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12 Tips to Help Keep Your 2-Stroke Running Strong


Rob@ProX

We know this is technically 'Thumper'Talk, but we also know a lot of you have 2-strokes in your garage. We want to share some tips we put together that will help you prevent and diagnose potential problems with your 2-stroke engine. Bookmark this maintenance guide to help keep your bike on the track or trail and off the bench! Article by Paul Olesen.

Two-stroke engines have a storied history of being finicky beasts. If you’ve been around two-strokes for any length of time, you’ve probably heard stories that start and end to this effect: “It was running amazing...then the next thing you know - it blew”. We find these stories interesting, and empathize with the unfortunate owners or riders who tell them. At the same time, however, it is often wondered if there were any signs that could have predicted the fateful date with destruction.

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Photo: Steve Cox

We're going to discuss and share a number of observations and diagnostic tests that can be performed to help identify whether or not your engine is going to leave you in the unfortunate role of the broken engine storyteller. While many operators are insistent that their engine gave up without warning, this is often not the case. We’ll start by going over observations that can be made with the engine running, and progress into diagnostic tests that can be routinely made to assess engine health.

Recognizing Symptoms

Startability

Does the engine struggle to start when kicked, but is more prone to coming to life when the electric start is used or when the machine is bump started? Poor starting under normal conditions is not an inclusive sign that the engine is doomed to a spectacular failure, but it is a sign that something is amiss. Carburetion or injection issues are possible, but the bigger potential issue to be aware of resides within the cylinder. 

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Worn piston rings can cause incomplete sealing, resulting in lower compression and more difficulty in starting.

Worn piston rings or reed valves that are no longer sealing properly may be the cause of the poor startability characteristic. When the piston rings don’t seal properly, the engine doesn’t build good compression, so when kicked, the engine struggles to come to life. Similarly, if the reed petals are damaged or broken, less air will be trapped in the cylinder. Push the machine or use electric start, and the compression event is shortened via faster rotational speeds, which may be just enough to bring the engine to life. 

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Damaged or worn out reed petals will allow air to leak out, creating less cylinder pressure and starting/running difficulties.

 

Inconsistent Performance

Does the engine struggle to hold a tune, or seem like the jetting constantly needs attention, despite relatively stable atmospheric conditions? Sporadic running is not always a death sentence, but should be investigated further. A dirty carburetor or worn spark plug can contribute to this behavior, but the problems that can lead to catastrophe are worn engine seals or gaskets. Stator side crank seals, leaking base gaskets, or intake manifold gaskets are all examples of seals that will result in air leaks which can lean out the air fuel ratio. Lean air/fuel ratios when running at full power can result in excessive combustion temperatures, which can melt a hole in the piston or seize them in the cylinder bore.

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Ignoring the possibility of a bad gasket or seal isn't worth the potential damage, especially with the affordability of OEM quality gasket kits from ProX.

 

Gearbox Oil Consumption

Loss of gearbox oil is abnormal, and in all cases should be able to be traced back to leaking seals or gaskets. In the unlikely event the bike tipped over or cartwheeled, gearbox oil can occasionally exit via the gearbox/crankcase breather. If the gearbox is losing oil but the leak path cannot be identified externally, there is a good chance the drive side crankshaft seal is leaking and allowing the gearbox oil to migrate into the crankcase. During the scavenging process the oil is transferred up into the combustion chamber and burned.

Tracking down a leak like this and finding you need new crankshaft seals will commonly turn into a bottom end rebuild job. If you're going to tear into the bottom end to replace seals, that same amount of wear the seal experienced could be evident in other components as well, including your crankshaft and bearings. Not only does ProX make it easier to tackle a bottom end rebuild with our rebuild guide (10 Tips for a Dirt Bike Bottom End Rebuild), but one of the most recent additions to the OEM-quality replacements parts lineup are complete crankshafts. Dropping in a complete ProX crankshaft paired with a main bearing and seal kit is an affordable option for reliable performance.

