How do I compression test an NSR125?

[scoobiemandan]

Hi,

Very new to bikes so excuse my ignorance. Recently bought me an NSR125 with little history and no idea whether a top end rebuild has ever been done. It's currently sitting at 21,000 (possibly km as an import). Don't have any running issues at present but would like to check for piece of mind.

Done some searching but found different answers which never really helps from a newbie perspective. Anyways, could someone please explain the steps (easy for an idiot to understand) on how to do a test? Also, is there any particular bit of kit I require or is the following okay for the job?;

https://www.halfords.com/webapp/wcs/stores/servlet/product_storeId_10001_catalogId_10151_productId_199553_langId_-1_categoryId_255216#tab3

Thanks for reading peeps
____________________
IN THE GARAGE: 2001 Fox Eye Honda NSR125R, 2004 Subaru Legacy GT Spec-B, 2013 Ford S-Max 2.0 Titanium X Sport

[Fisty]

You take the spark plug out fit the compression tester in the hole where the spark plug was and then turn the engine over.
____________________
Quietly and consistently taking the piss.
TL1000R | Hayabusa | ZXR400 | TL1000S | Bandit 400 V
Fatter and faster than Fret

[scoobiemandan]

I suppose I asked for that

Thanks but do I need to ground the spark plug, if so, how? Do I need to open full throttle? Cold engine or hot engine?
____________________
IN THE GARAGE: 2001 Fox Eye Honda NSR125R, 2004 Subaru Legacy GT Spec-B, 2013 Ford S-Max 2.0 Titanium X Sport

[jeddy11]

No its a compression test that is all to make sure your cylinder inst leaking air pressure so take a plug out and pull the ht leads off the other plugs(if there was any) as said plug the compression tester in the hole then turn the bike over your not trying to start the bike up so no throttle etc needed as the engine turns over your compression tester will go up and a reading will be made.

You will now know if your pistons/rings/cylinders are good or not..
____________________
Fuelly My Z1000SX
cbt 06/08/11 mod1 (second go) 01/08/12 mod2 21/09/12
Varadero Viking YBR125>Varadero125>ER6F>Z1000SX !!!

[scoobiemandan]

Thanks Jedda but how is air gonna enter the chamber if the throttle isn't open and couldn't I damage the ignition system without grounding the ht lead?

Just wanting to do this the safest and best way
____________________
IN THE GARAGE: 2001 Fox Eye Honda NSR125R, 2004 Subaru Legacy GT Spec-B, 2013 Ford S-Max 2.0 Titanium X Sport

[weasley]

The compression tester will have instructions, but typically you do have the throttle wide open when compression testing, to eliminate the vacuum effect of a closed throttle (plus allowing the engine to turn over easier/faster).

And don't worry about the spark plug.
____________________
Yamaha XJ600 | Yamaha YZF600R Thundercat | KTM 990 SMT | BMW F900XR TE

[Teflon-Mike]

[scoobiemandan]

I tested the compression before reading your post Mike but with throttle shut. Came back as 115psi!

I'm assuming that the top end needs a rebuild soonish based on that but not really sure what the psi should be! I've read it should be circa 125psi!?

No paperwork with the bike to say it had had a rebuild and is showing 21k on the clock, I'm thinking this is KM as the bike's an import so it's most likely due one!
____________________
IN THE GARAGE: 2001 Fox Eye Honda NSR125R, 2004 Subaru Legacy GT Spec-B, 2013 Ford S-Max 2.0 Titanium X Sport

[P.]

I'd say as long as its over 110, you are ok, but start saving.

You want over 130 really.

[Kickstart]

Hi

Throttle wide open and redo it.

From memory it should be 10 bar +/- 2 bar (so ~145psi give or take 29psi, so min 116psi).

All the best

Keith
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Hugh Farking Cant]

Them Gunson (and also the Draper) gauges are not renowned for their accuracy.
Alright for checking differences between cylinder though.

115 psi on closed throttle is very good if that is actual pressure.

[scoobiemandan]

[Hugh Farking Cant]

[stirlinggaz]

[alains]

i agree 120 PSI is a minimum
you need at least a ring set change (plus gaskets,pin,bearing) which wont cost a fortune
see david silver spares
if you're doing so , i can guide you through the process
____________________
the worst day fishing is better than the best day working

[scoobiemandan]

We're moving to morrow so things are a bit up in the air at the moment but I shall be redoing the test and having throttle full open and see if that makes a difference and do it maybe three or four times to get an average. I'll report back when I do.

I got a nephew who's just rebuilt his Aprilia engine so I've already told him I'll be accosting him when the time comes
____________________
IN THE GARAGE: 2001 Fox Eye Honda NSR125R, 2004 Subaru Legacy GT Spec-B, 2013 Ford S-Max 2.0 Titanium X Sport

[c_dug]

A two stroke rebuild is easy-peasy as long as you can follow some instructions
____________________
I am a bellend, I am a man of constant sorrow, I am a gummy bear, I am a rock.

[scoobiemandan]

Aye, read up on it. Seems easy enough. It's all about finding a day or two where I won't be needing the bike in case I come across other parts I need upon inspection.
____________________
IN THE GARAGE: 2001 Fox Eye Honda NSR125R, 2004 Subaru Legacy GT Spec-B, 2013 Ford S-Max 2.0 Titanium X Sport