RD 125 LC compression test should be.....?

[bladerunner]

Thought I'd ask the mass mind of BCF what the expected cold PSI reading for an old 2T 125 should be? Pre powervalve if that makes a difference!

its been sat in my shed for 8 years after it was donated from the last owner who had run it into the ground, occasionally at speed but passed his test and part ex it for my fireblade. Long story short just about everything was shot,missing or just worn out and the DT125 motor in it now was kindly donated from a guy in scotland and being an unknown motor I slung it in and see if it falls apart 90 miles done the water pump seal failed which was to be expected but hey the rest seems good like it ticks over and its pretty quiet.....too quiet to be reading 60 PSI or so I thought
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Current bikes...cbr929, KDX200's, Rd125lc mk2, RGV250's

[Teflon-Mike]

Two strokes have bloomin great holes in the sides of the cylinder....
The 'compression' then tends tgo only start being made when the piston is past them... a-n-d motor is spinning fast enough that pressure dont 'leak' soo much.

There's often two, or three 'quoted' compression ratio's for two-strokes.
a) Calculated... same as a four-stroke, the swept volume of the cylinder compared to volume of combustion chamber abovge the piston crown at TDC... this is often a LOT higher, when quoted than any actual pressure you might measure
b) 'Effective' Compression ratio... it wont start squashing gas until the piston past the ports, so you get something 'like' the swept vo0lume of the cylinder above the ports, maybe 2/3 or 1/2 the displacement (So if pot has 125cc displacement, only something like 80cc of that will get squashed after the ports closed) compared to the volume of the combustion chamber, at TDC.... to get it you measure the cylinder pressure when cranked at nomonal power making revs, then compare what A:B compression ratio would give you that compression in a 4T... you can do it from the vgolume above the ports compared to combustion chamber vol, same 'sort' of fudge.
c) 'Cold-Cranking-Compression'.. again, an effective, worked out from measuring actual cylinder pressure at cold-cranking revs, compared to the pressure you'd get if it was a four-stroke... some-times known as 'work-shop' compression ratio, as its the guide for what you should get on a compression tester.

Oh-Kay..... on a four-stroke.... typical quoted compression ratio's are in the order of 10:1. On a two-stroke... common that things like the Haynes will quote something more like 6:1... this is because the piston is only trapping that gas above the ports, and if you see a ratio in that order, its likely an effective, cold-cranking or work-shop figure.
If you see a compression ratio, often in the manufacturers claims up in the 11, 12, or even 14:1 realms... that will be a straight calculated CR, the combustion Chambers vol compared to swept vol of the cylinder, and actual pressures in the pot are likely to be no-where near.. at that sort of almost diesel CR the thing would be knocking like a fctard if it ran at all!!!

The 'answer' you want is something like 6:1..... that would be the sort of work-shop C/R, and I think my old air-cooled RD was around that on the dial of a car C/R tester, and in the book, where it was quoted at I think 6 point-something : one.. a work-shop reading, rather than a theoretical.

Hint.... on an engine that only displaces 125cc per pot.... C/R readings are rather 'vague' at the best of times, the cylinder and combustion chamber so small in relation to the volume of the spark-plug hole.... whether you use a push-in type tester, and how hard you hold it in, whether you use a screw-in type, and how much volume is in the pipe or adapter, all make a much bigger difference than on a typical car engine, which will have a swept volume of around 400cc, but the volume of the spark=plug hole and or compression tester union wont be any different..... So DO NOT take a C/R figure as gospel on a little engine. They are notoriously inaccurate on smaller four-strokes, and on a two-stroke with holes in the side the pot, almost completely useless.

You ALSO have the small issue on a two-stroke, that there's compression both sides of the piston; crank-case compression as well as cylinder compression. And you are more likely to 'loose' crank-case compression, that can escape through knackered crank-case seals or rusty reed-valve seats etc, as well as past rings on a worn bore.....

If the motor runs? Well, that would probably be good enough for me!

What actual compression tester reading I might get from the thing, PROBABLY wouldn't tell me much, and certainly enough to decide whether it needed a re-bore and new piston/rings, or whether I need bury into the bowels and replace the crank-case seals.....