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ProX crankshafts are assembled with double-forged, Japanese steel connecting rods, as well as Japanese big-end bearings, crank pins, and thrust washers, all manufactured by OEM suppliers.

Excessive Smoke After Warm Up

Since the engine is burning pre-mix oil we have to be careful here, because blueish-white smoke is a normal occurrence of two-stroke engine operation. However, excessive smoke after warm up can be an indicator of a couple problems.

  • Blue smoke exiting the exhaust pipe after the engine has warmed may be a sign that gearbox oil is burning in the combustion chamber. While I would never encouraging sniffing your exhaust, combusted gearbox oil will have a different odor than the normal pre-mix oil the engine is using.
  • White smoke exiting the exhaust pipe after the engine has warmed may be a sign that coolant is burning in the combustion chamber. The root of this problem is typically a leaking cylinder head gasket or o-rings.

Excessive Coolant Exiting the Overflow Tube

While it is common for coolant to exit the overflow tube when the bike has been tipped over or when it has overheated, it should not occur regularly. Coolant blowing out the overflow tube is another good indicator of a leaking head gasket.

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Note where your coolant overflow line runs, so you can keep an eye out for overheating issues.

 

Coolant Weepage

Dribbles of coolant exiting the engine around the coolant pump are indicative of a faulty water pump seal. If left unattended, the entire cooling system will eventually empty, causing overheating and an incredible amount of damage.

Excessive Top End Noise

Isolating top end noise in a two-stroke is easy since the only moving component is the piston assembly. Discerning what is normal takes a trained ear and familiarity with the particular engine in question. However, audible cues often present themselves when components wear or clearances loosen up. The most common noise associated with a two-stroke top end is a “metallic slap”. This is commonly referred to as piston slap, and is a result of the piston rocking back and forth in the cylinder bore as it reciprocates. This phenomena is normal, but the intensity of the slap will increase as the piston skirt and cylinder bore wear. Left unattended, excessive piston slap can result in failure of the piston skirt.

Check out our complete 2-stroke top end rebuild guide here.

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Excessive piston slap can cause damage to the piston and weaken the skirts. It's important to check piston-to-wall clearance when installing a new piston to ensure a long operating life. 2-Stroke pistons fitted with skirt coatings also help reduce friction and operating noise.

 

Diagnostic Checks & Tests

Engine Coolant

Coolant contaminated with black specks can often be traced back to a leaking head gasket or o-rings. Combustion byproducts are forced into the coolant system due to the high pressures in the combustion chamber during the combustion event. These black specks will often float and show themselves as soon as the radiator cap is removed.

Gearbox Oil

The composition of the gearbox oil can provide a lot of clues as to what is happening within the engine. For starters, what color is it and what is in it? Oil that appears milky is a good indicator that moisture is finding its way into the gearbox oil. The most common culprit is a faulty oil side water pump seal.

A keen eye can spot various metallic particles within the oil itself. Aluminum will appear silvery gray. Bronze particles will have a gold shine. Ferrous particles will be dull and are often more discernable by dragging a magnet through the oil. Accumulation of all of these aforementioned particles will be normal in small quantities, but excessive amounts of any of them could be cause for concern. Fortunately, since gearbox and power cylinder lubrication are separate the number of causes for problems is limited and more easily pinpointed.

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Other than changing your gearbox oil regularly, keep an eye out for metallic particles, as those can be a sign of accelerated wear on internal parts.

 

Cylinder Leak Down Testing

While less commonly prescribed on two-stroke engines, performing a cylinder leak down test is by far one of the most definitive diagnostic procedures that can be performed to determine the health of the piston rings, cylinder bore, and cylinder head seal, whether gasket or o-rings. If any of the previously mentioned symptoms are observed, a leak down test is almost always a great next step.

