,cheers.

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rosscla
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Sun Dec 01, 2013 10:46 pm

If you have the head off just now it's far easier, and much more accurate to measure down from the top face of the barrel using the depth gauge on a Verneer caliper to the top of the exhaust and the top of the transfers and use one of the online calculators. Degree wheels are a faff and it's very difficult to be accurate to even a degree.

http://lambretta-images.com/archive/porttiming.php

Other calculators are available.
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tavspeed
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Sun Dec 01, 2013 10:49 pm

sound,will have a look at that method,will post up measurements for further advice,cheers all.

tavspeed
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Sun Dec 01, 2013 10:58 pm

is piston crown to top of barrell at tdc,how far piston protrudes or sits below top of barrel at tdc,my piston protrudes about 1mm,what value would this be in the calculator

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rosscla
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Sun Dec 01, 2013 11:01 pm

-1mm, presuming ypu're using a thickish head gasket to get the squish right?
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tavspeed
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Sun Dec 01, 2013 11:03 pm

cheers rosscla

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rosscla
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Sun Dec 01, 2013 11:09 pm

Mine measures up like this

exhast 32.1 185.6
trans 45.9 128
inlet 96.5 155
ph 69.66
bd 28.8

I think mine had about 0.7 out the top of barrel to piston at tdc on a 60mm stroke with no base gasket, the casing has a very high gasket face.
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Darrell Taylor
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Sun Dec 01, 2013 11:11 pm

if its +1 out of top then it will lower port timings so make sure the calculator has the same effect as some are confusing

on your 60 stroke best to run no base gasket ,also check piston skirt doesnt touch down on case at bdc if 107 rod normally 1mm/1.5mm needs taking off piston skirt also be aware that as its been tuned before it may have had the base gasket face skimmed so a check that the spigot doesnt bottom in case be good
Last edited by Darrell Taylor on Sun Dec 01, 2013 11:21 pm, edited 1 time in total.
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