Why do pistons burn up? Hirth Pistons

[Go Go Diablo]

Every year come late winter/early spring, I get phone calls about pistons burning up in these Hirth motors. Even I get frustrated sometimes because I just don't know what to tell people because it seems they have everything right, yet pistons still give up. So this post is about Pistons and how they burn up!

Where and how do pistons burn? My experience has been that the pistons always get hot on the exhaust side. This makes sense as the cold air and fuel mix is entering the intake side and cooling that piston half. The top of the pistons seem fine, but the exhaust side almost seems to melt like shown.

I am interested in your feedback as we talk about this topic, so please share your thoughts!

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[Go Go Diablo]

First lets talk oil.

The first thing I ask people who inquire about a burnt piston is "What gas to oil mixture are you using?". This question often brings a variety of answers and often leads to another question - "What type of oil are you using?".

The dialog goes something like this:

Me - "What gas to oil mixture are you using?"
The befuddled fella on the other end of the phone - "50 to 1"
Me - "The manuals say 16 to 1, why are you mixing at 50 to 1"
Him - "Because I use Amsoil and the guy selling it says they run go carts with this oil in the summer at 100 to 1 without issue"
Me - "Go back to your Amsoil representative and ask him if he will pay for your new piston and cylinder!"

I'm no lubrication expert. I'm no expert in Amsoil or any other product. What works for me is to stick with something that worked back in the days. Regular 2-cycle oil for air cooled snowmobiles. Not injector oil. These Hirth motors run hot and when it gets above 30 degrees F, they run even hotter. I've been using this oil for years. It states clearly on the back that it is blended for air cooled 2-cycle snowmobiles.

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Regarding the ratio, I mix my gas to oil at 25 to 1 for regular winter driving. If running on a warmer day, richen up the mixture a bit to 20 to 1 or more. The more lubrication the better when running warm.

[Go Go Diablo]

Also before we get too far into this discussion, let's talk motors.

When people are calling me, a guess with little data to back it up is the Hirth 190R motors with electric start burn up far more often than the Hirth 54R motors with manual start. If this is true, I'd suggest some reasons to back it up:

- The motor is significantly larger in which it may hold more heat
- The flywheel fin design may not be pulling as much cooling air through the motor and past the cylinder and head
- There is far more equipment under the hood of a 503 which again may hold more heat or reduce air flow under the hood
- Some 190R motors have larger ports in the cylinder which means they conceivably produce a little more horse power which could also make them run hotter
- Recoils on the motor reduce air flow into the motor

[Go Go Diablo]

Motor seal & gasket conditions also have a big part in the longevity of your motor. I get calls each year from people who get Diablos and the motors have not run in years. Some motors are free and some motors are stuck but coaxed in to turning over with all sorts of techniques and once they turn over, then sometimes people think their motor is good to go.

One challenge these motors have is they lay on their sides with the intake up. Any moisture at all goes down around the piston and lays on the exhaust side of the cylinder and rusts. All I can envision is a layer of rust in the cylinders just got wiped clean by the piston rings and now a owner fires it up and away they go thinking everything is hunky dory.

Another issue the 54R motor have is that one of the seals was installed backwards at the factory. Although it may have sealed for 20 years when it was new, it almost assuredly leaks by after 50 years.

So a motor leaks air and has rust in the rings and right from the get go, odds are against it running for a long time. My suggestion to everyone is not to be afraid to pull your motor apart and do a little preventative maintenance to it. Hone that cylinder clean, replace gaskets and seals, and then pressure test the crankcase to make sure the motor is leak free.

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[Go Go Diablo]

The last thing I can think of regarding motors is to discuss the cylinder and head and the concept of coating or painting them.

I've seen some motors restored with the cylinders painted or clear coat put on the cylinder and head to make them shiny clean and pretty. When I see this, I wonder if this paint or clear coat acts as an insulator reducing the ability of the cooling fins to dissipate the heat.

This is a cylinder head that had been clear coated. It came off a motor that melted the exhaust side of the piston. It looks like the clear coating darkened and actually tarnished due to the heat.

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Anybody have an opinion?

I like to sandblast the cylinder and cylinder head so they are squeaky clean. I can't help if they rust or oxidize, but I am sure they give the fins the best chance to remove heat from the motor.

[BillNH]

I don't see any reason to paint a cylinder head, it is just a coating of insulation. If your sled is going to be a display model, and only ride from trailer to show line and back, paint away, it will look good. Very few ,VERY, air cooled 2 strokes come from the factory with painted heads or cylinders, it defeats the ability of the cooling fins to transfer heat in the limited air flow available. If someone is running a go-cart, etc, in the summer, they are going faster than us, moving a lot more air, and probably don't have a close fitting hood. I try not to run an air cooled sled at much above freezing as I have had problems with this issue in the past. Modern Fan cooled sleds are a different animal.

[Go Go Diablo]

Thank you for those comments Bill!

Lets talk carbs for a bit.

The typical setting on the HR carbs on the Hirth is the low speed out 3/4 turn and the high speed out 1 to 1-1/4 turn. For most temperatures, the high speed out 1-1/4 turns is too rich and the motor will not "run out" as people say. Not sure how to describe that, but when the high speed jet is tweeked in ever so slightly inward at the right spot, the motor just tends to rev up like you would expect it too and that is where you typically run the setting at.

During cold temperatures, that might be as little as 3/4 turn out for the high speed jet which is pretty lean. I've never had someone call me and say "I burned my piston driving around in 10 degree F temperatures", and even I have run my Diablo pretty hard in colder temperatures without issue.

But when it warms up, you have to be thinking about this high speed setting and back it out a bit. Yes, you will sacrifice performance, but hopefully it is one more tool in your toolbox to stop your piston from burning up.

Again, I am interested in what others think about this high speed jet setting.

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[Barrie2777]

I'll never forget the words of Henriot Filion "it is the gas drawn into the cylinder which keeps the piston cool". Henriot was a long time employee of Farmers Supply and was in charge of the service department for husski. Since most 252 jlo have a fixed high jet, husski riders take the rich mixture and live with it. I can not recall ever hearing of a burnt jlo piston. I agree that your burnt piston problem is too lean a mixture. More oil won't save the piston....and avoid riding for a prolonged time in plus temps.

[BillNH]

Oddly enough, the only engine I ever burned the piston in was a JLO 292, in a 69 Arctic Cat. It was about 40F outside, and I was doing hi speed runs across a long field. Young and Dumb come to mind. It did melt the entire exhaust side of the piston. I rode with others who have burned pistons in several brands of engines, especially the single cylinders that are cooled by the fan blades on the flywheel. This style of engine just does not move enough air, in an enclosed area, to cool the thing in warmer temps.

[Go Go Diablo]

Barrie and Bill - I appreciate the comments.

I too have been told that the gas/air pulled into the intake cool the piston. One would think the intake side of the piston is cooler than the exhaust side and that makes me wonder about clearances or the lack of when pistons get hot. How can the piston possibly stay round with heat on one side and cold on the other?

I would not necessarily say that more oil will save the piston, but I certainly would say that not enough oil will mean not enough lubrication and thus wear and tear on the piston. I go back to the original Diablo Owners Manual saying gas/oil mix should be 16:1. Even I do not run that rich, but if it is warm outside, maybe we should be richening up our mixtures.

One thing I thought about this weekend was motor orientation. The Hirth Motors are on their side in a Diablo. The weight of the piston and connecting rod is down toward the exhaust. Still yet another reason why they could be running hot on the exhaust side.