A leak down test pressurizes the engine’s combustion chamber and compares the amount of pressure going into the combustion chamber to the pressure that is retained. Pressurized air is administered via the spark plug hole and two pressure gauges are used to make the comparison. The piston is positioned at top dead center. Air exiting the combustion chamber can then be traced back to the piston rings or cylinder head seal.

Compression Testing

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A compression test can be an tell-tale indicator of the health of your top end components. Be sure to compare your reading the manufacturer's recommended compression measurment.

 

A compression test aims to quantify how much pressure builds during the compression event. A compression tester which is connected to the spark plug hole consists of a pressure gauge and a one way check valve. The engine is kicked repeatedly or turned over a number of times using the starter. The resulting pressure that is recorded can then be used to assess the health of the cylinder bore. Low pressure readings can then be attributed to problematic piston rings or leaking cylinder head seals.

Crankcase Leak Down Testing

A crankcase leak down test is utilized in order to assess the sealing integrity of the crankcase and cylinder. Personally, this is one of my favorite tests to perform because of its ability to isolate a number of potentially problematic seals and gaskets all at once. Components such as crank seals, base gasket, and power valve seals can all be checked to determine if they’re leaking.

In summary, a crankcase leak down test is performed by sealing the intake manifold, exhaust outlet, and any power valve breathers. Then the crankcase is pressurized under low pressure. Typically, the goal is to retain the pressure in the crankcase over a set length of time. Loss of pressure is indicative of leaks, which can then be traced to their cause.

Preemptively replacing components before the engine suffers a major failure is both safer and more affordable than dealing with the problem after the engine has stopped working entirely. Most problems that can occur within the two-stroke engine can be mitigated by servicing components such as pistons, rods, rings, bearings, seals, and crankshafts. Many riders dread the thought of having to service these items due to the excessively high costs associated with OEM or premium aftermarket parts. Fortunately, brands such as ProX offer a comprehensive lineup of OEM-quality components at reasonable prices, many of which are produced by OEM suppliers. Depending on what you need to service, components such as piston kits, connecting rods, crankshafts, bearings, gaskets, and seals can all be found in the ProX catalog.

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Replacing components as part of preventative maintenance can save time and money, especially with the availability of affordable, OEM quality parts.

Find ProX parts for your machine here.

Discussing specific time intervals in regards to when things should be replaced is futile. The reason is simple: different engines, maintenance practices, and applications will all have different intervals. Installing an hour meter on your engine so that you can log the number of hours the engine has run can be one of the most insightful ways to establish maintenance and replacement intervals specific to your engine, riding, and maintenance habits.

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ThumperTalk has a large 2 stroke following.🤙 We'd change the name, but 20 years of branding is tough to change (and probably not too smart either)! 😉

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It’s too bad there isn’t better info out there on leak testing, a leak test on a 2-stroke can be a life-saver but you have to know what you’re doing. 

Edited by Faded
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1 hour ago, Rob@ProX said:

Why do you say that?

Because, go look at all the threads of the Pro-X Pistons grenading.....  Look at all the guys that are unhappy with your products. Then you'll understand...

 

Here's one for you. The rest you can keep searching on your own...

Edited by FLO_Rida

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19 minutes ago, FLO_Rida said:

Because, go look at all the threads of the Pro-X Pistons grenading.....  Look at all the guys that are unhappy with your products. Then you'll understand...

 

Here's one for you. The rest you can keep searching on your own...

If you read through that entire thread, it looks the mechanic admitted the bore was too loose for the size piston that was installed, causing too much piston to wall clearance, and therefore, piston slap. I'm not saying it's never the parts fault. Of course there's going to be some bad pistons that make it out in the market here and there, which we try to minimize as much as possible, but no company is perfect and no one can ever manufacture every single part perfect. However, there are so many other variables at hand with a piston failure that can cause the problem, it can never be assumed one thing is at fault until everything has been considered. 

