Question about 2003 MZ 125 SX

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My MZ RT125 is not starting due to low compression. The bike is about 7 years old and is not in use very often. Would this problem be due to sticking valves?

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Refresh my memory, but I thought that bike was a 2 stroke, and if so, there are no valves to stick!
If you have low compression, it would mean your rings are shot, or there's a hole in the piston, or the piston and rod are no longer attached to each other.

Posted on Mar 14, 2011

  • spike_stef
    spike_stef Sep 07, 2011

    its a four stroke single ! thought these guys were supposed to be proffessionals...


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The age old adage holds true here: If it aint broke don't fix it. :)

If the bike is still running good and you have taken good care of it there is no reason to overhaul it. There are many bikes out there that have done well over 100k miles and are still running strong. There have actually been dyno test ran on bikes with over 100k miles and they made better horse power than they were rated at from new. This is attributed to slightly looser bearing clearances with the engine being well broken in.

I would recommend overhaul if you start having problems. These issues would include, low power due to low compression, excessive oil consumption, excessive smoke from tail pipe, excessive fuel consumption. Even then I would recommend good diagnostics and if the issue is found to be internal then I would recommend an overhaul.

Oct 23, 2015 | Motorcycles

2 Answers

HI i have bajaj 150 old model i was not getting pickup on my bike so i was suggested to change clutch plate and pressure plate by mechanic and i have changed clutch plate pressure plate and also changed...

Is the smoke dark (blackish) or bluish?

If dark, your carburetor could be running rich, causing the smoke, and losing power due to choking the engine (too much fuel-to-air mixture)

If so, might need rejetting.

Jan 22, 2014 | 2011 Bajaj Pulsar 150

2 Answers

I am restoring a 1972 MAR 250.At present it will not run due to lack of compression.Can you tell me if faulty crank seals would have an effect on compression?Thanks,Gary

it would more than likely be piston rings if your crank seals have gony you would still be able to start it but it would smoke really bad

Jan 26, 2011 | Ossa Enduro Phantom 125 Motorcycles

1 Answer

I have a ktm sxf 250 2006 model and when its hot it is very hard to kick start even with start assist spray and sometimes will not but it will roll start and also when you are at top of each gear mainly...

Using "Easy Start" or other ether based engine starting aerosols will damage the engines in the long run, reducing compression due to engine wear as the aerosol removes the oil traces off the engine bore and stripping the aluminiun from the pistons. If it were an old diesel engine, not a problem.
Check your petrol mixture settings and compression. As if the compression is low it will be hard to start.

Jan 15, 2011 | KTM 250 SX-F Motorcycles

1 Answer


Hi, Jpif4 and the usual suspects are:
1. Fuel tank empty.
2. The fuel tank has old dead gas.
3. Fuel tank bottom contaminated with ethanol sludge, dirt, water, rust, etc.
4. Fuel supply valve/petcock turned off.
5. Fouled spark plugs.
6. Engine flooded as a result of overuse of the enrichner.
7. Vacuum hose to the fuel supply valve/petcock disconnected, broken, cracked, or pinched.
8. Fuel valve/petcock or filter clogged.
9. Fuel line to carburetor or throttle body pinched, kinked or blocked.
10. Carburetor float stuck.
11. Fuel injectors clogged.
12. A severely discharged or damaged battery, you need 12.5 volts or better and be able to pass a load test if necessary.
13. Check battery terminals for damage or corrosion, check the battery cables at "BOTH" ends for loose, corroded, or broken connectors, "INSIDE" and outside the cable harness, perform connector wiggle test and check cables with an ohmmeter if necessary.
14. Loose or corroded wire connection at the coil or plug between ignition sensor and ECM module.
15. Spark plug cables in bad condition and shorting check for spark leakage in the dark, cable connections loose, or connected to the wrong cylinders.
16. Ignition timing incorrect due to a faulty ignition coil, ignition module or MAP, CMP, CKP, O2, TPS, ETP, IAC sensors.
17. Faulty neutral, clutch lever, or kickstand safety switch.
18. Faulty fuel pump or fuse or relay.
19. A stuck bent or burnt valve.
20. Tilt sensor needs a reset.
21. Security system not disarming alarm needs a reset.
22. Check for engine trouble codes.
For more information about your issue and valuable "FREE" downloads that you will need please click on the blue links below. Good luck and have a wonderful day.
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Apr 28, 2017 | 1998 MZ Baghira

1 Answer

Bike wont start, battery good power to bike tries to start but wont, can push start to get running, whats wrong

If you can push start it it may be a low compression problem. Do a compression test, if its low follow with a leakdown test to determine if it is due to valve or piston leakage on a for stroke. If compression is good, try cleaning the carburators.

Feb 01, 2010 | 2001 Yamaha YZF-R1

1 Answer

Have an 1995 xr250r and it won't start. when its cold only push starts, when its hot only starts some times or if its pushed. Thanks any surgestions?

Low compression can cause this. Insufficient valve tappet clearance is the most common cause of low compression on this engine. So, check the compression and, if it is lower that about 130 psi, have the valves adjusted.
Also make sure the kick starter actuated compression release isn't sticking after the kick starter is operated.
Another thing that can cause it to be difficult to start is a plugged up carburetor. If the bike has ever sat for more than about a year, then the carburetor needs to be disassembled and cleaned out.

Nov 15, 2009 | Honda Xr 250 Motorcycles

1 Answer

Looked at an america with very low miles. Great bike but exhaust pipes at head are very blue - more so that i have seen on others with much higher miles. Might this be an indication of overheating at some...

Triumph bikes are well made but the exhaust always suffer and turn blue due to heat .
Now this is common on lots of bikes but the good old twins have a high compression output and i would , just for your own sake , get the compression level out of the specifications and put a gauge on the bike . this is the safest way of telling if the rings are weak and done a lot of miles .
the color of the exhaust may be only telling that the metal thickness may not be adequate or poor chrome workmanship due to chinese metal .

Oct 01, 2009 | 2004 Triumph America

5 Answers

1997 yz 125 wont start, please help

2 stroke bike are very dependent on compression. and being a bike that is 12 years old my best educated guess is that the top end needs to be rebuilt. i usually have to have the top end done on my honda about every three years.

Jan 12, 2009 | 2003 Yamaha YZ 125

1 Answer


hi if its not been started for five years and it is trying to start but just won,t keep running get someone to tow you and jump start it because i suspect the compressions will be low due to a build up of damp carbon valves sticking all in all the seats on the head and valve will not be seating so if you persisist with the jump start it forces the valves onto the seats and will clear the debrie if it still won,t go check your compressions yates210456

Sep 30, 2008 | 2001 Volkswagen Beetle

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