A niff of petrol when I whipped the genny cover off WOULD give strong suggestion that was needed, dire running, uneven carburation, hard starting etc etc etc, would all be clues.....

And given the number of moving parts on a two-stroke.... Elcie and worse power-valve elcie motors tending to defy the notion, becoming as or more intricate than a simple push rod four stroke like a CG engine... but that asside, a two-stroke not being as 'involved' as a four-stroke, and so little that can really wear out...... I would be inclined to do full tear-down as course if there was any question; measure up crank, replace outer bearings, replace crank-case seals, clean reed seats, replace reeds, rebore and new piston/rings, and gaskets and be damned..... all done, its known 'good' base-lined for further fault-finding if needed.....

BUT.... if it runs... and runs half reasonably... I'd probably leave it until another day!!!
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[redeem ouzzer]

I’ve never found compression tests that conclusive on a 2t unless there’s next to none, in which case the thing runs badly or not at all. There’s no substitute for taking the barrel off and measuring piston clearance / checking the rings.
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Be a REAL MAN!

[stevo as b4]

Teff, your incredible!

You do realise that the person your preaching to about how two stroke engines work and how they differ with how compression ratios are quoted, probably knows more about two strokes than you ever will, and has had history of many high performance strokers too.

All he wants is the book figure for compression pressure measurement for an RD125LC. That's the one and only thing your speil failed to inform him of.

According to the RDLC forum, someone said 136psi was the book figure for an mk1 (pre powervalve) RD. That sounds quite possible as alot of Yamaha's tend to run lower compression ratios than other bikes. My KMX 125 is supposed to be around 170psi I think for a 7.8:1 trapped compression ratio.

[bladerunner]

Thanks guys Stevo your bang on point 136psi sounds good for an LC but as a ball park figure at least I have something to compare it to now

I used to test the RGV race motors when fresh built and use that as a gauge for issues or wear further down the line but plated bores running avgas CR ratios as well as the powervalves etc means it was not comparable to the humble little LC!

Tef- I did indeed pull the top end off today as well as the clutch casing as the water pump seal failed after 90 miles....hey this engine cost me £12.50 for delivery from scotland and apparently it last ran 4 years ago on a work bench so god knows when it was last running daily in a bike

I'd post pics of the top end if I figured out how to via facebook or whatever pic hosting site to use these days etc

long and short is its a DT motor (10V) rather than RD (10W) so porting may be different and whilst the head looks to have only carbon buildup from the miles I've done the top piston ring is rusty over maybe 1/4 its area so rough as buggery lol.

I pulled the old autisa big bore off the original motor that was in the bike and whilst on +2mm over size the last owner had fitted a recent piston and rings which look good and the bore also was still wet with 2T oil from when it last ran 8 years ago!!!!! so piston measures 68.00mm but the bore is 68.07 at the base.....larger than I would like by a lot its got no rebore option with the DT175 pistons fitted and I guess its going to slap a bit when cold but it'll let me finish the rest of the motor and chassis testing and hopefully pull past the current 70mph limit the bike was hitting.

watch this space
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Current bikes...cbr929, KDX200's, Rd125lc mk2, RGV250's

[bladerunner]

latest update for anyone in future searching this thread is....

65psi..std 10v motor had rusty top ring and hitech reeds that didnt seal by 1mm

86 psi fitting a worn autisa 175 cc on 2mm over bore but worn with a machined head with higher volume than std for the big bore

98 psi by fitting std steel reeds that seal!

110psi by fitting the std 125 head...tried Y2 model

115 psi with Y1 model head

120psi after honing the bore

160psi after lapping the head and barrel and fitting std head studs rather than the M8 stainless studding and annealing the copper head gasket

job jobbed now to fit the rest of the items together and run it
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Current bikes...cbr929, KDX200's, Rd125lc mk2, RGV250's

[Sister Sledge]

A staggering difference between then and now!

I do remember when tuning my old air cooled 2t 125s that initial rebuilt compression would change over the time of the first weeks running in - it would increase significantly (and then slowly disintegrate!)
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CCM 404 DS