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5 minutes ago, Rob@ProX said:

If you read through that entire thread, it looks the mechanic admitted the bore was too loose for the size piston that was installed, causing too much piston to wall clearance, and therefore, piston slap. I'm not saying it's never the parts fault. Of course there's going to be some bad pistons that make it out in the market here and there, which we try to minimize as much as possible, but no company is perfect and no one can ever manufacture every single part perfect. However, there are so many other variables at hand with a piston failure that can cause the problem, it can never be assumed one thing is at fault until everything has been considered. 

Very understandable..

 

 

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You make sure you read all the pages, don't assume or jump through....  There are some pretty reputable people saying that your product just isn't cutting it anymore. Sorry if this is news to you!  No offense to your either..  And I am not being sarcastic or rude, just trying to shed some light.  Maybe there was a bad batch who knows?

Edited by FLO_Rida

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12 minutes ago, FLO_Rida said:

You make sure you read all the pages, don't assume or jump through....  There are some pretty reputable people saying that your product just isn't cutting it anymore. Sorry if this is news to you!  No offense to your either..  And I am not being sarcastic or rude, just trying to shed some light.  Maybe there was a bad batch who knows?

No offense taken, we always take feedback seriously! This input won't fall on deaf ears...it will all be put in front of our Product Manager. We take pride in our reputation for quality so we'll put the work in to make sure we can continue!

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Just now, Rob@ProX said:

No offense taken, we always take feedback seriously! This input won't fall on deaf ears...it will all be put in front of our Product Manager. We take pride in our reputation for quality so we'll put the work in to make sure we can continue!

You know what?  Awesome. I'm glad to hear this. Not many companies would say what you just said. Some will argue till blue in the face backing up their product, (which they should) but not seeing the evidence right in front of them. Thanks. 

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1 minute ago, FLO_Rida said:

You know what?  Awesome. I'm glad to hear this. Not many companies would say what you just said. Some will argue till blue in the face backing up their product, (which they should) but not seeing the evidence right in front of them. Thanks. 

Of course. Nobody likes to hear "it's your fault, not ours." And it's not fair to say that without all the facts. It's only fair for all parties to do their due diligence to make sure problems are mitigated, including manufacturing process research on our end.

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14 hours ago, MGSEVENEIGHT said:

I think you'll find ProX is of good quality - aren't they effectively OEM spec anyway? 

Yes they are, usually the only time we vary from OEM spec is when there is a common problem with an OEM design, we aim to implement a fix in our part.

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Thanks Rob.  As you mentioned above, I think most cases of product failure are related to conditions other than the manufacture of the product itself (e.g. piston, conrod).  These engines take extreme abuse and there are many factors at play.  Granted, there will always be the odd outlier, but I have never known people to complain about ProX quality and have been pleased with ProX stuff from my own experience.  One example would be the KDX 220 piston which was prone to cracking a piston skirt (made by ART in Japan) - is this the kind of thing ProX would fix over OEM Spec?

Where do you make the parts? Or do you effectively distribute for OEM manufacturers? 

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On 8/4/2020 at 4:55 PM, MGSEVENEIGHT said:

Thanks Rob.  As you mentioned above, I think most cases of product failure are related to conditions other than the manufacture of the product itself (e.g. piston, conrod).  These engines take extreme abuse and there are many factors at play.  Granted, there will always be the odd outlier, but I have never known people to complain about ProX quality and have been pleased with ProX stuff from my own experience.  One example would be the KDX 220 piston which was prone to cracking a piston skirt (made by ART in Japan) - is this the kind of thing ProX would fix over OEM Spec?

Where do you make the parts? Or do you effectively distribute for OEM manufacturers? 

Sorry for the delayed reply, for some reason our original reply to this didn't post. But yes, if it's a commonly known problem like that, we'll try to tweak the design to address and fix it. Manufacturers for our parts vary by application, but a large majority of our 2-stroke pistons are still made by ART.

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